# Rigid (XPS) Foam Board Size for basement.



## fastmodena (Dec 3, 2012)

Starting my basement finish. Just got done tearing down the CRAP blanket insulation the builder left behind. New home. 

My plan was 2" PINK foam board against the concrete, followed by framed 2x4 walls, and R-13 fiberglass (non-faced) insulation in between the 2x4's. 

After research and the fact that non-faced fiberglass can only be special ordered in my area, I found Roxul and the R-15 Roxul costs about the same as the special order fiberglass. Not to mention it's in stock. 

I have searched google over and over (as well as forums) and my question still remains. Since the Roxul is R-15 would it be okay to go with 1" XPS board instead of the 2" that I was planning on? That would give me an effective R-20 which is okay for my area. 

My concern is 1" being too small as an effective vapor barrier to prevent condensation on the poured concrete foundation wall. I have searched all over and can't really determine the difference between 1" and 2" except the R-value and the 2" is a substantial premium. 

Your thoughts? Thanks!


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## Gary in WA (Mar 11, 2009)

Welcome to the forums!

I removed your link to another DIY site because the posts/facts/questions were identical.

The vapor barrier aspect of foamboard is less important than the warming of concrete to control condensation is. Building Science has changed an aspect of f.b. permanence with updating to that effect. The moisture can stay in the concrete/CBU wall without causing damage. 

Where are you located for the foamboard thickness required?

Gary


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## fastmodena (Dec 3, 2012)

GBR in WA said:


> Welcome to the forums!
> 
> I removed your link to another DIY site because the posts/facts/questions were identical.
> 
> ...


Thanks for the reply. I did not know I had any LINKS. Sorry.

Your right I am not so concerned about vapor but rather the potential condensation. I could not find a reference on building science to the size of XPS needed in my area. I live in northern Utah.


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## jklingel (Dec 21, 2008)

Northern Utah gets pretty cold. IMO, 2" is good, and the Roxul will work better than fg. Good call on that. Remember to air seal both the foam and the sheet rock.


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## Beepster (Oct 19, 2009)

You do not have unfaced insulation but you do have roxul on hand? Geez, that is just the opposite for me here in the Twin Cities.

I am only using 1" XPS along with unfaced insulation. I monitored the humidity level last summer and found 1" to be acceptable. You will have to read the Building Science links GBR provides. They state that humidity must be under a certain level (50%??) for 1" to be OK. With left over scraps I sealed all the rim joists with 3 layers of 1" XPS and caulk.

B


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## cube17576 (Jun 5, 2012)

I just bought faced insulation and peeled off the paper. It comes off very easy.


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## Gary in WA (Mar 11, 2009)

It appears you are in Zone 6? http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_par002.htm

Check below the map for the nearest large city listed...

Zone 6 requires R-10 continuous (foamboard), *OR* R-13 in cavity; footnote "c"; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002.htm So does Zone 5, so it really doesn't matter much.

If location is Brigham City with annual average low temp of 35* = 2" fb (R-10 XPS) would give you inside foam sheathing temp of 49* or 48%RH at 70* in the room at frost line and above grade. Since you are adding Roxul (R-15) in cavity, 1-1/2" fb = safe to 42%RH and 1" (R-5)= safe to 38%RH in room. 
Remember, Roxul doesn't loose R-value when wet as fiberglass does. And; sill sealer under p.t. bottom plate, fire-stop walls every 10' horizontally and top completely from floor cavities above, ADA drywall, air-seal/insulate rims, no gaps to insulation in cavity. Need links, just ask.

Gary


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