# Basement Wall Insulation



## Windows on Wash (Aug 30, 2011)

snucky said:


> How can you guarantee that the fiberglass stuff won't soak and have problems with moisture/mold? We don't live in a very wet climate here in Montana and yet I have heard several times that one needs to first create a reliable moisture barrier, such as 1.5" XPS sheeting.


The short answer is that you can't.

Putting an XPS skin combined with a stud wall and fiberglass is a much better option.


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## snucky (Jul 25, 2010)

thanks, that's sort of what I was thinking. Is there a minimum width for the XPS board? Would 3/4" suffice?

Also, should I leave an airgap between the XPS and the 2x4 wall?


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## Windows on Wash (Aug 30, 2011)

1" is preferred.

No air gap. Use the framing to brace and hold the foam in place.


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## Gary in WA (Mar 11, 2009)

Compare your location to MN (similar), use the 2" foamboard, you don't want less as you are in Zone 6, Table 5.1: http://www.buildingfoundation.umn.edu/FinalReportWWW/Chapter-5/5-optimum-main.htm

Page 59, Fig. 41: http://www.buildingscience.com/docu...merica-high-r-foundations-case-study-analysis

You really need to compare the wall insulation with the wetness associated.

Gary


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## snucky (Jul 25, 2010)

thanks guys! We live in a 4b zone and not a lot of soaking going on very often. Besides the snow melt, May - June usually sees frequent rain. Beyond that, air is practically always dry here. 

If using the 2", do I still need the bat insulation in the 2x4 stud wall? Or could I use a skinnier method of framing and save some room space?

Thanks!!!


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## alecmcmahon (Apr 30, 2010)

snucky said:


> thanks guys! We live in a 4b zone and not a lot of soaking going on very often. Besides the snow melt, May - June usually sees frequent rain. Beyond that, air is practically always dry here.
> 
> If using the 2", do I still need the bat insulation in the 2x4 stud wall? Or could I use a skinnier method of framing and save some room space?
> 
> Thanks!!!


You'll need R-13 for basement, IIRC 1" xps will give you an R-3. ( tape all joints ) Then throw in some basic r-13 insulation rolls on top of that.

Personally, I wouldnt even consider going anything less then a 2x4 wall. You'll need room to run electric / plumbing / whatever. They'll be easier to work with and install over anything skinnier IMo


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## alecmcmahon (Apr 30, 2010)

edit, misread - thought you meant skinnier framing.


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## Gary in WA (Mar 11, 2009)

Montana the State is: 

5A Worth 5A Passaic 5A Dutchess 4A Barry 5A Macon 4A Wright 4A Salem 5A Erie 4A Barton 4A Madison 5A Somerset 6A Essex 4A Bates 4A Maries*
Montana* 6B (all) 

From: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec001_par001.htm 

Your insulation requirements: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002.htm

You could go with strapping on the foamboard, remember to check with your local AHJ for the possible fire-stopping every 10’ lineally along the wall. http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec002_par031.htm


The basement foamboard thickness is based on temperature per location. Reducing the thickness is fine as long as you control the basement humidity, pp. 10: http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems



Gary


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## snucky (Jul 25, 2010)

besides achieving a sufficient R value, I also am looking to compensate the out-of-plumbness of the walls. Over the whole 8' they're about 1-1/2" to 2" out of plumb. Here's what I think I'll go with

a) Foam board airtight against foundation wall to create a moisture barrier and some insulation
b) no air gap
c) 2x4 studs with R-13 bat insulation to achieve plumb walls and convenient space for wiring

the question is: What's the minimum thickness of the foam board that will create a moisture barrier. People in this thread seem to indicate that's 1"?


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## Chemist1961 (Dec 13, 2008)

*hybrid*

Snucky, I did a hybrid combo, foil faced foam above grade, hi density open cell below grade. Sealed all joints and corners with spray foam and tape. The idea being the wall can still breath upwards to allow humidity to vent above grade. (When completed No VB will be used behind drywall).
Then aligned my studs inside and used Roxul R13.5 rock wool which is not affected by moisture... discovered this method on buildingscience.com site. 
Be sure to check and seal and insulate your RIM joist and plate thoroughly before you start the walls. This is the biggest heat loss zone in most homes. I sealed my bays individually and insulated to R25+


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