The Use of a 3 mil Plastc Sheet | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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CHARLES MAURO Posted: Aug 15, 2009 05:38 PM	The Use of a 3 mil Plastc Sheet Hi Mason, My customer has a crawl space under a new addition to their home. It currently has 2 small windows and a concrete floor. I am proposing to spray closed cell foam to the rafters in this space to achieve an R-Value of 30, place plywood over the two small windows and also spray the entire crawl space cement walls from the sill plate to the cement floor (over the window's plywood). They are requesting that I also apply a 3 mil plasic sheet over the entire cement floor as to further prohibit any moisture from getting into this area. Do you see any benefit to the plastc, and is my proposed spraying of this crawl space on track? Thanks
mason Posted: Aug 15, 2009 09:14 PM	You can check out the floor by taping plastic to the floor for about a week. If there is not moisture on the underside of the plastic, then it is not neccesary to add the plastic.
CHARLES MAURO Posted: Aug 16, 2009 08:53 AM	Thanks for the helpful information and quick reply! Charlie
jimcoler I have over 10 years of experience specifying and installing open and closed cell spray foam. I've sold my business but I'm still selling for the new owners and consulting on large and custom specific jobs. I've expanded my knowledge into t Posted: Aug 17, 2009 12:48 PM	The plastic is a quick and cheap belt and suspenders for the crawlspace. This may be attached to the basement somehow or have ductwork in this space which may create condensation issues and corroded ductwork down the road. Plastic is cheap for the crawlspaces and it won't hurt anything. If the concrete is emitting moisture any time of the year, you've potentially blocked it in and won't let it out. So, this drives the RH up in this space and depending on surface temperatures of things in this space, you could get condensation which is harder to get out and more costly than just moisture vapor in the air.
Posted: Aug 17, 2009 06:30 PM	,,,4 mil poly is the thinnest for a vapor retarding membrane,,,3 mil makes a fine dust cloth,,,, most use 8 on dirt crawlfloors,,,i would probably do the same here,,,
mason Posted: Aug 18, 2009 09:52 AM	Good point Foamdude. I always used at least 6 mil poly for a floor overspray protection. The thinner film tears very easily.

Author

Comments

CHARLES MAURO

Posted: Aug 15, 2009 05:38 PM

The Use of a 3 mil Plastc Sheet

Hi Mason,

My customer has a crawl space under a new addition to their home. It currently has 2 small windows and a concrete floor. I am proposing to spray closed cell foam to the rafters in this space to achieve an R-Value of 30, place plywood over the two small windows and also spray the entire crawl space cement walls from the sill plate to the cement floor (over the window's plywood). They are requesting that I also apply a 3 mil plasic sheet over the entire cement floor as to further prohibit any moisture from getting into this area. Do you see any benefit to the plastc, and is my proposed spraying of this crawl space on track?

Thanks

mason

Posted: Aug 15, 2009 09:14 PM

You can check out the floor by taping plastic to the floor for about a week. If there is not moisture on the underside of the plastic, then it is not neccesary to add the plastic.

CHARLES MAURO

Posted: Aug 16, 2009 08:53 AM

Thanks for the helpful information and quick reply!

Charlie

jimcoler

I have over 10 years of experience specifying and installing open and closed cell spray foam. I've sold my business but I'm still selling for the new owners and consulting on large and custom specific jobs.

I've expanded my knowledge into t

Posted: Aug 17, 2009 12:48 PM

The plastic is a quick and cheap belt and suspenders for the crawlspace. This may be attached to the basement somehow or have ductwork in this space which may create condensation issues and corroded ductwork down the road. Plastic is cheap for the crawlspaces and it won't hurt anything.

If the concrete is emitting moisture any time of the year, you've potentially blocked it in and won't let it out. So, this drives the RH up in this space and depending on surface temperatures of things in this space, you could get condensation which is harder to get out and more costly than just moisture vapor in the air.

Posted: Aug 17, 2009 06:30 PM

,,,4 mil poly is the thinnest for a vapor retarding membrane,,,3 mil makes a fine dust cloth,,,,
most use 8 on dirt crawlfloors,,,i would probably do the same here,,,

mason

Posted: Aug 18, 2009 09:52 AM

Good point Foamdude. I always used at least 6 mil poly for a floor overspray protection. The thinner film tears very easily.

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