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Community and Q&A

Good or Bad Idea? Adding Rigid Insulation to Top of Vented Roof?

bradesp | Posted in Energy Efficiency and Durability on

I have an older cottage I live in that is 1 hour north of Raleigh near the NC / VA state line in Zone 4.  The summer heat is brutal and when we renovated 15 years ago, we did our best to improve the homes comfort.  One thing we did was remove the roof decking and the old WET fiberglass insulation (from condensation issues INSIDE the house) and we spray-foamed 3-4 inches of closed cell foam… leaving 1.5-2″ of venting per cavity.  The rafters are only 2×6, so we couldn’t pack in any more insulation then tat.

Now that we live in this home full-time we we’ve noticed that our upstairs Heat Pump (brand new Mits Inverter system) struggles to keep up with the thermostat setup on particularly brutal days.    So it got me wondering… Would it help in the summer with radiant heat coming in through the shingles on the roof to possibly re-roof and add an additional 2 inches of rigid Poly or EPS foam to the exterior before putting new shingles down?

Also, would this possibly screw up the venting?


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  1. canada_deck | | #1

    I think you'd be better off by increasing airflow through the cavity. How big are your soffit vents? Perhaps a cupola with a fan could help to pull more air through that cavity.

    Or if you are thinking of going through that much trouble, take a look at installing solar panels on your roof. That will absorb the heat and give you power.

  2. JC72 | | #2

    It won't work due to the atmospherically open air gap between the ccSPF and the underside of the roof deck.

    Seal the air gap from the outside and you'll gain some benefit but I don't know how much. Current set up has a R-value of about 22 which is about half of what's required per code.

    Conversely, tear off the roof and roof sheathing to expose the ccSPF underneath. Fill the gap with more foam, then reattach the sheathing and add rigid foam on top.


    Convert the asphalt shingles to shiny metal. That'll reject a lot of radiant heat.

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