Rigid Insulation – Cedar Sidewall
GBA Editor
| Posted in Green Building Techniques on
.
Has anyone seen a workable detail for exterior rigid insulation with white cedar shingle siding? The only option I have come across is to sheath over the rigid w/ 1/2″ plywood? I’m hoping to find a more economical solution, but assuming there may not be…
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Chris,
Seems to me you will need a solid nailing surface like cdx plywood, this is what I used the last time I used cedar shingles. I think you have the detail right in your post.
Cedar shingle roofs were traditionally installed on "skip" sheathing with interlaced strips of felt, and there's no reason you can't install sidewall shingles the same way - on horizontal furring strips laid out on centers the same as the exposure of the shingles. In fact, cedar shingles will perform better and longer if they can breathe to the backside.
I used 3/4" plywood and roofing felt as an underlayment, the rigid has no bracing ability and the plywood takes care of this.
Chris,
For my own home, I used rough-sawn diagonal boards for sheathing, followed by a layer of housewrap. On top of the housewrap I installed a layer of EPS foam. Then horizontal 1x3 strapping, 5 inches on center; then cedar shingles.
The system has been in place for 30 years. I have opened up a few walls for remodeling, and the wall is dry and pretty.
Martin that is a pretty interesting assembly...was/is your wall "breathable" on the inside also?
Would you use Foil faced sheathing if building today?
What about foil faced and the horizontal strapping?
It seems like the EPS and the horizontal strapping is cohabitating just fine.
I am in the process of residing my 70 year old house in the same way; After stripping off the old siding, we are blowing in Denspack cellulose from the exterior. We then add 2" rigid insulation over the existing board sheathing and air barrier. I am then installing a continuous layer of OSB and Homeslicker and then the shingles and new trim. I was originally planning on 1 x 3 strapping, but at 5" on center it leaves a 2" gap, which proved to be too much work, when the OSB was less expensive and faster. I cut the Homeslicker drain screen to 2" strips on the chop saw and apply it like horizontal battens.
John,
Back in the early 80s, interior poly was all the rage, and that's what my home has. But I'm in a cold pocket -- about 9,000 HDD -- so interior poly may be appropriate here. In any case, all of the walls I've opened up look fine.
I don't believe that rainscreens really need to drain. There shouldn't be much water in a rainscreen gap, so horizontal strapping works fine. Any water that gets behind the siding evaporates quickly through the cedar shingles.
I think the houses with 4 inches of exterior polyiso are going to work fine, as long as there is no interior polyethylene.
Jon Wyman,
"Quicker and faster" doesn't always equate to better (in fact, I would say it rarely does). The Homeslicker you added is really protecting the highly vulnerable OSB rather than the shingles, which are durable in almost any application (as long as they can breathe in both directions).
The more processed our wood materials, the more susceptible they are to moisture damage, mold and rot. Sawn lumber nailers would not have required the protection of a drainage plane (viz. Martin's house), while OSB can hardly survive without it.
I don't use OSB anywhere in construction, and I'm constantly amazed at how popular and common it's become. Penny wise and pound foolish, as far as I'm concerned.
Whoops. Meant to say "cheaper and faster".
Chris,
I faced the same question on my own house that I am building and decided to forget the rigid exterior foam and use a double stud wall assembly with dense pack cellulose, sheathing, WRB and 5" spaced 3/8" Raftervent strips (corrugated plastic) for a drainage plane. This made life easier and my math said it was less expensive than enough inches of foam to get the R-40 wall I wanted.
Anything beyond 1 1/2" of foam on the exterior is a pain to work with and this adds only an R-10 at it's best. In a cold climate you really need to consider a thick wall system, a double wall or a wall like Robert suggests, a separate thermal layer. In a really efficient house the main heat loss areas are the windows and ventilation air, if the walls exceed either of these two by any degree you need to rethink your wall design.