foil faced in stud bays
A building inspector is busting my chops about having put 2 layers of Tuff-R by Dow in the stud bays. From the outside in of this north wall in Western Mass I have overlapping Masonite Siding (existing), 1/2 inch exterior plywood sheathing, 2×4 stud walls 16″ o.c.’
2″ Tuff R foil faced both sides insulation, 1″ Tuff R, 1/2″ airspace, 1/2″ sheetrock, primer, paint. He says moisture may get between the two layers of Tuff R and cause rot. Tuff R is isocynate or something like that. I researched in on Dow’s web site and it has a low moistrue permeability but does not discuss tight fitting sections of it in stud bays…. I’ve got a Dow representative working on it for me but any literature anyone can site about my nonconventional use of it? Thanks
Replies
Poet,
There is the chance that interior air could migrate to the inside surface of the 2" Tuff-r, but in your climate, the warm side of that 2" board should still be above the dew point. I would be more concerned with the space between the 2" and the plywood, as any warm air that gets in there will condense in the winter.
I do this type of insulation technique sometimes, and I fit the foam just a little loose, and foam the edges with a foam gun to stop air leakage around it.
Steve
He is trying to apply the principles he learend for FG batts to the foam installation.
He is kind of like an old man who still peeks at the Playboy centerfold even tho the only thing that it still applies to is his own imagination
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How tight fitting?
Personally I would seal the edges for foam.
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
If the foam board is tight to the framing, moisture migration shouldn't be a problem as it would require air movement (can't diffuse through the foil facing).
Of greater concern would be the thermal bridging at the studs. I would suggest applying foam board over the studs before drywall. This will not only reduce the thermal bridging but also create a more complete air/vapor barrier and probably make the inspector happy as well.
Where in W. MA are you? I lived in Greenfield for many years.
Solar & Super-Insulated Healthy Homes
Hi,
I live in Hinsdale, Ma. About 15 miles east of Pittsfield.
Riversong? Canoeist? Where do you live now?
As this sheetrock is all buttoned up tight over these stacked layers of Tuff R foilfaced I can't easily add a layer of foilfaced.... I'd rather justify (with technical data )what I did however I am finding that I did a rather unconventional application. Personally I think it's not a bad installation or use of the material as fiberglass lends itself to thermal cycling since it's air permiable....I know that the studs themselves have low R values but overall the wall has a higher R value because of the foam and the it has a low permeability to water vapor.... anyway I'm searching for technical data. Thanks for your thoughts! Dennis Pollock
Riversong? Canoeist? Where do you live now?
Yeah, done a lot of wilderness canoeing. Used to lead Outward Bound trips in the Boundary Waters of MN and CAN. Learned to canoe in the subactic of central Labrador.
I'm in the Mad River Valley of north central VT.
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Canoed the Boundary Waters? What was your favorite route?
I did one week long trip two years ago, loved it, and will go back within the next few years. I put in at Mud Row (spelling) up through the Horse River to the falls, up crooked lake, past Saturday-Friday-Thursday Bays, and then back south. The wolves were awesome and the fishing great.
I took that route three (I think) years ago. Camped at the site directly below Basswood Falls for a few days. Lay awake at night listening to the water rush. On the way out, the level of the horse river had dropped sufficiently that we had to walk and pull the canoe for a good portion of the way. Strange what a couple of inches of water level difference can do.Sorry about furthering the hijack;)
The Horse River was dry in several places along the route when we took it as well. Just a boulder field in several spots, but otherwise paddlible (is that a word).
Do you have a favorite route?
Favorite route... every one I've taken.
Winter camping at Gillis and Tuscarora. Caught a 10 lb Walleye on Seagull, a 43" Northern on Caribou, a little lake near the East Bearskin entry point. Hiking part of the Kekakabic trail. A couple of unauthorized forays into Quetico (shh, don't tell). Indian pictographs on Basswood. I've never had a trip that wasn't thoroughly enjoyable.
Edited 3/4/2008 9:51 pm by ptp
Yes, my wife and I loved our trip. We kept it really light, so with all the portages we could make one trip. Even the one mile long one was no big deal. My pack and canoe came in at just about 100 pounds, and my wife carried another 40 or so. Light and Fast. We covered the ground and fished and fished. We did have a bear rearrange our gear at one camp as we were out fishing. Of course no food was left available so he was not very interested in anything.
We will go again. We are also looking for other canoe/camping opportunities. We did a one week trip in the Bowrons in BC. Awesome.
Moogie, and others.....
Those were some heavy duty artilcles but I'll print them out and do the calciulations. I think the idea is that polyisocynate is considered vapor impermiable and 3" of it in a stud cavity, pressed together, mean that there is a certain amount of R value which may or not be enough to keep temperature on living space side of the stud cavity up above the dew point, hense, one wants to haver water VAPOR in the 1/2 in air space behind the sheetrock and not LIQUID water. If it's water vapor it can not permeate the insulation and can not condense in the insulation as it gets to the dew point somewhere on it's way toward the outside where the air pressure difference is pushing it. If the temperature on the interior face of the foil is too low, the water vapor will condense and roll down and wet the sole plate, causing mold, rot, nasties. THANKS to a great group of canoeists, fisherman, computer and paddle savy.
Hey, Riversong, what was the name of that river you did, probably not the George as it's not a circuit. Why did you stop going up there? Still canoeing?
that picture: Route 63? into the Adirondaks form the route 87 or on the Kankamengus Highway in the whites?
It's the Mad River of NH, though the scene looks a lot like my own Mad River here in VT.
See, Waters did the same as me, two layers of Tuff R in a stud bay, tight or spray foam to tighten up any area that's not tight.... now I'm trying to justify it to the building inspector.
That's just like my job, Waters.
Haven't done it yet. But I think you're spot on.
Invite your BI to read these posts."Research is what I'm doing when I don't know what I'm doing..."
I think you have a problem any time you use more that one moisture barrier. You can trap moisture and cause all kinds of bad stuff.
You get out of life what you put into it......minus taxes.
Marv
What Piffin said.
Seal the edges of the foam and be done with it. The inspector is worried about condensation forming between the layers, but between the layers shouldn't have a great enough temp differential to allow condensation on one hand, and on the other hand if it's sealed on the edges how's moisture going to get in there anyway?
As to how to handle the inspector, that's a different story. Best bet is to have the manufacturer tech support give their opinion and see if he'll go for that. Otherwise you might have to request a different inspector be assigned to your project. If that doesn't work I'd file a formal request with the building department. If it's worth it to you and nothing else works, an engineer can write up the specs and put his stamp on it.
Best of luck
Beer was created so carpenters wouldn't rule the world.
The only problem you have is the inspector. Whoever invented the "double vapor barrier" terminology should be taken out and shot, since it leads to this sort of idiocy but is otherwise meaningless.
Whoever invented the "double vapor barrier" terminology should be taken out and shot, since it leads to this sort of idiocy but is otherwise meaningless.
Not "meaningless" at all. But it depends on the elements of the wall system.
We don't even use the term "vapor barrier" anymore, since it's misleading. Most building materials are vapor permeable to some extent, and the industry standard is that any material with a perm rating of 1 or less is considered a "vapor retarder".
Nothing short of a welded-seam pond liner is going to keep ALL moisture out of a wall system (not to mention the moisture that's built into the wall during construction - e.g. KD lumber at 19% MC).
So any wall system should be able to both minimize moisture penetration and dry to one side or the other. A double vapor retarder can create problems, particularly if water can wick (as with concrete foundation walls) or moisture-laden air can enter the cavity (as in stud walls with unsealed utility penetrations).
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Thanks, I'm keeping track of all these responses and I'm in cantact with the manufacturer to see what they can come up with. Thanks.
I'm drawing a blank on were I saw it, but somewhere there is a specification for what R-value is needed across a given delta-temperature at a given minimum outside temperature for the warm side to be above dew point.The only thing I took away from it at the time from memory was that in our climate (and yours) 1" of foam (R6.5) was a little marginal, but 2" was way more than enough to keep the inside surface above dew point under normal conditions. I Maybe someone else recalls seeing that somewhere...Steve
Easy enough to "map" the temperature gradient in any building envelope, such as: 100904.25 or 100904.36 and then use a psychometric chart to determine dew point.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 3/3/2008 10:49 pm ET by Riversong
I would think that kind of homework ought to be able to get any reasonable inspector on board.Steve
Robert,Thanks for the links to those posts. I missed that thread. Very valuable info.Steve
Those look like some great and helpful graphs. I'll print them out and use them for the defense. Hey, Riversong.... what's this about subarctic Labrador? Is that still a part of your life? I've stopped at Menihek 3 times and spend 3 different weeks in the country west of there going as far as Howell Lake. This summer (early August) we're planning a trip to and down the MacPhayden River. Where have you been in Labrador? And don't you love it?
Dennis Pollock
Where have you been in Labrador?
I paddled a loop north out of Schefferville, which is the end of the railroad out of Sept Isles. Four weeks on the water. Haven't been there since 1984.
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How many miles total? Good fishing? Wildlife?
This is a midthread hi-jack on a parallel topic, if I may...
Say you've got a classic old house. 2x4 walls, zero insulation, lath and plaster. And now for the kicker, no sheathing and no tarpaper--just siding, which is 1x8 oldg. fir boards cut in a double 'v,' nailed right to the studs.
Say now you're going to remodel a room--tear out the lath and plaster and start from scratch.
How would you insulate?
One more kicker--this is Oregon, one wall faces south storms and it's WET for 8 months a year. Right now things dry out just fine with NO insulation!
Your opinion?
"Research is what I'm doing when I don't know what I'm doing..."
We have that situation on almost every house I work on here. I've done spray foam directly on the backside of the siding, and alternately 2" foil-faced foam, sometimes two layers foam, some times one layer and then strapping the wall horizontally with 2x4's then meshing over and blowing cells on top of the 2" foam.Steve
That is probably one of the few situations in which I'd recommend sprayed closed-cell foam, since it has to serve as weather barrier as well as air barrier and insulation within a shallow cavity.
But I'd also install 1" foil-faced foam on the inside of the studs to cut the thermal bridging, and drywall over it, perhaps on horizontal strapping to create a radiant air space.
I might even consider spraying a sealer on the inside of the siding before insulating, to protect the wood from possible rain infiltration.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
thanks to you and mmoogie."Research is what I'm doing when I don't know what I'm doing..."
Hi,
I'm stumped. Here's as far as I've gotten in the calculations: TUFF-R Polyisocyanurate has a permiability of 0.03 I think which means 0.03 grams? (of water?)/square foot/hour/level of mercury vapor. Correct me if I'm wrong with any of that or what follows. According to BuildingScience.com DSD-106 "Understanding Vapor Barriers," Any material withg 0.1 perm or less is a class I Vapor Retarder and is also vapor impermiable. I think the foam in the foil sandwich is closed cell, right. R values in the application in question include (heading from inside to outside) inside air film:0.68, 1/2" drywall: 0.45, 1/2" air space: R=?, 1" Tuff-R: 6.5, 2" Tuff-R:13, siding: 0.94, and outside air film: 0.17. That's a total of 21.74 R without the 1/2" air space between the Sheetrock and the 1" Tuff-R.
If it's zero degrees farenheit outside and 70 degrees inside, Delta T is 70 degrees; What I want to find is temperature on the interior most face of the 1" Tuff-R. That is to say, is it above the dew point if Delta T is 70 degrees F. I'm looking at it this way, I don't care about Ts (the temp. on the interior surface of the exterior sheathing). I just want to know if the air in that 1/2" air space between the sheetrock and the 1" Tuff-R is going to condense, drip down, create a dry vacume or negative air pressure and suck more moisture into the cavity and produce a cycle of wetness. Can you please help me with this Riversong? Thanks so much.
I did read the attachments and look at all those formulas and that's as far as I could get.
Dennis Pollock
Not that it matters, but 1 perm = 1 grain of water per sq.ft. per hour per inch of mercury pressure differential.
Conventionally, any material less than 1 perm is considered a vapor retarder. There is no such thing as a vapor barrier short of a pond liner. Just like insulation with heat, vapor retarders only slow the diffusion of moisture. But foil is about as good a barrier as there is.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 3/10/2008 11:40 pm ET by Riversong
What you need now is an estimate of apparent RH at each level, given the vapor retarding characteristics of the various layers. Would be a similar chart.
If your view never changes you're following the wrong leader
That would be true if all layers were permeable and there were a vapor pressure differential.
But, since there is a virtually impermeable layer at the foil facing (and assuming no leakage points), I think it's fair to assume that the air space on the outside of the drywall will equilibrate with the inside RH.
Beyond that air space, I think it would be impossible to model because of the hygroscopicity of wood studs.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
But you've insisted again and again that all layers ARE permeable.The only way to address this situation analytically is to create the table of dewpoints (assuming 30-40% inside RH) at each boundary, then compare those dewpoints to your table of temperatures. Only when the dewpoint at a boundary is above the computed temperature is there the potential of condensation.
If your view never changes you're following the wrong leader
But you've insisted again and again that all layers ARE permeable.
Picking nits? Foil-face foam board has a dry cup perm rating of about 0.01, which - as I clearly stated - is virtually impermeable (just as an insulation with a U-value of 0.01 or R-value of 100/in would be a virtual heat barrier).
As far as your "analytical" approach, care to model it for this wall section?
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 3/11/2008 2:58 pm ET by Riversong
> As far as your "analytical" approach, care to model it for this wall section?Would need the numbers for everything. But to provide an oversimplified example:A dewpoint of zero is about 6% RH at 70F. So figure 40% inside and 6% (equivalent at 70F) outside. 34% difference. If you have two membranes of equal permeability, one at the inner wall surface and one halfway through the R value of the insulation (and perfect permeability elsewhere), you can figure the %RH (70F equivalent numbers) to be 6 + (34/2) = 23% at the midway membrane. That works out to a dewpoint of about 30F.But we placed the second membrane at the halfway point in R value and so in temperature. The temperature there is 35F. Since the temperature is greater than the dewpoint, no condensation will occur. (Though of course in practice 5 degrees isn't much of a safety margin.)
If your view never changes you're following the wrong leader
So figure 40% inside and 6% (equivalent at 70F) outside.
Why would you assume 6% RH outside? It would be necessary to use real case outdoor RH, which is almost invariably much higher than 6%.
And you didn't analyze the proposed wall section, but rather a purely hypothetical and meaningless example.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
A dewpoint of zero (the presumed outside temp) corresponds to about 6% at 70F. By using RH at a fixed temp I avoid having to translate terms to PPM or whatever for absolute humidity.
If your view never changes you're following the wrong leader
And your example was meaningless without the corresponding humidity table.
If your view never changes you're following the wrong leader
And your example was meaningless without the corresponding humidity table.
I had said, "But, since there is a virtually impermeable layer at the foil facing (and assuming no leakage points), I think it's fair to assume that the air space on the outside of the drywall will equilibrate with the inside RH."
Either refute this statement, or retract the one above.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
You had two layers of foam. You didn't offer any hypothesis for the dewpoint between the layers. And clearly the dewpoint on the outside of the outer layer will equate to outside dewpoint, within a small delta.
If your view never changes you're following the wrong leader
I didn't have two layers of anything - the OP did.
Since the foil-faced foam board, if properly sealed, is a near-perfect vapor barrier there is no moisture migration or condensation plane beyond the first layer of foil.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
So why did your chart treat them as separate layers? And imply that there would be condensation between the two layers?
If your view never changes you're following the wrong leader
Edited 3/13/2008 2:23 am by DanH
So why did your chart treat them as separate layers? And imply that there would be condensation between the two layers?
That's a generic spreadsheet for any envelope section, and it is intended to show the temperature gradient between layers and the potential for condensation (in terms of indoor RH and temperature) based solely on surface temperatures.
If the two layers of foam are not well sealed to studs, then there is potential for moisture to diffuse between them. My above comments, however, were conditioned on proper sealing of the foam board.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Is this the article you recalled? With it one can calculate for one's location the minimum exterior foam sheathing required to prevent moisture condensation in the wall assembly on the inside surface of the foam.
http://www.buildingscienceconsulting.com/resources/walls/Vapor_Barriers_Wall_Design.pdfIf it ain't broke, don't fix it, just open it up to see how it works!
It's a good article, but oversimplifies the issue - as it acknowledges:
"Using this approach the thermal resistance of the interior finishes and exterior cladding and associated air films are ignored along with the thermal bridging effects of the wall framing."
And it uses average Dec//Jan/Feb temperatures and specific indoor relative humidities.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Maybe this will help:
http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/?topic=/buildingphysics/moisturecontrol/watervaporcontrolbasics/drying/main_topic