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Locating ducts in attic underneath a thick layer of cellulose

rob kohaus | Posted in General Questions on

Building a two story house with basement in climate zone 5A. Attached to the “main” portion of the house is a one story “wing” with a laundry/mudroom and the garage (this portion is built on slab…insulated with EPS to ~R-22). Walls are R-48. Raised heel (16″) trusses in the attic with blown cellulose to at least R-60.

My plan was originally to keep all ductwork to the slab “wing” inside the conditioned envelope and soffit around the ducts.

Problem is – – – I neglected to share this particular detail of the house construction with my wife. I let her know about this plan today and she let me know that she dislikes soffits.

So now I’m looking at solutions to make her happy. My realistic options are probably as follows…..

1.) Maybe go with exposed duct work. This is the lesser of two evils (soffit versus exposed) for her if the ducts MUST stay in the conditioned envelope.

2.) Turn the attic into conditioned space by use SPF on the underside of the roof deck. This isn’t my favorite option due to the huge cost involved.

3.) Install the ductwork in the attic.

I’m leaning towards option #3 and wonder how big of an efficiency penalty would result. If all ducts were insulated and well sealed and run against the bottom cord of the 2×4 trusses…..

I thought of it this way. At R-3.4 per inch and hitting R-60 minimum I’m going to be blowing this stuff in 18″ deep minimum. A 4″ duct insulated to R-6 sitting on a 2×4 truss will still have about R-36 of insulation between it and the air above the insulation.

While obviously not the same as keeping the ducts below the ceiling, I think there wouldn’t be a huge energy penalty….and it keeps the wife happy.

Any input, advice or other ideas are appreciated!

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  1. GBA Editor
    Martin Holladay | | #1

    It's possible to bury attic ducts under a deep layer of insulation, but the approach can be risky in a hot, humid climate. Here's the risk: during the summer, when the ducts are used for air conditioning, the outer jacket of the ductwork will be cold -- especially if the ducts are buried under thick insulation. (The added insulation tends to keep the duct jacket cold.) Since your cellulose is air-permeable, it won't prevent humidity in the outdoor air from contacting the cold duct jacket. The predictable result is condensation and dripping.

    If you want to take your suggested approach, the best insulation to use to encapsulate the ducts is closed-cell spray polyurethane foam, not cellulose. That will increase your costs, of course.

    For more information, see Keeping Ducts Indoors.

  2. wjrobinson | | #2

    To me it depends on how long your air conditioning season is. Where I live it is short and would bury the ducts in cellulose. I have sprayed a roof with foam, worked out great but prefer not to do so.

  3. rob kohaus | | #3

    Thanks Martin. Runs aren't too long so probably a Froth Pak would do it. At about $400 or so it would probably cost less than the labor and materials involved with soffiting in the ducts.

    AJ - air conditioning season really varies with each year. On average its 3 months (June, July, August).

    I wonder if the same idea could be applied to running insulated PEX plumbing in the attic as well.

    Thoughts on that?

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