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I have a 50 year old home with a vented slate roof. The HVAC system is in the attic.

E6rfyTNKwt | Posted in GBA Pro Help on

The HVAC system is in the attic…..not efficient. Is it reasonable to spray foam the attic ceiling below the slates, and seal the attic? I’m trying to make the home more comfortable and save money on

I have not got a definitive answer from my builder, and I don’t want to regret something down the road.

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  1. GBA Editor
    Martin Holladay | | #1

    You don't want to spray the underside of your sloped roof assembly, because your roof probably has skip-sheathing (a type of sheathing that allows you to see the backside of the slates from the attic).

    If you really want to use spray foam to create a sealed, conditioned attic, the first step is to install a layer of rigid foam (I recommend EPS or polyisocyanurate) in each rafter bay, up against the skip sheathing, to protect the back of the slates from the gooey spray foam. Once the layer of rigid foam is installed -- fussy and time-consuming work, by the way -- you could then install spray foam.

    That way, the sticky spray foam won't glue the slates together, and you'll still be able to perform future roofing repairs.

    Remember, though, that spray foam is not the only possible insulation. You could also install ventilation channels under the slates, and another type of insulation (for example, cellulose) between your rafters.

    Read more here: Creating a Conditioned Attic.

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