Re-roofing a flat roof in Utah
Hello,
I’m re-roofing our flat roof single-story home in northern Utah — dry summers and sometimes snowy winters. Air humidity is very low and we get about 19″ of precipitation, much of which is snow. We have about 2650 ft2 of conditioned space (walk-out basement) and used 510 therms of natural gas last year for heating at a cost of about $500 with a forced air gas furnace that is probably < 75% efficient. The interior of the house is tight based on a blower door test. The roof has just enough pitch to drain water and has 3-4 foot overhangs from the edge of the house. A few years ago the previous roofer said they cut 2 x 10s or 2 x 12s joists at angles to create the slope. Under the overhang between each joist all around the house is a 2″ x 3″ ventilation rectangle covered in metal mesh w/ roughly 1/4″ squares – like 50 or more of these. The layers from the top down: 1. Tar and gravel 2. Plywood 3. The joists that run north-south: * the southern 3/5 of the area is dense packed with cellulose and no air gap * the northern 2/5 have fiberglass bats with about 2″ gap above them * I think (but have no idea for sure) that there is a horizontal joist running east-west at the “peak” between the two joist sections, i.e. in theory airflow is isolated between them. 4. Plywood 5. Drywall I have lived here 9 years and we have had no indication of water spots, mold or other damage that I can see. The limited cellulose and fiberglass I have seen is dry. I need to re-roof and am strongly considering a membrane roof with rigid insulation underneath, over the roof decking. None of the roofers are proposing to remove the decking unless something needs to be replaced. So I wouldn’t know exactly what is happening underneath and the current insulation would remain in place. My questions are about avoiding moisture problems (primarily) and energy efficiency (secondarily) – $500 for heating doesn’t offer much savings no matter what I change. 1. Is the information I’ve read on “warm roof” flat roof layering in the UK considered best practice in the US if building from scratch? What products are typically used for a vapor control layer? http://www.superhomes.org.uk/resources/insulate-a-flat-roof/ http://www.greenspec.co.uk/building-design/timber-flat-roof-insulation/ 2. If my house appears to be functioning okay from an interior moisture perspective (with my limited view), will the membrane system change anything over the tar and gravel? Presumably tar and gravel doesn’t breath any more than a membrane would. Other thoughts? 4. One roofer proposed removing the gravel, but keeping the tar, and putting the rigid insulation and membrane over it, only replacing decking as needed. Is this a good, bad, or neutral idea? Would the tar act as a vapor control layer? 5. With all the ventilation, the northern 2/5 is a cold roof and the added rigid insulation won’t do much. On the southern 3/5, I guess it might. It costs $800 or more to add 1-2″ more rigid insulation (from the standard 1/2″). Is it worth it? 6. Given the current plans (keeping the existing insulation, decking, and adding rigid insulation with a membrane), if you had to propose a low-cost and/or a high-cost alternatives or things to investigate, what would they be? I’m thinking of anything simple like do moisture drill tests to blow in insulation on the northern 2/5 to drastic like removing all the decking and insulation and plugging the ventilation under the eaves to make a true warm roof set-up. Thanks for any thoughts. Mike
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Replies
Mike,
Here is a link to an article that discusses this type of roof: Insulating Low-Slope Residential Roofs. The article should answer most of your questions.
A few general points:
1. Your roof is not a good candidate for the vented approach, since you don't have enough rafter height for an attic. You want to create an unvented assembly. That means that you will need to seal all of the soffit vents to prevent air from entering your roof assembly.
2. You'll need to add a sufficient quantity of rigid foam insulation above the top layer of roof sheathing, as described in the article I linked to.
Concerning your questions:
Q. "Is the information I've read on ‘warm roof’ flat roof layering in the UK considered best practice in the US if building from scratch? What products are typically used for a vapor control layer?"
A. Without going deeply into the weeds, here is my short version: Listen to the advice on GBA, not the advice on web sites published in Britain. If you install an adequately thick layer of exterior rigid foam, you don't have to worry about vapor control. All you need to worry about is airtightness.
Q. "If my house appears to be functioning okay from an interior moisture perspective (with my limited view), will the membrane system change anything over the tar and gravel?"
A. I'm not sure what you mean by "the membrane system." I'm guessing that you mean "new roofing." New roofing plus added rigid foam will "change something," because your roof will perform better (be better insulated) with new roofing and new rigid foam.
Q. "Presumably tar and gravel doesn't breathe any more than a membrane would."
A. Correct. Both types of roofing are air barriers and vapor barriers.
Q. "One roofer proposed removing the gravel, but keeping the tar, and putting the rigid insulation and membrane over it, only replacing decking as needed. Is this a good, bad, or neutral idea? Would the tar act as a vapor control layer?"
A. It's a good idea. Since new rigid foam will act as an air barrier and a vapor barrier -- as will the new roofing -- the vapor permeance of the "tar" (actually asphalt) is irrelevant. There is no way that water vapor will move through this assembly, no matter what you do about the "tar" (asphalt).
Q. "With all the ventilation, the northern 2/5 is a cold roof and the added rigid insulation won't do much. On the southern 3/5, I guess it might. It costs $800 or more to add 1-2" more rigid insulation (from the standard 1/2"). Is it worth it?"
A. You have to add rigid foam above the roof sheathing to keep your roof sheathing safe, as explained in the article I linked to. Skipping an adequate thickness of rigid foam in not an option that you should consider. In your climate zone (probably Zone 5 or Zone 6), you'll need either R-20 or R-25 of rigid foam, as explained in my article.
In all cases, you need to seal the ventilation openings to prevent cold air from entering your roof assembly.
Q. "Given the current plans (keeping the existing insulation, decking, and adding rigid insulation with a membrane), if you had to propose a low-cost and/or a high-cost alternatives or things to investigate, what would they be?"
A. I'm not going to suggest any high-cost alternatives. Every homeowner should seek out a contractor who provides an affordable solution. That said, you have to follow the rules provided in my article.
Perhaps by membrane, Mike means a TPO or similar roofing material.
Thank you for the response Martin.
Steve - Yes, when I refer to "membrane" I mean TPO. Not having to remove the tar saves about $2000, but limits my ability to remove the decking and see what's happening underneath.
The difficulty is how to manage the overhangs which are uninsulated. See my drawings. I would put the rigid foam and TPO over the tar (minus the gravel). Then on the north side of the house I could seal the vents but the overhangs and the air gap over the fiberglass are uninsulated. On the south side, the overhangs and a bit of the area over the walls are uninsulated.
Is sealing the vents alone and putting the rigid foam on top enough? The large, uninsulated overhangs seem to circumvent the new rigid insulation unless I seal the joists horizontally or fill the overhangs with insulation, no?
What is the best way to seal the vents from the underside? They have fairly well stapled wire mesh on the inside of the plywood.
The correct way I would guess is to remove all the decking, insulate or seal the overhangs, seal the vents, then put on the rigid foam and TPO. But that's $2000 to remove the tar and $2000 to replace the decking and $1000-2000 to insulate or seal the overhangs between the joists.
The simplest is to seal the vents alone, leaving the cellulose and fiberglass alone, and then put on the rigid foam and TPO. Perhaps good enough?
Thank you for your help!
Mike,
The best way to access these soffits is from above. You don't have to remove all of the existing roofing and sheathing -- just enough (above the soffits) to gain access to the soffit area from above.
Ideally, you would seal the vertical areas above the exterior walls with rectangular pieces of rigid foam, installed in an airtight manner. Alternatively, you could seal this area (between the exterior wall top plates and the existing roof sheathing) with lots of two-component spray foam -- an approach that is faster but which has a higher materials cost.
Martin,
On residential flat roofs, is it standard practice to NOT install a substrate layer of exterior plywood or gypsum board to protect the rigid foam?
Steve,
The answer to your question -- whether or not to install a so-called "cover board" -- depends on several factors, including the requirements of the roofing manufacturer, the density of the rigid foam, and the preferences of the roofer. Most experts recommend the use of a cover board.