Installing venting on a cathedral ceiling
Hi,
Owner/builder doing a garage conversion/remodel, finishing the upstairs. I’ve been reading the articles on GBA so far, all very helpful and educational, thank you.
With this project, unfortunately budget has been a driving factor for many project choices so far, though trying to do the best I can within constraints. I am using mineral wool batts to insulate the cathedral ceiling. I bought Owens Corning Raft-r-Mate to vent the the ceiling, there had already been a ridge vent and soffit venting on the garage.
We’ve already installed most of it (it’s not the easiest to work with – it tends to break fairly easily, but it was the cheapest product and you get what you pay for), but there are some gaps. Reading the installation instructions from OC online, it actually recommends leaving a 2″ gap between vents (http://www.owenscorning.com/NetworkShare/Residential%20Insulation/20314-raft-R-mate-Attic-Rafter-Vent-Brochure.pdf).
I had been overlapping them, though there are spots where the vents didn’t quite reach the next one in line. Any suggestions on how best to install this product?
I live in climate zone 6 (Vermont), and I will be adding a vapor barrier between the insulation and drywall.
Thanks,
-Eric
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Pictures of installed raft-r-mate
Eric,
You get what you pay for. Raft-R-Mates are built like cheap styrofoam coffee cups, and are almost worthless.
For more information, see this article: "Site-Built Ventilation Baffles for Roofs."
My guess is that you will want to continue with what you've got, even though the venting is less than optimal. If you do that, you shouldn't leave gaps between each length of foam baffle. You should tape the seams of the baffles with housewrap tape to limit air leakage.
Martin,
Thanks for the response. I've realized this in retrospect. I hadn't seen the site-built ventilation article until after I had already purchased and installed much of the raft-r-mate. I would have gone that route if I had known earlier.
If it's likely to cause significant problems down the road (ie rotting the roof decking), I am willing to tear out all of the raft-r-mate and call it a loss and go with this alternative. That would be a significant loss of time and money on materials at this point though, so if I'm able to reasonably minimize the risk of this being an issue down the road while still keeping the already installed venting, I'd like to do so.
If I tape the seams and ensure a good vapor barrier between the drywall and the insulation, do you think that should be sufficient? Or should I just tear it out and do it another way. I guess what I'm asking is how relatively important is it for venting to really be air/vapor tight, if there is sufficient vapor/air barrier elsewhere?
If this is more likely just an issue of loss of efficiency in sealing the building envelope, and the likely worst outcome is a somewhat higher heating bill, then unfortunately it's what I can afford at the moment, but I'll have the piece of mind of not having to worry about replacing a rotted roof. [I know, not the sort of thing one should post on a green building website, but it's the reality of building on a tight budget.]
Thanks for your feedback,
-Eric
Eric,
You can leave the Raft-R-Mates in place if you want. The most important thing you can do to minimize the chance of moisture problems is to create a tight air barrier on the interior side of the insulation. (An air barrier is far more important at this location than a vapor barrier.)
Most builders create an air barrier at this location with ordinary drywall. Pay particular attention to air sealing at electrical boxes and any penetrations, and make sure there are no recessed can lights.
Once your drywall is installed and taped, you can verify the performance of your ceiling air barrier if you want using a blower door. As a vapor retarder, you can install vapor retarder paint on the drywall.