Insulation on a low slope, built-up roof
We’re going to make some renovations to our 1920s Baltimore rowhouse (solid masonry walls), one of which will be replacing the low-slope built-up roof, likely with an EPDM membrane roof. There’s a small space between the interior ceiling and the roof, (3-4′ at the front of the house, ~1′ at the rear) which is not accessible.
The current configuration of the roof is (from outside to inside):
1. Built-up asphalt
2. Homasote or similar insulation layer
3. Plank roof decking
There’s also some sparsely distributed insulation on top of the interior ceiling.
We recently got a quote from a roofing contractor who suggested the following setup (from outside to inside):
1. EPDM membrane
2. 2″ poly-iso foam board insulation
3. Plank roof decking
My questions are:
1. Does EPDM over foam board make sense? It doesn’t make sense to me to have 7″ of foam board to meet R-38, and I’d be concerned about walking on the roof with that setup.
2. 2″ clearly isn’t enough insulation, would it be better to eliminate the foam board and insulate the underside of the decking with something like fiberglass batts or open cell foam? I’m concerned about using closed-cell foam since the EPDM is a vapor barrier, and spray foam tends to be expensive.
3. Any other options? Protected membrane roof w/ ballast? Insulating the interior ceiling with unfaced batts or cellulose?
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Replies
Eric,
You can do it either way -- you can install insulation in your attic, or insulation on top of your roof sheathing. Just be sure that:
1. The R-value of the insulation meets minimum code levels.
2. You address thermal bridging at the walls of your attic.
It's possible to install EPDM on top of rigid foam insulation.
Thanks Martin, I'm thinking that adding 7" of insulation above the deck is probably a non-starter, as it adds some complexity to mounting the gutters etc.
I'll probably go with blowing in cellulose above the interior ceiling. Are there any issues associated with having a gap between the underside of the roof decking and the top of the insulation? The attic space is not currently vented.
Eric,
Firestone has extensive details and drawings for their EPDM low-slope products, it's a very good reference to see what a manufacturer recommends: http://www.firestonebpco.com/roofing/epdm/rubberGard/
As Martin said, thick layers of poly-iso with EPDM bonded to the top layer is a very common system.
I have 5 inches under epdm and it is easy. This is by far the simplest solution to your problems.
I am not going to suggest you ignore code, but mostly you are reroofing, insulation is merely a happy accident. They most likely would not even ask, but check with the code official!
They make tapered foam for just the kind of edge issues you are speaking of.
worst case, do you think redoing gutters and drip edge is going to be more complex than trying to insulate above the ceiling? I don't .
A new roof needs all new flashing drip edge etc etc.
Look at it this way, you are either getting a cheap roof to go with your insulation or cheap insulation to go with your roof.
Iso board is like 50 cents a square foot inch[is that a unit of measure?] so just do your math.
Make sure they are doing a "fully adhered", that they stagger the foam seams in both directions.
http://www.modulrts.com/tapered-polyiso.php
tapered iso board
If I am thinking right 2% amounts to an inch
oh, and .06 epdm instead of .045
There's another type of roof that I love for retrofit - sprayed-on polyurethane.
Let me count the ways:
1. You can usually avoid tear-off.
2. All the roof penetrations are flashed "for free".
3. The R-8 insulation is free. Polyiso sheets can be added as needed under the spray foam top layer, or they can spray an extra lift.
4. It's a white roof, not a black roof, and that's earth-friendly and A/C friendly.
5. Usually costs half as much as any of the other options.
Caveats:
1. Quick inspection needed yearly. (true for ALL low slope roofs, but mandatory for foam)
2. Recoat every 5 years.
3. Don't use foam if your hailstorms with > 3/8" hail happen more often than every 20 years.
Well there's "cheap" as in inexpensive and then there's cheap as in shoddy, one doesn't necessarily mean the other. The very big benefit to batts or blown-in cellulose is that I can DIY it, which I'm less comfortable with doing for an EPDM roof.
I spoke briefly to an exterior poly contractor. They hadn't had much experience doing residential, but he seemed willing to do it. Never got an estimate out of him though. The EPDM/2" of polyiso foam board was about $7k, if I recall correctly (14' x 60' roof).
Also, since rooftop decks are reasonably popular in our neigborhood, would using exterior foam (either board or spray) preclude putting up a deck that bears on the roof surface?
Post images of your roof and I will give you an idea of whether it is a DIY project.
Remember that 7k is for insulation and a new roof.
If you have a deck that "bears on the roof surface" then what holds it down? Now you need an engineer.
I've always had trouble finding a good polyurethane roofing contractor, I guess that's the fourth caveat. But yeah, it would be about $3500 for that roof when you find someone.
Again, the sprayed poly is the only easy way to flash in the short columns needed to support a deck.
Fortunately I am an engineer. Unfortunately I'm a water/wasterwater engineer, not a structural one.
To be clear, I'm not actually installing a deck, but could conceivably do so in the future. Floating deck on sleepers is pretty popular around here. The ability to handle foot traffic w/o a deck would also be useful.
There are numerous solutions to putting a deck on an EPDM roof. A EPDM roof cna handle some foot traffic,and they make walking pads for anticipated traffic.