How do you go about insulating an unvented flat roof? Due to the height limit, etc., the roof needs to be flat (1/4" per foot) and I'm thinking of using TJI 11 7/8" roof joists. (80 mil. IB PVC Roofing & 1/2" roof OSB sheathing above, 5/8" gyp. bd ceiling below) The minimum requirement is R-30. What are some of the optimal combinations of rigid foam above the roof sheathing and closed cell spray foam below the sheathing? Thanks in advance

Need more info:

1 - What building climate zone are you located in? I assume since it is a flat roof you are NOT in snow country.


Open cell spray foam is still vapor permeable. Water vapor can penetrate the foam and then condense on the cold side of the roof and create moisture/rot problems on your OSB sheathing and joists. Closed spray foam is "better" since it has better vapor impermeability properties when applied at > 6" final thickness.

Spray foam is a tricky animal because it has to be applied in proper layers. Too much during application time and it can cause a "spontaneous combustion" type of fire (well documented phenomena). Wrong mixture and it can off-gas and become a festering pile of goop that has to be removed.

As the saying goes: Black colored vehicles are not a color option but a career choice. The same goes for flat roofs. They are a maintenance issue for the life of the home. They must be meticulously detailed during the building phase. Parapets must be kept clean and roof drainage must be kept closely monitored. If not, you will have roof leaks and serious problems. With flat roofs all roof penetrations must be detailed with a belt & suspender approach.

Flat roofs for residential homes is a low desert southwest style of building because there are no snow loads and rain is far and few between. Even then, I have observed many flat roofs that are <5 years old that leak during the rainy/monsoon season. Flat roofs are common on warehouses (Lowes, HD, Costco) but walk into those stores during a heavy rain and you will be hard-pressed NOT to find water dripping from the roof. 

Thanks Lbear, The climate zone is 7. We never had snow in the area. The roofing of choice is IB Roof (PVC) as it has good records in previous projects. Our local codes prohibit the use of open cell spray foam for unvented roofs. Do 3" rigid foam (R-15) over sheathing and 6"+ closed cell spray foam (R28+) under sheathing work?

Posted By pinakes on 19 Feb 2015 10:12 PM
Thanks Lbear, The climate zone is 7. We never had snow in the area. The roofing of choice is IB Roof (PVC) as it has good records in previous projects. Our local codes prohibit the use of open cell spray foam for unvented roofs. Do 3" rigid foam (R-15) over sheathing and 6"+ closed cell spray foam (R28+) under sheathing work?

I think you misunderstood. There is no way you are in a Zone 7 building area as that is Northern Wisconsin, Northern Minnesota on the Canadian border and all of Alaska except the most arctic portions. Not only would you get snow but very bitter cold winters.

Look at this map to see your climate zone:
IECC Climate Zone Map

On your map, it's 3. I was looking at our local map designation. (Coastal is 7 in our state) Sorry.

For a Zone3 climate the 2012 IECC requires a minimum of R-38 for the roof. Your roof build-up would have a total R-Value of around R-36 + R-12 = R-48 which is above code.

Here is info on the spray foam: GBA Roof Spray Foam

With your build-up (from interior to exterior = 6" closed spray foam - roof trusses - OSB SHEATHING - 3" EPS). Water vapor will NOT get through (which is a good thing) from the interior 6" UNLESS you left gaps or air leaks in the foam and sheathing. At the same time it is important to note that if the sheathing gets wet it CANNOT dry to the interior with 6" of closed spray foam.

Now with the 3" of exterior EPS there is some very slim drying potential for the sheathing to dry to the exterior. Main point would be to NOT let the sheathing get wet in the first place. I am not familiar with IB Roofs but from my limited Google research it seems like a good product. As long as it doesn't let moisture get past it and therefore past the EPS into the sheathing, you will be OK.

If your OSB sheathing gets wet it will NOT dry out to the interior and drying out to the exterior through the EPS will be very slow if not unlikely. If the latter, it would cause sheathing rot over time.

I think the buildup would work for a Zone 3 climate. If the roof was pitched I would not lose an ounce of sleep over it but a flat roof always makes me uneasy but as long as you detail it PROPERLY and watch the roof penetrations, parapets and drainage, you will be fine.

Consider using only EPS insulation above the OSB - no spray foam. This will allow the OSB and joists to dry to the interior.

With 11-7/8" TJIs you could meet code with 1" of polyiso above the roof deck and 9" of half-pound open cell foam between the TJIs. At that thickness the vapor permeance of the spray foam would be about 3-5 perms, which is plenty of drying capacity toward the interior- a Class-III vapor retarder roughly equivalent to standard latex paint, sufficient for meeting the requirements of IRC 2012 Chapter 8:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

If local codes prohibit half-pound foam, use cellulose, and just fill it on up. The R5 min exterior foam requirement is based on a total R of R38 for wintertime dew-point control, which is 13% of the R on the exterior. With 11.7/8" of cellulose you'd be at no higher than R44 in the cavity, which means you'd need at least R6.6 on the exterior, bringing the total to R51. Cellulose is dirt-cheap, less than 5 cents per R-foot, compared to EPS or polyiso at about 10 cents per R-foot.

So 1.5" of EPS (R6) on the exterior would run $0.60 per foot, plus ~12" of damp sprayed cellulose ( R44) would run $2.20/foot, so you'd be in for under $3/foot for R50+.

To do a code-min R38 as 6-7" of polyiso above the roof deck would run about $4/foot. It would outperform R38 between joists, but not the R50+ TJI stackup.

To do R30 above the roof deck as 5" of polyiso it would run about $3/foot, in the same cost range but potentially higher than the R50+ roof.

If you did it with proportionally more EPS or polyiso above the roof deck it becomes more expensive, since it's also more complicated and labor intensive to do a partial-depth cavity fill, particularly in a TJI.

If you used blown fiberglass for cavity fill it would be higher-R, and somewhat more expensive than cellulose (in my market anyway), and you'd have to bump that to 2" of EPS above the roof deck to maintain the same R-ratio.

Get quotes- my guess is that either 1.5 EPS over the roof deck with a full fill of cellulose, or 2" of EPS with a full fill of Optima / Spider / L77s probably cheaper than less than full fill, and WAY cheaper than a 5-7" of closed cell polyurethane moisture trap.