Duct insulation in unfinished attic

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Old 08-13-16, 11:01 AM
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Duct insulation in unfinished attic

My house two heating "zones," downstairs with central a/c and oil-fired forced warm air. Upstairs is heat pump. The the air handler and ductwork for our heat pump is in our unfinished attic. We typically only use the upstairs zone during hot or cold extremes, otherwise the downstairs keeps things pretty nice throughout the entire house. During extremes, though, the system struggles. Today we're looking at 95F ambient outside, 105-110F in the attic, and the HP is running non-stop to keep things at 80F inside. I'm betting that it's due to the minimal insulation on the ductwork.

The main trunk line in the attic appears to be rectangular duct wrapped in foil-faced fiberglass with a marked R value of R-5.7. There are round duct branches off of that to the rooms that appear to be wrapped in foil-faced bubble wrap. The register boots themselves also have the R-5.7 foil-faced fiberglass.

I know that the foil-faced bubble stuff can technically be used as insulation, but proper installation requires spacers to create air gaps, etc. I have no idea if that was done or not, but if I had to guess based on the quality of workmanship elsewhere in the house, I'd say no. There was a partial roll of it left in the attic, too, and it appears to be the thin 1/4" stuff, not the thick 1" stuff. So basically, radiant barrier wrapped around ductwork.

I have also noticed during other work in the house that there's some evidence of condensation forming around the register boots and staining the ceiling in spots around the register grills.

Would it be a good idea to replace that insulation with foil-faced fiberglass at R-8?

The other idea I had is, what about removing the current insulation, building a type of enclosure around the duct, and spray-foaming around the ducts with a 2-component spray foam? It would be expensive, but at nearly R-7 per inch, I could get much higher R-values.

What are your thoughts?
 
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Old 08-13-16, 12:24 PM
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I doubt moving from R5.7 to R8 would make any noticeable improvement especially for as much work as it would be.

If the location of the duct work would allow, your best solution would be to move it into the conditioned space by removing insulation under it, building an enclosure over it from foam board, air sealed and taped, and then covering the enclosure with as much additional insulation as possible. In this case you're not moving the ductwork into conditioned space, you are moving conditioned space to include the ductwork.

If that won't work, the foam idea would work, but remember you will likely need a combustion barrier over the foam as most codes don't allow exposed foam.
 
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Old 08-13-16, 01:10 PM
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Sorry, my brain short circuited. I meant replace the unknown R-value bubble wrap on the branch lines with the fiberglass R-8. The R-5.7 on the trunk line would either stay, or be replaced by the foam. I just don't know if the bubble wrap stuff is doing much good at all?

Currently, the attic floor has blown rock-wool insulation between the joists and then 2x12 planks as flooring (1890's house). So the duct work is run along the top of the flooring. I don't know that it would be feasible to bring the conditioned space up around the duct.

Was basically thinking of framing mini-walls around the existing duct, type X drywall, and fill up with spray foam 4 inches deep or so on all sides of the duct. Of course, need to do volume calculations on that. May very well cost thousands in spray foam, at which case the return on investment isn't worth it.
 
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Old 08-13-16, 01:44 PM
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More likely airflow/leakage issue than insulation. You should compare the temperature of the air coming out of the shorter ducts to the longer runs to see what the effect truly is.

In an old house if it is 95 outside and only 105 in the attic then you are probably cooling that space unintentionally.
 
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Old 08-13-16, 02:42 PM
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Yeah, we're probably cooling it by having maybe R15 (if we're lucky) between the second floor and the attic.

All of the registers were reading between 69-71F when I checked just now.
 
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