Roof Assembly Question: Foam Board Insulation Ok Directly Over Rafters/Purlins?
Hi,
We are going to be building our home using an unconventional hybrid “Barndominium” type construction method that will use steel columns and trusses with wooden 2×6 girts and 2×10 purlins/rafters @ 24″ O.C.. The building is cross braced with metal tie rods under the ceiling and metal portal frames inside the walls, and therefor does not require any roof decking.
Question:
Will Polyiso foam board insulation be ok to install directly on top of the 2×10 purlins without putting down roof sheathing first? We have no snow load requirement, just 20 psf roof load.
Unvented Roof Plan: (Top Down)
Metal Roof, Underlayment, 7/16 decking, 2″ Polyiso foam board, 2×10 Rafters, Fiberglass batt or Rockwool in cavity, Sheetrock.
I looked up Polyiso compressive strength and found 16-25 PSI. Would this not be enough?
Thank you
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Replies
You'll have to do the calcs PSI wise, my guess you are fine. The bigger issue is that it will be very hard to build. I would not walk on that roof as it will be hard to know if you are directly over a purlin or not.
Compared to the overall cost of a house/barn build, the extra bit of decking cost is pretty much noise. Even if you go for something minimal like 3/8, it will still be a much more solid surface and easier install.
I'm guessing the roofing metal was meant to install directly over the grits which means it can span. If this is the case you can save a bit of cost by putting 1x4 strapping over the rigid instead of the 7/16 decking.
Some metal roofs (standing seam with clips) can also be installed with steel bearing plates that can go over the rigid. The metal roof can then screw to these. Check with your supplier.
You will lose much if not all of the shear value installing like this. This is important as lateral loads travel through the roof diaphragm to get to braced or shear walls. I would install your foam on top of your sheathing, then see if your roof manufacturer allows use of long fasteners.
In rough numbers, you'll have about 18 square inches of bearing area (1.5 inches x 12 inches) per each 2 square feet of roof area (at that 24 inch purlin spacing if the purlins on edge). The unsupported foam doesn't have any appreciable bending or shear strength between the purlins, and compressive strength requires a supportive substrate underneath, so the purlin area is the only thing that matters without some sort of decking under the foam to distribute compressive loads.
Moving the 7/16 deck to underneath the sheet foam would be a more traditional way to deck the roof, even though you don't need the shear strength. That would more-or-less solve the issue of falling through the foam as well, but would also require that the metal roofing panels are capable of being installed directly over strapping, etc.
Not sure if you're planning on using a lighter gauge roof panel that requires a solid deck directly underneath for support.
Thanks for the input guys. As far as the installation, sorry I should have mentioned, was planning to choose a nailbase product with insulation glued to sheathing so it goes on at the same time. My first choice would be to use Zip R12 insulation sheathing up top since I am already planning to use Zip R6 to wrap the walls and thought I could extend it up around the rafters and tape over the top seam, connecting wall to roof to get a good air seal and continuous WRB. (Am doing a cathedral ceiling with unvented roof assembly)
As far as evaluating the insulation compressive load, and determining if it's strong enough, do I simply try to exceed my code, 20 PSF live load? This has always been confusing to me because someone walking on a roof is going to far exceed 20 lbs of force ( ie. 200 lb man standing on one foot). But using 20 PSF, so is that only (20psf /144 sq inches in 1 sq ft = .138 psi)? Vs. foam board rating of 16-25 psi?
Looking at it another way, so taking the 2 square feet roof area and the small 18 square inches of bearing area as Chris_in_NC pointed out/calculated above. And assuming a 20 PSI rating on the foam. (18 sq. in x 20 PSI = 360 lbs per 2 sq. ft). Does that look right, and is that enough load bearing capacity?
There is also your dead load that bears onto the foam at the purlin, which in this case would be your metal roofing, underlayment, nailbase panels (the full panel including the foam), etc.
Loosely (for the foam), 16psi * 18 sq in = 288 lb, so that's your theoretical upper limit per 2 sq ft of roof for your live load, dead load, and whatever other loads.
The 200 pound man is usually considered part of the 20 psf live load. His weight will be distributed over a larger area because of the nailbase stiffness, but again you've got 16psi foam that is only bearing on the purlin edges, so you don't want to get close to the limit of anything because of the fairly low compressive strength. I believe the compression ratings for foam are based on a percentage limit of thickness compression, but I can't remember what the percentage limit is.
I won't give an opinion about whether that's enough capacity, as I'm not a structural engineer and don't know the rest of the types of loads (if there are any) that would need to be considered.