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Ducts in Unconditioned Attic

DigitalDadGoesAnalog | Posted in Energy Efficiency and Durability on

I have a 1960’s ranch-style house with gas furnace in the attic. There’s currently about R19-worth of insulation in the floor of the attic, which should be R59 according to the zone I live in. Utility bills are very high and I also have noise issues from low flying airplane traffic. I’ve read more articles than I care to admit to try to decide on the most cost-effective way to improve insulation and soundproofing, and haven’t found any ‘silver bullet’ solutions yet.

Open- or closed-cell spray foam seemed like the obvious solution at first, but since this isn’t something I can DIY and there appears to be a risk of even professional installations failing (e.g., failing to cure, trapping moisture under the roof & rotting the roof, trapping moisture in the attic, etc.). Even if it goes to plan, it sounds like I may need mechanical ventilation (HRV/ERV) in the living space and dehumidification in the semi-conditioned attic space. Taken together, this option seems too expensive / risky to do correctly, notwithstanding any additional concerns about flammability or VOC’s from using foam.

Having ruled that out, a bog-standard loose fill blow-in at a fraction of the price seems like the best choice. But this leaves two problems: (1) doesn’t necessarily help much with noise and (2) the ducts are still in the unconditioned attic which can have extreme temperatures.

How can I ALLEVIATE (1) and (2) without spray foam? Here’s what I came up with:

1. Add batts of insulation in the attic ceiling between the rafters (e.g. rockwool), in addition to the loose-fill. Even if this doesn’t help much with the temperature swings, perhaps it would help with sound proofing?

2. Closed-cell spray foam the ducts, as explained here:
The idea is that this would help air seal the ducts as well as mitigate the disadvantages of being in the unconditioned space.

What do you think?

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  1. user-6623302 | | #1

    Move/change the HVAC system?

  2. user-2310254 | | #2

    Jonathan may have the right idea. If your floor plan is fairly open, you could remove the furnace, abandon the ducts, and install mini splits (and gain AC). You could focus on air sealing and adding inexpensive blown-in insulation.

    You could then monitor indoor conditions to determine if you need an HRV/ERV and/or mechanical dehumidification. You might or might not need to invest in these systems.

  3. JC72 | | #3

    How cheap do you want to go? Air seal the doors, windows, ceiling, first then add more insulation second? Other than pulling the duct system out of the attic (expensive) or converting to an unvented attic I'm thinking the word "cheap" will not be applicable.

    How many years left on the roof? Reason - Might be better to take a hybrid approach with reclaimed rigid foam on the roof and batt insulation within the rafters. Turning your vented attic into unvented.

    As far as the sound from aircraft From my experience windows have a lot to do with that.

  4. DigitalDadGoesAnalog | | #4

    Thanks for the great suggestions!

    @John: The roof is a couple years old. Do you have any resources on the reclaimed rigid foam approach? I assume this means re-doing the roof, so probably not something we want to pursue just yet.

    Cost aside, I agree that changing the HVAC system would be ideal. Unfortunately, I think we'd probably be looking at $30K+ to install ductless mini splits everywhere (3000 sq/ft house). Also, the existing HVAC system is fairly new and in good condition (recently inspected), so it's hard to justify making that change now.

    It's good to know that windows have a lot to do with aircraft sound, and indeed we have very old wood frame windows (single pane). There are storm windows over them, but I don't know if they do much. Are there any good resources on how to improve the insulation / air sealing of existing windows? Or is replacing them the best recourse (which would be another $30K+)?

    1. JC72 | | #5

      Yes it would be a re-roof. As for resources you'd have to call around in your area but since you're nowhere close to to a re-roof it seems somewhat moot.

      As far as air sealing your doors/windows I would take a look at whatever programs your state utilities offer. Replacing windows is obviously very expensive.

  5. walta100 | | #6

    First let me say the smart move is to undo the poor choice of locating the HVAC in the attic but most will not consider that.

    You did not say where you are but R59 sounds cold. If the out-door humidity is not high for months at a time and or you don’t need a lot of AC burying the duct work in insulation could be a good option.

    The other option is to condition the attic and it only slightly better than well sealed ducts in a vented attic. The truth is R60 insulation on the rafters is dam near impossible and certainly unaffordable. Some people play Russian roulette with mold and rot and call it incapsulated attic, I don’t see that a better option.


  6. PBP1 | | #7

    A relatively inexpensive option is foil-backed duct insulation (R-8 or more). I have air handlers in conditioned space and I have covered the ducts with such insulation, was pretty easy. As to air traffic noise, I lived five miles from O'Hare (directly under a landing path) and now live five miles from a small Western town airport (directly under a landing path) and it's worse than O'Hare (lower altitude approaches). I have spray foam and batts, double pane windows, not sure there's much one can do.

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