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Old 07-20-2018, 12:39 AM   #1
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Adding insulation to a block wall


I have a house in Maryland (climate zone 4) that has a finnished basement. The exterior walls are plaster over cement blocks, with 1x2 furring strips nailed to the blocks and wood paneling nailed to the furring. The drainage outside was bad, and both the furring and the paneling show water damage on the back wall.

I've got a contractor who is going to excavate down to the foundation and waterproof the outside, but I am going to deal with the inside myself. I've already pulled off the paneling, and there is some white mold.

I'm going to use 2" XPS foam insulation and drywall. I'm just not clear on how to layer it to avoid moisture problems. My best guess is to glue rigid XPS foam right on the concrete blocks, then glue new treated furring strips to the outside of the foam, and screw the drywall to that. The blocks and plaster should be protected by the waterproofing the contractor is doing, and in any case can handle some moisture okay and dry to the outside of the house. The R10 Owens Corning Foamular 250 board will act as a vapor barrier and thermal break, so I won't get condensation inside of the wall when it is cold outside. The furring strips and drywall should be able to dry to the inside of the house. I plan to use the Great Stuff can kind of foam top and bottom to close any gaps.

Does that make sense? Am I missing anything? Is there a better way to build it?

I'd apprecate hearing any comments or suggections you have.
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Old 07-20-2018, 09:40 AM   #2
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Re: Adding insulation to a block wall


Can furring strips be anchored by foam?

I think they would have to be anchored to the concrete wall through the foam.

Or you could just build a 2x4 frame after putting the foam. This frame can take additional insulation and with a thermal break you wouldn't need a plastic vapour barrier; though it may be required by code, these vapour barriers don't at all belong in basements especially with a thermal break.
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Old 07-20-2018, 09:49 AM   #3
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Re: Adding insulation to a block wall


What's to keep condensation from forming between the xps and the concrete block above grade just as it did before?
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Old 07-20-2018, 10:45 AM   #4
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Re: Adding insulation to a block wall


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Originally Posted by user_12345a View Post
Can furring strips be anchored by foam?
Great question! The furring strips just have to carry the weight of the drywall . . . and whatever you hang on the finished wall. I was thinking I could glue the strips to the foam, and the weight would be spread out enough that it wouldn't be a problem.

But when I tried to look up the strength of the glue, I came across this gem - the way I proposed to build the wall puts the romex too close to the drywall. To get at least 1.25" between the wiring and the back of the drywall, I'll need to put the furring strips on the block wall, glue the foamboard to the strips, then attach the drywall directly to the foamboard using glue and screws that go back to the furring strips.
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Old 07-20-2018, 11:10 AM   #5
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Re: Adding insulation to a block wall


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What's to keep condensation from forming between the xps and the concrete block above grade just as it did before?
If I understand it correctly, the 2" XPS foamboard serves as a thermal break, so the damp warm air outside never meets a cold surface.

Since the above grade part of the wall showed no sign of water damage, I think we can rule out condensation as the problem. The water damage on the existing wall was caused by ground water that gets left against the subgrade part of the wall when it rains. There is a hill behind the house, and I think the original swale got filled in over time. Hydrostatic pressure drove water through the plaster and blocks. The drainage will be fixed by a contractor who specializes in it.

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Old 07-20-2018, 02:27 PM   #6
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Re: Adding insulation to a block wall


I don't think foam insulation performs well with a gap.

The gap can allow for convection currents.

You also would have to seal the gap between the insulation and wall and it would never be perfect.

Why?
When you put insulation, you make the wall much colder so the risk of condensation forming goes up a lot. Foam right on the wall prevents this by stopping all air movement, vapour barriers to do a point but only above ground with drying potential to the outside. (yet are still used in basements by people who don't know better)



Unless space is at a premium, what about framing the wall? Or even using thicker furring strips?

For furring strips, I would use longer anchors that go through the foam and into concrete. I imagine they have specific nails and self tapping screws for concrete - with the right tool driving them shouldn't be a problem.
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Old 07-22-2018, 11:03 PM   #7
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Re: Adding insulation to a block wall


Thanks so much for all the help and advice. I've come to the conclusion, supported by many sources and reinforced here by user_12345a, that the foam needs to get glued directly to the wall. And to meet code for the romex in the wall, I will need a 2x4 frame to allow the required distance between the wires and the drywall. I had hoped for something less expensive that would not make the wall 5" wider, but I think this is the best solution.

I found a diagram on Green Building Advisor:

http://www.greenbuildingadvisor.com/...n%20detail.jpg
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Old 07-23-2018, 08:16 PM   #8
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Re: Adding insulation to a block wall


I guess the furring strips will serve as a circulation system between the block wall and the xps foam right? Keeps the moisture between the wall and the foam
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Old 07-24-2018, 01:06 AM   #9
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Re: Adding insulation to a block wall


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I guess the furring strips will serve as a circulation system between the block wall and the xps foam right? Keeps the moisture between the wall and the foam
No, no furring strips. I've concluded based on several sources that it is best to have the XPS foam glued to the blocks with no space between. The concrete blocks will tolerate moisture without damage and slowly dry to to exterior. The foam will act as a vapor barrier to block the outside moisture from reaching the inside, and the thermal break provided by the foam will prevent warm inside air from condensing inside the wall when it is cold outside. That is the way I understand it, anyway. The 2x4 framing will go on after the foam is up and provide enough space to meet code for the required distance between the drywall and the Romex runs to the electrical outlets.
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