Why Are Basements Warmer/Cooler in Winter/Summer?

I just measured the temperature in my unfinished basement and it’s been mid to high 60s all weekend, though the temperature outside has been teens and 20s. And it’s not because of the furnace being down there, because it does not vary significantly on different sides of the basement. And the reverse is true in summer.

Question is whether this is simply the result of the ground being insulation combined with the heated (or cooled, in summer) house or whether the ground itself actually provides some heating or cooling.

The difference would be if I insulate the basement from the ground. Does that make it more or less energy efficient? My first thought is that it’s even more energy efficient, but then I started to wonder. (The question is how to structure the heating/cooling should I finish the basement.)

Your furnace will give off heat in the winter, from air leaks and radiation from the duct work. In the summer the leaking air will be cooler from the AC. It’s not the ground causing it to be warmer in the winter. You’d be better off insulating the floor.

Another data point: I have an unfinished room in my otherwise finished basement which has a sheetrock wall on one side, and poured concrete on the other three. I put some insulation in the ceiling and sheetrock wall to help keep it cool in the summer. We use it for food storage, since it’s colder than anywhere else in the house. The temperature ranges from about 58 degrees in winter to about 68 degrees in summer.

The temperature of the ground does fluctuate: In the winter, the ground will be colder than you’d like, and in the summer, it’ll be hotter than you like. It just doesn’t fluctuate as much as the air does. So you will get benefit from insulating your basement; it just won’t be as big as the benefit from putting insulation elsewhere in your house.

The ground temperature drops to a pretty steady 50-55 F once you get several feet down. (Obviously, the depth varies somewhat). I have a lower level that is half underground and you can really feel the difference, especially in the summer.

But you’re not going to get into the 60s from the ground alone - you’ve clearly got some significant heat coming in from the house. My guess is that the cooler heated air from the house is descending into the basement while the warmer heated air rises.

I’m having trouble understanding you. Are you sure you said what you meant to say?

Well, assuming that the temperature you’d like is close to the average yearly temperature. I suppose that in much of the inhabited world, the optimum temperature is warmer than the average temperature, in which case the ground might be colder than you’d like year-round. Either way, if the ground is off the optimum in either direction, it’ll help to have insulation.

[QUOTE=ZenBeam;12063122 In the summer the leaking air will be cooler from the AC. It’s not the ground causing it to be warmer in the winter. You’d be better off insulating the floor.

That would be a really great trick at my moms unairconditioned house…

Below the frost line the earth stays around 55°. The frost line varies by region, with Anchorage being lower than Louisville, for example.

Your basement is largely below grade and is thereby insulated from the direct effect of the weather----much better insulated that an R13 stud wall.

So…you have 4 walls that are largely insulated from the weather----and as the wall gets lower the insulation increases. Someone mentioned “several feet”; although in Midwestern cities like Dayton, Ohio the frost line is probably less than 3 feet—probably 30" or so. In most places the frost line----the point where the earth will be a constant 55°----is probably less than 36".

On top of these 4 below grade walls is a conditioned space. In other words, the “roof” of the basement------your first floor------is heated and cooled. (Contrast that with the “roof” of your top floor-----your attic-----which is in direct solar load.)

In short, you’re giving up little heat to the outdoors in the winter, and accepting very little heat (from infiltration) in the summer.

I was once told by a park ranger that the temp in a cave is pretty much equal to the average of the surface temp for a year, not sure how true or applicable that is.

But that leads to another question. Suppose I finish the basement, I would be blocking off the ductwork. That would not be a problem in obsolute energy efficiency, but might lead to a problem in planning.

Because I’m thinking if the temp is already mid to high 60s on the coldest day of the year, I might not need much if any heating at all, especially once the walls are insulated. But if this will change once the ductwork is sealed off, that might not be the case.

The core of the question is: how do you figure out what you’ll need after you change things around?

[The floor of the basement is about 5.5 feet underground, and there are 12 inch concrete block exterior walls.]

If you did nothing, right now would you say the basement is more usable in winter or summer?

Why would duct work be sealed off?

Hard to say. My wife thinks it’s less pleasant in summer than in winter, but she is very sensitive to heat, and she doesn’t spend much time down there anyway. My kids spend a lot more time in the basement and they seemed to disagree with her. I’ve never measured the temperature until this winter so I don’t have a summer reading.

Because when you finish a basement you generally box it in - you don’t leave exposed ductwork in a finished basement.

I live in a garden (AKA basement) flat and it is always warmer than one would think.

In the Summer if we have a hot spell it takes about two days to cool down and in the Winter if we get a cold snap, it again, takes about two days to really feel the cold.

I often have to open the windows in my flat in the dead of Winter 'cause the apartment gets so hot

You get plenty of cold air down there in summer if you run the A/C a fair amount. If you do, there is no doubt about it: Insulate the basement. You don’t need the cool earth to help the basement stay cool, since the cold air from upstairs will keep it plenty chilled it settles in the basement.

Insulating it makes it warmer in winter… But it doesn’t negatively affect it in summer.

There has to be a ground effect…

The last house I owned, in the early part of the summer it would be nice and cool down there, even when sweltering outside. Toward August though, it would be warm/hot down there.

Nov…cold outside, nice and toasty warm in the basement.

Feb…DAMN it’s cold down here!

{I live in MN - fyi}