At my wit’s end: Flat roof insulation, Part 2
This is a follow-up to a previous question I posted a week ago, “AT MY WITS END: FLAT ROOF INSULATION…..I’ve been calling my roof a flat roof this whole time, but it has a 3″ pitch. One end of the ceiling(near soffits) is 8′ and the other end is 8’3”. 12′ long from soffit to attic space. I don’t know if that constitutes as a flat or pitched roof. Anyway I called my bldg. inspector and he advised me to put in some soffit vents and then drill through the cross beam into the attic to create a vent channel into the attic. My plan as of now is to install baffles to the sheathing leaving approx. a 2″ gap and then installing 2 layers of ROXUL (R-30) underneath baffles and then drywall. The bldg. inspector says code for existing construction is R-38 in my area, northern NJ. I’m not too worried about only getting R-30 because he originally suggested R-19 before I told him I’m planning on deepening my rafters. My ceiling should be air-tight in that I’ll only have a ceiling fan and a central a.c. vent in it, which should be easy to seal. A issue I have with this is that we had an extremely humid summer here this year. So humid that I have mildew growth on the outside of my soffits for the first time ever. I’m a paint contractor and I’ve seen plenty of houses over the years with mildew on the outside of houses even though they were exposed to sun and plenty of air/wind flow. So how is it possible that a 2in. space below my roof sheathing, with little soffit vents, is going to somehow prevent mildew growth on the sheathing in humidity? Anybody have any input on this, good or bad is welcome.
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Replies
Tom,
Q. "Anybody have any input on this, good or bad?"
A. It won't work, because a slope of 3 inches in 8 feet means that your pitch is 3/8 over 12. That's a low-slope roof. There isn't enough of a height difference between your soffit and your ridge to create air movement.
All of this is explained in my article, "Insulating Low-Slope Residential Roofs."
The same article also includes information on safe ways to insulate your type of roof.
Well it blows my mind that a bldg. inspector AND a insulation contractor both recommended the system I stated in my question. I've absolutely run out of avenues to go down.
Contractors and inspectors just quote the “rule book”, which are the code books. They often don’t understand the reasons why the code says what it does, and they rarely understand the science well enough to know what to do when they come upon unusual situations like yours. It is the job of engineers to know the “why” so that they can design specialized solutions to unusual problems.
Your roof has so low of a pitch to be essentially flat. I had a very similar problem to yours in my own house except it was more of a slope, but my roof came into a vertical wall so there was no peak. It was a massive mold farm, the worst I or my contractor had ever seen. The roofers insurance covered the mold remediation (they sandblasted the moldy sheathing and then sprayed it). We went with closed cell spray foam to avoid the need for venting. The ccSPF also encapsulated everything that has previously been moldy which was a good thing too.
Bill
It's possible that the height of the attic would create the needed stack effect to drive air flow. The danger is that the suction might suck air in other soffit vents preferentially over sucking it through the long channel of the flat roof. Maybe if the ridge vent open area is much larger than the soffit vents other than the ones on the flat roof. Can you say more about the attic configuration?
As far as how to prevent mold where the outside air is humid enough to grow mold, I don't know. You can hope that the sun on the roof will help dry it out, but it sounds like that's not always enough. There's been some discussion of treatments of the wood surface that might help, but I don't know. There are high-tech active ventilation control systems for attics that monitor humidity inside and out and vent only when it will help more than hurt, but that depends on that system continuing to be reliable for decades, and I'm not sure it's commercially available in the US.
Hi Tom -
I have worked on buildings in your climate with low slope roof assemblies: I can't agree with Martin more: DON'T try to ventilate; you will introduce more opportunity for moisture problems than you will drying potential, promise.
Peter
Thank you Peter. Forgive me if this seems ridiculous, but if I went with a unvented approach and left the rafter cavity space empty(6") and then put 6 inches of rigid foam nailed to the face of the rafters. Then drywall on top of the that. Wouldn't this work? I would think there wouldn't be any air getting into the rafters so condensation wouldn't form.