Condensation @ screw tip – exterior foam
I have a retrofit to do next week, and I am bothered about condensation on the tips of screws. Two parts that concern me are…
Adding two x 2″ exterior XPS, with Tyvek (or left over GAF Deck Armor — 16 perms) held under 1x furrings.
Adding a TPO roof membrane over 3.75″ of polyisocyanurate (2″ +1.5″ lapped + 1/4″ high compressive strength)
In each case the interior finish cannot be damaged.
My concern in both cases is where the screws penetrate the sheathing they present a condensation risk.
Since I cannot damage the interior I cannot cap any screws that miss the studs. In the case of the wall maybe I could back out screws that feel like they missed the studs, but I don’t view that as reliable. In the case of the roof there needs to be a pattern for wind uplift requirements that means it is not possible to hit the beams even 25% of the time. The roof has a mitigator in that 1/4″ of high compression strength polyiso will be adhered on top putting R1 between the cold and the screw heads.
The climate is Long Island — 5600 heating degree days, average low 20F.
I’m still worried about condensation on the screw tips.
I am considering fully adhering the roof (more work more cost and no non-VOC adhesive available).
I am considering backing out screws that miss wall studs, and/or fitting 1×4 or 1×6 at 16″ oc over the sheathing/studs to present a bigger target and only penetrating 0.5″ of the 0.75 sheathing (for 1.5″ pullout total), and/or using less screws and more glue (more messy on vertical surface and more work).
I am not happy with any of these ideas.
So I’m looking for advise.
Replies
what is the interior finish???
is there cavity insulation???
how proud will the screw tips be???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
The wall interior finish is sheetrock. The walls are 2x4 with fiberglass batts (only about R7) and then AirCrete pumped in (a cementeous foam).The roof interior finish is sheetrock, with fiberglass batts in between 2x6 rafters. But I would like to put dense pack cellulose in there. The screw tips would be at least 1/4" proud, but could be 3/4" proud.
so how do you figure that there would be threat from condensation on the screw tips???
I did read that correctly didn't I???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
did you you know that filling out yur profile some is a plus???
and... BTW... welcome to BT...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
how much condensation are you expecting???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
1) Why condensation... The screws are conductive, thus when the head of the screw is exposed to say 20F (average low temp in Jan here), the tip will also get cold. If the tip of the screw falls below the dew point of the vapor in the warmer air just inside the sheathing the vapor will condense (just like in hot humid summer weather when condensation occurs on cold water pipes).2) How much condensation -- if it causes enough to get mold that's too much. Mold can grow in very isolated/small areas, so I presume that localized condensation on screw tips could be a problem.3) Profile filled out.
so the heads of the screws are exposed to the weather???
I believe we need to hear from others here... it'll be several hours before they begin drifting in...
with a bazillions and bazillions of installed fasteners there are out there that have thru penitration.... this is the first time I've ever heard of this being a concern... never saw it to be an issue either...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
In the case of the wall the heads are exposed to the cold. They hold the furrings on (under vinyl siding).In the case of the roof, the heads are protected by 1/4" of R1 material glued on top before the membrane. But R1 is not much, especially with snow siting on top sinking the heat out.I had been pointed to this:
http://sustainable.cchrc.org/docs/REMOTE_Manual.pdf
Page 36 shows heads being covered, but this is with the interior open (I cannot damage the interior sheetrock). However, the link is also for a colder climate.
Mark,
Welcome to Breaktime... It is possible that the concern is more of a problem in the EXTREME climates...
Thorsten Chlupp was involved in the document referenced....
He may have some insight.... I will contact him and see if he wants to comment.
I believe on the Mass retrofit discussed at GBA that the interior walls were not opened up and at least they (BSC)had no concerns.
John Brooks
I am hoping that my climate 5600 heating degree days and 20F average low is my mitigating factor here. But I don't know of a good way to assess this.The way I see it, if screws miss the studs, that basically brings in tiny spots at only a little above 20F of highly conductive material (steel) which should cause condensation.If you are able to ask Thorsten Chlupp or any folks at BSC that would be great. Work starts on Monday, and we'll get to this maybe Wednesday, I don't have long to get the final screws purchased and details worked out. But even if it misses my date there will be a record here for others to find.http://www.greenbuildingadvisor.com/homes/old-house-gets-superinsulation-retrofit
Says
Walls: existing cellulose-filled 2x4 studs; 4-in. of foil-faced polyisocyanurate foam outside of sheathing(R-39)So the implication is it was not a problem - or it was a problem resolved with an undocumented detail; like ensuring only hitting studs which I don't think is reliable.
Mark,
The roof condition was solved with the spray foam....
So the implication is it was not a problem - or it was a problem resolved with an undocumented detail; like ensuring only hitting studs which I don't think is reliable.
Or Noboby considered it !
"What me worry" AE Neuman
My guess is that if you make a good effort to hit the structure...the rest will not be significant...just a WAguess....wires could be a problem too ;-)..try to hit the studs
I do not have a contact at BSC .. but sometimes Joe will answer your questions...sometimes not
maybe you could Contact Alex (the retro owner)... he seems friendly
he was all over that job.
For there to be condensation, there needs to be moisture. Most moisture comes from humidity of the infiltrating air into the cavity. If you seal the outside with foam board, there will no air movement, so very little moisture penetration, particularly with additional cavity insulation. Further, the cellulose will absorb some of the moisture and let it seep back to the interior, as long as it stays warm and you don't have an impermeable inner surface.The amount of cooling will be very low. You seem to be worried about an occasional miss. With the little bit of moisture that could possibly reach the errant screw, there couldn't possibly be much condensation, which would probably migrate away and be immediately warmed and revaporized.The problem with moisture is when the humidity condenses and drips away in a leaky area, so the next round of moist air is ready to release more condensation. Moist air will not reach your sheathing unless you don't seal the foam board.---mike...
Infiltratring air (i.e. air from cold outside) is not humid as it warms up. As infiltrating air enters e.g. a wall cavity, the temperature goes up (of the air). As it does, the relative humidity drops significantly, therefore, there is no moisture problem. It's the warm moist air in the house that enters the cavity (exfiltration) that is the potential problem. If that air is allowed to hit a cold surface before the humidity diffuses (like it does in a well ventilated attic), then condensation can occur.
You've got the concept all mixed up. Moisture moves from the warm interior to the cold exterior. Even when it is cold and raining outside, the humidity drops as it warms up, and as it does so, there is less and less chance that it will condense (as it needs a cold surface to condense on).
Yeah, up here in the Frozen North I've never heard of this being an issue.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
or here...
interesting thing is kraft paper barrier and vynal siding exterior...
this is a set up...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
Think how many vinyl jobs have been done by ham-fisted guys using 10d commons to hang it. Yet I've never heard of this being a problem.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
don't need to...
2" roofers thru sheathing...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
so were we had???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
I think he is worried about the wrong issue.
In a relatively mild. Long Island, climate. I would be more worried about insects and foam on the exterior than the possible minute quantities of moisture on an occassional missed fastener.
But what do I know, I live in a harsher climate like Danh and have never seen the problem he is alluding to!
ditto on the alluding...
harsh here to..
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
While I suspect you are right I don't KNOW that you are right.This set me thinking ....
I looked up thermal conductivity of steel, and it is more than an order of magnitude less than copper, but nearly two orders more than wood.So what would really satisfy me....
1) 3D thermal analysis of a steel screw in 4" of XPS with a 40F temperature gradient
2) Experimental data
3) Data from houses build this way, where moisture sensors were located close to screws.I'm not likely to get either by tomorrow.
So here is what I did...My own less than scientific experiment, but enough to give a good indication...
1) Put a 4" steel screw across two cups in my kitchen, fill one cup with ice water (~32F), lean head of the screw into ice water.
2) Put a second 4" steel screw into 3" of XPS with the head in a cup of ice water and the tip protruding 1/4".
3) Cook leftover turkey soup in kitchen with the extractor fan on low, and within 8' of screws. This is a worst case humidity level, and maybe unrealistic, but the roof in question is in a kitchen area. Note it is important that the soup is turkey soup.
4) Eat soup.
5) Go back and check screw tips for condensation and second cup for any drips.So far using an infrared temperature probe I can clearly see a temperature gradient in the screw without XPS. Also I can see condensation within 2" of the head, but there is only a glossy sheen of water, growing to nearly enough to form 1 drip after an hour. Whereas the outside of the ceramic cup with the 32F water is soaked. After nearly 2 hours the condensation is over 3" of this screw, but still not enough for 1 drip to form and hard to see in the 3rd inch. This appears to be a steady state.The tip of the screw embedded in 3" of XPS has clearly cooled, but is much closer to the temperature on the warm side of the XPS than the cold side (measured with temp probe) it has no condensation so far on the tip.Here is what I think based on what I've read in the many replies and my kitchen counter experiment.* In my climate (Long Island average low 20F in Jan) this might be a problem if only 2" of XPS is used.* With 4" of XPS it should not be a problem in this climate, especially in non kitchen / bathroom areas.* In an extreme climate like Alaska it might be a problem if only 4" of XPS was used. Thus the suggestion that I referenced ( http://sustainable.cchrc.org/docs/REMOTE_Manual.pdf - p36 ) of capping the protruding screw tips, if you have access to interior of sheathing, this is a good idea in Alaska.Thanks to all those that chimed in.- Mark
here in Colorado it gets a lot colder than -20...
and what yur worried about has never shown it's self to be an issue.. we deliberately add humidity during the winter to the point of near saturation... we use visible condensation as the the indicator of "too high" and dial back from there...
so don't what to tell ya otherwise...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
see my posts for clarification. I've no idea how this concept got mixed up and why some of the 'old cogers' didn't step in to discuss this. It looks like maybe your experiment verified what I said.
It only takes a psychrometric chart to show one what happens in this case. Once you understand that and walk through it a bit, it becomes clear, although a psch chart isn't the most simple thing to use, it can show the 2-3 properties of air clearly if you understand how those 3 parts work (i.e. sensible temperature, relative humidity, and dew point).
It will only be a problem if you are going to boil turkey soup INSIDE the wall cavities where the tips of these screws reside! If you build the house right, there will never be enough moisture in contact with them to be able to frost up.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
I agree with Piffin:
> It will only be a problem if you are going to boil turkey soup INSIDE the wall cavities where the tips of these screws reside!
The experiment was flawed in two respects. Saturated air will not be blowing across the screw tips. You have stated that the wall cavities are filled with insulation. If there were condensation on the screw tips, the local humidity will be high, and moisture would want to migrate away from, not toward, the few errant screws.
Secondly, not in your favor, the insulation in the cavity will drop the temperature at the screw tips because you will not have as much warming from the interior. The interior humidity level which would cause condensation at the sheathing is much higher in an uninsulated cavity with exterior insulation than it would be with more cavity insulation.
Lastly, you sound like an engineer in another field overdesigning the details of your wall. Go to buildingscienceconsulting.com and search the site for Research Report RR-0410 Vapor Barriers and Wall Design by Lstiburek. It has thermal formulas you can play with and examples of different wall compositions.
---mike...
Coastal Construction had a series of photos of moisture damage caused by poor construction techniques last year. One of the photos was of a plywood chimney chase from the inside. Staples had been used to install the stucco lath and tips of all those staples were dripping water from condensation. It was quite mess.
exposed staples to elements???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
Read it again. The lath was stapled in place. Then stucco over that.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
that's a common way to install lath..
use a roofing stapler...
now read what I said again...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
you said... Staples had been used to install the stucco lath and tips of all those staples were dripping water from condensation. It was quite mess.
I said... exposed staples to elements???
so those exposed staples (tips) were open to the elements???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
They didn't show the outside of the chase but would assume from the context that the chimney had stucco applied and this problem cropped up later.
The staple tips were inside the wall. Not what I'd consider "exposed to the elements".
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
not wall...
chimvey chase....
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
Right, but the inside is not "exposed to the weather".
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
chases are generally unconditioned and internally they suffer the swings in weather short of percipitation..
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
Right, but they're not "exposed to the weather", any more than the inside of a barn is.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
so now were doing a screw check on barns???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
Did you see the article and photos? Not my area of expertise, might be my area of stupid even but wouldn't the inside and outside of a chase like that be pretty close to the same temperature and humidity? Where did the condensation come from? Heat loss from the living area?
Nope, didn't see the article. A situation like that could have a lot of variables -- leakage from above, water soaking through the stucco, moisture from the house, moisture from the smoke, moisture evaporating out of soil below. I don't think it would be a situation that one could draw any general conclusions from.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
and it's still not like the wall of a conditioned space..
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
But it's not "exposed to the elements".
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
about the only element it isn't exposed to is direct percipitation...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
The point of the picture was to not use so many staples as they could become focal points for condensation. He apparently saw it as a common problem.
That is actually the wrong way of 'solving' that kind of problem.
The thing to do is to build the cchase properly and control the moisture gatting into it from living space.A wood chase should be insulated like any other exterior wall of the house, but is often ignored in that respect, then the joints between the metal appliance and the trim insoide are not sealed, so you end up with a high draft situation sucking warm moist air up into a cold space with colder exterior surfaces. The moisture in the air WILL condense someplace!So removing staple tips only means it will condense on something else.The solution is the same with any other condensation problem in a house, control heat, insulation, and air flow.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
At my son's previous house in Colorado we could hear a booming sound every now and then. I finally traced it to the galvanized cap on their chimney chase moving when the wind blew over it. Probably 35' feet high on the outside wall and not a drop of insulation anywhere. This was a pretty high end house built by a builder who has built thousands of homes in the area. I'll bet everyone of them is built without insulation in the chase.
which builde???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
I'm having black out. Let me see if I have their website bookmarked. This was 5 or 6 years ago and that's heavy lifting for my brain.
take yur time...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
I insulated the chase, but I wonder about it. Because it's near a chimney, you can't have flammable materials exposed. Insulation was mineral wool, but no internal vapor barrier. Air barrier would then be external only, plywood sheathing and Tyvek.But because the chase is walled off from the main room, it will be cooler than room temp, therefore, more likely to reach a condensation temp in the bays. It's really hard to seal off the chase from the interior space, so moist air will get in.I'm about to put on a chase cap and overmantel. Should I remove the mineral wool, lay down a vapor barrier in the bays, then re-install the mineral wool? The idea would be to keep moist air from getting to the sheathing, and let moisture breathe back to the interior. And if the moisture doesn't dry, at least the sheathing will be protected, and maybe I'd only need to replace the insulation.thoughts? 6 mil poly?and while I'm asking... the chase cap would be one layer of metal on top of the open chase. It's an attic fireplace, so no between-floor blocking. I was thinking about adding a metal "firestop" a few inches from the top of the chase to put at least one airspace buffer between the chase cap and the rest of the chase cavity.any other ideas?thanks,
---mike...
no- do not remove the mineral wool insulation to do a VB. The VB belongs to the warm side of the insulationNow I don't know whay you say it is hard to seal the chase offfrom migration of interior air.As for insulating at the cap/cover, what sort of chimney pipe/flue goes through this?I also have a hard tiome picturing what is meant when you say that this chase is near a chimney. A chase IS a sort of a chimney. Is there an existing masonry chimney and you built a wooden chase near it?
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Piffin suggested:
> no- do not remove the mineral wool insulation to do a VB. The VB belongs to the warm side of the insulation
But what kind of VB can go inside the chase? I don't think I can put anything flammable, in case it comes loose and lays against the metal chimney. The insulation guys used mineral wool in the chase, but left it exposed to the interior. I don't think exposed mineral wool or FG does very much, since it's exposed to a large cavity and it won't stop air flow. (Building inspector liked mineral wool; specified for between-floor penetrations.)
> Now I don't know why you say it is hard to seal the chase off from migration of interior air.
It's a manufactured, zero-clearance, wood-burning fireplace set into a wood-frame chase. I don't think the firebox itself is vapor-proof or that well sealed. Perhaps I can caulk the sides and top, but I guess I'd have to fashion a thin strip at the base in order to be able to caulk that edge. The overmantel was going to be board and batten, possibly removable for A/V equipment. Then there's a side portion that eats into a closet for the external air supply, and which I'd have to seal from the outside.
> As for insulating at the cap/cover, what sort of chimney pipe/flue goes through this?
I also have a hard tiome picturing what is meant when you say that this chase is near a chimney. A chase IS a sort of a chimney. Is there an existing masonry chimney and you built a wooden chase near it?
There is a double-wall galvy chimney out of the top of the fireplace unit. There is a wood-frame chase around it with somewhat of an airspace, but not too big. The steel chimney pokes through the chase near the top, with a factory chimney cap on top.
---mike...
So you have a class A mineral packed metal flue AKA Metalbestos type. You can have combustibles within 2" of that.
You wood framing is a combustible.Here you are ultra concerned and worried that the screw tips could attract condensation but you can't get a vb fastened well enough to stay in place????At the cap, use plywood sheathing, then 2" EPS foam to within 2" if the pipe, then the metal cap. You could do mineral wool or a FS spacer right at the pipe too to slow draftiong there. This is a ploace that comonly does have condensation problems. Often appears to be a leak at the storm collar above.Some places require gypsum bd firestop lining the interior of the chase all the way up. You could do that over a VB on those walls to alleviate your concerns about it coming loose.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Piffin:
> So you have a class A mineral packed metal flue AKA Metalbestos type. You can have combustibles within 2" of that.
Actually, vented air gap between the double metal walls - no insulation between. But I believe the 2" still applies.
> Here you are ultra concerned and worried that the screw tips could attract condensation but you can't get a vb fastened well enough to stay in place????
Screw tip condensation was the OP; I agreed that shouldn't be a problem. Then someone turned to staples in a chase, and I further hijacked the thread with my questions.
> At the cap, use plywood sheathing, then 2" EPS foam to within 2" of the pipe, then the metal cap. You could do mineral wool or a FS spacer right at the pipe too to slow drafting there.
I'd think the plywood and foam wouldn't do much with a 2" gap around the pipe, unless it was sealed against the metal cap. What do you mean by a firestop spacer? A sheet metal donut attached to the plywood that goes tight to the chimney pipe, or a full metal plate from walls of chase to the pipe (which is what I was thinking originally)? Can I lay EPS (not XPS?) on the metal, thereby insulating all but the 2" around the chimney? It would be easier to make an insulation sandwich below the top of the chase, then attach a simple metal cap over the sealed area. You suggested mineral wool can touch the chimney pipe?
I must say I don't get the 2" rule. You can put metal against the chimney, then attach that metal to combustibles within 2". Won't the metal conduct the heat to the combustibles, bridging the 2" spacing?
> Some places require gypsum bd firestop lining the interior of the chase all the way up. You could do that over a VB on those walls to alleviate your concerns about it coming loose.
It's somewhat hard to maneuver in the chase, which is close to the chimney. There might only be 2" clearance at the diameter of the pipe; I'll need to check. What VB would you suggest? Poly? Kraft paper? Paint the gypsum before putting it up? (Isn't the paper face of gypsum combustible? Does that matter?)
I'm just wondering if the code put the 2" rule because it sounded reasonably good, rather than for good sound reasoning.
thanks,
---mike...
There was a lot of stuff burning up this thread - so I didn't keep track of the characters and that you had done a hijack. So I might still be confused, and ain't gonna read back thru all sixty some posts to clarify who is who."I'd think the plywood and foam wouldn't do much with a 2" gap around the pipe"But it will. The only place left for condensation to appear is immediately around the pipe, and that will be kept warm enough to prevent it when the unit is in operation.Now about that pipe - either you have described it wrong, or you have the wrong pipe. There are two kinds of clas A metal pipe. One is double walls and mineral packed between for a solid insulation situation.The other is a triple walled that employs air movement in opposite directions in the two different chambers to keep the outside of the pipe cool and safe.If yours is only two walls with no separation between them, it is class B pipe and not suitable for solid fuel burning appliances.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
@Piffin
> > "I'd think the plywood and foam wouldn't do much with a 2" gap around the pipe"
>
> But it will. The only place left for condensation to appear is immediately around the pipe, and that will be kept warm enough to prevent it when the unit is in operation.
I guess that assumes the chase cap is sealed to the plywood. My concern is that most warm air will find its way into the chase and condense inside, either walls or cap. I didn't know much about VB's etc. when the chase was built.
What VB should I use? It's hard to get into the chase now, with the chimney installed. The original manual didn't mention insulating the chase at all, and now the new version suggests vapor barriers, insulation and taped drywall.
I was thinking about putting up 1/4" Hardibacker over the exposed unfaced mineral wool (which seems would have little actual value on its own. Or maybe it would just breathe so much that there won't be moisture problems, even if it doesn't provide much insulating value. Could I just paint the Hardibacker as a VB (or would it be too hot for it?) or should I put a poly or Kraft paper or housewrap on the studs before the Hardibacker?
> Now about that pipe - either you have described it wrong, or you have the wrong pipe.
I have SL1100 pipe as specified by the manufacturer - Heatilator. I believe it's a two-wall, flue/liner with an air gap, which apparently is not up to Canadian code. At the base there are vents around which there is a box with a flange for 4" tubing which connects to an exterior cold air vent.
BTW, how should I seal that box to the firebox? Is there a certain high temperature caulk? I think there was some metalized tape used to seal the chimney sections; is there a certain high temperature metal tape?
thanks,
---mike...
There is a GE silicone rated for high temps up to 2000°FWithout pictures I'm having a hard time keeping up on all this one. The flue is installed, but not the firebox???
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>The flue is installed, but not the firebox???The metal chimney pipe (double-walled, flue+liner) is connected to the top of what I'm calling the "firebox," the main ZC, manufactured, metal fireplace which is installed in the chase. The chimney goes up through a wooden chase for most of the way, with a small amount of pipe exposed. The top of the chase is plywood with a tarp over the unfinished top. The inside of the chase has unfaced mineral wool in the stud bays.Still to be done as part of the base installation:
- on top of the firebox, the chimney cold air vent assembly needs to be secured to the top of the box.
- the metal chase cover needs to be fabricated and installed.
- the chimney cap needs to be installed
- finish work on the interior: mantel, surround, overmantel, etc.Concerns:
- exposed bays in the chase
- no VB for the mineral wool batts
- chimney air vent system should be sealed or cold air will be seeping into the chase, and moist warm air could be drawn into the chase to escape through the leaky areas.
- moisture in warm air in the chase would rise, hit the single-layer cold metal chase cap, and condense
- convection loop created in the chase
- leaks would prevent the chase from warming sufficiently for conditioned space and would cause moisture to be drawn into the chase by exfiltration where it could condense anywhere in the cool chase.
- I've completely hijacked this thread. :-)---mike...
I am downloading the heatilator install manual to check some thingsHow tall is this chase and what are the interior dimensions of it? And to further clarify, is the chase continuous from top of firebox unit to the top outsde/above the roof?Is there a firestop spacer or insulation shield at the ceiling level?
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Edited 9/21/2009 3:24 pm ET by Piffin
Thanks Piffin,It's an Icon60 model installed in the 3rd floor attic, so the ceiling is the roof. The chase is at one cheek of a gable dormer with ceiling/roof height of about 7' (I think). The chase necks down past the firebox to a square with interior dimensions to just give 2" clearance at the widest parts of the chimney pipe. That should be 13" + (2 x 2") = 17" internal. 17" + (2 x 3-1/2" studs) + (2 x 1/2" sheathing) + (2 x 3/4" trim) = 26-1/2" exterior.There is no firestop, as it was in the attic - it's a clear shot to the top of the chase. The height of the chase is the minimum recommended by the installation manual in terms of total height, which also met height over nearest roof surface, but I don't have the dimensions handy now. Actually, I think it might be a little short; overall height, not straight section of pipe, was 18' I think, but that seems more than I recall. I think we were counting on being in the attic (sealed with spray foam walls and roof) to have hot air from the rest of the house rising and pushing out the chimney to prevent downdrafts.---mike...
Well, this gets more unigue with every post!
;)Height of this thing
Do you realize that in addition to height above roof and ridge line, this calls for minimum of a total 18'? That probably inccludes the appliance itself, because the generic standard is for 14' of pipe above the stove or FP.This is not necessarily an absolute, but if you have less, it is more likely that you will have poor drafting/draw, and occasionally downdraft in some conditions.The location at cheek of a dormer sounds like this chase is located at the bottom of a roof valley for the dormer, or in the valley itself? That makes it hard to gaurantee against leaks if so.Back to the original Q about sealing against air and vapour leaks...You obviously don't have room to get anything IN the chase over the insulation. Type X fire rock would have been nice, mudded and painted with a oil base sealing paint.Now, the best choice IMO, if I understand all this right now, is to fashion a fire stop spacer in two U-shaped pieces of metal to fasten to bottom of the rafters around the pipe, and caulk that. Do NOT caulk the pipe to the chase cap above. Caulk out there to the storm collar onlyThis IS a triple wall pipe, BTW, you just don't get to see the inner wall very easily. They make the screws in joints appear optional. I would definitely use them on the pipe.
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Piffin noted:
> Do you realize that in addition to height above roof and ridge line, this calls for minimum of a total 18'?Yeah; the framer thought that was more than necessary, and it would have been really high since it's in the attic - about 10 feet above the roof line. [However, after things were underway, I realized the project lead was semi-brainless and couldn't handle anything slightly unusual.] I measured the chimney/firebox: 11-1/2" of chimney, 3 feet inside the firebox.I read part of the reason for the long chimney is that when fireplaces are on lower floors of old houses, heated air leaks out of the upper floors and is replaced by cooler air from the bottom floors, and an open chimney is a low resistance path for cool air to enter the bottom floor as the fire dies down. Since the attic fireplace is at the top of the house, and the attic is sealed with spray foam, the chimney would be a low resistance path for my expensively conditioned air to leak out when the fireplace is in operation.For the firestop, you suggested placing it at the rafter level because of code, or to keep the the chase warm below it and cool above it, or because it's about halfway, or to keep the sealed area from being too small, or another reason? It might be easier to access from the top of the chase, maybe a foot down. I could then add some material like 1/4" Hardibacker to the sides of the mineral wool to stop convection flows through it to the top of the chase. It seemed you were suggesting caulking the firestop to the chimney pipe, right?Regarding your question about leaks, do you mean water dripping down the roof and getting caught on the edge of the chase? Then yes, the original framer didn't understand how water flows. [Like when he put a tarp over the roof opening for the dormer before a storm, he tacked the folds in the tarp to catch water and direct it toward the unsupported opening.] The chase is halfway down the dormer cheek. I'm having the siding/carpentry crew add a triangle on the upper side leaning against the cheek and chase walls to direct water past the chase and down the main roof plane.Oh, to another poster. Part of the reason for this type of fireplace, in addition to the looks, sounds, smell, and feel (moving warm logs around) was for back-up heat if the power goes out. I was wondering how the plasma screen would generate warmth in that case. Burning plastic inside the house doesn't seem like a good idea. And why would I spend money to get a TV anyway, much less a really expensive one? Or are you one of those looks-trumps-reality people?---mike...
I never heard what you repeated about the draft and long chimney situation, so I don't know if there is anything to that or not.But I have installed over a thousand wood stoves ( used to have a stove store) and I know that in some cases, a short chimney will not draw.Since yours is high, you are probably OK, depending on prevailing wind direction and trees or other surrounding structures that might effect air movement etc.But I would want a test fire with that height in that install before finishing all the rest of the chase out.The reason I suggest the firestop spacer in that location is to act as a VB to keep moisture from rising into the chase - that was your original primary Q. as I recall.
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> The reason I suggest the firestop spacer in that location is to act as a VB to keep moisture from rising into the chase...Hmm... good point. I could seal the chase walls below the FS spacer and hopefully keep moisture out of the upper part of the chase, and then I wouldn't need a VB for the mineral wool. Probably wouldn't want a VB - just let it all breathe above the FS.Just checking - it's okay to seal the FS spacer to the chimney. You originally said to leave a 2" gap around the pipe (with plywood and EPS) and it would be okay.Good point about checking before wrapping up. I think I'll leave the chimney cap off until the test so I could add another SL1100 segment if needed.thanks,
---mike...
You leave any combustible materials back 2" from the pipe, but a piece of metal is not combustible on this planet - not yet anyways.OK to caulk it to the pipe near the stove like this.It is up high at top of the chase that you do not want it caulked d=tight to any stationary portion. The reason is that if it expands taller from heat, and then cannot shrink back down again, it will pull itself apart at one of the joints or lift off the top of the fireplace which is considered mis behaving and cantankerous
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It's a manufactured, zero-clearance, wood-burning fireplace set into a wood-frame chase. I don't think the firebox itself is vapor-proof or that well sealed.
Warning...Fireplace Rant...
Most (almost all)fireplaces are such energy hogs....
This is another reason just to avoid Fireplaces and CHIMNEYS all together....
It is like puncturing your roof with a leaky culvert that adds an incredible amount of surface area to the thermal envelope.
Even when the fire is burning ... wasted energy...
365 days a year wasted Energy.....Air leaks and thermal leakage galore.
Get one of those DVD's of burning logs...Use the Plasma TV to heat your house...at least when you turn it off..... it is not wasting as much energy as the CHIMNEY
Edited 9/19/2009 1:57 pm by homedesign
@homedesign
> Get one of those DVD's of burning logs...Use the Plasma TV to heat your house
Yeah, we installed a wood burning fireplace because we think the convenience of a video of a fire would be better than a fake fire from a gas log, in addition to being safer, healthier, and less carbon emitting. Thank you for thinking of things you assume we didn't agonize over before making our decision. Sometimes, you just need to do certain things.
At the start of the project, I rationalized that since we tighted the whole attic with high density spray foam, the chimney could be part of the controlled fresh air exchange. It was a little before HRV's were hitting their stride.
Our bungalow is missing the living room fireplace (you can see where it was supposed to go) because the plot is too narrow and the chimney would prevent full access to the backyard garage. Bungalows need fireplaces to be complete.
---mike...
So. there is no vp on the warm side of either roof or walls?
What about a plan to reduce interior moisture levels?
The warm side has paper/kraft facing on the fiberglass batts.
But I don't like to rely on that. It is just too easy for the vapor retarder to have gaps, and I can be 100% sure there are gaps.In the case of the roof, the top membrane in impermeable. So I had thought of ripping out kraft faces batts between the 2x6 rafters, and put dense pack cellulose in there to make more breathable to interior.
You need to be commended for trying to build the perfect system.
The easy answer is don't miss. Snap lines where you need to screw or nail.
Can you explain this sentence; In the case of the roof there needs to be a pattern for wind uplift requirements that means it is not possible to hit the beams even 25% of the time. What good is a fastener doing if it does not hit the substrate? Are they holding the finished roof to the sheathing?
Mike
Small wheel turn by the fire and rod, big wheel turn by the grace of god.
Ruff Mike, that's exactly it. The screws hold to the roof deck (in this case 3/4" T&G).Here is an example screw pattern:
http://www.specjm.com/files/pdf/4x4_Roof_Insulation_Fastening_Placement.pdfSo it is like a chain:Rafters <--> deck (with nails) <--> polyiso (with screws) <--> membrane underlay R1 (with glue) <--> membrane (with glue)The weakest link is holding the deck to the rafters with nails.
And a chain is only as good as ......?
So I don't really understand when the screw patterns become more dense than the nails holding the deck. But I guess it depends on the underlying roof/deck.
I may be missing something, but I'm not 100% sure why you are concerned about the dewpoint.
On the roof, you have ~R24 outside of the sheathing and R20 on the inside.
On the wall, you have R20 outside of the sheathing and ~R17 (R7 for the FG plus R10 for the foam?).
Is this right so far?
If so, I would be inclined to think that the temperature of the fasteners poking through the sheathing would be above the dewpoint.
I used an excel spreadsheet I made that references a formula from a Building Science paper. I have attached the spreadsheet below along with the BS paper and the pdf of the roof and wall dewpoint analysis.
I would not bet the farm that my spreadsheet is 100% right, but I think I wrote the formulas correctly. Of course, if anyone finds a mistake, let me know.
Edit- I did not attach the Building Science paper to keep my attachment size down. You can view the HTML version here, or the pdf version of the file here. The formula can be found on page 15 of the document.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
Edited 9/15/2009 11:53 am ET by JonBlakemore
My point is not about the dew point on the sheathing, which the analysis does quite well. But the dew point on the tips of the screws. The tips of the screws will be colder than the sheathing just next to them. Thus condensation may occur on the tips of the screws (not on the sheathing).If I had the means to do a 3 dimensional heat transfer analysis of a steel crew in 4" of XPS with 20F on one side and maybe 50 or 55F on the other side (that being the temp of the sheathing) I would have my answer.
Yeah, you're right.I read the part about the higher conductivity of the steel screws in your posts but totally glazed over that point.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
If you have a condensation problem there on tyhe screw tips protruding, you have an overall problem with the way this was built.
I know the problem you are concerned with, but you do not have the kind of extreme cold that makes it apparent.
There should be a VB or vapor retarder on the interior surface that prevents this anyways.
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If I am reading this right, you should have no concern, really for condensation. These are the screws on the exterior of the construction, not the inside. Condensation occurs when the warm interior air will hit e.g. a cold fastener head (exposed on the inside) that maybe penetrates through insulation to the cold exterior.
Your example is the other way around. A head of a screw exposed to the cold and warmed by the interior. People fasten exterior materials all the time w/ exposed fastener heads ... no issues.
Either I'm reading something wrong or you got the concept misunderstood.
I've seen ice balls form on the points of roofing nails in an attic where lots of moisture (from warm air) didn't get vented quick enough and ended up freezing on the cold points of the nails.
As cold air comes in contact w/ the warm nail, the RH goes WAY down and no condensation occurs. I think you are worrying about nothing.