Green way to insulate cathedral ceilings?
GBA Editor
| Posted in Energy Efficiency and Durability on
After much debate, I’ve decided that spray foam is not an option I’m willing to explore. Too much room for error. Can I use cellulose on cathedral ceilings? What are my options? I will be making venting channels from the eaves to the ridge and putting something on top of that.
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I assume you mean you will be installing insulation below the vented channels.
What is your roof framing composed of - rafters (what size?), trusses (how deep?). Was eave clearance between wall top plate and roof deck created to allow full depth insulation (high heel)? And where are you located - what is your climate zone? This will determine minimum R-values.
I support your refusal to consider spray foam - it is problematic on a number of levels. And I support your decision to vent your roof - it improves both performance and durability in several important ways.
Cellulose is perfectly appropriate for a vented cathedral ceiling, as long as a tight air barrier separates the thermal layer from the conditioned space, and as long as the vent channels are built in such a way as to channel potential roof leaks down to the eaves and away from the insulation.
But, to get the required thickness and R-value, you may have to build a secondary ceiling frame with standoffs from the primary rafters. This is also an excellent way to reduce thermal bridging.
Thanks for the reply. I will be installing insulation below the vented channels. Anybody sell 12' channels out there?
The rafters are 2x6. Ton's of eave clearance and fully vented soffet and ridge. All new rough sawn lumber roof deck and rafters under a black steel south facing roof that get's really hot. . I hope it never rots and wonder if I should protect it with something before I nail up the channels.
What types of cellulose? Loose so I can do it myself? What's is a good air barrier between the insulation and 3/4 T&G?
You didn't say what climate zone you're in.
I'm in the Northeast. 5a on the hardiness zone map. Thanks
Hardiness zones are in reverse order from IECC climate zones (http://reca-codes.org/IECC%202006%20Climate%20Map.jpg), but it looks like you're probably in zone 5 in both cases.
The current IECC insulation standard for climate zone 5 is R-38 ceiling, so you'll need 10"-12" of densepack cellulose. That will require that you build a sub-rafter ceiling, suspended from the 2x6 rafters with either plywood or solid wood gussets that can be side nailed.
There are many manufactured roof vent baffles - Accuvent makes a nice set that includes a soffit baffle and extension lengths ( http://www.brentwoodindustries.com/spg/building-products/index.aspx), though you can also build them yourself from ¼" hardboard and 2" nailing strips.
You don't need to do anything to protect the rough-sawn roof decking - it's likely to be more durable than any plywood or OSB roof sheathing.
You have to have a blower to install cellulose, and a good one and the proper technique to get the prescribed density of at least 3 pcf.
The best air barrier for a wood cathedral ceiling is drywall. Install ½" DW with 4" gaps every 8' with window screen or filter fabric behind the DW at the gaps. Then cut holes in the screening to blow the cellulose and fill the gaps with 3/8" drywall and tape the seams. Make sure that the ceiling to wall corners are taped as well and all penetrations are sealed. Then install your wooden ceiling boards.
Great advice.
I think I'm going to skimp a bit vs. the IECC standard and just screw the DW to the 10ft rafters, leave a 4in gap at the peak and blow the cellulose in using your screen idea. It's a small space with a big wood stove, so I should be ok. Just using it on the weekends. I'm going to forget about using T&G since I'm going with DW. Thanks!
On March 10th Harold Orr presented a report on his retrofit of a 4 plex in Regina,SK to SESCI-NAC. He covered a low sloped roof (no room to get in appropriate insulation) with a poly vapor barrier and then built up a new roof decking with crisscrossed 2 - 2 x 6s and 2 x 4s which he covered with OSB, tyvec (?) and metal roofing. In other words he treated the roof like a vaulted ceiling without adequate space for insulation. Powerpoint slides are at http://www.solaralberta.ca/seminars.php .
Thanks Frank.