Basement vent -- off topic perhaps

Discussion in 'HVAC Heating & Cooling' started by mar3232, Apr 30, 2013.

  1. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Off topic maybe but who better to ask than an HVAC pro.

    I have a really nice, tight (no windows) half basement in my house and have been using a dehumidifier down there in the summer -- works great.

    I've been concerned about fresh air getting to the basement and it just so happens that the guy who built the place installed a very robust aluminum vent (opening is about 12 x 8) and this is mounted at the top of the basement wall, goes through ground outside and then vents a few feet above ground in a nice rainproof fashion.

    I have a couple of questions --

    When I opened it, (I had it sealed), I immediately felt warm outside air coming into the basement, which solves the fresh air problem BUT sure doesn't help the humidity situation.

    I read somewhere that if you actually create a vent that pulls air from the basement floor, that this can then act as a dehumidifier ? true?

    It would be easy to extend this to the basement floor and it would be nice to save the electricity on the humidifier. but ?
  2. Dana

    Dana In the trades

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    Location:
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    "Sub-slab depressurization" vents designed for radon remediation will dry out basements when groundwater is the primary moisture source, but summertime outdoor air in Indiana will always be adding, never subtracting from the moisture burden. It usually requires a 50-150watt blower to get there, so it's not exactly going to save you on the power use end.

    If you're air-conditioning the first floor, actively exchanging the basement air with conditioned space air allows the AC to purge basement moisture without adding to the sensible (temperature only) cooling load the way a dehumidifier does.

    If you have ground moisture wicking through the slab and walls using masonry sealers will slow it down considerably, reducing the load on the dehumidifier. But in most basements (even windowless basements) air leakage at the foundation sill is still a large (often the largest) source of basement humidity. If the basement needs the dehumidifier even in winter ground moisture is likely to be a big chunk of it, but if it's primarily a summertime problem, it's air leakage.

    If your basement has no wall insulation and only fiberglass or nothing on the foundation sill & band joist, it's worth treating that. Even a flash-inch of closed cell spray foam over the band joist & foundation sill sealing it to the foundation is a good enough air seal, but it's worth adding some real R-value there too. you can't just put up a studwall and stuff batts in it or you'll have mold issues due to both ground moisture trapping and wintertime condensation/frost. The cheapest way to get there is with an inch of rigid EPS (white bead-board) foam trapped to the foundation by a non-structural studwall that's insulated with UN-faced batts (either rock wool or unfaced R13 fiberglass "sound deading" type batts.) To keep the bottom plate both warm & dry you need to put an inch of foam between it and the slab too. You then need to spray-foam seal the band joist & foundation sill to the top of your wall foam.

    An inch of EPS with all seams foam-sealed is sufficiently air tight to keep air-transported moisture from creating problems and is vapor open enough that the foundation can dry toward the interior, albeit slowly. At ~R4 is has sufficient R value to keep wintertime condensation in check for your climate, and the batts/studs won't take on wintertime moisture from the interior, they stay warm enough to stay dry.

    Alternatively you could put 2-3" of foam against the foundation held in place by furring through-screwed the foundation with TapCon's 24" o.c. on which to hang the half-inch gypsum (required by code as an ignition barrier for the foam.)

    Either way you do it, insulating & air sealing the basement will make the basement both warmer & drier, year-round, and will reduce the duty-cycle on your dehumidifier.
  3. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Dana --

    thanks so much, I've printed that out because within the year, I plan on putting up stud walls and finishing the basement.

    so it sounds like (especially after get the walls up and insulate) that exchanging the basement air with the air conditioned air on my first floor (ranch house anyway) is actually better than the dehumidifier?

    there are floor vents in the basement that I sealed off.

    maybe a little turn on/turn off fan (cycling) to draw the air up as well? or come to think about it, a fan that pushes the upstairs air down into the basement would be better?

    thanks.
  4. Dana

    Dana In the trades

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    Location:
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    A dehumidifier converts a latent cooling load (moisture) to a sensible cooling load (air temperature), plus adds the heat o the energy used to turn the compressor to the room.

    That's fine in the winter, but if the only time you're running into basement moisture issues is the summer it's adding to the whole-house load of the central AC, which could be providing that dehumidifcation function somewhat more efficiently, since it's generally higher in efficiency, and the compressor motor heat is dumped outdoors, not inside the house.

    But if you don't air condition the first floor there's little difference between ventilating with conditioned space air vs. outdoor air in summer- the air will have about the same dew-point/moisture level. In an air conditioned house the air will have a dew point below that of the temperature of the basement slab, so you won't be approaching a mold-inducing levels near the floor, but if ventilated with outdoor air it will often enough have a dew point above the temperature of the slab, and the moisture levels near the floor and lower parts of the walls will be extreme.

    Even the tiniest fan would likely move enough air between floors to keep up with the moisture load in the basement if the band joist & foundation sill are well sealed (as well as any dryer vents, etc.)

    The size of the heat loss from uninsulated basements is usually underestimated by homeowners, but it's usually more than 15%, sometimes more than 25% of the total heating bill. A poured concrete foundation is typically a bit less than R1, so for every square foot it loses 1/R, or 1 BTU per degree of temperature difference between the interior and exterior. A typical 2000' 40 x 50' rancher with 2' of exposed above grade foundation had about 360' of exposure. When it's pretty cold, near your 99% outside design temp, say, 15F outside and 55F inside that's a 40F difference. Times 360' that becomes ~15,000 BTU/hr, which is probably more heat loss than all your windows & doors combined.

    Most reasonably-tight 2000' homes with double-paned windows (or storm windows) in IN without foundation insulation will have a whole-house heat load at the 99% design temp of somewhere around 40,000 BTU/hr, which can be reduced to under 30,000 BTU/hr by insulating the basement. The basement then "coasts" in the low to mid-60s F all winter unless intentionally heated, hardly changing at all with the outdoor temperature. How well it works in YOUR house depends on the specifics of how it's built, the amount of above grade exposure, the air leakage, subsoil temps etc, but it's a pretty big uptick in thermal performance & comfort no matter what- your floors will average a degree or two warmer during the mid-winter coolth.
  5. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    This place of mine is very unique -- I should post a photo to show you. The entire flooring system is concrete and steel. It's apparently an industrial flooring system, the house was built in 1961.

    There are concrete blocks that lay in steel "T" beams (picture a T upside down) -- the blocks look like they're about 16 x 8 but there not conventional cinder blocks. On top of that, they actually poured concrete that serves as the upstairs floor.

    The guy who lived here wanted the basement to be a bomb shelter. ! (duck and cover).

    It's somewhat larger than a half basement.

    I have no heating down there but really don't need it and I heat with a wood stove and a couple of space heaters (putting in a couple of minisplits this year for ac + heat pump).

    Have some nice south facing windows, so I stay very comfortable in winter -- I'm thinking the concrete floors may help ? releasing heat at night ?

    The outdoor temp and indoor generally are 40 degrees apart before the sun goes down which is fine for me and generally just a few hours of wood heat keep me warm all night.

    I'm surrounded by woodlands so humidity is an issue and the only reason I use a humidifier in the basement is beacuse of some electronic parts I have down there.

    I need to just experiment with this basement/upstairs connection. It seems that leaving the basement door shut all year is a good idea -- right now anyway, the 2 spaces are completely isolated from each other -- maybe it should stay that way. I do need fresh air from outside though and thought I'd put a fan that come on once a day for 10 -15 minutes might be good.
  6. Dana

    Dana In the trades

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    Location:
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    The high mass floors are definitely evening out the diurnal temperature swings and moderating the solar gains to good benefit. When you insulate the basement it'll be even better, since the mass-floor won't be giving up as much heat to the basement.

    The evapotranspiration from the local woodlands have surprisingly little effect on your summertime basement humidity- it's really a fact of life of living in Indiana (or anywhere east of the Great Plains), and the larger air-mass currents sweeping north from the Gulf of Mexico. Even when the winds are from coming down from the Canadian midwest, it's coming across large forested & agricultural landscapes as well as the Great Lakes, and is still not very dry, though drier than the Gulf air.

    Mini-splits "play nice" with woodstove & passive solar since they modulate over a wide range of output, and run VERY efficiently at part load.

    Unless you have a radon issue or other sources of indoor air pollution in the basement there's no real NEED for fresh air down there when unoccupied, and using outdoor air in summer just introduces an indoor air pollutant, namely excess moisture.
  7. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Thanks Dana. I'm anxious to get the basement finished -- glad to hear about the concrete floors, I know they have to be helping.
  8. Retro_Lou

    Retro_Lou New Member

    Messages:
    2
    Location:
    Northern Virginia
    A fan may just be the solution to your problem. Depending on how much of the humidity you really want removed or if it's just an issue of having air passed through the room. Helping to circulate the air may make the space more breathable but if the humidity causing moisture damage is what you're concerned about than that's entirely different all together.
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