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My basement is CMU blocks and concrete floor.

I had water issues, so I waterproofed it with an interior french drain, drainboard at the edge and a sump pump. This wouldn't take care of dampness so I added a dehumidifier.

I plan to finish the basement and here's my understanding of how I should go about it:

I think I'll avoid vapor barrier so I don't get mold problems. It gets quite humid around here. I'm in Washington DC on the east coast and if I interpret correctly the following: http://www.house-energy.com/Basements/Interior.htm, I shouldn't use vapor barrier. I have a few minor cracks in the wall maybe I should fill those may be not.

Rigid extruded polystyrene installed directly on the wall. Two inch R10 perhaps tucked into the drainboard of the french drain. This way any moisture will go down into the drain.

Leave some space (inch or two) then build the wall from the ground so it's completely detached from the wall.

Not sure what I'd do with the floor; maybe would insulate it maybe not. Height is only 7ft so any inch of height is precious.

Here's an article about this in BuildingScience.com: http://www.buildingscience.com/documents/reports/rr-0309-renovating-your-basment

This is all theoretical, I've never done anything like this, so I'm looking for feedback. Also, would I need insulation on the ceiling? I like the idea of spraying fire-rated foam, is it an overkill? Guess, I'd need soundproofing if nothing else.


Since I'm starting with the walls, here are my first-priority questions:

  • Fill cracks or not?

  • Vapor barrier or not?

  • 2 inch rigid foam or just 1 inch and then more insulation next to the dry wall? (I'm thinking all insulation better be away from the dry wall to avoid any condense around it)

  • Do I tuck it into the drainboard?

  • Do I glue the insulation or is there another method of attaching?

  • Use spray foam to insulate the parts of the wall between the studs? Or is the rigid foam going to be fine? (I'm thinking the spray would fill all gaps, therefore'd be better)

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I don't understand why people insulate basements in the mid-west. I live in Ohio and I insulated the rim joists for heat loss only. The earth simply does not get that cold in our area of the world. In the dead of winter, open your register vents in your basement, and I think you will be surprised at how warm it gets. The earth is a pretty good insulator. I agree with your move of not using a vapor barrier. Insulation as well is just a spot to collect moisture. I studded my walls out 4 inches from my block and left the top open dressed up with crown molding to allow moisture to escape. –  Evil Elf Oct 29 '13 at 13:22

3 Answers 3

up vote 4 down vote accepted

Why would you need insulation on the ceiling in a basement? Since the ceiling is touching a heated space itself, there is no need if you will then be using the basement too. Insulation there should be done only for soundproofing, unless you have radiant underfloor heat installed, in which case, the insulation should have been done already as part of the installation. So insulation really only offers soundproofing value. So the question is, how much noise will be generated there?

Counterbalance that with the rationale for leaving this space accessible. I've often enough needed access to the underfloor regions of my home, for wiring changes, plumbing changes, repairs, etc. So any insulation may be better done as fiberglass batts anyway.

As far as the walls go, filling the cracks is a reasonable idea, IF they are stable. If the walls have moved due to frost heaves, etc. and might continue to move, I would ensure they are stabilized before you cover the wall over. A buckling foundation wall is not something good to have under your house.

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That's a good point. In fact, I may leave the ceiling completely exposed. But since the first floor is hard wood you hear everything that happens upstairs from the basement. Some soundproof would be nice. So this is an open question for now. My first priority are the walls... –  Peter Q Dec 26 '10 at 19:21
Then I would add fiberglass batt insulation stuffed into the cavities as insulation. It will allow access as you want it. –  user558 Dec 26 '10 at 21:03
The only reason I prefer to stay away from fiberglass is humidity and mold. I'd take every measure to eliminate humidity (dehumidifier, etc.) but I'm still concerned... –  Peter Q Dec 26 '10 at 21:42
woodchips, thanks for commenting on the cracks. Some of the cracks are due to ice heaves (although they don't look too bad to me). How would I stabilize the walls? From the inside? My thinking was that the only good way to do this would be from the outside... –  Peter Q Dec 27 '10 at 1:52

I am in the same situation, except I don't have the interior french drain (or "drain tile"). If I did, then I would do this:

  • moisture barrier (e.g., that heavy plastic sheeting with dimples in it) on the block wall, with the bottom tucked into the drain board.
  • a layer of rigid foam insulation on the room side of the moisture barrier.
  • spray foam the rim joist area, covering the tops of the rigid foam.
  • 2x4 framed wall as you described.

I have seen conflicting info on whether or not to put a layer of rigid foam under the framed wall. I know - it seems like it would get squished - but I saw it from a reliable source (which I cannot find just now... I thought it was buildingscience.com).

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Under the wall framing, you place a thin foam sill gasket that intentionally gets crushed. This stops drafts and helps to block moisture wicking up from the concrete. –  BMitch Oct 29 '13 at 12:30

I am in a similar situation, and I believe you are correct.

In some cases a vapor barrier helps, but in some cases it does not. I installed rigid foam insulation on the basement wall, and then strapping, and then blue (mold and moisture resistant) gypsum drywall sheets.

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