Spray foam under cedar shingles
andy_engel
| Posted in Energy, Heating & Insulation on
Situation: New red cedar shingle roof on purlins open to the attic. Older home, attic would be useful space if conditioned. Homeowner wants to have spray foam installed under the roof. I like spray foam, but don’t think it’s a great idea in this case. I think the thing to do is either nail proper vents to the bottom of the purlins, or use rigid foam and spacers to create an air channel, then spray foam below that.
Thoughts?
Andy
“Never try to teach a pig to sing. It wastes your time and annoys the pig.” Robert A. Heinlein (or maybe Mark Twain)
“Get off your dead
and on your dying feet.” Mom
“Everything not forbidden is compulsory.” T.H. White, The Once and Future King
Replies
People often ask questions on the forum where the circumstances are not what most of us would ordinarily see. Answering them is often just conjecture. Since a lot of us New Englanders have been dealing with ice dams lately, the idea of skip sheathing under cedar roof shingles isn't something I would recommend. I don't know what kind of R value you can get in the rafters. If you are losing heat through the attic, it's going to cause problems up here. R-38 is becoming the minimum. I would expect snow melt and water penetration even under the best of conditions. I'd go with full roof sheathing and ice and water shield whether you did the shingles over cedar breather or on strapping. You owe me 2 cents.
Beat it to fit / Paint it to match
You're dealing with ice dams here too, eh?
While we do full I&WS on all our roofs now, if the roof is properly insulated and well-vented then ice damming should not happen very often. Andy's down south in CT, so proper insulation is probably R-30 to R-38, easy to achieve with 6" of closed cell foam.
It sounds to me like the roof is already there, and probably not coming off anytime soon. Given the choice between spraying tight to the underside of the shingles or leaving an airspace, I would leave an airspace. Sure they might have a leak or two at some point but at least with an airspace there is a chance for stuff to dry out. When it's time to re-roof I would look at full sheathing and I&WS as you suggest, on cedar breather or, preferably, strapping.
Ice dams are a perpetual problem in cold snow country. The amount of snow and cold temps have certainly changed in our area over the last few years. These winters are more like the ones of my youth. Connecticut can be a lot warmer but they are not immune to an old fashioned winter.I sure wouldn't spray foam on the under side of shingles. I don't think a small channel of air circulating will overcome the loss of heat but I would want it, if just for the hot season and the effect to the roofing. You have to have some breathing room.In a loft or cathedral situation an option might be the addition of foam board under the drywall. You could gain an extra 10 points with 2" and have a continuous surface that offers a thermal break over the framing members. The issue isn't normal conditions but those unusual years when lots of snow combines with near zero temps. I would rather err on the safe side when you have the opportunity to do it.I've never seen skip sheathing on a residential roof up here. If you do have ice issues you might as well have a hole in your roof. With the lack of available ventilation space I'd make sure that as little heat escapes as possible. Spray foam is good but with a hot roof, you are bound to have framing shrink or move and places for the air to get through. When the light fixtures fill up with water, it isn't a good thing.Beat it to fit / Paint it to match
Walter (theslateman) posted pics from a Vinalhaven job he did recently with full I&WS roof deck and strapping over it. I haven't seen many skip sheathed roofs here either but have seen a few. Old barns are pretty much skip sheathed with the knot holes and gaps between boards.
Mike nailed the situation. The roof is in place. The polyiso detail is similar to what I did in my own attic with cellulose (but that's got asphalt shingles). That makes for a total R-value of maybe 25. I've never had an ice dam, but the house in question is oriented differently. I'm concerned that a northeast facing valley is bound to dam no matter what we do. This one's a tough call.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future Kingue
Andy,
You might be able to get some opinions at http://www.sprayfoam.com
Lots of knowledgeable sprayfoam pros over there -- look at the Q&A Forums.
Billy
Edited 2/5/2009 9:36 pm ET by Billy
Andy, re your T.H. White quote.
When the brother and I were teenagers, my father would misquote it as: "Not everything forbidden is compulsory." There always seemed to be a few choice expletives bracketing it though.
Having dealt with my own teenagers, I now know what he meant.
Excellent! I never reversed that, but as the parent of teenagers myself, I agree it's bloody perfect. You're also the only other person I've encountered who knew the quote.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future King
Thanks all for the responses. Without wading into the fire controversy, I got the official word from the Cedar Shingle and Shake Bureau. Because cedar shingles dry from the bottom as well as the top, a minimum 1 in. vent space is required below them. No foam contact.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future King
Regular propa-vents would melt and/or crush, but you could use this:
http://www.brentwood-ind.com/spg/accuvent.htm
Or make up shallow U-shaped channels from 1-inch polyiso, tape the corners or poke a nail through the joint every 24" or so, and tack up to create a channel like you suggest. Either way it would be a shame to lose the value of a purlined or skip sheathed roof under the cedar by spraying it tight with foam.
Are there already soffit vents or would it be easy to create some?
Andy,
I'm currently working on a 1790's house in souther RI with red cedars on skip sheathing. The homeowner's having serious ice dam issues on the north facing valley at the back ell. The opposite valley gets decent sun exposure and shows some signs of leaking, but not dripping through the ceiling and down two stories to the sills at the first floor where we are working now. (I now need to fix the roof before I can install the kitchen). Anyways, a couple of thoughts: I would leave as much space between the skip sheathing and insulation as possible to allow for drying. A totally airtight insulation would be their best chance at avoiding potential ice damming from heat loss, especially at the top plate to rafter area. If they go with the spray foam, I'd look more closely at the closed cell as it doesn't seem to hold water like open cell. Good luck, this seems to be one of those tough situations where your navigating between what the customer wants and the realities of building science.
Here in the Northwet, we have a lot of cedar roofing. That laid on top of plywood has a much shorter life span than does that on top of skip sheathing. The former holds water longer and doesn't have an opportunity to dry as well. Even cedar rots, especially if it's second growth. As such, you want air movement, which, I suspect, you already know.
Nonetheless, insulation would be great, regardless of what kind you use, as long as you can insulate the attic from the roof, leaving the air gap between, thus also reducing the problem of ice dams.
A few asides:
1) The only difference between siding and top grade roofing is price and a switch of tags. Don't pay more for siding. The two are one and the same. If you don't believe, go spy on a mill having large quantities of roof when it gets an order for siding from the broker. It's not uncommon to see the roofing tags come off, to be replaced with siding one. Of course, then the price goes up.
2) Cedar is not bullet proof, with regard to nature. It can last far longer than comp and such, if installed properly and taken care of. For example, cedar dries out, just like any other wood. When it does, it shrinks and cracks, degrading it. Too, when dry and brittle, walking on it cracks it. The solution is simple - old technology. More specifically, use oil (non-hardening, never use sealer type protective applications on cedar roofs and such). Chevron sells a shingle oil, but you could even get away with any non-hardening oil. The trick is, forget factory directions and thin it with paint thinner, turpentine or naphtha (whichever you can get cheaper-the only difference is flash point). Thinning will cause deeper penetration and saturation (as long as it will suck up oil, it's fair game to keep adding). In the end, your shingles will not absorb water (risking freezing and cracking), will not rot, and will remain resilient in the summer heat, so will tolerate being walked on. The only thing that should kill well cared for shingles, or shakes, it erosion. As such, a 3/4 inch shake should outlast you.
For those concerned about fire, the thinner will evaporate off, leaving the oil behind. Too, try starting a fire with dry cedar, then try it with oil soaked cedar. I usually have better luck igniting untreated cedar. Of course, this is not to say you should not be aware of the fire hazards associated with wood roofs, but there are ways of addressing those too.
"Cedar is not bullet proof, with regard to nature. It can last far longer than comp and such, if installed properly and taken care of."
Unfortunately you can no longer say that about the cedar shingles that available NOW (today) with any decent raw material apparently going overseas. Roofers in our area (central east coast) are generally no longer roofing with cedar shingles unless demanded for just this reason.
Jeff
Edited 2/6/2009 8:04 am ET by Jeff_Clarke
I have to agree. The grading rules have loosened a bit and what is now tolerated wouldn't have been in the past. For us in the Northwet, it's just a matter of going to the mill directly. They get to bypass the brokers and make a bit more, while you still get a good deal. It's not hard to cut a deal with mills, since most are pretty small operations, to make sure no second growth and only minimal seconds get tossed into a bundle.The time of year you buy can make a difference. In the hot summer, mills have to keep their cedar under sprinklers to keep it from drying and cracking. In the winter, they have to worry about wet wood freezing and checking. So avoid extreme weather, settle for only Grade A, and don't tolerate second growth and todays cedar is just the same as yesteryears.
G'mornin' Andy!
The foam would be fine for everything except the cedars 'cause they could then not dry to the backsides. That is why they are laid over skip-sheathing.
So I'm with you - provide a breathing channel first, then spray closed cell to your heart's content.
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Thanks for the comments and suggestions. I'm going to visit the spray foam forums, but you've all pretty much echoed my concerns. And, the Cedar Shingle and Shake Bureau's website has no detail for a hot roof with skip sheathing, which is a strong indicator.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future King
I'd also consult a BO re: the potential fire hazard aspects of doing this. Cedar shingles are a often a fire problem in themselves ... combining that with spray foam ... can only imagine a roof fire ... OMG.
Depends on the foam - ours (NCFI) is <25 <400 - if you hold a torch to it you can't ignite it.
http://ncfi.com/
Jeff
It would depend on the material. What is NCFI?
North Carolina Foam Industries - see link.
Uh ... didn't do an exhaustive check at the site, but I did find some information
This product is in the urethane insulation family - normally when I hear that ... it means a potential fire safety issue (i.e. it would require protection like from drywall).
The installation specifications say fire protection is required and that a 1/2" drywall will provide the necessary fire safety protection for this stuff.
Maybe I'm missing something?
Different local building codes treat this differently. 1/2" drywall is needed over foam in habitable space but when you're talking about a roof deck and an attic that is not habitable the 1/2" drywall is usually not needed... but codes vary on this issue.
Billy
I understand. And I've never really been sure about how the codes deal with e.g. foam insulation in concealed situations (e.g. attics). I thought most 'regular' codes like the IRC have required all foam be protected by a fire barrier "in substantial contact" with the surface of the foam. I thought this referred to foam exposure no matter where it was (concealed or not). But there is where my expertise is well ... fuzzy. Maybe someone else will comment.
I'm not an expert here either, so we'll wait for some real code experts to come along.
But these guys have some observations:http://www.sprayfoam.com/mnps/fullthread.cfm?threadid=5770&mnforumid=2&mnboardid=6&startat=1
Billy
Edited 2/7/2009 5:53 pm ET by Billy
BUSTED!
That statement about polyurethane foam not burning is misleading. I can see the lawyers already drooling.
Flame spread less than 25? Sure ... it drops to the floor of the test tunnel, where it's not considered ... even though it's burning like rocket fuel.
Now ... if you can show me a fire resistant assembly that's UL listed for 1 hour without burying the stuff in drywall, you might have something. Or, perhaps, a roofing test where the exposed foam stands up at least as well as Class C asphalt shingles. Then I might accept some sort of 'fire resistant' claim.
Cedar shake makers have also made claims as to the fire resistance of their 'treated' products. Some have even asserted the resistant of old, soggy, molding shakes. Nice try; there's still all those decades between when the salts was out and the moss sets in.
For ANY foam, the rule is simple ... in anything resembling a horizontal application, bury it in gypsum.
For ANY foam, the rule is simple ... in anything resembling a horizontal application, bury it in gypsum.
http://www.airkrete.com/ This stuff really will not burn. It is not a polyurethane foam it is a cement foam.
Maybe they really have made a "cement foam." If they have, they're on to something. I still want to see some fire-wall type testing, though. I've seen the E-84 test manipulated too many times. While they describe it as 'cementitious,' they also assert that the composition is 'proprietary.'
"Depends on the foam - ours (NCFI) is less than 25 and 400 - if you hold a torch to it you can't ignite it."
I have to agree with Reno on this one.
I looked at the ESR's for both Insulstar and Sealite. They *do* allow the foam to be installed in attics and crawlspaces, but not without significant warnings.
If you're Chief Building Official allows this in your jurisdiction, that's great. But there are a lot of areas that will not allow the foam to be installed in the manner which you speak of.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
"But there are a lot of areas that will not allow the foam to be installed in the manner which you speak of."
Maybe re-read thread? I didn't suggest installation of any kind.
I only mentioned that when an ignition source is held to the foam we have installed (tested in my own hand) that it does not ignite. At all. Or smolder.
Also, you should be aware that there are exceptions in the IRC for coverage of foam in other specific areas, such as along rim joists, etc.
Jeff
Edited 2/8/2009 11:16 pm ET by Jeff_Clarke
the problem with foam is it insulates so well it magnifys the heat. If you have it in the attic you must cover it with an intumescent coating or drywall and no access. If you had the foam on the top of the roof you not would need the fireproof --weird
In real life I think foam helps slow fires because lack of air flow
anybody who says you dont need to cover foam is stretching the truth
except the mudsill and minor stuff
<If you have it in the attic you must cover it with an intumescent coating or drywall and no access.>That's not the case in our area. During an insulation inspection, I asked about foam as fireblocking, since the wall cavity is completely full. He thought with foam, fire blocking may go the way of the poly vapor barriers, bye bye.http://www.tvwsolar.com
I went down to the lobby
To make a small call out.
A pretty dancing girl was there,
And she began to shout,
"Go on back to see the gypsy.
He can move you from the rear,
Drive you from your fear,
Bring you through the mirror.
He did it in Las Vegas,
And he can do it here."
I'm sorry ... I'm still skeptical. The web site you referred me/us to says this is a urethane product. It says it needs fire protection.
You say you've personally seen flame held to this raw insulation and it will not burn.
I hope you realize that what you are saying/suggesting is generally COMPLETELY contrary to what people have come to believe and know. Codes have been created around this knowledge about urethanes. And in 1 or 2 sentences, you expect us to buy what you are saying? With all due respect, you can't expect us to either understand you or buy into what you are saying.
Something is missing here that either we aren't getting, or you aren't revealing or saying. If we are missunderstanding what you are saying, I think there is something that you aren't telling us. I'm not the only one 'not getting it here', so I this puts it back on you to fill in any blanks that we might be missing.
I'm not trying to be sarcastic or mean, either ... you need to put yourself in our shoes.
I didn't realize I was 'on trial.'
If the foam doesn't fit, you must acquit!
Look, all I am saying is that I had the 'rocket fuel' impression too ... but after trying to ignite a chunk of it with a butane grill lighter (without success) I was fairly impressed.
That's ALL. No smoking foam gun ...
Email me your address and I'll mail you a piece to try and burn.
Yes, an ignition barrier is required under many circumstances. You can read the ICC Eval here if you want - http://www.icc-es.org/reports/pdf_files/ICC-ES/ESR-1615.pdf
Here are some exceptions from the IRC 2006 NJE that you might find interesting, and they should (correctly) convey the impression of increasing acceptance of low-flammability foam in building construction:
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The NCFI foam at <25 Flame Spread <450 Smoke (per ASTM E84) complies with R314.5.11 above for omission of the ignition barrier in the example given.
Jeff
Edited 2/9/2009 6:20 pm ET by Jeff_Clarke
Hey, I had to ask ... thanks for the food for thought ... so ... I'll read a bit ... busy, though for a couple of days. I'll be back (me an Ahnold). Maybe we'll all learn just a bit more eh? Can't hurt (although my brain feels full right now).
Well all righty then. That's a lot of code language to absorb (I'm exhausted) ... best I can say is that it requires a fire barrier, etc. etc. When it comes to the flame spread and smoke density, I am admitedly a little rusty ... all I remember is that urethanes and polystyrenes were not allowed w/out drywall in "substantial contact" with the insulation (eliminating e.g. hidden voids I think).
Maybe 'your' insulation doesn't support combustion, but does support the development of too high smoke density (the biggest fire hazard is smoke). So ... I'm not an expert. Frankly I've lost track of the point, here. I think it was our reaction to your claim that this urethane does not readily combust on its own.
Maybe there is someone that is more of an NFPA expert. I could call a fire protection engineer friend of mine if I can remember in my busy day. Maybe he'll help us understand this issue a bit more.
I have held a match to Demilec... it burned while the flame was on it, went out when it wasn't... didn't smell too good, though.http://www.tvwsolar.com
I went down to the lobby
To make a small call out.
A pretty dancing girl was there,
And she began to shout,
"Go on back to see the gypsy.
He can move you from the rear,
Drive you from your fear,
Bring you through the mirror.
He did it in Las Vegas,
And he can do it here."
You must be from the west. Our cedar roofs are usually so wet that they're about as flammable as slate. <G>Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future King
Yeah, I am ... isn't everyone? ; ) You mean there is an east?! Yikes, I always thought it was just a myth!
It's hard to find the East just now because it's been under a foot of snow since about last June, but it's here.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future King
Would the permeability of open cell foam work in your favor?I'm assuming there's not a tarpaper underlayment. I'd think a lot of the foam sprayed directly under shingles would either find it's way out, or displace a bunch. Especially be the case with closed cell.http://www.tvwsolar.com
I went down to the lobby
To make a small call out.
A pretty dancing girl was there,
And she began to shout,
"Go on back to see the gypsy.
He can move you from the rear,
Drive you from your fear,
Bring you through the mirror.
He did it in Las Vegas,
And he can do it here."
Open cell has half the R value.
Jeff
Not really. 3.5 vs 5.5-6 when the cc hits it's degradation max. Way less expensive, also.http://www.tvwsolar.com
I went down to the lobby
To make a small call out.
A pretty dancing girl was there,
And she began to shout,
"Go on back to see the gypsy.
He can move you from the rear,
Drive you from your fear,
Bring you through the mirror.
He did it in Las Vegas,
And he can do it here."
I doubt there's a clear answer to that one. It would depend on the time of year, the delt-T, how wet it was outside v. inside... Probably it would be a wash. I think that closed cell with a vented airspace above would be fine, although just how that would work in the valley is an open question. The skip sheathing on this roof would allow some airflow, but enough? I don't know.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future King
I can't answer for your situation.. however I can tell you that I don't have any problem with Ice dams on my roof..
MY roof is SIP's which are a "hot" roof like you propose. If the insulation is thick enough the roof won't have problems. If on the other hand it's a shallow pitch and the insulation can't achieve the needed R value I suspect there will be an issue with Ice dams..
Hi, you also have to be aware that the foam can expand and force up the shingles.
I have had that happen more than once on slate roofs that we had repaired.
Had calls about slates sticking up in air, foam had pushed up through gaps on boards, dislodging slates.
would be less likely on cedar, but can happen,especially older roof.