Unconditioned AC ductwork
GBA Editor
| Posted in Energy Efficiency and Durability on
Some of my AC ductwork is in the unconditioned space of my attic. I wondered if I could add a layer of spray foam to it or relocate the ducts (no easy task).
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Replies
There are several approaches you can take with this problem.
If you do decide to use spray foam be sure to get prices from several suppliers and to compare the cost of open vrs. closed cell foam.
You will also want to look at the cost of adding an inch or two of foam or another radiant barrier coating to the underside of the roof while you are doing the work.
It will likely be more cost effective to bury it all with a deep and thick layer of cellulose after you apply good duct sealant (white "duct mastic") to the exterior of the duct connections and ceiling boots as well as to the plumbing penetrations and the drywall-to-wood intersections at all interior and exterior top plates you can get to. Just don't do the cellulose until after you have done as thorough a sealing job as you possibly can.
It would be good to add one more option to the cost analysis, which would be to use spray foam to seal the attic and bring it inside the conditioned envelope. This is not always achievable in a remodel as it involves closing off the soffit vents and covering the top of the exterior wall plates and the under side of the roof with 5-10" of spray foam. In many markets this is still very expensive, so it's best to get multiple prices from as many foam applicators as possible if you want to explore this option. Don't be surprised if estimates vary by 100% on this.
Scott,
It's great that you are making plans to insulate your attic ducts. One caveat: be careful about using vapor-permeable insulation there. The best option is closed-cell spray foam. If you use a vapor-permeable insulation like open-cell spray foam, blown-in fiberglass, or cellulose, moisture from the hot, humid attic air can diffuse through the insulation and can condense on the cold attic ductwork, leading to mold or dripping ceilings. It doesn't always happen -- variables include the moisture content of the attic air, the permeability of the insulation, and the temperature of the ducts -- but it can happen and has happened.