Rigid insulation, furring strips, and metal roofing
Spbreslin
| Posted in Energy Efficiency and Durability on
I’m considering a roof design that would place all the insulation above the roof deck with no insulation below. I would use polyiso with a top layer of EPS as recommended in your blog post on the topic. The project is in Climate Zone 4 (Seattle area) and I want to have an open beam look in the ceiling. Placing a layer of fluffy insulation below the roof deck would needlessly complicate the roof assembly. What are the potential problems I could have if I installed all the insulation above the roof sheathing, which will actually be the finished ceiling?
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Replies
Stewart,
I'm not sure how we can anticipate problems. The most important point is to find a contractor familiar with this approach. If you can find a contractor who's done a SIP project, that contractor should understand. Using either SIPs or nailbase may be your best approach, although it's also possible to stack up the foam yourself without using SIPs or nailbase.
My question was prompted by your article about using rigid foam insulation above the roof deck. Included with the article was a drawing illustrating a method using layers of polyiso with stacked 2x4's at the eave. However, the drawing assumed an attic below with additional batt insulation. The assembly that I would like to use would be (top to bottom) comp shingle roofing, Zip System sheathing with taped seams, 1" of XPS foam per your recommendation in the article, over 4 layers of 1-1/2" polyiso, over a finished ceiling layer of phenolic birch plywood with taped seams, over 1-3/4" LVL joists. The problem with using SIP panels or nailbase in this design is the lack of a suitable finished surface. I can't see any drawbacks to this solution but I can't find anything on GBA or the net that directly addresses this design.
Stewart,
It's perfectly possible to install nailbase or SIPs above a layer of phenolic birch plywood, if that's what you want to do.
Here is a link to a relevant article: How to Install Rigid Foam On Top of Roof Sheathing.
In that article, I wrote:
"If you plan to install rigid foam above your roof sheathing, you have two choices:
"Option 1: You can install all of the insulation above the roof sheathing (in which case the rigid foam has to meet minimum code requirements for ceiling R-value); ...
"If you choose Option 1, your rigid foam will be fairly thick:
In Climate Zone 1, you’ll need R-30 of rigid foam (about 8 or 8.5 inches of EPS, 6 inches of XPS, or 5 inches of polyiso);
In Climate Zones 2 and 3, you’ll need R-38 of rigid foam (about 10 or 11 inches of EPS, 8 inches of XPS, or 7 inches of polyiso); and
In Climate Zones 4 through 8, you’ll need R-49 of rigid foam (about 12.5 or 14 inches of EPS, 10 inches of XPS, or about 9 inches of polyiso).
"... If you don’t want to create a site-built sandwich of rigid foam and roof sheathing, you might want to consider installing structural insulated panels (SIPs) or nailbase instead.
"A structural insulated panel is a sandwich panel made out of rigid foam (usually EPS) faced with OSB on both sides.
"Nailbase is similar, except that nailbase has OSB on only one side of the foam.
"If you decide to install SIPs, you should follow the recommendations of the SIP manufacturer. However, SIPs come with a few quirks. SIP seams are more vulnerable to air leaks than the seams between rigid foam panels in site-built assemblies, because it’s impossible to stagger the seams with SIPs. Some SIP roofs have experienced OSB rot near the seams; this occurs when indoor air has access to air channels that connect with the upper layer of OSB at the seam. To address these potential problems, installers should (a) always tape SIP seams on the interior and seal SIP seams with canned spray foam; and (b) strongly consider installing ventilation channels above the SIPs, especially in cold climates.
"Nailbase is available in a variety of thicknesses from several manufacturers. For example, ABT Foam sells 9-inch-thick nailbase panels rated at R-33.
"If you can’t find nailbase with a high enough R-value to meet your needs, you can install nailbase above a layer of rigid foam. This method has an important advantage: it allows you to stagger the seams between the two layers."
In a Seattle climate there is no point to using polystyrene on the outer layer, the way it sorta makes sense in much colder climates. Even at Seattle's ~25F outside design temperature the mean temp through the foam layer is going to be north of 45F, a temperature at which matches the performance of XPS, and it will outperform XPS whenever it's warmer than that (which is 99% of the time.)
Martin's R-value numbers are a bit off, since you don't need anywhere near R49 to meet code with continuous foam due to the fact that there is no thermal bridging of the insulation by rafters. Meeting code on a U-factor basis it only has to duck under U0.026, which is ( 1/U0.026= ) ~R38.5 "whole-assembly-R". You get about R0.8 credit for the combined interior & exterior air films, about R0.9 for the roof deck itself (assuming 3/4" thickness, R0.55 if half-inch), and R0.55 for a 9/16" nailer deck above the foam and about R0.5 for an asphalt shingles & #30 felt layup.
That leaves a requirement of only R35.75-R36 for the continuous insulation, not more. Using typical 2lb roofing polyiso rated at R5.7/inch (it's tested slightly differently from insulation for other applications) it's do-able with just 6.5" of foam, not 9". Using lower density foil faced stuff you'd get there in 6".
See:
https://up.codes/viewer/utah/int_residential_code_2015/chapter/11/re-energy-efficiency#N1102.1.4
https://www.jm.com/content/dam/jm/global/en/commercial-roofing/brochures/RS-5131_2014LTTRBrochure.pdf
Using reclaimed foam it's actually pretty cheap. These folks in Kent will sell you ~3000 board-feet of used polyiso for $650.
https://seattle.craigslist.org/see/mat/d/rigid-foam-insulation/6330847376.html
For 6.5" you'll need 6500 board feet of roofing polyiso per 1000 square feet of roof area. With used foam allow 10-15% of extra for scrap from sheets that are too damaged to use.
With a half-inch nailer and 6.5" of foam you'll need 8" (8.5" if self-drilling) pancake head timber screws to through-screw the nailer to the structural roof deck. ( eg: Spax XTM312-8000 there are others.) The screw points may present a cosmetic issue if you're using the roof deck as the finished ceiling. Using the fastener spacings and types recommended by nailbase panel manufacturers works. eg:
https://www.hunterpanels.com/polyiso-roof-products/installation-and-fastening/1438-h-shield-nb-application-guide-1/file
Thanks Dana, that was very helpful.
Stewart
Hi,
I am starting my exterior foam roof deck on my tiny house. I read the "How to Install Rigid Foam On Top of Roof Sheathing" It was very helpful.
I am going to do 4 inch of polyiso on top of my roof deck. Then a high temp ice and water shield membrane, then going to install either 1x4 or 2x4 furring strips on the diagonal for my standing seam metal roof above that, to allow for ventilation. Its a low slope roof and for added water protection I was thinking of coating the furring strips with a waterproof sealant of some sort. I have not been able to find any information on people waterproofing furring strips for a roof application but this seams like a good idea to me?
Also once I screw the furring strips onto the trusses and roof deck with long screws through the foam. is it recommended to still use the same long screws for attaching the metal roofing to the furring strips?
Thanks,
Alex
Alex,
If you are planning to use diagonal strapping I'd draw out how it actually vents. Where does the air enter and exit at the eaves, gables and ridge.
My own feeling is that since your roof doesn't rely on ventilation, it would be best left unvented. Consider using sheathing instead of furring, moving your underlayment up above it, and fastening the metal roofing directly to the plywood..
Hi Alex.
It's not common for furring strips for ventilated rainscreen siding details or used under metal roofs to be waterproofed. Because the furring strips are in a ventilated space, they will be able to dry if they get wet. As you know, because you have rigid foam over your sheathing, you'll need screws long enough to fasten the furring strips to the roof framing. The roofing will be fastened to the furring strips.
There are a few GBA members with extensive experience with metal roofing who will probably be able to offer more insights for you.