How do I build a rigid insulation roof on my shed roof cabin?
cdiggins
| Posted in Green Building Techniques on
I am building a shed roof on my NW (Seattle area…yes rain and moderate temps) cabin, 16 x 18 and I want to use rigid insulation so that I can have exposed beams and recycled car decking on the inside. So the car decking will go on top of beams, and from there I am unsure about vapor barrier, thickness and kind of rigid insulation, then air flow, and then metal roofing. And last detail for trim , etc.
Any suggestions where I can info on my project?
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Replies
Christopher,
Would you tell us the slope of your roof? A lower slope roof makes the detailing EVEN more critical in preventing water vapor penetrating your roof assembly potentially rotting the building materials.
As I am not a builder (the foundation is being dug on my first house in 2 days), I will try to help answer your question by gathering information from similar previous questions.
Looks like the R-value you want to achieve is R-38.
http://apps.leg.wa.gov/wac/default.aspx?cite=51-11-0625
So if you use polyisocyanurate rigid foam insulation which has an R-value of 6-6.5/inch, you would need 6 inches to reach code minimum. One prominent building scientist, Dr. Joe Lstiburek, wrote about his experience using one thick layer of foam on his barn in this article, "Foam Shrinks, and Other Lessons". He advises using at least 2 layers taped between layers with seams staggered to prevent gaps in the foam boards.
http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks
Poyiso is considered the most environmentally benign of the foam products (as in less global warming potential) and has the highest R-value per inch. There are a number of ways you can apply it - as foam boards, as nailbase or as a SIP. If you are doing the building yourself you will probably want to use foam sheets which usually have a foil facing.
Here are 2 similar discussions:
https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/21528/low-slope-metal-roof-over-car-decking-and-rigid-poly
Insulating a Cathedral Ceiling on Old House Web http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks
There are lots of opinions about where to put roofing felt, additional layers of plywood sheathing that I am not really qualified to answer. I'm sure someone will weigh in on the topic tomorrow.
Good luck with your cabin - it sounds nice.
Christopher,
If you are installing metal roofing, you won't need OSB or plywood roof sheathing -- just 2x4 purlins (nailers), 24 inches o.c., installed above the rigid foam (parallel to the ridge).
You can use your rigid foam as your air barrier if you want; this only makes sense if you have at least two layers of rigid foam with staggered seams. If you go this route, you should tape the seams of the foam. (At least one of the layers of foam needs taped seams, or two if you are a belt-and-suspenders type.)
If you are worried about foam shrinkage and its effect on air tightness, you can install a separate air barrier membrane between the car decking and the rigid foam if you want. That might be MemBrain or one of the similar air-barrier membranes from Europe that are sold by Small Planet Workshop and 475.
Hi Martin and Lucy,
Thanks so much for your replies. More info- slope is 3:12 pitch. So I am not worried about drainage.
Also, I thought that I did not need plywood for sheer strengthening if car decking was used. I got some recycled car decking 2x6 T&G fir and planned to use as flooring on main floor (only 1 floor) and as exposed ceiling/ roof and under rigid insulation. In my previous build, my main home, the county building dept allowed me to use this configuration on the 2nd story floor for sheer instead of plywood. I haven'e checked with them on this yet. I am designing now.
I am confused about ventilation. Some people are saying, you Martin, that a vapor barrier is not necessary and airflow up the slope is unnecessary as well? Please explain.
Thx, again, Christopher
Martin, sorry...just reread your post... so you are saying then that the rigid insulation acts as vapor barrier and and no ventilation required up the slope as one would need with standard bat/ fiberglass insulation?
My local building supply recommends Thermax 5", i think. And 1x4's , 24o.c. wouldn't be enough for purline's.
Thx, Christopher
Christopher,
For information on why you don't need a vapor barrier, check out these three articles:
Do I Need a Vapor Retarder?
Vapor Retarders and Vapor Barriers
Forget Vapor Diffusion — Stop the Air Leaks!
For more information on all of the different ways you can build an insulated sloped roof assembly, see this article: How to Build an Insulated Cathedral Ceiling
I read it Martin...that answers my questions about ventilation. Can I use 1x4's for purlines instead of 2x4s? Seems like if I use short screws through metal roofing they would just barely make it through purlins. And what do you think of sheer provided by car decking to replace plywood?
Christopher,
Q. "Can I use 1x4's for purlines instead of 2x4s?"
A. Yes.
Q. "What do you think of sheer provided by car decking to replace plywood?"
A. Talk to an engineer if you are worried. Remember, the metal roofing also provides racking resistance (assuming, of course, that the strapping and metal roofing are properly screwed in place).