R-Value question. (Basement)

ryanmcl

I'm planning to do 2"XPS in the basement framed over with 2x4's. However 2" XPS only has a R-Value of 10 and my code calls for R11.

What is the recommended procedure to get to R11?

I see some people doing unfaced batts and others saying batt insulation does not belong in a basement.

Beepster

I did 1" XPS framed over by 2 x 4 with unfaced R 13 batts.

B

spring3100

I would use ROXUL in the basement,2 by 4 construct would give you R-25 (10 from the XPS,and 15 from the ROXUL),seems like overkill unless you live in like Nome or something.
How about putting a vapor retarder on the walls (Drylock or the like) framing within a half inch of the walls (but not touching),then stuff the cavities with ROXUL.
XPS is a great vapor barrier,but its pretty expensive.

Beepster

Drylock?

2" XPS is expensive. 1" is more manageable.

Your poo pooing the extra cost of XPS but recommending Roxul???

B

itsreallyconc

1 good thing about ' drylok ' type mtls is that you can color them by buying pigment from sher-wms,,, 1 more is that when they fail its easy to see the damage ooops, guess that's 2 its always easier doing moisture test BEFORE starting to spend $ & invest labor

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if you hear it from a guy in the apron store, be VERY suspicious the mtl/method will work,,, when it time to build something together, they won't answer phones NOR help
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ryanmcl

I have been reading that 1" foam is not sufficient to act as a vapor barrier. It must be 2" minimum. Is that an accurate statement?

itsreallyconc

hang 6mil plastic sheeting & don't worry about vapor,,, conc OR block, you won't have to worry provided the exterior's been properly waterproof'd,,, bldg code ONLY requires 3mil dampproof coating tho hardly sufficient impo

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if you hear it from a guy in the apron store, be VERY suspicious the mtl/method will work,,, when it time to build something together, they won't answer phones NOR help
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Beepster

Somewhere in one of the many links that GBR provides from Building Sciences (in the Insulation section) there is a statement that you can get by with 1" if the humidity levels stay low enough in the summertime). I can not provide that for you now, but do a little more searching in that section and see what you find.

B

spring3100

YES,and in leiu of,not including XPS,read before you write

HomeSealed

No plastic
Here is a page from GBR's book :
http://www.buildingscience.com/docum...lation-systems

Great read.

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spring3100

Great advice if you like mold

Gary in WA

I think I answered your R-10 already.... R-11 is not in the Codes for continuous insulation for a basement, your inspector is wrong; Chicago, IL; http://www.reedconstructiondata.com/...odes/illinois/ SO, IECC 2009 is your state-wide code with no local amendments...from that link.

Under that code, 10/13 is required in a basement;

"c. "15/19" means R-15 continuous insulated sheathing on the interior or exterior of the home or R-19 cavity insulation at the interior of the basement wall. "15/19" shall be permitted to be met with R-13 cavity insulation on the interior of the basement wall plus R-5 continuous insulated sheathing on the interior or exterior of the home. "10/13" means R-10 continuous insulated sheathing on the interior or exterior of the home or R-13 cavity insulation at the interior of the basement wall." From; http://publicecodes.cyberregs.com/ic...9_4_par004.htm

http://www.rssweather.com/climate/Illinois/Chicago/ Averaging your low temps for Dec, J, F = 7*F With 1" XPS (R-5) your foam, inside face would be 31*F at "frost line" and above grade, using R-15 (Roxul) 3-1/2" thick at 70* room temp. with 23% RH (room air) giving condensation there.

With 2" f.b., 39*F f.b. temp., safe up to 32% RH at 70*F, much better...

Gary
PS. MN with 42" "frost depth" and above grade--- 1" f.b and f.g.= condensation at/above 18% RH, need to dehumidify the basement air.

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