# attaching poly board on concrete block wall



## Windows on Wash (Aug 30, 2011)

There are foam specific construction adhesives that are foam compatible.

Is the existing wall CMU block or concrete?

You do not need a vapor barrier in your case and certainly not on the cold side wall as that would be incorrect placement.

Apply the foam of sufficient thickness and seal all the seam in the foam.


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## joecaption (Nov 30, 2011)

http://www.loctiteproducts.com/p/12...iew/Loctite-PL-300-VOC-Foamboard-Adhesive.htm


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## acuts1 (Apr 25, 2012)

Windows on Wash said:


> There are foam specific construction adhesives that are foam compatible.
> 
> Is the existing wall CMU block or concrete?
> 
> Apply the foam of sufficient thickness and seal all the seam in the foam.


-CMU
-What do you seal the foam with?
Thx


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## acuts1 (Apr 25, 2012)

What is a good tool for cutting foam; utility knife just seems to shred the foam?
thx


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## acuts1 (Apr 25, 2012)

Windows on Wash said:


> There are foam specific construction adhesives that are foam compatible.
> 
> 
> You do not need a vapor barrier in your case and certainly not on the cold side wall as that would be incorrect placement.
> ...


Regarding the above:
I am installing a bathtub/shower, 1 of the walls will be the exterior CMU with the poly foam board. My plans were to use 6mil vapor barrier behind CBU on both the wood stud wall and the CMU wall. 

My question is do I still install the vapor barrier on the cold wall, is this going to create issues?
Thx


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## Windows on Wash (Aug 30, 2011)

acuts1 said:


> -CMU
> -What do you seal the foam with?
> Thx


Spray foam is good or regular foam compliant sealant if the gaps are tight.

Foam usually gives you a bit more fudge factor because it is near impossible to make all perfect cuts and joints.



acuts1 said:


> What is a good tool for cutting foam; utility knife just seems to shred the foam?
> thx


Sharp utility knife works well and you have to cut the foam in multiple passes sometimes to get a clean cut. If you are cutting a bunch of it in a row, some folks use a circular saw but it make a bit of a mess.



acuts1 said:


> Regarding the above:
> I am installing a bathtub/shower, 1 of the walls will be the exterior CMU with the poly foam board. My plans were to use 6mil vapor barrier behind CBU on both the wood stud wall and the CMU wall.
> 
> My question is do I still install the vapor barrier on the cold wall, is this going to create issues?
> Thx


No vapor barrier at all. Not only do you not need a vapor barrier, but the vapor barrier always goes on the warm side wall. In the humid South, that would be the outside wall.

With the foam installed tight to the outside wall and sealed properly, you should not have any moisture diffusion coming through.

Put the foam tight to the wall and be done with it. I would also be cognizant of what type of shower liner combo you are using (i.e. durock with kerdi, etc). If any moisture gets behind the shower, it won't have a place to go to make sure it is a tight wet wall design.


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## dwcopple (Apr 1, 2012)

i cut mine with an electric kitchen knife. a bread knife works well too if you have one handy.


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