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Old 07-19-2015, 01:45 PM   #1
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Delicious Delicious (moisture) Sandwiches


Anyone an engineer type? Familiar with moisture sandwhiches?

My new porch from the inside-out will be:

sheetrock, polyethylene sheeting, denim insulation, expanded polystyrene (white, 3/4"), osb, tyvek extruded polystyrene (pink 1/2"), vinyl siding.

Curious if anyone thinks or has any information that will help me determine if the EPS - OSB - Tyvek - XPS layering will cause moisture sandwich problems.

I'm thinking it'll be okay now, but before I do it and close everything up, better to see differing opinions.
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Old 07-19-2015, 03:57 PM   #2
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EPS is very vapor open so it can dry to that side.

Where is the home located?

Typically, foam on two sides of the wall is a bad idea no matter the foam.
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Old 07-19-2015, 06:17 PM   #3
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Fargo, ND. So... super hot summers. super cold winters.

From what I've read XPS is as vapor permeable as Tyvek, if not more; and EPS is more vapor permeable than XPS, so its like osb wrapped in 3 layers of Tyvek; that bad?
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Old 07-23-2015, 07:24 AM   #4
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Ghengisconrad (love the name by the way as that is what I call my one pup...Ghengis),

Gary notified me that you had replied. He is great to have as a sort of extra notifying system when he "polices" the boards.

That is likely the definition of a moisture sandwich depending on how the XPS is skinned.

See the perm table here.

If the XPS is poly skinned, you have a nightmare waiting to happen. General building procedures are moving away from poly on the interior but I do know that certain parts of Canada still require it.

http://www.buildingscience.com/docum...roperty-table/

Last edited by Windows on Wash; 07-23-2015 at 07:26 AM.
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Old 07-24-2015, 11:44 PM   #5
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1. Poly should NOT be used for your location. 4-5 cooling months with the AC on will give condensation trapped in the wall, esp. when using denim (cotton) that holds water in it to rot the wood framing.

2. Use a polyamide sheet or asphalt-faced FG for the vapor barrier, OR increase the exterior foam thickness to go without one; http://www.certainteed.com/resources/30-28-080.pdf

Either R-10 or R-15 foamboard in your Zone 7; http://www.buildingscience.com/docum...r-requirements

http://www.reca-codes.org/codes2006/North%20Dakota.pdf

2x4 or 2x6 wall? Gary
PS- Tyvek (drainable-- or stucco-wrap, loose nail siding per instructions not compress foam) should be on exterior of foam, then window install.
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