# Attaching XPS to Concrete



## Windows on Wash (Aug 30, 2011)

Plates and concrete screws work and you can usually find a cheap sealant that is non reactive to the foam that will help maintain a physical bond with it. Just test out a few pieces and find something that is inert with the foam.


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## Bud9051 (Nov 11, 2015)

What are you going to cover that rigid with. Codes often require a layer of drywall or other thermal barrier over the insulation. In that case adding strapping provides a place for the drywall to be attached and it secures the foam while some dabs of adhesive cure.

Also provides a small gap for wiring, although I prefer more space.

Bud


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## Nealtw (Jun 22, 2017)




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## Abuc (Aug 28, 2018)

Nealtw said:


> https://www.youtube.com/watch?v=zDNyjR1SHUQ


This is what I plan to do. Looks much cleaner and easier than adhesive.


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## Abuc (Aug 28, 2018)

Bud9051 said:


> What are you going to cover that rigid with. Codes often require a layer of drywall or other thermal barrier over the insulation. In that case adding strapping provides a place for the drywall to be attached and it secures the foam while some dabs of adhesive cure.
> 
> Also provides a small gap for wiring, although I prefer more space.
> 
> Bud


I just started reading about the fire hazard. I have read that the Dupont Dow blue board has been certified to use without covering. Don’t know about other brands. I was going to attach the XPS board directly to the concrete and then construct a 2x4 wall with Sheetrock. There would be a 1” or 2” gap between the XPS and the studs. Can’t find much info on fireproofing XPS. Articles say it has to be covered (except the blue board) but they don’t say if the sheetrock has to be tight against the XPS. I will have fireblocking at the top of the wall.


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## Nealtw (Jun 22, 2017)

Abuc said:


> I just started reading about the fire hazard. I have read that the Dupont Dow blue board has been certified to use without covering. Don’t know about other brands. I was going to attach the XPS board directly to the concrete and then construct a 2x4 wall with Sheetrock. There would be a 1” or 2” gap between the XPS and the studs. Can’t find much info on fireproofing XPS. Articles say it has to be covered (except the blue board) but they don’t say if the sheetrock has to be tight against the XPS. I will have fireblocking at the top of the wall.


 As long as there is drywall between the foam and the living space you are fine with that plan.
What he doesn't talk about it the top plate of the wall should be wider and go all the way in to the sill plate on top of the concrete and that can be plywood or OSB or 2x?. We do that first.


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## Bud9051 (Nov 11, 2015)

I am aware that Dow Thermax has a foil facing that has been approved to be left exposed in some applications but have not heard of the blue board being approved. Can you post the link you are reading?

As Neal said the 2x4 wall with drywall will do the job, just use pressure treated at the bottom with approved fasteners at all points and no need for the gap. The studs can be installed directly against the rigid. Only need for a gap is to be sure wall is vertical if foundation isn't.

Bud


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## Nealtw (Jun 22, 2017)

Bud9051 said:


> I am aware that Dow Thermax has a foil facing that has been approved to be left exposed in some applications but have not heard of the blue board being approved. Can you post the link you are reading?
> 
> As Neal said the 2x4 wall with drywall will do the job, just use pressure treated at the bottom with approved fasteners at all points and no need for the gap. The studs can be installed directly against the rigid. Only need for a gap is to be sure wall is vertical if foundation isn't.
> 
> Bud


I you use sill gasket, no need for treated wood and worry about rusting drywall screws


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## Bud9051 (Nov 11, 2015)

Sill gasket is a bit thin. I prefer the 1.5" PT bottom plate with my drywall set 1/2" up providing more protection from water. Nothing is perfect and the sill gasket is probably good in most cases.

Bud


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## Abuc (Aug 28, 2018)

Bud9051 said:


> I am aware that Dow Thermax has a foil facing that has been approved to be left exposed in some applications but have not heard of the blue board being approved. Can you post the link you are reading?
> 
> As Neal said the 2x4 wall with drywall will do the job, just use pressure treated at the bottom with approved fasteners at all points and no need for the gap. The studs can be installed directly against the rigid. Only need for a gap is to be sure wall is vertical if foundation isn't.
> 
> Bud


The wall needs to come out far enough to clear waste plumbing. My contractor likes to use PT on the bottom plate so I will go with that. Will look for the link.


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## Abuc (Aug 28, 2018)

https://www.buildsite.com/pdf/dowch...Plastic-Insulation-Technical-Notes-400113.pdf

https://www.dupont.com/industry-upd...ica-now-produced-with-bluedge-technology.html


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## Bud9051 (Nov 11, 2015)

The gap is more of a problem when the foundation is not insulated. Moisture entering through the concrete as vapor at the bottom is pushed upward to where it finds the cold foundation at the top exposed to the outside. There it deposits its moisture and a ice begins to accumulate. 

you will have the nice layer of rigid so minimal moisture and nothing should be below freezing.

Bud


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## Nealtw (Jun 22, 2017)

Sill gasket is cheap and stops water from wicking up the wall 

https://www.homedepot.com/p/Owens-C...ti-Use-Ridged-Sill-Plate-Gasket-1FS/202084452


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