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Community and Q&A

Will insulating my basement make the house hotter in the summer?

RoseWhale | Posted in General Questions on

Sorry if this has been asked before, I couldn’t find anything specific to my situation on the forums.
Our house has no AC, only a heater. The basement is honestly fine during the winter, not too cold at all for us.
It’s also the only room in the house that is even remotely bearable during the summer. It usually gets up to 30°C/86°F *inside* the house during the summer and it’s miserable. Our basement is unfinished at the moment, but my husband’s parents are insisting on insulating the basement walls. I understand it keeps the home warmer during the winter and will save money from needing to use our heater less, but I’m worried that because we have no inside source of cool air whatsoever and that the heat just fills the house so easily during the summer, that it’ll just make the heat upstairs even hotter? We were hoping to just stay in the basement during the summers for the most part once it has walls and is properly sealed because opening windows at night, using fans, etc does not help in any way upstairs. But now we’re concerned that the basement will just be warmer too and we honestly really like the current basement temperature year-round and don’t want that to change.

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  1. Expert Member


    The answer to this is somewhat location dependent. If you wouldn't mind sharing a general location, we'll be able to help nail down an answer. Some of the details (if known) would help too. Approximate house size, basement size, wall construction layers, etc.

    1. RoseWhale | | #2

      We are located in Calgary, Alberta Canada. The summers have been getting hotter each year, with the outside temp getting close to or exceeding 30c fairly consistently.
      Square footage of our house is approximately 1100sqft (according to landlord's wife so that might not an exact measurement).
      Basement size is the same floorspace as the house with about 2 feet of the wall being above-ground.
      I'm not 100% sure about the wall construction layers (or what you're looking for exactly)? For the basement specifically I believe it is concrete, wood frame, and then they plan to insulate that and then add on a layer of drywall. And the entire basement will be sealed due to previous tenant water and pet damage.
      We have also had a problem with leaks (about 3-4 times in the last year, sometimes due to pipe issues, once from heavy rain).

      1. Expert Member


        That should be enough detail, thanks! For the wall construction layers, from outside to inside, I was really only asking if there was any insulation and what type of waterproofing existed on the outside of the wall, if any. There's a lot of topics on this site about insulating basements, I'll put a few links below. They're mostly concerned with controlling moisture, not really the heat, but it's something that it sounds like you'd need to address at the same time. If you install a fibrous insulation like fiberglass and it gets wet, you'll get yourself into trouble quick with drywall, wood, and mold.

        Can you tell us a little more about the outside of the house? Are the gutters in good shape and diverting the rainwater away from the building? Is the grading sloped away from all sides? Any standing water next to the foundation? Just trying to establish if it's a ground water source or a drainage issue.

        Tell us a little more about the planned 'sealing' of the basement too.

        Sorry for all the questions, but the more we know the more we can help!

        1. RoseWhale | | #4

          Thank you for the link!
          I'm honestly not sure what waterproofing measures were taken for the outside of the walls- if any. The house is 60+ years old and I don't think any major renovations to the outside or the foundation have been made. I'll ask later though (it's 12am right now so I can't contact them until later in the day).
          I believe the gutters are in good shape, they were also just recently cleaned with a pressure washer (after our last leak, the in-laws took care of the gutters to try and divert water away from the house and so far it seems to have worked but we haven't had the basement inspected yet).
          The ground outside the house is angled towards the house/the house is lower than the ground around it, but thankfully we're in the process of fixing that via adding dirt and mud-jacking the ground and surrounding sidewalks in the near future. We've added several bags of dirt around the house to try and level the ground that's directly touching the foundation/walls for right now.
          There is no standing water near the house thankfully.
          As for sealing the basement, I'm not sure what the exact process is. But the previous tenants took very poor care of the house, and during renovations before we moved in, the ground level floors were slightly damaged and reeked of animal urine, so the floors were sealed before the new vinyl flooring was put in. As far as it's been explained to me, the same thing will be done to the basement floors and walls because of the same reasons, especially where there is any mold present from the water damage (it was flooded with a few inches of water for a few days before the previous tenants told anyone). I think they mentioned using Shellac sealant? Unless that's just for wood. And then using some kind of product called "kilz" I believe? I'm assuming the basement & masonry waterproofer, from the sounds of it. Sorry, we've been letting the in-laws handle all of the plans unquestioned until recently so I'm not 100% sure what the steps are or products being used. I'll try to get more solid info when possible.

      2. user-2310254 | | #10


        You mention a "landlord's wife." Will the landlord perform this work, or is it something you are considering funding? If it's the latter, I can't see how fixing the basement will be a good worthwhile.

  2. Expert Member

    That's all helpful information so far! Let me and the others ponder on it for a while and I'm sure one of us will come up with at least something marginally helpful :).

  3. ohioandy | | #6

    Rose, let's not lose sight of your basic question: "will my house get hotter in the summer if I insulate the basement?" The answer is theoretically yes, since you're adding thermal insulation to the part of the house that touches the relatively cool soil. BUT, there's a lot more going on here. Once you start "conditioning a basement" using sound advice from building science, you WILL end up with more comfort. Maybe the best course of action is first understand the science of what's at play, and then manage your expectations.

    Insulating a basement that has a history of dampness--let alone flooding--is bound to cause bad problems. See the related basement articles on this site, and fix the water problems and air leaks before embarking on any insulation scheme. There's a contingent of folks here who argue that a basement is the worst thing you can put under a house. They're not wrong, but I gotta say I love mine despite the expense and hassle.

    Your basement feels cool in the summer for a couple reasons. First, it's a total contrast to the upper levels which, from your description of this 60-year-old house, are poorly insulated and probably quite air leaky. It may be a wise investment to also think about beginning to airseal and insulate the top half of the house, which will pay off in both heating and cooling seasons.

    Secondly, your basement is a summer refuge because it IS cooler. Like a cave. Subsoil temperatures next to uninsulated walls and floor keep it that way. But it's a damp cool thanks to water leaks and vapor diffusion through the concrete. Few people want to live in a cave. If you work at conditioning your basement properly, you'll lose some of the benefit of that cool soil, but the inside air will be drier. Drier air feels more comfortable than damp air, even at a higher temperature. Plus, a drier basement is less moldy and musty, and the whole house benefits.

    1. RoseWhale | | #7

      I was looking around at some of the articles on this website, and I'm not sure what you mean by "conditioning the basement properly"? Is that just part of the insulation process? Could you elaborate? (Or point me in the direction of a resource because all I see online is about air-conditioning)
      Sorry, very new to this so I'm just trying to get more information. I appreciate your help.

      1. ohioandy | | #8

        Sorry about the jargon. I'm saying "condition your basement" as a catchall term for doing the things required to make it livable. Another term might be "bring it into the envelope", referring to layers that separate a house's interior from the outside. In your house, like most old houses, what exists of these layers wasn't typically extended to the basement. Basically, conditioning a basement requires beefing up some or all of those four control layers (water, vapor, air, thermal) and then adding heating/cooling and ventilating the space. Some of this is hard and expensive; basements can be a hole in the ground where your money goes.

        The moral of the story is: if you have limited funds and are looking for a way to improve your basement, don't start with insulation. Spend it on keeping water out first, and then blocking air leaks second.

  4. DC_Contrarian_ | | #9

    If it's 30C out and you don't have air conditioning, presumably you're going to have the windows open. What you do with the basement isn't going to make a difference.

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