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David G. Axt, CDT, CCS, CSI, SCIP
Senior Member
Username: david_axt

Post Number: 2006
Registered: 03-2002


Posted on Thursday, March 09, 2023 - 11:33 am:   Edit PostDelete PostPrint Post

I received an interesting email from Stego asking the question whether the vapor barrier should go above or below the underslab insulation.

What do you recommend?
David G. Axt, CDT, CCS, CSI, SCIP
Specifications Consultant
Axt Consulting LLC
Phil Kabza
Senior Member
Username: phil_kabza

Post Number: 750
Registered: 12-2002


Posted on Thursday, March 09, 2023 - 12:25 pm:   Edit PostDelete PostPrint Post

I have always expected the underslab bapor retarder sheet to be below the underslab insulation, so that it would technically protect the insulation from the higher moisture content of the soil. I don't see any advantage in placing the insulation directly on the soil.
Phil Kabza FCSI CCS AIA
SpecGuy Specifications Consultants
www.SpecGuy.com
phil@specguy.com
David G. Axt, CDT, CCS, CSI, SCIP
Senior Member
Username: david_axt

Post Number: 2007
Registered: 03-2002


Posted on Thursday, March 09, 2023 - 02:07 pm:   Edit PostDelete PostPrint Post

Phil,
Do you specify the insulation joints to be taped to prevent concrete from oozing down the open joints?
David G. Axt, CDT, CCS, CSI, SCIP
Specifications Consultant
Axt Consulting LLC
Phil Kabza
Senior Member
Username: phil_kabza

Post Number: 751
Registered: 12-2002


Posted on Thursday, March 09, 2023 - 03:01 pm:   Edit PostDelete PostPrint Post

Doesn't seem like a bad idea.
Phil Kabza FCSI CCS AIA
SpecGuy Specifications Consultants
www.SpecGuy.com
phil@specguy.com
RH (Hank) Sweers II RA SCIP
Senior Member
Username: rhsweers2

Post Number: 33
Registered: 08-2005
Posted on Saturday, April 01, 2023 - 06:25 pm:   Edit PostDelete PostPrint Post

David:
Per Joe Lstiburek and the "Perfect Wall" (or in this case, the "Perfect Slab"), the insulation control layer is at least graphically indicated to be below the vapor-control layer. But it's sort of mixed in with the other "control layers", so for all I know (and I don't really) it could be interchangable . . . re:https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall

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