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How to insulate attic rafters with foam spray

2hTv7FfiGP | Posted in Energy Efficiency and Durability on

My wife and I own a 2 story home that has a partial 1 story with the rest 2 story and both share the same attic space. I live in the midlands of South Carolina which has very hot and humid summers and winter low averages of around high 20’s to low 30’s. I am interested in adding foam spray to the underside of my roof to prevent heat loss/gain through the roof to the attic. I have had 2 contractors come in to give me a quote and told me how they install the foam. One of the contractors stated that the job would have 5.5″ of open cell foam between the rafters and around 2″ of foam encapsulating the rafters to prevent the thermal bridging. The second contractor stated that it would include 2″ of closed cell foam between the rafters only without any foam sprayed to the rafters. My question is in my area with our weather, would my rafters need to be encapsulated or not to prevent condensation or does my weather conditions really not threaten any condensation from happening?

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  1. GBA Editor
    Martin Holladay | | #1

    You are in climate zone 3. According to the 2009 IRC, the minimum R-value for a ceiling in your climate zone (for those following the prescriptive code) is R-30.

    Contractor #1 is suggesting the installation of about R-20 of foam.

    Contractor #2 is suggesting the installation of about R-13 of foam.

    Neither contractor is proposing enough foam to meet minimum code requirements. Obviously, contractor #1 is offering a better layer of insulation than contractor #2, but neither one is getting you what you need.

    If you choose open-cell foam, you need at least 8 1/4 inches of foam.

    If you choose closed-cell foam, you need at least 4 3/4 inches of foam.

    And yes -- encapsulating the rafters to reduce thermal bridging is a good idea. Don't forget to check with your local building inspector to find out the local requirements for thermal barriers. Your foam may need to be protected with drywall.

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