Thermally broken
I am planning to use 2″ of rigid foam to the edges of my rafters on my cathedral ceiling to get me 11″ of cavity and a thermal break. I can use polyiso or EPS for the break. The polyiso will be a little higher R-value, but costs more (not a big deal since we are just doing the edges). We plan to vent the ceiling and dense pack with cellulose. Some questions come to mind:
1) Can the netting for dense packing be secured to the foam board effectively? The netting is normally attached to the studs, but in order to get the maximum cavity insulation, it would be nice if the netting could be fastened to the foam board.
2) If so, which is better, polyiso or EPS? If not, is there a workaround?
3) Which rigid insulation will be better for drywall? I know they will need to use longer screws, but is one more rigid than the other, so as to provide a better base for the drywall?
4) Finally, some of the valleys in the ceiling do not have good ventilation channels, so I am considering cut and cobble in those sections to keep the condensation issues to a minimum.
On last note, in this area good spray foam people at reasonable prices are non-existent, so I have limited options.
Thanks,
Norman
CZ 3A, Greenville, SC
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Replies
Norman,
First, we need a clarification. Are you talking about cutting the rigid foam into thin strips that are 1.5 inch wide, and tacking the strips to the interior side of the rafter edges? I think that's what you are talking about.
Briefly, the easiest way to make sure that you can attach the Insulweb fabric for dense-packing is to install 1x4 strapping on the interior side of your rafters (after attaching your foam strips).
Here is a much bigger concern, however: If your roof has valleys, you can't make the roof into a vented assembly. You have to use an unvented approach. For more information, see How to Build an Insulated Cathedral Ceiling.
Finally, the cut-and-cobble approach is risky for poorly ventilated roofs. For more information on the risks, see Cut-and-Cobble Insulation.
Yes, strips of foam on the interior side of the rafter edges.
I understand the use of the 1x4s perpendicular to the rafters. How far apart should these be? Is this more effective/economical than just putting up fabric covered poly iso board on the entire ceiling and dense packing through openings?
As far as the unvented portions of the ceiling, I was thinking of hitting them with closed cell foam about an inch deep with dense pack cellulose on top. Does that sound right for CZ 3A?
The issue I am running into in my area is finding an insulation contractor who can work with me to build the tight house I want.
Norman,
It sounds like you want to use the flash-and-batt (or flash-and-fill) technique for your unvented cathedral ceiling. In your climate zone, all you need is a minimum of R-5 of closed-cell foam for a flash-and-fill roof assembly -- so your plan to use one inch of foam will work. Here is a link to an article with more information: Flash-and-Batt Insulation.
One other point: When you wrote, "I was thinking of hitting them with closed cell foam about an inch deep with dense pack cellulose on top," I think you meant to write that the cellulose will be underneath the closed-cell foam, not on top of the closed-cell foam.
Norman,
Q. "I understand the use of the 1x4s perpendicular to the rafters. How far apart should these be?"
A. Sixteen inches on center is normal.
Q. "Is this more effective/economical than just putting up fabric-covered polyiso board on the entire ceiling and dense packing through openings?"
A. If you plan to hire a cellulose contractor to install the cellulose, talk to your contractor -- ask what approach the contractor prefers. If you decide to install a continuous layer of polyiso on the interior side of your rafters, along with 1x4 strapping on the interior side of the polyiso, you don't need fabric. The cellulose contractor will just make some holes in the polyiso and insert the hose through the polyiso.
For more information on this technique, see How to Install Cellulose Insulation.
Do I need the strapping if I use long screws to secure the drywall to the studs through the foam?
You need the strapping to restrain the insul-web or polyiso during dense packing, otherwise it's going to bow out/buckle (or shred, if webbing).
Webbing is usually stapled directly to the framing at fairly close spacing to have sufficient retention against dense packing pressures- you can't just staple it to foam board, which has very low staple retention.
It would take a LOT of caps-screws to retain foam board against dense packing pressures, and with 24" o.c. rafters it would take 2.5-3" foam board to provide sufficient rigidity to keep the surfaces fairly flat. 1x furring would ease that by quite a bit.
I understand the issue. My evolving plan currently looks like this:
- Ventable sections: 1" vent channel, dense pack cellulose with netting attached to rafters. With 9" to work with. that gets me R-32 with no thermal break.,
- Non-vented sections: 1" closed cell foam, dense pack cellulose with netting attached to rafters. With the same 9", that gets me R-38 and no thermal break
- Add 1" fiberglass backed poly iso (R-6.1) to both assemblies, giving me R-38 in the vented sections and R-44 in the non-vented.
Assuming equal amounts of each type of cavity (vented and non), I should get around R-40 - R-44,which should be good for CZ 3A No need for 2" poly-iso..
Strapping over that.