Potential Moisture Issues with a Conditioned, Non-vented Attic
We live in CZ 4 (Fairfax County, Virginia) and we are currently having a custom home constructed. We’re the owners; so our technical knowledge is limited, but I think we may have a potential construction issue that could cause moisture and mold issues for us in the future. Do the non-living spaces (enclosed roof truss assemblies) in an un-vented, conditioned attic need to be conditioned and ventilated to avoid moisture and mold issues?
The “attic” (under the gables of a craftsman-style bungalow) was designed to be conditioned and un-vented and contains both living spaces (bedroom, bathroom, family room) as well as non-living spaces. Both types of attic spaces are located inside the thermal and air boundary (see attached drawing). We plan to is to use closed spray foam to insulate (sufficiently thick to achieve R-38 so to be code compiant) the underside of the roof deck as well as air seal the the attic. The roof frame is comprised of engineered roof trusses. The attic will be serviced by a dedicated HVAC system located inside the living area. When we asked the HVAC contractor his plan to condition the attic spaces, it informed us the attic equipment wasn’t sized to service the thermal loads or ventilation needs of the non-living spaces of the attic, and its duct design doesn’t include trunk/branch ducts or supply and returns to “condition” and ventilate the non-living spaces.
If the non-living spaces need “air”, what’s the most practical and cost effective solution? Increasing the capacity of the HVAC equipment and adding ducts at considerable cost, or will adding roof/wall exhaust fans to ventilate the non-living spaces be enough to control moisture?
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I'm no expert so hopefully somebody more qualified will chime in, but from reading this website for many years, my understanding is that if your attic is fully sealed up with R-38 closed cell spray foam, you should be fine. I believe the need for dehumidification/conditioning arises when open cell is used.
I wouldn’t be surprised if the HVAC is oversized for the actual loads. Contractors aren’t known for delivering accurate Manual Js. How large is the home and what are the specs for the HVAC system? Also, you probably should be shooting for R-49 in the attic, which you might be able to achieve at lower cost with a flash and batt approach.
With close cell foam above the roof deck you should not have problems provided your interior partition walls are not insulated. The heat loss through an uninsulated wall (R3) should be able to keep most of the attic spaces pretty close to house temperature.
The larger attic space used for storage should get some conditioning, since this is inside your thermal boundary, you don't want it to turn into an inbetween space where humidity and RH is different than the rest of the house. You don't need much, 50cfm per 1000sqft is plenty.
The HVAC system for the house needs to be able to handle the loads of the full house. By having the attic inside the conditioned envelope, it means that it is part of the house. Even if you don't run ducts there you are indirectly supplying heat and cooling to the place which your HVAC needs to handle no matter what. There is a very good chance the HVAC equipment is way oversized (most tend to be if sized by HVAC tech) so this should not be an issue. If anything, by going through a bit of up front design work you can reduce the size of the equipment and ducting.
For more on load calculations, see https://www.greenbuildingadvisor.com/article/who-can-perform-my-load-calculations.