Low slope roof: retrofit insulation inside low slope roof trusses or re-roof and insulate from above?
Hi all,
First, as I always say, I’m a DIYer, but fairly hardcore and not an idiot.
I have a house with a low slope epdm roof (tucked behind parapets so some slope is possible) badly drained by scuppers. Some previous idiot cut holes in it for a swamp cooler and it leaks a little there but nowhere else. The epdm roof sits on 20″ 2×4 trusses. There is R-30 of fiberglass insulation in there but the house performs very badly in the summer. The truss space is unventilated.
I’m currently doing some remodeling and wanted to pull down the drywall ceiling and install ~R46 or ~R53 in mineral wool batts in the truss cavities. However, in doing some research, I’ve learned that air permeable insulation in an unventilated flat roof is apparently a bad idea? I’m not sure I understand 100% why.
So now I’m looking at some alternatives, somewhat in order of what I perceive as cost:
- Leave it alone (current winning proposition but I know I’ll hate that this summer). $0
- Add the batt insulation anyways. Zone 5b and dry here so maybe it would be ok?. $1000 (DIY labor)
- Rip out the batt insulation. Have 2″ to 4″ of closed cell foam blown in against the roof deck. Replace fb batts with new mineral wool batts (I’m not reinstalling old fb … cloud of fibers). $3000 (pro spray, DIY drywall/batt)
- Add ventilation to the truss space (not supposed to work with flat roofs) and reinsulate with more batt insulation. $?
- Replace the roof (it has ~5 years left in it possibly and likely dates to 1987) and add board insulation (polyiso? mineral wool) on top. $5k to $15k?
- Get crazy and replace the flat roof with a trussed pitched roof and conventional uninsulated ventilated attic. $15k to $35k?
Your opinions appreciated.
Replies
If you have a swamp cooler you likely live where winter temps rarely drop below about 30, and humidity is generally quite low. In such circumstances you probably have very little chance of condensation problems above the insulation.
The edge of swamp cooler terrain...
In fact, I'm on the front range of the Rockies. Swamp coolers are common, as are winter temperatures below 30. I believe our average winter temperature is 39 F and our heating degree days ~5500. Zone 5b.
I agree about the condensation as I've done some investigating of the original structure. 35 years of batt insulation behind a vapor barrier and no ventilation and I don't see any signs of deterioration in the plywood deck (at least from below).
Some other internet info and sites suggest these facts:
- I should have R-38 minimum in my climate (I agree)
- A flat roof should not be ventilated (it doesn't work apparently? I can accept that.)
- An unventilated roof should not have air permeable insulation
- If retrofitting insulation into an unventilated low slope roof, the insulation should either go on top or needs to be closed cell foam against the bottom of the roof deck.
However, I'm confused why some of these things are true.
In terms of air permeable insulation, isn't the issue how well the vapor barrier between the theoretical vapor load of the interior (in this dry climate, I'm not convinced it is easy to overcome the available water carrying capacity of the dry air) and the roof decking works? Assuming a perfectly installed vapor barrier on the interior and an unventilated roof, what does the air permeability of the insulation have to do with it?
In terms of closed celld foam, I'm confused as to the value. If the vapor barrier to the interior is well-installed and the vapor barrier to the exterior is well installed (rubber + plywood is pretty vapor tight), then where does the condensation come from? The trapped air? Well, yes, it has some vapor and if the roof deck is cold and the air can move through the insulation to the cold roof deck, it could condense against the bottom of the plywood which could absorb some of it I suppose. Does anyone actually think that the bottom side of a piece of plywood without something to trap the condensation against it will absorb dangerous MC levels? That seems crazy to me.
I've seen some articles about roof deck failures (greenbuildingadvisor.com) in Phoenix. However, that is kind of a special climate, right? Huge air conditioning load, white epdm roofs that reflect most heat, osb decks. I'm guessing some of those roof are TPO as well...
It seems to me that in a dry western climate with black epdm roofing in a heating dominated climate with a plywood deck and an interior vapor barrier, that an unventilated roof with batt insulation will perform just fine?
So why ask? Well, I'm aware that if I significantly increase the R value of the ceiling, the roof deck will get colder. And if I rip out my vertical swamp cooler penetrations (the current plan), then any ventilation occurring as a result of those penetrations will be eliminated. Also, I've been encouraged to abandon my vapor barriers in this climate. In other words, I don't currently have a problem but if I remove my polyE, remove my swamp cooler ducts, and improve my insulation, I might create a problem.
One last comment if you have read this far:
The swamp cooler ducts could be removed from the interior space but terminated ~1" below the roof deck and get a vent cap of some kind. They are, essentially, pre-existing dog houses. I'd probably need some 'soffit' penetrations (vertical walls, 0" overhand so those would wall penetrations) to create air flow which would also neccessitate putting an air barrier above the batt insulation (non trivial in corded trusses)... Opinions?
Vapor barrier question
You're right about one thing you said - you are certainly no idiot.
When you read that article(s) at Green Building Advisor, was it written by them? Or was it sourced from maybe Building Science Corporation?
Vented/unvented/condensation planes midway through the wall (R2000)/dry to the exterior/dry to the interior...there are enough opinions out there to make your head spin.
If you haven't already dug through Building Science Corporation's website, do it. Far and away the most thorough, down to earth, consistantly logical information I have found. I just hope they're right on about 90% of their calls, I drank the cool aid.
From your comments, your main displeasure with the roof system is that your house is too hot in the summer, but ok in the winter. In that case I would suggest that you look into a white reflective coating to apply to the EPDM, which will reduce the roof surface temperture from about 180° F to 130 or lower depending on how well the coating covers the black EPDM. This could reduce your roof heat load by about half. You can roll on the coating yourself. I would go to roof supply house and not a big box store for the coating. Also, make sure the coating you get is recommended for EPDM by the coating manufacturer.
I would also work to get the leaks fixed before the coating is applied as the coating will make future leak repairs more difficcult.
Ted