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Adding insulation to a slate roof

Tim White | Posted in General Questions on

I bought an old Victorian home in southeast PA (zone 4/5) and the roof currently only has R-19 fiberglass batts in the rafters with no baffles. Some areas have loose cellulose in the rafter bays with 1/4″ plywood screwed to the underside of the rafters to hold the insulation.

The attic is finished and conditioned (it’s currently my kids’ playroom), so I don’t want to insulate the attic floor.

The roof is slate with nickel-gap sheathing, and the rafters are 8″ deep.

Reading through some of the articles/forum posts here it sounds like I ought to provide a 1″ or 2″ air gap behind the sheathing with baffles to provide ventilation and prevent wind washing. Once those baffles are in and sealed, would it make sense to re-install the R-19 fiberglass and then put another 2″ rigid board onto the underside of the rafters, and finish with drywall? Is a sandwich of 2″ rigid, 5.5″ fiberglass, 2″ rigid an OK assembly, or should I re-organize those layers? Or throw out the fiberglass and replace it with either mineral wool or additional layers of rigid? 

I assume that removing the fiberglass and cellulose entirely and replacing it with rigid baffles and spray foam would be the *best* solution from an efficiency standpoint, but just doing some back of the napkin math, that’s looking like about 6x to 8x more expensive. Considering this is a large roof (over 2,000 sf to cover), those costs really add up, and I don’t think the improvement in R-value and air sealing is worth it compared to the options above.

Open to suggestions you all might have, including which type of rigid to use in this situation. I’m leaning toward polyiso which I can get locally for about $0.20 to $0.25/bdft.

Thanks,

Tim

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