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Thread: HVAC ductwork to basement

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  1. #1
    DIY Junior Member
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    Default HVAC ductwork to basement

    My house was built on a hill, and under the house on the lowest part of the hill, there is a 13'*28' unfinished basement. No HVAC, accessible only via an outside door, and we use it only for storage. Since we're in north Florida, the basement has a really nasty humidity problem, and anything down there winds up rusty or soggy. Life expectancy of a screwdriver bit down there: 1 year.

    The house's HVAC ductwork is in the attic, so tapping that directly to run it into the basement is more or less out of the question.

    I had the following thought, and I'd like to hear if it's a good--or stupid--idea:

    - Cut hole in floor of HVAC closet and put floor air register in floor.
    - Use a rectangular-to-round adapter, and run 4" semi-rigid metal duct through the crawlspace to the basement--about a 10' run.
    - At basement, install directional damper, duct booster fan, and ceiling diffuser.

    Thus, the booster fan would draw in conditioned/non-humid air from the interior of the HVAC closet [vicariously from the house via the grille in the closet door] and use that air to control the humidity in the basement.

    The only issue I could see is that, when the air handler runs and is sucking air up, that it would be competing against the duct booster fan. The workaround I had in mind was to put an airstat in the air handler's intake, so that when the air handler runs, the basement duct booster fan stops--and the directional damper would close the duct.

    Thanks!

  2. #2
    DIY Member Lightwave's Avatar
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    It is unlikely that your main floor AC has the spare capacity to dehumidify your basement in addition to conditioning the rest of your house.

    Consider a dehumidifier or a separate air conditioner for the basement.

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    Quote Originally Posted by Lightwave View Post
    It is unlikely that your main floor AC has the spare capacity to dehumidify your basement in addition to conditioning the rest of your house.

    Consider a dehumidifier or a separate air conditioner for the basement.
    Yeah...I've used a dehumidifier down there in the past when I'm painting furniture to speed up the drying. I was looking for alternatives to a dehumidifier and its associated energy use.

    One other option I looked into was getting a GE GeoSpring heat pump water heater and ducting the heat pump's cool/dry output into the basement. Yes, I realize it would get especially cold in the winter, but I care more about humidity than temperature. This is much more a property protection issue more than personal comfort. :-)

    Thanks

  4. #4
    In the trades Dana's Avatar
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    First things first:

    Have you sealed the foundation wall & slab in the crawlspace & basement with a masonry sealer, to limit the capillary draw?

    Have you pressure-tested the basement to ensure that air infiltration has been minimized to the extent possible? (Have you foam sealed the foundation sill?)

    Are there combustion appliances (hot water heaters, gas-fired dryer, furnace, etc) in that space, with open flues?

    Have you laid down a poly vapor retarder in the crawlspace floor, and mastic-sealed it to the foundation walls a foot from the floor, held in place with furring?

    Are the above-grade exterior walls insulated? (Higher basement temps increase ground-vapor drives.)

    Given the high latent loads of FL & typical levels of AC oversizing found throughout the industry, the first-floor AC probably IS capable of dehumidifying the basement & crawl with capacity to spare, as long as they're reasonably air tight to the exterior, with some measures taken against ground moisture. It need not be driven by the the air-handler for the AC directly- floor grates in opposite corners of the basment and a dehumidistat-operated blower to exchange air between conditioned-space and the basement or crawl is sufficient. Keeping it under 65% relative humidity is best for mold-hazard control.

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    OK...You've asked some good ones. Here goes:

    "Have you sealed the foundation wall & slab in the crawlspace & basement with a masonry sealer, to limit the capillary draw?"

    No. The fun part is that the previous owner "finished" half of the basement with drywall walls and ceiling, an interior door, and a vinyl floor. We use it for "clean" storage like furniture; the other unfinished/concrete block/concrete slab floor, we use for "dirty" storage like the wheelbarrow, lawn care tools, etc. Oh--and every time it rains hard, water gets in under the outside door. I need to do some French drain magic at the door...

    I don't know whether or not the previous owner sealed before "finishing", but the finished room definitely treats its contents more humanely than the unfinished half. No mold has shown up on any drywall.

    "Have you pressure-tested the basement to ensure that air infiltration has been minimized to the extent possible? (Have you foam sealed the foundation sill?)"

    I have foam sealed, but just around the door assembly. It looks like the basement had double doors at one time, and someone put in the current single door with about a foot of clapboard siding on either side. All perimeter walls in the basement are concrete block, meeting a concrete floor, so really no sill to foam seal. There's also a double-pane window in an unfinished wood frame down there; I should seal that.

    "Are there combustion appliances (hot water heaters, gas-fired dryer, furnace, etc) in that space, with open flues?"

    None. FWIW, there are no gas lawn care tools or gasoline down there either.

    "Have you laid down a poly vapor retarder in the crawlspace floor, and mastic-sealed it to the foundation walls a foot from the floor, held in place with furring?"

    Nope.

    "Are the above-grade exterior walls insulated? (Higher basement temps increase ground-vapor drives.)"

    They're concrete block; no clue if they're filled with insulation. The ceiling has pink fiber roll insulation between the joists, insulating the house upstairs. All this insulation is in excellent condition. The house was built in 1980.

    "Given the high latent loads of FL & typical levels of AC oversizing found throughout the industry, the first-floor AC probably IS capable of dehumidifying the basement & crawl with capacity to spare, as long as they're reasonably air tight to the exterior, with some measures taken against ground moisture. It need not be driven by the the air-handler for the AC directly- floor grates in opposite corners of the basment and a dehumidistat-operated blower to exchange air between conditioned-space and the basement or crawl is sufficient. Keeping it under 65% relative humidity is best for mold-hazard control."

    The crawlspace is _not_ airtight--there are a few concrete-block-sized vents to the outside. The basement does not have said vents. FWIW, the crawlspace has a plastic tarp held to the ground with buckets filled with gravel, again, not at all sealed.

    If I went that way, I'd want to put the floor grates on the floor inside the air handler closet; I'd rather not have floor grates visible in my house--and have to arrange my furniture around them.

    Thanks for your help! I guess it's time for me to open up my massive DIY book!
    Last edited by mahohmei; 04-26-2010 at 06:05 PM.

  6. #6
    In the trades Dana's Avatar
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    If the crawlspace has air-communication paths between it and the basement, the basement is essentially at outdoor humidity levels due to those exterior vents in the crawl. Crawlspace ventilation to the outdoors was conceived to be a way of letting ground moisture escape and put into building codes on this theory. While that theory may hold true in cooler drier climates, in places hot and humid as FL venting crawlspaces to the outdoors INCREASES rather than decreases the average crawlspace humidity. Read this article for details on converting itl into a sealed-conditioned crawlspace (and why.) You may be required by code to keep the vents ,making them operable, opening only when you need to for some reason (say, for drying after a bulk-water flooding event.) Insulating the walls with an inch or two of XPS rigid foam (pink/blue, not beadboard or any poly/foil faced stuff) rather than merely vapor-sealing them with poly will earth-couple the building, decreasing both the sensible-AC & heating loads. Either leave a 3" gap in the insulation between the top of the foam and the foundation sill as a termite-inspection strip (those suckers will tunnel through foam sometimes), or nail 6" of copper flashing to the foundation sill extending down between the foam and the foundation to deny them an unseen path to wood.

    The plastic tarp on the floor of the crawl is a good start. If it's long enough at the ends, sealing it to the foundation with duct-mastic before installing any wall insulation or vapor retarder will increase it's effectiveness once the vents are sealed.

    The vinyl flooring in the finished basement is an effective capillary break & vapor retarder for ground moisture on the floor. On the sub-grade finished walls behind the drywall let's hope they at least applied some masonry sealer to lower the capillary draw, but do NOT put any vapor retarders such as vapor retardent paints, vinyl or foil wallpapers on the drywall or you'll create mold condtions in the studwall.

    Where the exterior walls are exposed and completely above grade, acrylic or silane/silcone based masonry sealers both inside & out will reduce the capillary draw from any rain or dew -wetting yet still allow the masonry to dry from vapor permeation. When sun hits wet masonry the vapor drives skyrocket. Insulating the interior with semi-permeable or semi-impermeable foam (1-2" of XPS), or impermeable foil or poly faced EPS beadboard or iso reduces the amount of vapor permeation through the wall into the basement. (Again, don't use the impermeable stuff below grade, or you'll end up with higher moisture content in the masonry, which could rot the foundation sill and/or cause the above grade portion to have efflorescence & spalling issues.) To meet fire code, foam insulation needs a thermal barrier against ignition- half- inch drywall works.

    Gluing the foam board to the masonry with purpose-made foam-board adhesive in a ~18" pattern of walnut sized blobs, then through-screwing thin furring (even ripped down 3/8" plywood works, or standard 1x furring is fine if the wall thicknes is OK with you) into the wall to hold it in place gives you something to mount the drywall to. (Ignition barrier may be required in the crawl as well.) Devising a spacer method to keep maintain a 1/4" gap between the masonry & foam will allow it to operate as a cavity-wall, giving those high vapor-drive conditions a place to vent the moisture to other than the foam, and any liquid water that finds it's way in then falls to the floor. This is more important if you use vapor-impermeable rather than semi-permeable foam, and more important on sides where the sun would actually hit a rain or dew wetted exterior wall. It's less critical elsewhere- the glue blobs usually give you about a 1/8" cavity if you don't press too hard, and wait for it to set up before applying the furring for the wallboard.

    A window fan can be used to pressurize/depressurize the basement to find & fix all of the air leaks. Stack effects in even 2-story houses are significant, which will draws air out of the attic depressurizing the house, drawing air into the basement. When hot the humid outdoor air drawn in cools in the basement & crawl, it's relative humidity goes up, which is likely to be more than half your basement humidity problem. Blocking the flow into the basement (and out of the ceiling on the upper floors) breaks up the stack effect slowing the flow, reducing both latent & sensible AC loads.

    Air sealing the ceiling to the attic is step #2, but with ducts in the attic it's often both easier & better to foam-insulate the underside of roof deck with 4-6" of open cell foam & close off the attic ventilation (especially if this is a circa 1980 house with the typical 27 recessed lighting cans that would need to be boxed over to air seal it.) A ceiled insulated attic puts the ducts completely inside the pressure boundary of the house, reducing air-handler-driven air infiltration, and rendering duct leaks relatively neutral. In a ventilated attic sealing & insulating the attic ducts is far more important. Radiant barrier between the ducts in addition to duct insulation is also worthwhile. (If you ever go the sealed attic route, don't let some foam installer tell you that you need to remove the attic floor insulation, or that "(x)-inches of foam is all you need". R30 of foam is expensive, and any fiberglass or cellulose/whatever on the attic floor will continue to provide signficant benefit, and causes no problems in cooling dominated climates. In cold climates you'd have to play the R-ratio game to limit wintertime condensation potential, but not in FL.)

    [Edited to add] Another easy-reading discussion of vented vs. unvented crawlspaces lives here. Even if you don't have mold problems currently in your crawl, it's undoubtedly adding to the latent-load on the AC and keeping the basement at higher humidity in your climate zone.
    Last edited by Dana; 04-27-2010 at 11:13 AM.

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