# Thickness of XPS Foam



## Corey47 (Oct 8, 2012)

I am finishing of part of our basement and am having troubles with the last few details about insulation on the walls.

The basement walls are below grade and when I got my building permit they said the insulation needs to be a minimum of R13.

I had planned on using XPS foam board applied directly to the walls with the seams taped and spray foam used at the top and bottom. Then a small gap and a 2x4 frame that will get filled with R13 insulation batts and greenboard over that.

The question I cannot seem to find an answer to is the thickness of the Rigid foam. The few places I found mention of rigid foam applied like this said to use 1.5", but they did not include the R13 batts as well. Because of space loss and cost, was hoping to go with 3/4" or 1" instead, but did not know if the foam would still serve the purpose of keeping the cold away from the concrete at those thicknesses.

Thanks in advance for any help with my dilemma.


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## Windows on Wash (Aug 30, 2011)

1" is as low as I would go and put the studs (if the wall is plumb) tight to the foam.


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## Gary in WA (Mar 11, 2009)

Where are you located?

That will establish the foam thickness.

Gary


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## Corey47 (Oct 8, 2012)

Nebraska


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## jklingel (Dec 21, 2008)

Why go to all the trouble and then weinie out on the foam? Put in 2" and enjoy the place. For batts, try to get Roxul instead of fiberglass. The also make cotton and cellulose; all three are good and no itchy glass shards in your skin and lungs.


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## Gary in WA (Mar 11, 2009)

2" will work! Or, find the lows of Dec., Jan., and Feb., follow the second 1/2 of this article, enlarge the picture links at the end; http://www.greenbuildingadvisor.com/blogs/dept/musings/are-dew-point-calculations-really-necessary

That will tell you the thinnest f.b. with the average R.H. in your basement without condensation wetting the fiberglass or wood framing.

Gary


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