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Community and Q&A

Proper way to insulate an unvented slate roof?

ARF9VudJeZ | Posted in Energy Efficiency and Durability on

Hello GBA,

Any best practices on the proper way to insulate a home with an un-vented slate roof?

Keeping the envelope defined as the attic floor would dictate that the attic needs to be vented. I am struggling to figure out how one would go about installing adequate attic ventilation where a slate roof is present.

Re-defining the envelope to the rafters, one would likely use a vapor impermeable spray foam. I am dubious about spray foaming the underside of an 80 year old roof….

Any thoughts or experiences?


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  1. GBA Editor
    Martin Holladay | | #1

    There are several ways to insulate such a home. The best way is to insulate the attic floor.

    If you want to insulate the sloping roof, you definitely don't want to install spray foam against the back side of the slates. You would first have to create a vent channel under the slates by installing 1x1 sticks at the top of each rafter bay, followed by stiff material like rigid foam or plywood. At that point, a variety of insulation materials could be used to insulate the rafter bays. Once the rafter bays are filled, you can install rigid foam under the rafters to address thermal bridging.

  2. ARF9VudJeZ | | #2

    Thanks Martin.

    Would the attic actually be vented with this configuration? Soffit vents/ridge vent? Or would this vent channel just allow air movement on the underside of the slate, but with no real exhaust capability?

  3. GBA Editor
    Martin Holladay | | #3

    Slate roofing is far from airtight. Such roofs are ventilated to a high degree, even without soffit vents or ridge vents, because of the cracks between the slates. The driving forces are sunlight (which creates temperature differences) and wind.

  4. kfitzpat337 | | #4

    I realize this is a very old thread but I'm hoping someone can chime in on my slate roof attic insulation questions. I have a slate gambrel roof with closed decking and I'm exploring attic insulation options. It is a walk up attic with tongue and groove decking on the floor and loose fill underneath in the 2x8 joist bays. There are 3 double hung windows in the attic that I have been using for ventilation. I have ruled out applying spray foam to the underside of the roof due future serviceability concerns with the slate. I am inclined to lay down batts on the attic floor since this would be inexpensive and reversible if I wanted to finish the attic space in the future. Is there any value to this approach or is it a waste of time and money?

  5. kfitzpat337 | | #5

    I should add a couple of details:

    1) I am located in central Connecticut
    2) The 5 ton fan coil unit and ductwork are located in the attic space
    3) My primary motivation for adding insulation is to prevent ice damming. Currently battling ice dams that cause leaks at a dormer and valley.

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