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Community and Q&A

Insulate ducts in conditioned basement?

runner9 | Posted in Energy Efficiency and Durability on

I have been air sealing my ducts as best I can with mastic. I now have the first 4 feet or so of supply duct from the furnace sweating pretty good whenever it’s on. This is all in the basement, which we use as another family room like area, don’t use it as much as the first and second floor, but a fair amount.

In the summer it’s freezing with the air conditioning on and the ducts sweat for a few feet, as mentioned.

In the winter, however, it’s still a little chilly, certainly not overly warm.

I’m planning to air seal the rim joists, but I can’t put insulation on the cinder block walls (already covered with drywall, inaccessible, etc)

So, this makes me think that adding an R6 duct insulation will help with the sweating and cold basement in the summer but will take an already bit chilly basement in the winter and make it colder.

Is this thinking correct, and if so which way do I go: insulate the ducts or just air seal them?

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  1. GBA Editor
    Martin Holladay | | #1

    The solution to the sweating (condensation) issue, as you correctly guessed, is to insulate the ducts. Make sure that you use duct insulation with a vinyl jacket -- the vinyl jacket is essential to keep the insulation dry. Carefully seal the seams in the duct insulation with an appropriate tape.

    If your basement is cold during the winter, you can install a register in one of your hot air ducts (supply ducts) if you like. Select a register with an integral damper, or install a take-off and short duct serving the register, with a damper in the duct. That way you can control the air flow to the register in your basement.

    The more air sealing work you can perform, and the more you can improve the insulation level in your basement walls, the more comfortable your basement will be.

  2. charlie_sullivan | | #2

    And if you don't mind your basement being chilly in the winter, there's no need to heat it.

  3. Expert Member
    Dana Dorsett | | #3

    Air sealing the rim joists an anywhere else you can in the basement is likely to help (but may not entirely solve) the sweating duct issue too. But CMU walls can leak a lot of air, even below grade. Anywhere you have access to the CMU it's worth air-sealing & insulating, even if it's not the whole basement. Any duct, electrical or flue chases that can leak air through the basement ceiling need to be air-sealed too, especially those that extend to the attic, since stack effect drives are otherwise a major part of the air infiltration into the basement. Air sealing both the basement AND the attic can reduce stack effect driven infiltration considerably.

    If you're never going to insulate the basement from the interior, you can often add 2-3" of EPS on the exterior, digging down 1.5-3' below grade. The EPS can be glued and cap-screwed to the CMU, and protected with a cementicious purpose made stucco-like material suchs as QuikKrete Foam Coating. (In termite country coat the whole thing, all the way to the bottom of the foam and backfill with compacted sand.) At the top of the foam you'll need some Z-flashing lapped with the WRB drain plane to direct bulk water out rather than between the foam & CMU.

    It's likely that when you had leakier ducts there was regular displacement of the basement air with drier air conditioned air, but now that they're tighter it become important to reduce the amount of humid outdoor air infiltration, which is a primary cause of basement humidity in summer. Running a room dehumidifier in the basement may take care of that, and lower the overall latent cooling load of the house. It's probably worth monitoring the temperature & humidity in the basement, and seeing at what room temp & RH the duct sweating goes away.

    If the ducts are undersized for the air handler, as you tighten up the ducts the total volume of air going through the evaporator coil goes down, which lowers the temperature of the exiting air. This may be part of why the symptom is appearing now. If they're TOO undersized it's not uncommon to end up with frost clogged coils when tightening up the ducts.

    Small supply & return registers in the basement close to the air handler feeding the furnace room a bit of conditioned air would...

    A: increase the air volume going through the coil, increasing the temp of the duct,

    ... and ...

    B: displace humid basement air with dry air-conditioned air.

    If done with the appropriate supply & return balance this may be the right solution once the basement walls are as air-tight as you can make them. Even though it would impart some sensible cooling to the basement, the latent cooling load of the basement is the problem, and it doesn't take a huge number of air changes per day to make a big difference. You can close the basement registers when the cooling season is petering out, or leave them open- your call. Even though this may seem like you're defeating the duct sealing, by doing it in a balanced fashion rather than random leakage at random locations, it's still reducing air-handler driven outdoor air infiltration.

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