Supply and returns in sealed attic in hot/humid climate
I have a new construction house on the coast in South Carolina which has been spray foamed with closed cell in the crawl space( at the subfloor), closed cell in the walls, and open cell at the roof line. In hot/humid climates, I have read there needs to a small amount amount of conditioned air released into the “sealed” attic to slightly dehumidify and circulate the air. The rate mentioned was 1 CFM per 30 sq ft of floor space. This is the same amount the codes require for sealed crawl spaces in NC.
I consulted my HVAC contractor and he installed a 4″ supply with no dampener and a 4″ return in the attic space. My concerns are:
1) No dampener on the supply to limit the amount of air introduced
2) The return does not meet fire code as the attic is not rated for fire
3) No filter for the return.
My question is what is the best way to handle the air in the attic and does the return violate the fire code?
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I don't know about the fire code issues but I agree that a damper on the supply would be helpful, although you'd need some flow measurement equipment to adjust it properly. An alternative would be to monitor humidity and adjust it gradually over many weeks until there's just enough flow to keep humidity reasonable.
If you decide to install spray foam at the roof line, you are bringing the attic inside the conditioned space of your house. Now your attic is just like your living room or bedroom. It's inside, not outside. Of course, the code requires the spray foam to be protected by a layer of 1/2 inch drywall. Once you do that, your inspector shouldn't have any fire code issues about the ducts going to your attic, because your attic is no different from your bedroom.
As this document discussing conditioned attics (a document from the U.S. Department of Energy) notes, "All foam on the interior should be protected by a fire-rated material, such as ½-inch gypsum board."
Problems occur when builders ignore the need to protect the spray foam with 1/2 inch drywall. I'm guessing that's what's happening at your house. In cases like that, code inspectors worry that you will get a fire in your attic, and the fire will spread through the ducts. The best solution is to cover the spray foam with drywall. It can be argued that building codes already require the drywall, and that builders who skip the drywall have been getting away with a code violation.
Joe Lstiburek discusses your dilemma in an article called Cool Hand Luke Meets Attics. In that article, Lstiburek writes:
"None of the International Code Council Evaluation Service (ICC-ES) Evaluation Reports for spray foam insulations allow this type of application if there is “real” air change or communication with the “occupied space”. Unless, the spray foam is covered with gypsum board. ... There is an option. We can install a smoke detector in the return duct that is coupled to air handler and a fire alarm so that in the event of a fire the system is shut down. We do this commercially, we need to do this residentially. ... The best way to handle it in the short term is to go to your Chief Building Official and tell him/her that you are going to add a supply and return with the smoke sensor alarm set up and plead for approval. The Chief Building Official has the authority to accept this. Be nice."