Good assembly for cathedral ceiling with blown-in insulation?
Tristan Roberts
| Posted in Energy Efficiency and Durability on
This is a hypothetical question but it’s something I want to consider for my next house.
I like the natural aspects of cellulose insulation, and I like cathedral ceilings — what are good ways of making the twain meet? There are advantages to rigid foam, e.g. eliminating thermal bridging (and I noticed that all or almost all of the cathedral details provided on the site use rigid foam), but can cellulose be used and obtain similar performance? How should it be framed?
Thanks in advance,
Tristan
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Hi Tristan - the construction details in GreenBuildingAdvisor.com do not often specify the type of cavity insulation in walls and roofs because there are simply too many options. Cellulose is a good selection for cavity insulation, with a couple of points to consider:
1. Although more air tight than other loose fill insulations, cellulose, even dense-pack, does not qualify as an air barrier. So you will need a continuous air barrier right up against or contiguous with the cellulose.
2. At about R 3.6 per inch, it may be challenging to get all the thermal resistance you need in the cathedral space, depending on your climate and structural roof framing. Just as with walls, I prefer exterior rigid insulation on roofs to "warm" the framing cavity and in that way manage interstitial or moisture that works its way into the roof assembly. But then this means an unvented roof assembly; in my opinion, not a bad thing. I recommend going back to the Details section of the website and taking a look at the unvented series of roof details in the 4-05000 and 4-06000 series.
Thanks, Pete. This is very helpful. The most interesting thing about what you said is that dense-pack cellulose is not an air barrier. I am surprised about this -- I think the conventional wisdom is that it is. It wouldn't be the first time conventional wisdom is wrong, of course. Do you have any data on this?
Tristan,
I have a cathedral ceiling in my house, but I did not make it to the actual roof line. I dropped down with a scissor truss type system so there is space between the cathedral ceiling and the actual roof.
If I had extended the thermal envelope of the house to the roof line, this would have then been in conditioned space. Since I did not, the ceiling is insulated with batt insulation. A recent thermographic inspection indicates that I should install an air barrier between the cathedral ceiling insulation and uninsulated attic space. Live and learn.......