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In an old house with plaster walls there is a large room on the first floor with wood floor. The walls are not insulated. The basement has no ceiling, so you just directly see the subfloor of the first floor. There is no insulation.

What is more effective, insulating the walls on the first floor (with blown-in insulation from outside holes), or attaching some insulation under the subfloor?

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It's far more important to insulate the walls, if you must choose. Far more heat is lost through the walls than the floor, for several reasons:

  • The walls are exposed to the weather. Wind and rain extract heat much more quickly than the stagnant air in the basement does.
  • The basement is partially insulated by the Earth. Assuming it's partly or mostly below grade, relatively little heat transfers through the foundation.
  • Most of the heat in your home is above the floor. Since heat transfer rates relate directly to temperature differential, the highest rate is in the upper portion of your walls.

It probably doesn't need to be said, but if your attic isn't insulated well, that's your highest priority.

  • Fully agree was going to type a similar response but you covered it well+ – Ed Beal Feb 12 at 22:03
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If your walls are not insulated, then the walls need to be. If the walls of your basement are insulated then no insulation in the floor joists should be installed. If your basement is not insulated, then you should have insulation with Kraft Faced paper installed with the paper side against the floor of the upstairs. Or have closed cell spray foam installed in the floor joists. You can have someone come in and foam the walls to ensure a complete fill of the wall cavities. That would be the most efficient way of insulating your room.

  • I agree, but you haven't really said why. We can infer that you think more heat is lost through walls than the floor, but you might elaborate on that as it's the crux of the question. – isherwood Feb 12 at 16:29
  • I also agree the walls are exposed to the outside the basement in the ground is some what constant. – Ed Beal Feb 12 at 22:01
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Floor insulation is not going to make the room significantly warmer by itself, but if your basement is unconditioned, it can help eliminate drafts. Underfloor insulation can also help prevent heat loss by retaining heat in the winter and preventing cool air from falling into the floor below during the summer. This can also help by creating a vapor barrier, preventing any moisture from reaching the bottom of the wood floor above. Fiberglas insulation is also an excellent fire retardant.

I would start by using some roll insulation with insulation supports for the floor.

Insulation Supports

Regarding wall insulation, I suppose that depends on where you live. I live in southeast USA. I have done a lot of remodeling here and have seen a lot of mold issues where people have insulated behind plaster. I have also seen instances where that hasn't happened, primarily in the mountainous areas near my home. I know that plaster walls were designed to be able to dry themselves. I also know that plaster is nearly double the thickness of standard 1/2" sheetrock. Therefore, the "R" value of a plaster wall would be at least (probably more than) double of drywall.

Consider these issues before insulating the walls:

Seal cracks in the perimeter of your attic access hatch, around the inside of windows, at light switches/outlets and door trim. Repair any damage to the plaster walls..

  • Actually I was told that the "science" is going away from insulating basement ceilings. – Manu Feb 8 at 19:50
  • While you indirectly answer the question by referring to floor insulation, you don't explain why you think that's more important. – isherwood Feb 12 at 16:28
  • Further explanation has been added in my edit. – Jerry_Contrary Feb 13 at 19:37

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