Did I build my roof incorrectly?
I am the Owner/ GC for my house. I also live outside of Austin, Texas. I directed my roofer to overlay the outside OSB decking, with StormShield, an adhesive water barrier, instead of felt. On the inside of the roof, I had Open-Cell foam sprayed 6 inches thick, and sealed the inside of the attic space.
When you look at my house from the outside, it’s possible to see the ‘outline’ of the attic area. It’s as if there is a visible temperature variation apparent, from the insulated portion of the house, than from areas where the spray foam is absent (like the garage).
I am concerned about this, as I don’t want a humidity build-up to occur in the attic, and mold to form and or grow.
Monitorings of the interior of the house, versus the attic area, show there to be a 20-point variation in the humidity levels; the attic is always higher.
I recently had my HVAC guy come in to install an intake, and an exhaust of conditioned air, into the attic. So far, the results are inconclusive.
Help me guys, -Did I do something wrong, or is this ‘visual footprint of the house’, a non-issue?
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Replies
Stephen,
Your description is unclear. You wrote, "When you look at my house from the outside, it's possible to see the 'outline' of the attic area."
So what are you looking at? Are you looking at your peel-and-stick roofing underlayment (the StormShield)? Or are you looking at your roofing?
If your roofing has already been installed, what kind of roofing is it? Asphalt shingles? Steel panels? Concrete tiles?
When you say that you can see an outline, what does that mean? Is the underlayment or the roofing a different color? If so, is it darker or lighter? Is the area moist or dry?
StormShield is vapor-impermeable, unlike asphalt felt (which is a "smart" vapor retarder). You have chosen to cover your entire roof with a vapor-impermeable membrane. That may or may not be a problem. I'm not a fan of this approach, because it absolutely prevents any moisture from drying out to the exterior. However, it is certainly true that most types of roofing prevent drying to the exterior in any case, so the impermeability of the underlayment may not really matter.
There have been reports of problems with damp roof sheathing when roofs are insulated with open-cell spray foam, as you have. The issue is complicated; here is a link to an article that discusses it: Open-Cell Spray Foam and Damp Roof Sheathing.
My guess is that, in your climate, your roof will eventually dry out and turn out OK -- as long as you can use your air conditioning system to reduce the humidity in your conditioned attic. Keep pumping the cool dry air up there until things stabilize.
In Austin Texas the climate (US climate zone 2) isn't cold enough for wintertime moisture accumulation in the roof decking to reach mold-hazard levels with your stackup. During shoulder seasons heating by the sun the vapor drive is high enough that there will be moisture cycling to/from the roof deck over the daily cycle, which may explain some of the high attic humidity readings.
How high ARE those readings, and are they relative humidity, or absolute humidity (dew point or wet-bulb temp) readings?
If RH, what is the temperature to which they are relative?
If there is air leakage between the outdoors and attic (this can be determined with blower-door testing) the summertime humidity in the attic could be quite a bit higher than the rest of the house, due to the high summertime outdoor dew points in that climate.
A separate but issue is that R20-R22 is well below code minimum for US climate zone 2 (R38 is IRC 2012 code-min- it used to be only R30, not sure which rev. of the code your location is working under)
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
If the high humidity readings are due to solar moisture pumping at the roof deck, and NOT outdoor air leakaget, assuming you're at only 6" due to 2x6 rafters, you could kill two birds with one stone by putting 2x4s cross-wise to the existing rafters and installing unfaced R15 batts, but with a layer of MemBrain smart vapor retarder between at the original rafters, detailed as an air barrier. With that stackup you meet code on a U-factor basis even if the center-cavity R may be shy of R38. (Since you would have an R15 thermal break over the 2x6 rafters, you would be under U0.030. See TABLE N1102.1.3 in the IRC further down the page in that link.) The smart vapor retarder becomes a "humidity diode"- it releases any moisture being driven off the roof deck rapidly when the relative humidity on the foam side goes high due to the moisture being driven out of the roof deck by heating. But when the roof cools overnight the RH behind the vapor retarder falls and it becomes more vapor tight, restricting the rate at which moisture from the attic side can get back in. If the humidity issue is solar driven moisture pumping, that symptom should subside fairly quickly. It's pretty cheap stuff- ~$100 for an 800 square foot roll if purchased online, sometimes cheaper through distributors, but it's probably hard to find in your area.
If it's an attic air leak situation, fix the air leaks, and monitor it a bit more before adding insulation. It's surprising just how often the soffits don't get air sealed when foam retrofits are performed. Verifying it with a blower door and theatrical smoke machine makes it DEAD obvious where those leaks are, but even less dramtic blower door methods work.
If you have deeper rafters there are other ways to deal with both the R-value/U-factor and moisture pumping issues.