Hello folks,

I own a 1956 house in Washington, DC, with two low-slope roof planes, each about 900 sq ft. It’s time to re-roof, not only because the modified bitumen layer is worn, but also because there is too little insulation in the rafter space. The original 3 1/2″ of fiberglass batts, after nearly 60 years, has little insulating power left. 

We will do the insulation work from the exterior, because the ceilings throughout are in perfect condition – hand applied plaster over old gyp board (not button board).

The plan is to tear off the bitumen (two layers) and the board sheathing, pull out the old fiberglass, inspect for and repair any damaged framing, install a skylight over the kitchen, and add wiring for a few new ceiling lights. There is no attic. Roof framing is 2×10, 16″ o.c., 1:12 pitch. There are generous soffit overhangs on all sides.

I plan to install TPO membrane as the finished roof surface. There are very few penetrations: one chimney, one plumbing stack, one (future) skylight.

My main question is about insulation. Local code requires R-49 in the roof, which can be met with a combination approach above and below the roof deck.

I have considered blown in insulation (cellulose, loose fiberglass, denim?) in the rafter bays,  with air space above the fill and vents at the eaves top and bottom to allow for air movement and to prevent excessive transfer from the hot roof. Alternatively, I’ve thought of using spray foam in the rafter bays against the ceiling plane (applied from above), which would provide a better air seal, and allow for dead air between top of insulation and bottom of sheathing.

Then plywood sheathing, and additional foam/ply or foam/OSB nailbase sheets above the sheathing, for additional insulation and/or air movement before finishing with the TPO. 

This is an unusual roof for these parts, and I have one chance to do it well. I’ve talked with one insulation contractor so far, and have a list of LEED advisors I can consult with. Right now I’m trying to narrow the choices, and gain from the assembled experience here.

A basic sketch of the roof is attached.

 

I hope to do the work in late September … many thanks.