plastic sheeting over closed cell foam | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

Q&A Forums

plastic sheeting over closed cell foam Post New Topic | Post Reply

Author	Comments
hykerob Posted: Aug 19, 2016 03:48 PM	plastic sheeting over closed cell foam I'm having closed cell foam insulation sprayed between the rafters of my vaulted ceiling in NH, 5.5" thick directly to the underside of the plywood sheathing. The rafters are 2x10", so there will be a few inches of space below the foam. Do I need to install poly sheeting before installing sheet rock? One builder tells me I need poly to prevent moisture from condensing on the bottom surface of the foam. The other tells me poly will just trap any moisture in the ceiling between the poly and the foam, since there is no air flow. who is correct and is poly necessary or dangerous? Also, are there any concerns with using traditional figerglass batts (and poly) in the walls, from the roof down, and is there any special transition necessary between the fiberglass in the walls and the foam in the ceiling? I only want to do this once. thanks, Rob.
mason Posted: Aug 19, 2016 04:52 PM	Rob, Your builder does not know closed cell foam, its physical properties and how to use it. No, the plastic is not required and could possibly work against the foam doing its job properly. The closed cell foam is a relative vapor retarder, with a permeance of around 1 to 1.5 per inch, it allows a controlled flow of water vapor through the material while insulating the outside temperature from the inside temperature. This keeps the dew point from being achieved within the assembly. Throughout the year, moisture vapor flows into and out of building materials depending on the temperature inside and out and the relative humidity. With the closed cell foam, it allows a small amount of water vapor to flow through the assembly so that there is not a sufficient amount of water vapor at the dew point within the assembly. 5.5 inches if sufficient in your area to prevent condensation and to separate the inside and outside temperatures. Go to spray foam.org and download the Moisture Transmission Paper from SPFA. It describes moisture vapor transfer and how closed cell foam works to reduce the potential for moisture.
hykerob Posted: Aug 24, 2016 02:21 PM	great. thanks for the help.

Author

Comments

hykerob

Posted: Aug 19, 2016 03:48 PM

plastic sheeting over closed cell foam

I'm having closed cell foam insulation sprayed between the rafters of my vaulted ceiling in NH, 5.5" thick directly to the underside of the plywood sheathing. The rafters are 2x10", so there will be a few inches of space below the foam. Do I need to install poly sheeting before installing sheet rock? One builder tells me I need poly to prevent moisture from condensing on the bottom surface of the foam. The other tells me poly will just trap any moisture in the ceiling between the poly and the foam, since there is no air flow. who is correct and is poly necessary or dangerous?

Also, are there any concerns with using traditional figerglass batts (and poly) in the walls, from the roof down, and is there any special transition necessary between the fiberglass in the walls and the foam in the ceiling?

I only want to do this once. thanks, Rob.

mason

Posted: Aug 19, 2016 04:52 PM

Rob,

Your builder does not know closed cell foam, its physical properties and how to use it.

No, the plastic is not required and could possibly work against the foam doing its job properly. The closed cell foam is a relative vapor retarder, with a permeance of around 1 to 1.5 per inch, it allows a controlled flow of water vapor through the material while insulating the outside temperature from the inside temperature. This keeps the dew point from being achieved within the assembly. Throughout the year, moisture vapor flows into and out of building materials depending on the temperature inside and out and the relative humidity. With the closed cell foam, it allows a small amount of water vapor to flow through the assembly so that there is not a sufficient amount of water vapor at the dew point within the assembly. 5.5 inches if sufficient in your area to prevent condensation and to separate the inside and outside temperatures.

Go to spray foam.org and download the Moisture Transmission Paper from SPFA. It describes moisture vapor transfer and how closed cell foam works to reduce the potential for moisture.

hykerob

Posted: Aug 24, 2016 02:21 PM

great. thanks for the help.

You need to login to reply to this topic. Please click here to login.