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Rigid foam on top of roof

user-228058 | Posted in Energy Efficiency and Durability on

Hi, I’m looking at remodeling a 1986 cape. It has R-19 in the roof rafters with Proper-vents from the eave to a unconditioned attic. The house has a shot cedar roof on 1×4 purlins. I am thinking of adding a layer of rigid foam (XPS or polyiso) on top of the 1x4s after stripping the cedar. I would then run a layer of furring strips parallel with the rake followed by 1/2″ sheathing and standing-seam metal. Thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    Don't forget to seal the air intakes and air outlets of the ventilation channels above the R-19 fiberglass. These ventilation channels should be sealed so that the upper insulation layer and the lower insulation layer are not separated by a cooling airstream. If you have an "unconditioned attic," do you have any plans to insulate the rafter bays in the attic area?

  2. user-228058 | | #2

    Hi, Martin Thanks. Yes I plan to add a sealed hatch, and blown in cellulose.At the bottom of the vent channels I will install foam blocks from the top. At the top of the attic plane, I will stop the exterior foam and fur it thicker. The air from the exiting soffit vents will flow above the ridgid foam,up the roof and into the unconditioned attic and out the existing gable vents. I,m thinking of using two layers of 2" xps. the only downside is a thicker looking roof.I think this strategy will do great things for most cape homes poor insulation. More comments Thanks .

  3. user-593033 | | #4

    Why not eliminate the venting ie a hot roof. Venting is totally overrated.

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