Help thinking through re-roof and insulation questions
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 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 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 panels above the sheathing, for additional insulation and possibly 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.
Replies
Unless the roof has leaked a lot, the old fiberglass has not lost much insulation value -- it just never was that good to begin with.
Venting will not do a lot of good. You're not in a cold climate where you need to remove moisture (from inside) in the winter.
It's pretty much normal for a "built up" roof to include several inches of foam board anymore. The flat roofers should know how to do this pretty well.
Thanks for the help Dan H. ...
I appreciate you taking the time to answer. The roofing contractor I've chosen does mostly commercial work, and a beautiful zinc standing seam roof in my n'hood. I know he can handle the foam board. One reason I want to do the carpentry/woodwork is that the rafters are probably not right on 16" layout, so the sheathing will all have to be special cuts. I want to do that, and do it carefully. Above a good clean deck, I'm ok with roofers doing the foam, membrane, and trimmings.
As for cold climate ... we had a doozy last winter. I'll gtry to incorporate some air space somewhere in the layering.
Much obliged ...
Why?
DavidPatton wrote:
I'll gtry to incorporate some air space somewhere in the layering.
Much obliged ...
Why would you do that?
As a buffer ...
... so radiant heat doesn't have as much effect down into the rooms during the intense summer. Is this completely off base? There is no need to fill all 10" of the rafter bays with blown-in foam. So there will be a dead air space of almost 5" betwen top of foam and bottom of sheathing. Plus at least a couple of inches of rigid foam boards on top of the deck, and under the final membrane, should temper the heat extremes, effectively insulating the air and insulation in the rafter bays.
seems like your aiming to add a radiant barrier
I think the best way to reduce radiant heat gain is to reflect it...and dead air is a poor reflector of radiant heat.
You can, of course, carry away heat gain within thermal mass by positive venting (convection), but then you would be short circuiting your built-up thermal insulating "sandwich" of spray and sheet foam with a vented air gap in between layers. Bad idea.
I recommend cooridinating your insulation and radiant barriers to effectively address conductive and radiant transfers of thermal energy.
Other than cost, I suppose, there's no downside to filling up the entire available space with closed cell foam. That extra insulation will defeat solar gain better than passive air convection. And, install a white/reflective membrane.
A reflective membrane does no good without air space next to it. (Or at least this is what I've been told.)
Dan, it would seem to me that having all that space above the roof to the outer limits of the cosmos would be enough. Maybe not.
It sounds to me like you've got the right idea: take off old sheathing, foam the insides with closed cell, put on new plywood sheathing, and top with a membrane. This workk should be able to be done in just a few days by a good contractor.
My best advice: don't necessarily hire the lowest bidder. Ask around for recommendations. This may be a job for a commercial contractor if your house is unique to the area. Flat roofs are common in commercial work. As you probably know, the work won't be cheap.
Thanks for lending an ear, sapwood ...
I appreciate the confirmation you offer. I've chosen a roofing contractor experienced with commercial (and flat) roofs. No, he won't be cheap, but he's careful and thorough. What's that saying, "In for a dollar, in for a dime" ...? I only want to do this work once, and plan to stay in the house, so it's bite the bullet ...
Cheers ...
I met with an insulation expert yesterday
And he suggested filling the rafter bays with open cell foam (Icynene e.g.). Shouldn't be a problem with vapor transmission, shouldn't be a problem with radiant transmission with a white TPO top membrane. I liked his approach.