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Basement very cold what can I do

MsHyacinthRose | Posted in GBA Pro Help on

I have a new home that is well build and very well insulated…very low gas heating bill for size of house.  I have two furnaces one on the upper and main floor.  The main floor thermostat serves both the main floor and basement.  House is very well built and uses very little energy.  The problem is the basement is 10 degrees colder than the main floor.  Usually 58 to 60 degrees.  It is fairly large basement; finished, carpet and air vents in the ceiling. The return vents are in the ceiling and are at the back of the basement.  No basement door.  Any ideas on what I can do to warm up the basement. No one wants to use it.  I do have two space heaters but would probably need 5 or more to warm it up. Would adding a door help. I’m open to adding more heating.  Note I am putting in solar panels this year.   Appreciate any advice.  Thank you.

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  1. Dick Russell | | #1

    The experts on this forum will need a lot more information. Where is the house (what climate zone, or general state/town)? Is the basement largely below grade? Describe the basement wall structure. What does "finished" mean, and by that I mean to ask if there is just paneling or something like that, or is there insulation against the basement walls. Is there insulation beneath the slab?

    If there is no basement door, I wonder if air from the basement ceiling vents is largely returning via the return vents from the upper level. Is there any chance the basement return vents are partially blocked?

  2. Steve Knapp CZ 3A Georgia | | #2

    I suspect you have a problem because the basement and first floor are on the same zone. Unless your systems were professionally designed by an independent engineer, they are probably oversized and not running efficiently. For comfort, you really want systems that run a long time at low speed. If they are just turning on for a few minutes and raising the temperature to the set point, there isn't much of a chance for the basement to reach a comfortable temp.

    This is all speculation, of course. What does your HVAC person say? Has he/she proposed putting the basement on its own zone?

    I had a similar comfort issue in my last home, and the HVAC company was able to solve it by adding a motorized damper and additional supply to the space. It served to oversupply the area when the system was running. A local control allowed me to open or close the damper if needed. That was a pretty simple and inexpensive solution. That said, my systems were professionally designed and sized for comfort.

  3. MsHyacinthRose | | #3

    Steve, thank you very much for your advice. I'm not terribly construction minded but it was a custom built home. I did have the HVAC people out who put the system in and they didn't really have any advice expect to say the basement is always colder, which is fair but not 10 degrees. I'll reach out to another company.

    Dick, thank you also. I'm in the midwest, zone 6 in flower terms. It is a walk out basement with a large patio window and a few regular windows that are above floor. All good quality. By finished I mean dry wall and conventional ceiling per bob villa .--had to google that :) The basement stairs have a landing and there is a return air vent on the landing. I don't know if there is insulation against the basement walls but the house is well insulated in general. The builder finished the basement. I didn't build the house so wasn't involved in the details but I'll ask.

  4. Expert Member
    Akos | | #4

    90% of cold basements are from air leaks at the rim joist area. Unless somebody has done proper blower door directed air sealing, even a house that looks well built and insulated can have big leaks there. Stack effect in the house draws in cold air through these leaks causing the issues.

    Because of stratification this cold air than settles in the basement making it feel like a fridge. Most basement supply and returns are near ceiling level and they don't provide enough mixing to avoid stratification.

    You have two options. Fix the air leaks or add more heat. If you go down the path of more heat, generally you want a couple of registers down low near floor level. Make sure to put a damper on the trunk supplying these as you generally need to shut them off during the summer.

    A well sealed and insulated basement is quite warm, it should be within a couple of degrees of the main floor with little to no heat.

    1. Charlie Sullivan | | #6

      I agree that this is the most likely problem.

  5. Steve Knapp CZ 3A Georgia | | #5

    Here's a link to the climate zones for construction:

    Yeah, it all gets back to the system design. Most HVAC companies seem to use general rules of thumb rather than house-specific numbers. If you are not doing so already, you should buy an inexpensive hydrometer (something like this: and monitor humidity levels--especially in the basement.

  6. Doug McEvers | | #7

    An R-10 minimum under the basement slab does wonders for comfort. The earth connection is largely responsible for the temperature difference between floors. The basement should have its own zone with a motorized damper sending heat where it is needed.

  7. MsHyacinthRose | | #8

    Thank you everyone appreciate the advice. What are rim joints? Also what is blower door directed air sealing? I do appreciate all the advice. I will have someone come to assess and it is helpful to know if I am getting good advice. I assume if I have carpet, walls up and a ceiling that it would be difficult to insulate?

    1. Expert Member
      Zephyr7 | | #9

      Rim joists are the boards that run around the top perimeter of your foundation and tie the ends of the joists together. There are a lot of framing elements that tie together there, and it's a famous area for leaks. When I insulated mine, I could feel drafts in almost every joist bay by just holding my hand near the rim joist there. Leaks that you can easily feel with just your hand tend to be bad ones.

      Blower door directed air sealing means running a blower door (a big fan -- a blower -- that quite literally is mounted temporarily in an exterior door to your home), then using a smoke pen or similar to pinpoint leaks that need fixing. The smoke will show the leaking areas since the blower door will be either pressurizing (which makes the smoke blow out TOWARD the leaks), or depressurizing (which makes the smoke blow inwards, AWAY from the leaks). This is the best way to find any leaks, especially sneaky hidden ones.

      It's much easier to insulate before walls get closed up, but it's not impossible.


  8. Expert Member
    Dana Dorsett | | #10

    If you have an infra-red camera and a big window fan you can find the big leaks without a full blower door treatment. The $200 FLIR One that works with smart phones & tablet computers is more than sufficient for this task.

    If the rim joists and foundation sills are leaky it will be hard to ever make it comfortable down there. But in addition to the rim joists, sealing all penetrations of the attic floor/upper floor ceiling will also help. Holes in the top of the house let's buoyant warm air out, depressurizing the house, which sucks cold air in at the bottom of the house (basement.) Fixing both the attic leaks and basement leaks are job # 1.

  9. MsHyacinthRose | | #11

    Thank you everyone. I really appreciate all taking the time to offer advice. Do you think a door to the basement would help as currently there is no door at the entrance to steps or at the bottom to the basment?

    1. Expert Member
      Dana Dorsett | | #12

      An empty doorway at the basement is a pretty big hole between floors, well worth sealing along with al the other big and medium sized leaks at the basement ceiling, especially any chases running all the way to the attic. But it's far more critical and more effective to seal the basement's exterior walls/doors/windows to the outdoors. Air sealing a basement ceiling adequately enough to make a big difference is nearly impossible in a retrofit, whereas air-sealing the exterior walls all the way down to the slab, the crack between the slab & foundation walls, any sumps holes or cracks in the slab can actually get you there.

      But it's slightly more important and often easier to seal the attic floor/upper floor ceiling plane to limit that depressurization factor that's causing the basement to suck in outdoor air in the first place. Sealing both the top and the bottom of the "stack" in the "stack effect" lowers the overall drive- it's much more important than all of the air leaks in the intervening stories' walls.

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