# Cut and cobble or not for foam insulation



## akajoe87 (Sep 16, 2015)

I have never finished a basement before but I have built several houses in the past but I have never finished the basement. After reading I understand fiberglass insulation is out of the question so in regards to the foam board technique I have found this website http://www.familyhandyman.com/basement/how-to-finish-a-foundation-wall/step-by-step#step2, my question is can I cut and make the batons to fit in between the studs vs gluing the full foam sheet against the block wall to save space. I'm sure not doing it this way is a better seal but I'm looking to save space. Any suggestions?


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## Bud9051 (Nov 11, 2015)

One of the objectives of applying the foam first against the wall is to isolate the wood from the moisture. All concrete allows moisture vapor to pass. Depending upon your climate, you can use less rigid foam and then fill the cavities with batt insulation. The ratio of rigid to air permeable insulation must keep the inside surface of the foam above the dew point. There are charts to help with that choice depending upon your climate zone.

Air sealing the rim to concrete and all other seams that can potentially leak air from outside is top priority. In addition, as you cover your new walls, air seal to prevent humid basement air from having easy access to those cavities.
http://buildingscience.com/documents/digests/bsd-103-understanding-basements?full_view=1

Bud


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## akajoe87 (Sep 16, 2015)

Bud9051 said:


> One of the objectives of applying the foam first against the wall is to isolate the wood from the moisture. All concrete allows moisture vapor to pass. Depending upon your climate, you can use less rigid foam and then fill the cavities with batt insulation. The ratio of rigid to air permeable insulation must keep the inside surface of the foam above the dew point. There are charts to help with that choice depending upon your climate zone.
> 
> Air sealing the rim to concrete and all other seams that can potentially leak air from outside is top priority. In addition, as you cover your new walls, air seal to prevent humid basement air from having easy access to those cavities.
> http://buildingscience.com/documents/digests/bsd-103-understanding-basements?full_view=1
> ...


Ah I see. So the foam limits the condensation if against block and if the foam was between the studs condensation would occur on wood studs. I am in West Virginia. So you think I could get away with 1/2 foam and unfaced R13 fiberglass batts.?


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## Bud9051 (Nov 11, 2015)

Virginia shows climate zone 4: https://energycode.pnl.gov/EnergyCodeReqs/?state=Virginia
And this article shows R2.4 for a 2x4 stud wall with r-13 batt insulation:
http://www.greenbuildingadvisor.com...rmal-mass&utm_campaign=green-building-advisor
So unless you are in a cold spot you should be fine with 1/2". Using XPS (the pink or blue) would be better than EPS (the white stuff), but the difference is minimal.

I would not use a vapor barrier so everything would be able to dry to the inside. 

All of this assumes you have eliminated any water leakage and basically have a dry basement from the start.

Bud


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