Should I insulate between my basement and main living floor?

Question

I'm in my first winter in a ~140 year old home and attempting to improve efficiency / comfort.

The home has a concrete basement (originally sandstone foundation) which is conditioned space, but in practical terms the ceiling is too low for it to be living space. A home heating audit tells me that the basement should be insulated because it is a major source of heat loss.

I've investigated spray-foam insulation along the foundation wall and sill, but recently I've been thinking that for our purposes it may be more economical to consider the basement's ceiling as the 'envelope' to be insulated, with the following considerations / benefits:

• We don't care if the basement itself is cold - it is unoccupied
• fiberglass batting in the (exposed, accessible) joists will be much cheaper to install. Specifically, it won't involve hiring anyone
• the overall insulated cubic footage will shrink, which I presume can only be a benefit
• batting will also provide some soundproofing against the furnace + dehumidifier which kick in intermittently from the basement

Answer 1

So long as you don't have a humidity problem in your basement (you said it's conditioned space), the standard fiberglass batt insulation should be just fine.

But as many here will tell you, using fiberglass batt insulation in an unconditioned space such as a crawlspace is asking for trouble. Over time the fiberglass will pick up & hold moisture from the air, which causes it to lose it's insulating property and becomes a home for mold.

Answer 2

If the basement is conditioned, then it should be dry and the temperature of the basement would be intermediate between the living space and the environment. Therefore, you would not get much benefit from insulating the basement from the living space.

As reported above insulation can cause problems. A new building practice for attics is the "sealed" attic with no soffit vents for outside air to come into the attic nor roof vents to expel air. The attic is actually lightly conditioned by transfer of air from the living space. The insulation is on the underside of the roof decking and there is no insulation between the attic and the living space below. I am told that the temperature in such attics is intermediate between the living space and the outside.

If the hvac system transfers air between the living space and the basement, it makes absolutely no sense to insulate between them.

Answer 3

If it's an old house, the first thing you should do is look where it loses the most heat.

You can do that with something that makes smoke to look for drafts (like a vape) or, much more effective, rent a FLIR thermal camera and take shots of the house from outside on a cold night. This will show you where the outside surface of your walls/roof is hot, which means either hot air from inside the house is leaking through, or there is a lack of insulation somewhere. You can also use the thermal camera inside to look for cold spots on the walls, ceilings and floors. These can either be a draft of cold air or just bad insulation. You can do the same in the basement to look for drafts.

Once you've done that, focus your energy and money on the worst spots. This could be the basement, but it's better to check. If there is no draft in the basement, and it is underground, maybe you're losing more heat through the roof, and perhaps you should put a thick layer of fiberglass in the attic instead.

If you do insulate the basement, fiberglass batts on the ceiling are fine unless it's humid, as Steve said. Mice also love to nest in fiberglass batts. So expanded styrofoam panels could be a better option, although more expensive. If you do install fiberglass, I'd recommend a N95 mask and eye protection.

Answer 4

You don't need to insulate the top of your basement if it is conditioned, only the sides. Heat rises, so insulation between the basement and the main floor will prevent warm air from leaving the basement and traveling up into the main floor as easily, which in turn will require more energy to heat the main floor to your liking. Since you don't care if the basement is colder, there is no downside to this.

The fact that warm air rises is why most old houses in the northern US are two stories and a smaller footprint, compared to ranch-style houses further south and out west, where the climates are warmer. In short, smaller-footprint, multi-story homes are more efficient when it comes to heating. Insulating between floors only hampers this efficiency.

Continuing this train of thought, however, should make you realize you absolutely should insulate your roof or attic space very well, so that the heat doesn't escape the house unfettered altogether.