Better to bury ducts in attic insulation vs put in unconditioned space?
We live in Maryland just outside Washington DC. A huge tree fell on our house, crushing the roof. The house is a 2-story 5,000 sq ft colonial built in 1973, with a 2-zone HVAC system. The 2nd floor air handler and ducts are in the attic, which is unconditioned.
Before the tree strike, all the ducts were buried under the blow-in insulation, which worked well. During the restoration, my contractor is keeping the return under the insulation but decided to move the trunk supply line and suspend it about a foot above where the top of the blow-in insulation will be. He said this is standard practice and better than burying the supply in the insulation.
I’ve read that up to 20-30% of heater and AC output is lost running supply and returns through unconditioned attic space, and read the articles on this site on burying ducts in insulation. The county I live in enforces the 2018 IECC, so burying attic ducts is permitted, and according to my home inspector, what he sees almost universally in new homes built in the area.
Is moving the supply out of the insulation into unconditioned attic space standard practice in the mid-Atlantic? And is it a good idea?
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Replies
Not a professional HVAC person here. My understanding is that ducts buried under blown insulation can be a problem in the Eastern half of the country because of the humidity. Especially in summer the humidity can condense into water even if the ducts are buried and can cause problems. This is what I've read on GBA but I don't have any direct experience with that because I live in a dry climate.
Having said that, if you have had buried ducts in insulation in the past with no problems maybe that's apocryphal information, at least for your region. Others here should be able to offer more direct experience about burying ducts in your region of the country. Your HVAC person's advice to elevate the ducts in an unconditioned attic is incorrect and wasteful of energy. It might be a moot point if the air handler is already in the attic in unconditioned space.