Proper flat roof insulation without replacing roof
Hi,
Apologies in advance for the wall of text, but I want to make sure I outline my situation adequately.
Recently replaced our flat roof deck above a bedroom (~200sqft) with a vinyl membrane after the old one (with mod bitumen on top of it from a previous owner’s repair attempt) was leaking. The original construction was as follows, and the roofer simply replaced the vinyl membrane and rotted wood:
exterior-> interior: vinyl membrane, plywood deck, small air gap (<3″), insulation, poly, drywall. the air gap is vented to a soffit on one edge of the deck, but I suspect this is ineffective because there’s no other vent. The deck is on the 3rd story of a townhouse, with the bedroom below on the second.
I’ve read quite a bit about insulating flat roofs, and am aware that the ideal insulation would have been foam board on the exterior. However, due to the construction, foam on top of the roof deck would have raised it too much to access it from the top floor (too high for the door).
The original plan was to take down the water damaged ceiling, then spray foam the bottom of the roof deck. After doing some reading on spray foam, I’m now concerned about some of the potential health impacts, as my wife has fairly severe asthma.
Ultimately my question is, would it be better to use the cut and cobble approach on the underside of the roof deck, or replace insulation with roxul and install a smart vapour barrier and allow moisture to dry towards the interior. I am in climate zone 5.
thanks for the help
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Replies
B.P.P.
First of all, can you tell us your name? (I'm Martin.)
I don't recommend the cut-and-cobble approach.
You've backed yourself into a corner, and there is only one option: install closed-cell spray foam on the underside of the roof sheathing.
My advice: get your wife a hotel room for a few days. Choose a very experienced spray foam contractor. Make sure that the foam is installed properly, with adequate ventilation. Leave the windows open after the work is complete, and ventilate with a window fan for a few days.
B.P.P -
1. get rid of the one roof vent - You probably have already figured this out, but venting low-slope roofs is almost always a bad idea, introducing more moisture problems than venting is meant to solve.
2. Read this article on spray foam insulation: https://www.buildinggreen.com/blog/foam-place-insulation-7-tips-getting-injection-and-spray-foam-right. Any spray foam installer is practicing chemistry at your jobsite; it's complex operation requiring a system or program of real quality control. It starts with the company AND the installer showing up at your home both being third-party trained/certified.
3. With the PVC roof membrane on one side of your structural roof deck and closed cell spray foam on the other, the drying potential of this roof assembly is not good. Especially since PVC membranes are typically white in color, rejecting a lot of solar IR that can actually help drive any moisture down and through that closed cell spray foam.
Be very sure that you have flawless moisture management bulk water details for this roof. Any penetrations need perfect flashing and the roof should be inspected for wear-and-tear and subsequent maintenance over time. Note that a PVC roof deck has a service life in the 20-year range.
Peter
Hi,
Thanks for the replies (name is Brendan btw). I was hoping to avoid spray foam, but I guess I may have to find a good spray foam contractor. A couple of follow up questions:
1) since I am in a townhouse, would the neighbours have to vacate as well? if so, the project might be a non starter
2) what are some signs that the flashing might be wearing/failing?
3) it appears our cathedral ceiling on the top floor has a similar design as the flat roof (vented soffits, but with two square louvers). No apparent moisture issues so far, just wondering whether the previous owners just gotten lucky in avoiding moisture issues for 25 years?
thanks again, I appreciate the help
Brendan,
Q. "Since I am in a townhouse, would the neighbours have to vacate as well?"
A. No.
Q. "What are some signs that the flashing might be wearing/failing?"
A. The number one symptom would be a roof leak. Other than that, determining the condition of your roof flashing would require an inspection by a qualified experienced roofer.
Q. "It appears our cathedral ceiling on the top floor has a similar design as the flat roof (vented soffits, but with two square louvers). No apparent moisture issues so far, just wondering whether the previous owners just gotten lucky in avoiding moisture issues for 25 years?"
A. You haven't provided enough details for us to have any idea how this roof was detailed. We don't know the type of insulation, whether or not there is an air gap between the insulation and the underside of the roof sheathing, or whether or not there are any vented cupolas. For more information on the correct way to insulate a low-slope roof, see this article: "Insulating Low-Slope Residential Roofs."