Insulating for A/C

[MiddleMike]

Originally Posted by theholycow

The air might flow down to the basement IF you have an open door (the small amount of draft beneath a closed door becomes inconsequential at the temperature delta in question), but then that air doesn't flow into the earth. The air remains contained in the basement, and IF the earth is warmer than that air, it will conduct some heat into that air, depending on the R-value of your concrete/finished basement walls.

However, in reality, the basement is bound to be naturally much cooler. The earth and the thick concrete walls both act as high-capacity long-term ballast, and all through the summer my basement is nice and cool even if I haven't been using A/C -- and the front half of my basement is half-exposed with uninsulated wood walls instead of full-height concrete.

The situation in a car is far different. My guess would be that it really isn't strongly affected by temperature-related air density differences because the volume is so small, the height is so small, and the air is constantly moving, so insulation should be evenly distributed.

It will go down through any openings that allow air through, not just open doors. Why cool your basement with AC (assuming it's not finished) that you don't intend to use for basement cooling? Seems quite a waste, but easily and cheaply accounted for with a can or two of good-stuff and/or caulk and an hour or two on a Saturday morning. All leaks become consequential if there a lot of them spread across an area. That's why folks seal attics after all, and seal baseboards.

Basements are much cooler, without a doubt. That kind of points to how bad an insulator cinder block/concrete is, though it is true that you'll never be able to fully fight the entire planet earth when it comes to temp in a basement. Summer time it's fine since it requires zero to cool it, but in wintertime it's a bit of a PITA when it comes to saving energy especially if you have uninsulated floors (or rather, uninsulated basement "ceiling") and don't seal the sill/concrete joint. Big heat sink and energy waster.

I was talking about heat transfer, btw, not air movement, but wasn't very good at expressing that initially.

When you say the front half is exposed with uninsulated wood instead of cinder/concrete, you do mean that the wood is overlayed on the concrete/block right? Can't say I've ever heard of a wood-only basement (though my not hearing of them does not mean they don't exist, lol).