# basement insulation 1+2=3? xps additive?



## kree (Nov 11, 2011)

Hi,
For basement insulation in my area it seems that XPS is difficult to find in 3", so is there any problem with adhering 2" to the wall then 1" board to that?

The final layer will then be taped and sealed at the bottom.

I figure it may be easier to do that versus roxul in the joist bays?

thoughts?


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## Windows on Wash (Aug 30, 2011)

Foam board will stick to itself very easily.

Stagger the seam and tape the first layer seams as well.

At 3", you should have some mechanical attachment to the wall as well (i.e. furring strips).


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## Gary in WA (Mar 11, 2009)

R-10 continuous is minimum required for your Zone 5: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec001_par001.htm

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

Chicago has 6493 HDD compared to Minn. (7882 HDD) used in the test here where R-10 is fine, fig.5.6; http://www.buildingfoundation.umn.edu/FinalReportWWW/Chapter-5/5-optimum-main.htm

With the extra thickness reduces the perm rating from 2” (0.75) to 3” (0.56) which means the moisture will take longer to pass through, going longer into summer to fully dry out. That isn’t as concerning as the possibility of convective looping at each heating start-up until the foam board is warm through the drywall in the empty stud cavities. Roxul would prevent any convective loops from forming which heat the upper framing and rim joists. http://joneakes.com/jons-fixit-database/743
Without cavity insulation, you also have larger areas that would be affected by air leaks in your plate/concrete slab, drywall/framing, and slab/rim air spaces. Be certain to: sill sealer under the p.t. bottom plate for air/thermal/capillary break- http://www.ussteel.com/corp/construction/bulletins/ACQ%20Pressure%20Treated%20Lumber%20Constructin%20Bulletin.pdf
(especially if no plastic vapor barrier under the slab at install.).

Air seal the rim joists, use your f.b. scraps; http://www.buildingscience.com/documents/information-sheets/critical-seal-spray-foam-at-rim-joist/

Air seal the drywall (ADA); http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Fire-stop the top plate/joist cavities and every 10’ horizontally as per Code; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec002_par031.htm
Gary


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