Basement temperature

Discussion in 'HVAC Heating & Cooling' started by mar3232, Nov 15, 2012.

  1. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Figured I'd ask here, someone probably has a definite answer --

    I have a half basement that is really well built and has NO windows.

    It really is always comfortable down there and this time of year, (midwest winter) it always is about 8 to 10 degrees warmer than the main level and that is with the heat off. (Evenings is when I look at the thermometers -- when the sun is down).

    Fact is, I'm heating with a wood stove and a space heater this year (upstairs).

    If a basement has no windows and well built then it IS going to an advantage to let that warmer air come up through a few registers it would seem to me, maybe even with a very small on/off blower. In fact, I have a couple of register openings I sealed of and now thinking of opening them back up.

    In the winter only though -- too humid down there in the summer.

    You hear a lot of people complain about their "cold" basements in winter but I bet it's because of windows and exposed foundation and I have neither.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    A fully submerged basement tends to reach the average annual temperature, which varies in INdiana from around 53-55 degrees. Not sure why yours is so warm...must be some other source of heat going on there, or good insulation.
  3. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Not going to believe this but the guy who built it made it a bomb shelter in 1961 (like concrete is going to save you).

    Concrete everywhere -- even my flooring system is concrete and steel T beams. I finished it as an office space, nice and dry, carpeted and like I say, all winter long whenever I open the door to the base ment I get a hit of warmer air, so I think I'm going to go ahead and open those registers.

    After all, the thermometers don't lie. No heating source down there at all other than occasional dryer activity and a few CFL bulbs.

    Thanks -- I'll have to remember that -- basements should follow the outdoor temp.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    Submerged basement walls and floors anyways, tend to reach the average annual temp, not the outdoor temp, which in Indiana varies through a pretty big range. Now, if they are insulated well, and have a huge thermal mass, and there's a way to transfer heat into them during the summer, they can act like a huge thermal storage tank at least part way into the winter. Now, if you lived over a big hot spring, that'd be a different story!
  5. Dana

    Dana In the trades

    Messages:
    2,841
    Location:
    01609
    The deep subsoil temps reflect the annual averages, except in active volcanic zones:

    [​IMG]

    So for an IN location that subsoil it probably in the high-50s.

    The fact that there's an insulated and heated house above it will increase average air temp well above the deep subsoil average, particularly in the early part of the heating season.

    The above grade section of the foundation will respond to the hourly outdoor temperature swings, with maybe a few hours delay due to the thermal mass effects if it's a 2' thick wall, but IS a net heat loss that's worth addressing. A few feet below grade it will respond to the monthly outdoor averages, also with delays due to the thermal mass of the soil. The soil itself has an R-value as well as thermal mass, but that R value will vary by an order of magnitude depending on soil type and moisture content. If you insulated the exterior of the foundation with R8-R12 rigid EPS down to the local design frost line it would make a real improvement on the seasonal heat load of the house.

    The U-factor of 8-10" thick concrete is about 0.8BTU per square foot per degree F. For a 2' thick wall it would be about 0.4BTU/degree-ft^2. This compares to a U-factor for typical 2x4 fiber-insulated construction of 0.1 for above-grade walls. With 18" of exposed foundation and a perimeter of 150-200' the foundation could be more than 20% of your total annual heat load, since the 18" of exposed foot-thick concrete loses more heat than the 10 vertical feet of insulated 2x4 wall it's supporting. By insulating it on the exterior you would improve on the (already apparently pretty good) earth coupling benefits of the thermal mass.

    FWIW: The foundation walls of my house are only ~8" above grade, ~12" below grade, but they're insulated on the interior with R18 rigid foam, and it's not dropped below 69F down there yet this year, despite only heating the conditioned space to 68F (and then with overnight setbacks to 65F, not that it always reaches that temp.) As long as there isn't insulation under the floor, the floor itself is helping to heating the fully conditioned space. By the end of winter it'll usually hit 65F though. (Before insulating it would drop to the very low 60s.) My deep subsoil temp is about 52F.

    BTW: A foot or two of concrete WILL save you from a significant amount of radiation due to the radiation trapping cross sectional of the densely bound oxygen atoms in concrete. (A quarter inch of lead would do even better.) Nothing's going to save you from the heat of a direct hit. But in the event of a nuclear conflagration you might not really want to be one of the survivors, eh? (I'd personally rather go in a flash than dying of starvation and cold in the ensuing years-long nuclear winter scenario. YMMV.)
  6. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    my other post didn't show.

    anyway, very informative -- maybe I should open my registers only at night because sounds like my concrete floors are helping to trap that warm air.
  7. Dana

    Dana In the trades

    Messages:
    2,841
    Location:
    01609
    It won't make a whole lot of difference either way- just kept them open and let it convect. It takes a lot of convection to move much heat when the temperature difference is only 5F or so, since the air has very little thermal mass. That's why you need monster 1000cfm+ air handlers for even the smallest hot air furnaces. The greater the temperature difference, the greater the convective forces, but if you have only a few square feet of register you're unlikely to get even 10cfm unless the main floor is 20-30F colder than the basement.

    The primary heat transfer will be the several 100s of square feet of ~R1.5-R2 floor + subfloor. At R2 (U-factor = 0.5) and a 5F temperature difference every 100 square feet of floor is transferring 250BTU/hr of heat from the basement to the first floor. If that's a 1000' of floor it's 2500BTU/hr, which is about the same output as a 750-watt space heater.

    By contrast, the heat moved by convection at a 5F difference is a bit less than 0.1 BTU per cubic foot. (The actual BTU exchange is closer 0.018 BTU * cubic feet * temperature difference I just rounded up to 0.1BTU per cubic foot at a 5F differenc to make the math easy.) So even if it hit 100cfm (=6000 feet per hour) you'd only be getting ~600 BTU/hr out of the convective currents. In practice it will be much less than 100cfm unless you make some truly large holes in strategic places to enhance the convection.
  8. lane234

    lane234 New Member

    Messages:
    7
    Location:
    Texas
    hi, my basement is warm also... no damp so i do not have a problem with mold... i had not thought to pull air up... elaine.
  9. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Ok Dana -- sounds like it's 6 to 1 half a dozen etc.

    You really know your stuff, very interesting. If nothing else, I still may blow air up from the basement once in a while during the evenings (maybe a timed blower) just to refresh the air down there. Was thinking about that last night -- because it's so tight down there and no fresh air coming in, that's an issue I need to address.
  10. Dana

    Dana In the trades

    Messages:
    2,841
    Location:
    01609
    Foundation sills (even with sill-gaskets) and band joists leak a lot more air than you might think- usually way more than all of the window & door seals on typical houses.

    In an Indiana climate that leakage often results in higher basement humidity in the summer since the summertime dew points are higher than that of the air-conditioned interior air.

    Active ventilation isn't usually necessary in homes of that vintage (or most vintages, for that matter, but that may change in states where IRC2012 is becoming code, with a more restrictive leakage maximum), but periodic active air exchanges between basement and the conditioned space doesn't hurt.
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