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Community and Q&A

SIPs or nailbase with spray foam?

user-5885160 | Posted in Energy Efficiency and Durability on

I recently read an article from Building Science Corp regarding unvented ceilings. If I understand it correctly, if I use SIPs or Nail Base as the main insulation/structure of the roof, I can also add closed cell or open cell spray foam on the underside of the roof to increase the insulation. Is this correct? Is one foam better than the other? Could you also use any other type of insulation on the underside of the roof? I thought if you used insulation on the exterior of the roof, you needed to use an insulation that would allow the ceiling to dry inwards. I guess I am a little confused. Thanks for your help.

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to install spray foam on the interior of a roof that includes SIPs or nailbase, you need to specify open-cell spray foam, not closed-cell spray foam. Open-cell spray foam is vapor-permeable.

    As with all roof assemblies, attention to airtightness is important. The spray foam will help, but you want to make sure to do a careful job of sealing SIP seams and any penetrations like vent pipes.

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