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Thread: Adding an air return

  1. #1
    DIY Junior Member slippy field's Avatar
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    Default Adding an air return

    trying to work on finishing my basement and need to add one air return. Any advice on getting me started? I would now the first thing about tying into the main trunk or where to put the return trunk. I supposed since I am just going with one return, as close to center as possible in the large room would be best.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Are you primarily worried about heating or air conditioning? What you don't want is your return positioned so it can suck your supply in before it ends up being distributed around the room. Where are the supply ducts, in the ceiling or low near the floor?

    FWIW, I find that in my home, pulling the hot air out from the ceiling, whether heating or cooling, and running the fan on low gives me the most even room temperature. A/c ducts at the floor aren't the greatest, nor are heating ducts in the ceiling. They both can be made to work if carefully positioned and you have good returns and your fan is up to it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
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    Hopefully you've air-sealed and insulated the foundation walls in a manner that won't turn into a mold farm?

    Heating & cooling loads of insulated basements are quite modest in any OH climate, and insulated or not, basement zones are impossible to balance with above-grade loads.

    Jim's approach will likely work for you too. Simply ventilating the basement with air-conditioned air from floor above would almost always handle the cooling load of a basement. And with very low heating loads directly heating the basement may or may not be necessary with if it's insulated, depending on how well insulated any distribution ducts in the basement are, but even a modest amount of air drawn from a warmer spot upstairs would get you there.

    With a walk-out basement this changes, but only the rooms with significant above grade would need ducting, but the room-by-room heat loads & air flows would need to be calculated to make it balance reasonably with the first-floor zone.

    In my central MA home the air-handler & ducts are in the basement and not well insulated, but I have ~R18 in rigid foam on the foundation walls & band joist, and it stays ~68F there in winter unless we're using the wood stove to heat the air-handler zone, and even then it doesn't drop much below 65F, even when it's in negative single-digits F outside. If I cut in heating registers to the basement rooms it would likely cook down there just on the bleed-by leakage with the registers closes.

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    DIY Junior Member slippy field's Avatar
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    I do have a walkout in this basement. The floor plan is just one open room around 700 sq/ft. The 2 feeds are in the ceiling almost center of the room maybe 10 feet or so apart. The return trunk is right beside it but my only return vents are on the top 2 floors. I wasn't even sure I needed a return in the basement until someone told me I should put one in.

    Year around I go in the unfinished basement for odds and ends and of course its a little cooler in the basement in the winter but all in all the temp range seems decent. Up under the joists has been insulated agains the foundation and around the slider door. I planned on using the R-17 rolls against the foundation walls.

    The part about not putting a return too close to the feed vents makes sense. I just need to know if I need a return in my basement and the best place to put it.

    I appreciate all the info, thanks!

  5. #5
    In the trades Dana's Avatar
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    Fiberglass roll insulation have a pretty lousy history from moisture & mold management point of view. For poured concrete foundations it's better to either use 3-4" of rigid foam (expensive, at full retail- I used reclaimed roofing foam), or an inch or two of EPS (the macroscopic-beaded stuff of cheap coolers and coffee cups), trapped to the wall with a non-structural studwall with UNFACED rock wool or fiberglass.

    With R13-15 in the stud bays and 1" of EPS (edges & seams sealed with can-foam) the "whole wall-R" after factoring in the thermal bridging of the framing is about R14-15. With 2" of EPS it's about R20. Since EPS is air-impermeable but semi-permeable to water vapor moisture doesn't build up in the concrete, raising the mold potential at the foundation sill- it dries toward the interior. Yet the interior side surface of the foam stays warm enough to not have wintertime condensation/accumulation in the fiber side of the stackup, even without interior vapor barriers (which would otherwise trap ground moisture in the studwall.) Since it's a non-structural studwall you can isolate the bottom plate of the framing from the slab with an inch or two of EPS, which keeps it warmer (=drier), and provides a capillary break against moisture that might otherwise wick up through the slab. It also means the top plate can be a single-plate, not doubled. Insulating and air-sealing the wall foam to the foundation sill band-joist with an inch or two of closed cell foam is highly recommended (a couple of 12 board-foot FrothPak kits from a box store can do quite a bit if you're careful), since sill gaskets all leak, and most band joists weren't air-sealed during construction, making the foundation the largest single infiltration factor in most homes. (It's just one big long skinny hole, but adds up to 10s of square inches.)

    You could also use XPS (pink/blue/green board) or polyiso, instead of EPS. XPS is somewhat more vapor tight and more expensive than EPS, but it's also blown with HFC134a, which outgasses slowly over time, causing it to lose ~15% of it's R-value after a few decades. HFC134a is also a very potent greenhouse gas, about 1400x CO2 in global warming potential. Polyiso (and EPS) is blown with much more benign pentane (~7x CO2 GWP), but usually sold with foil facers, which can be an issue for the foundation sill unless the house was built with a decent sill gasket as a capillary break between the concrete & wood.


    If you have a good sill gasket, insulating the wall with a fire-rated Thermax using HDPE or nylon pancake-head fasteners can be much quicker & sometimes cheaper than an EPS + studwall method:







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