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I have an unfinished basement in Upstate NY in a large old house that helps in keeping the house cool in the summer. Would insulating it limit the cooling effect in summer?

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Wait... You want your house to be warmer in the summer? –  Tester101 Sep 6 '13 at 16:35
I don't see where he said he wanted to "limit the cooling effect in summer". –  Philip Ngai Sep 6 '13 at 18:13

4 Answers 4

Yes, insulating the unfinished basement would limit the cooling effect in summer and increase your cost of air conditioning. On the other hand, the uninsulated basement cools your house in the winter. The question is, which one would be less expensive if you want to keep your house comfortable in both summer and winter?

The only way of avoiding the tradeoff is likely impractical, you would need to build in an extensive air plenum behind the insulation which would allow you to extract cooling when desired by blowing air through the plenum. This is actually a form of geothermal energy.

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That cooling plenum idea is so bizarre.. I thought the same thing.. It would need to have some condensation recovery built in, perhaps sloping the plenum and draining to the sump, or using a condensate pump. –  HerrBag Sep 6 '13 at 17:45
It's unconventional but not unprecedented! Agree about the need to deal with condensation. campmackinaw.com/earth_tube_cooling_heating.htm –  Philip Ngai Sep 6 '13 at 18:12
Insulation slows heat transfer, it does not stop it. I live in a similar climate, my basement is lightly insulated and stays plenty cool in the summer despite on the hottest days blowing air up the stairway, drawing in hot outside air. Enough of that heat still leaks into the ground to keep the basement cool enough that one needs a sweater to spend any time down there even on the hottest days. In such climates, limiting heat loss is more important than increased cooling load which I expect to be negligible. (I have no active cooling at all) –  bcworkz Sep 6 '13 at 20:32

It may seem counter-intuitive to think insulating a basement will prevent it from being cool in the summer, but this has truly helped me keep my home cool during the hot months. The insulation keeps the warm air in, and the hot air out. It has definitely helped me stay cool in the summer and warm in the winter!

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First the term "basement" in the technical term is supposed to be used for a floor that is completely below ground level. Almost nothing in the US is built like this for residential. So I will use the term basement to mean many things but not for what it is technically supposed to be used for.

Working with a local company doing energy audits on basements and finishing more basements than I can remember to count I have taken a few things away with me. There are a lot of factors that should go into the design of your insulation. So to answer your question in its most basic form - yes adding insulation will make your basement warmer in the summer for sure.

List of factors:

  1. What is the variance of temperatures in your house during the most extreme weather (hot and cold). You are in NY so you can have 100F - -20F. So given your thermostat is set at 72F all year long what is your basement floor at when it is 100 or -20. There are specific remedies depending on the variance. With lower variances you need little to no insulation in temperate climate.

    Reasons for variations:

    • Type of basement walls.
    • Condition of basement walls
    • Thickness of walls and slab
    • how many windows
    • what percentage of the basement is above grade
    • others
  2. Climate - If I am doing a house in Toronto it is getting a ton of insulation. In Texas (yes there are basements in northern Texas) probably none. In between it gets tougher to say.

  3. How will the basement be finished? If you are drywalling a basement, then your basement and house will be noticeably warmer with drywall by itself. That factors in.

  4. What is the budget? In colder climates you would want insulation on the outside of the house. Spray foam is probably the most effective and leads to the least amount of mold issues. But is there budget for this? No use offering a solution that will not be used.

What do I suggest for a very temperate climate:

  1. Any part of the basement that is not "under ground level" gets insulation for sure. I offer three choices: Spray foam, rigid panels, or rock wool. So if you have a walkout - that whole wall is done.

  2. Keep insulating until you have diminishing returns. This is really hard to figure out unless you know an area well or it is the coldest part of the year. So for instance if I do a house in Kansas City, I would go about 1-2 feet below ground level. I just did this at a friend's house using rock wool. The inspector was impress too because he knew that the winters are short and that doing the whole wall would impact the summer temps.

  3. I hate putting insulation on the bottom 3-4 feet of a basement. First I find it is of little value unless you are in a very cold climate. Second I want a large air gap behind my drywall. Give moisture room to fall and evaporate.

  4. The main concern with insulating in any climate is to keep from having a huge temperature variance. That means in NY I would make sure that all of the edges of that basement were overinsulated in the the joist/crawlspace areas. I would be stuffing every nook with rock wool going 1-2 feet deep if I need. I would probably run it another 2-3 feet below ground level too. On a cold cold winter day I want the temperature in one of the upper corners of my basement (should be the coldest point) to be the same as the center of the basement floor.

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I did my entire basement with spray foam a couple of years ago. The basement is still cool in the summer, but not as cool. But it is now usable in the winter.

One assumption we might make is that fossil fuel costs will escalate more than electricity costs. If this is true, savings in winter heating costs will offset the additional air conditioning costs. And fundamentally, this is the question. How can I maximize comfort for the lowest total cost?

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