# Vapor barrier under slab?



## righter101 (Oct 31, 2013)

From the 2009 IRC:

R506.2.3 Vapor retarder. A 6 mil (0.006 inch; 152 μm) polyethylene or approved vapor retarder with joints lapped not less than 6 inches (152 mm) shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists.

Climate is NW Washington, so quite wet.

Anyone allow just R-10 rigid (closed cell) with Tyvek taped seams?

Yes/No? By policy or just tradition? or not allowed?

Slab is heated, residential space, none of the R506.2.3 exceptions would apply.

Just renewed my Sawhorse.  Thank you to everyone for keeping this forum up and running with some great code discussions.  I have really enjoyed reading such a variety of posts and opinions.


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## mark handler (Oct 31, 2013)

I have used Extruded Polystyrene Rigid Foam under freezer/refrigeration slabs.


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## righter101 (Oct 31, 2013)

mark handler said:
			
		

> I have used Extruded Polystyrene Rigid Foam under freezer/refrigeration slabs.


For insulation, or also to double as a required Vapor barrier?


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## mark handler (Nov 1, 2013)

Check the individual manufactures permeability.


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## Rider Rick (Nov 1, 2013)

Always 6 mil with R-10.

What would come up is some contractors would place the 6 mil under the R-10 and some would place the 6 mil over the R-10.

Never just R-10.


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## mark handler (Nov 1, 2013)

Also refer to geotechnical report recomondations


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## Rider Rick (Nov 1, 2013)

I read your question again.

I have never had anyone use Tyvek but I would be open to it if the manufacturer would ok it. Tyvek is a one way vapor barrier so it would need to be installed correctly.

My question would be why use Tyvek?


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## RLGA (Nov 1, 2013)

Tyvek is a weather barrier and, in some cases, an air barrier.  However, it is not a vapor barrier or a vapor retarder, and it is not approved for use in an under-slab condition.  I would use a product specifically designed as an under-slab vapor retarder.


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## righter101 (Nov 1, 2013)

mark handler said:
			
		

> Check the individual manufactures permeability.


I tried to find the data sheet for the tape and had no luck. I  have an inquiry to them already.  The foam board is A-OK but i am not sure about the two together.  Just wondering if anyone else had already figured it out yet.


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## righter101 (Nov 1, 2013)

Sorry for the confusion...

The pink foam board is the majority of the vapor barrier as well as the required insulation.  The issue then becomes the seams between the foam boards allowing moisture through.  Wondering if using TYVEK TAPE at the seams would work, not just using Tyvek under the whole thing.


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## righter101 (Nov 1, 2013)

See my reply to Rick.  R-10 pink board is the majority of the vapor barrier.  In theory if they had a big enough single sheet, with no gaps or seams, it would work.  They have individual sheets so the seams become the issue.  Tyvek tape is proposed.  wondering if anyone has allowed this or sepcifically disallowed it.  Working on getting the Mfg data specs on the tape.


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## RLGA (Nov 1, 2013)

If you are using Owens Corning insulation, then they have tape specifically for use with their XPS (extruded polystyrene) insulation.  Other insulation manufacturers may have their own tape products for their insulation.

I would not use the Tyvek tape, since it was never intended for under-slab use or for taping insulation panels.


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## Mark K (Nov 1, 2013)

Can anybody provide a code reference.


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## RLGA (Nov 1, 2013)

Mark K said:
			
		

> Can anybody provide a code reference.


For what?  Righter101 states in his original post the code requirement for the vapor retarder in the IRC.


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