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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
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    If this is going to drop the temperature in the basement, and if you live in an area where it can get very cold, also consider what might happen to your plumbing on the coldest days. I have a poorly insulated bump-out with some water & waste pipes running through it, and when it gets to -20F and colder, those pipes can freeze up.
    – Z4-tier
    Jan 29 at 18:23

4 Answers 4

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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.

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    I'll add that you will want to make sure any water pipes in the basement will still receive enough heat not not to freeze.
    – Evil Elf
    Jan 28 at 13:21
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    OP mentions their basement is conditioned space (and you acknowledge that in your first sentence). So I am not sure what the point of your second paragraph is.
    – TylerH
    Jan 28 at 15:24
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    @TylerH - Is the second paragraph wrong? If not, I don't see what the point of your comment is other than to nit pick.
    – SteveSh
    Jan 28 at 15:31
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    @SteveSh It's not wrong per se, but it's not relevant either. And could be confusing readers because it's information for an entirely different scenario than what this Q&A is. Since you're confused, comments on Stack Exchange sites are intended to be used for clarification requests or improvement suggestions.
    – TylerH
    Jan 28 at 15:39
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    @TylerH for what it's worth, I found the second paragraph useful because the same recommendation had been what was worrying me about my potential use. To see this problem explicitly mentioned alongside a more general "OK" for my specific case was comforting.
    – NiloCK
    Jan 28 at 16:38
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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.

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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.

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    Appreciate the comment on mice. As it happens, I have lots of N95 masks here!
    – NiloCK
    Jan 28 at 14:13
  • We did receive a professional audit of heating loss in the home, and the basement was identified. This intervention has the highest improvement / cost ratio of anything available right now.
    – NiloCK
    Jan 28 at 14:18
  • N95 is sensible for both types, when placing for the fibreglass and when sawing / cutting for the sytro.
    – Tim
    Jan 29 at 0:09
  • I used styrofoam panels to insulate my cellar ceiling. The first thing we noticed was that the floors upstairs were noticeably warmer when walking around barefoot. But this is an old house and we made it cosier in many ways. I did the cellar ceiling because the entrance hall floor (ceramic tiles) was so very cold in winter. My cellar is not heated or air-conditioned.
    – RedSonja
    Jan 29 at 16:28
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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.

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