Unvented cathedral ceiling
I’ve read some of the other threads about this but since they didn’t quite answer the questions I had, I posted this.
I have a ranch house built in the early 60’s here in central Ohio. It has a low pitched, cathedral ceiling with an interesting issue. The overhang has soffit vents that upon inspection enter into the roof cavities, but all the 2×8 cavities are blocked at some point before they reach the peak. So there is no air circulation under the roof after a point (maybe 10-12 feet from vent). This has caused some issues in areas closer to the peak with summer condensation at times and I’ve found remnants of moisture when I’ve remodeled in the past. And the way the roof is built there are two major beams (2 joined 2x10s) that run parallel to the peak on either side where I would imagine some thermal bridging is taking place due to the old and/or bad insulation.
What would be the best way to insulate a roof like this? I’m about to remodel another room where I’ll be removing ceiling drywall and will have to re-insulate. I’ve read about the “hot roof’ concept and wonder if this would apply here since I have no ventilation. Could I just fill the cavity with fiberglass and not worry about an air space since there is no ventilation anyway? And if I used fiberglass could I use plastic as a vapor barrier? I’ve recently read about vapor retardants that look like plastic but breath a little and wonder if that would be better.
Also, I know in the near future our rubber roof will have to be replaced and probably most of the decking. Could I at that point remove the current insulation and spray in closed foam from above as opposed to the traditional method of doing it from the inside? I’ve never read about doing a retro-insulation job like that.
Thanks for any input.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
A "vented" roof without a flow path is worse than an unvented roof, since it allows moisture in but provides no way to evacuate it. A roof with a vent channel but no bottom or top openings actually performs much better than either an unvented roof or a roof as you describe, since there is at least an air space for moisture to evaporate and diffuse out over time.
A vented roof is always more durable than an unvented roof, particularly one with a rubber or other impermeable roofing membrane, since it allows the sheathing and framing to dry if it ever gets wet. Additionally, a vented roof mitigates against ice dams and reduces summer radiant heat gain.
I would avoid fiberglass in this roof, as it is the worst performing insulation, and is far too vapor open, and definitely avoid a poly vapor barrier as that will sandwich the deck between two impermeable layers and create a moisture trap . I would either insulate above the decking (ideally with a vented nailbase insulation like CoolVent II, or build the ceiling downward with a secondary ceiling frame gusseted to the roof frame and densepack with cellulose. But I would also try to open the vent channel and install a ridge vent if that's possible.
John,
1. You can't use fiberglass for an unvented hot roof.
2. The best way to insulate your roof is by installing rigid foam insulation on top of the existing roof sheathing, followed by more roof sheathing. This work should be performed when you re-roof.
3. It's odd that you are experiencing summertime condensation. Condensation occurs when humid air encounters a cold surface. What cold surfaces do you have up there during the summer? Are there air conditioning ducts in your attic?
Thanks for the answers, they of course bring up more questions!
-In the short term, as I remodel this next room what should I put in the ceiling until it's time to replace the whole roof (maybe a few years)? Both for insulation and vapor barrier.
-In the long term when I replace the roof, I know I'm going to replace the current sheeting and take out the skanky insulation that's present now. If I use an above roof system, should I put anything in the cavities? If so what? I've read a little about Coolvent and my only concern is whether my pitch is great enough. It seems to be for steeper (3:12) slopes. The info mentions H-Shield NB, for less pitch, but I think I need to find someone locally who deals with this as there are questions about vapor barriers and installation a roofer might not know about. If I used something like this would it go over new sheeting? And once again, what about the cavities.
In terms of the summer condensation, you got me. The only thing I could think of is that I'm getting cooler air conditioned air migrating to the hot cavities that aren't getting ventilated. Weird that I don;t get anything in the summer.
John,
If you want to insulate between the rafters, you have two choices:
- Create a hot roof with closed-cell spray polyurethane foam, or
- Repair the ventilation channels so that they run continuously from soffit to ridge, and then install almost any kind of insulation below -- ideally insulation with enough R-value to meet minimum code requirements in your climate zone.
Repairing the channels would be problematic due to various reasons. The hot roof makes sense as long as I could spray in foam from above. Unfortunately I'm still stuck with my newest room renovation where it would be a waste to spray in foam from below, then have to replace the sheeting in that area a couple of years down the line.
Which is it?
Sorry, I meant to say it was weird I don't get any condensation in the winter!
It would be smart to determine what the "condensation" is, where it's coming from, and when.
The house just was never constructed to breathe. I've seen that after 45 years the insulation in the cavities is pretty useless. So I imagine hot air from the deck (that roof in the summer is scorching) is hitting the cooler air from the inside and had no place to go.
I have the same problem, but I live in atlanta, ga. 1955 house, soffit vents but no ridge. formerly a bulit up roof, now metal. Low slope 1.8 in 12. In the living room, when you look up at the cathedral ceiling you see roof decking. Its 1 x 6 (a real 1). the rafter are 2 x 8.
I have had an energy audit. they are saying spray it with open cell foam. then saying I can't plank over it... has to be sheet rock. why if closed cell is fire retardant? don't know.
Can I install rigid foam sheets, 2 or 3" thick, up against the planks with no airspace, seal with something then do planks over it so some rafters still show?
should we just block up those soffit vents? with what?
I misspoke above: I thought the open cell they want to spray is a fire retardant. they want to do open cell because it's not a moisture barrier so if leaks, you'll see them. but saying according to fire code, can't plank over it, leaving just dead air space between the new planks and the new foam.
the metal roof is 2 years old. slight leaking around the chimney last year when we had 20" of rain in 3 days. they sealed up the panels really well.