Layering Insulation subgrade

[darcar]

Does anyone see a potential issue with applying batt insulation over a wall that already contains insulation AND a vapor barrier? A portion of the wall is below grade...IMO i think there is a potential for condensation at the vapor barrier and that moisture saturating the batt insulation in contact with it potentially causing a mold situation.

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[righter101]

Our State of Washington has their own energy code. This is the language I cite when this issue comes up.

502.1.6.1 Vapor Retarders: Vapor retarders shall be

installed on the warm side (in winter) of insulation as

specified in the following cases.

EXCEPTION: Vapor retarder installed with not more

than 1/3 of the nominal R-value between it and the

conditioned space.

The most common scenario I encounter is folks that want to spray 1" of Corbon or similar on the exterior side, then put batts in the rest. The problem is that it creates a 2nd vapor barrier, on the warm side of the Corbon. The way the exception reads, they could put mostly Corbon, then the batts, so long as the batts only made up 1/3 the nominal value.

We have, however, accepted dewpoint calculations, specific to the project, showing that the dewpoint would occur beyond the wall, then accept that as compliant.

You are correct to have concerns. I don't know however if the IRC energy code addresses it.

 

[syarn]

yes depending on your geographical location.

per righter101; generally in the northern USA (zones 5, 6, 7, 8 and marine 4) 1405.3 IBC 2009 calls for vapor retarder on interior side of frame walls. u may have an exception for a below-grade portion of wall but that would be to NOT install a vapor retarder.

http://building.dow.com/na/pro-us/tools/design/steelstud2.htm

if ur are technically oriented there probably is a "science" to it with regards to R value of the wall, location, indoor temp & relative humidity that u could calc out to show an official maybe...

 

[Wooly]

The prediction of "dew point" is extremely complicated and the Dow dew point calculator is considered a steady state program. The ASHRAE handbook warns against using such calculations. WUFI is a complex 2D model that does a fair job of modeling interstitial conditions. There is a free download available from Oak Ridge National Laboratories. With all that said it simply isn't good practice to have a vapor retarder in any part of a below grade wall, period.