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We are remodeling our bathroom and are in the process of framing/insulating.

We need to insulate 2 concrete block walls but don't want to eat the already small space we have. Which solution could work?

Currently, the wall has 1x3 furring strips and we were thinking about using them to hold 2 inches insulation board—leaving some air pockets—then installing the thinner 1/4 inch Hardiebacker board and then Redgard the hell out of it for waterproofing.

My concern is the furring strips won't hold the backer board and the ceramic. Other option we thought of was using the Schluter Kerdi board and forget about insulation altogether, so Kerdi board attached to 1x3 furring and then titling.

Is there any other option that you can think of? Will any of the above ideas make sense? Thanks for asking. We are in DC (U.S.) area so summers are really hot but winters are mild.

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  • Does "2 concrete block walls" mean that a wall is double thickness of blocks?
    – wallyk
    Aug 12, 2017 at 23:32
  • There are 4 walls, 2 are concrete -facing the street and side of house- the other ones are just interior walls that i won't have to insulate -unless i want to make the space quieter.
    – Rose
    Aug 12, 2017 at 23:38
  • I have attached tile directly to both block & concrete walls with almost no air gap for insulation I don't think I would waste any space with furring strips and backer board.
    – Ed Beal
    Jun 13, 2018 at 21:40

1 Answer 1

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I'm about 2 hours south of you. Cut 1/2" styrofoam between those strips, use a wide flashing tape to cover the furring strips and lap over onto each piece of stryofoam to the left & right of the furring strip.

Then, Keep gettin up. Put you some good 1/2" material on the wall, for the sake of your tile. I suggest cement board, but you'll do what you prefer.

The difference between what I just suggested, and 2" of foam, will make no difference in perceived heat loss, or electric bill increase.

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    Thanks. I was concern about humidity and vapor -and mold. Reason why i was thinking on attaching the board -it could be styrofoam- to the furring strips instead of in between them and closer to the concrete wall.
    – Rose
    Aug 12, 2017 at 23:42
  • The flashing tape, being installed on stryofoam, will act as your vapor barrier.
    – NPM
    Aug 12, 2017 at 23:45
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    So the furring stripes will well hold the cement or the 1/2 hardy board? That was my other concern -i am guessing i might have to replace the 100 years old ones that i encounter when I teared apart the walls.
    – Rose
    Aug 12, 2017 at 23:49
  • They will, as long as they're installed adequately. I know that sounds snarky, but you'll just have to judge that for yourself. For comparison, I generally put a nail every 16", and use construction adhesive.
    – NPM
    Aug 12, 2017 at 23:53
  • I'd use PIR (eg Kingspan) rather than Styrofoam. I think you are mistaken about the difference between 1/2" and 2" too. Obviously the first 1/2" gives the most benefit, but the next three or four inches make a big difference too. (But my experience is in Cambridge and Germany, where the problem is cold, not heat.)
    – Martin Bonner supports Monica
    Jan 4, 2020 at 9:47

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