Flat roof vented or unvented 2012 advice vs 2019 – 4 roofers – 4 different approaches
I have an addition attached to back of house. House 3/12 pitch. Addition 1/2 to 12 pitch (flat) Addition is attached apprx 3 ft onto back of roof. Cathedral ceiling style, 2×6 rafters, batt insulation runs wall to wall. There is no insulation in the void in the roof over area. Atop is one layer shingles, purlins, metal roof. 15 ft of soffit on side, 40 ft soffit on back, no out take.
Different approaches are:
EPDM area with no vents.
TPO area and install 5 vents across the void area.
Install vents to vent the metal only.
I am sure you are aware I have condensation issues. Located in Zone 4 the humidity levels get high. Water has dripped through smoke detectors and areas of drywall that butt wall to ceiling damaged.
What is the correct course of action? The advice for 2012 is flat roof unvented period! Is this still true for 2019?
https://www.greenbuildingadvisor.com/question/insulating-a-flat-roof-ventilated-or-unventilated
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Replies
Hi MinVirginia,
Venting a low-slope roof like yours and providing an adequate thermal barrier is not likely. There are a few ways to make unvented low-slope roof assemblies work. They are all outlined in the article I've linked to below.
Condensation issues in a roof assembly are related to the dew point temperature, of course. But managing that isn't the only important factor. Air sealing to keep conditioned air out of the assembly is important. Managing relative humidity inside the house matters. And your choice of roofing can be a factor.
I was at a building science conference this week and attended two presentations on roof venting and insulation from Building Science Corp. It turns out that so called "cool-roofs," roofing materials that reflect light to keep the assembly cool, often don't provide the energy needed to keep the roof assemblies dry. That is one of the stated benefits of TPO, which makes me wonder if EPDM may be a better choice for you. Or, at least don't have a light-colored TPO roof installed.
Here's the article you should read: http://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
Brian,
Thank you so much for your quick reply. At 61 years old, I do not wish to get into contracting nor be taken for $9K-$19K for a roofing job which will not solve my issues or worse yet cause more!
Here is what I understand will be the best course:
Close the soffits
Install NO vents
Remove and replace any damaged batt
Be sure all batt fills area between sheathing and ceiling
ISO board - minimum R 15 value for zone 4
EPDM or grey TPO
Should they insulate area between rafters over roof void or would this whole area have to be filled?
Am I close to what a contractor should be offering or am I still being mislead with "this will solve your problem?"
Hi MinVirginia,
Did you read the article? It will answer you questions better than I can here.
To be clear, the polyiso in this case is installed above the roof deck. That gives you R-15 which is the minimum you should have outside. Fiberglass batts in a 2x6 rafter add R-19 for a total roof r-value of 34. In Climate Zone 4, code minimum is R49. Sometime in retrofits people don't hit the modern code minimums and inspectors let them get away with it out of necessity. In a risky roof assembly like this, I don't recommend cutting corners. You can add more exterior insulation.
Thank you. I tried the link. Could not get the article to open. I truly appreciate your responses. The intention behind my questions was to try to gain just a little knowledge in hopes a roofer wasn't cutting corners on me. As I mentioned, I had 4 roofers and 4 totally different solutions - all that did not seem to me that would really work! Again, thank you for your time.