Cedar shakes over foam
GBA Editor
| Posted in Energy Efficiency and Durability on
I am installing cedar shingles on a gable end wall over 1″ Dow Styrofoam over Tyvek Drainwrap, over 7/16″ OSB. Am I OK to attach the shingles directly against the foam with ring-shank siding nails through the OSB? Is there a better way? I am using premade “panels” of shingles on one gable end and single shingles on the other.
Thanks for any help and advice.
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Replies
Let me clarify, my above post. (note to self - read before clicking "post")
I realize I should have an air gap/rainscreen behind the shingles. However, because of the window/trim detail (1"thick) I am limited to at best a 3/16" gap. Not sure what material would be thin enough to use that way? Standard wood would just crack out i would think. 3/16" Plywood strips primed both sides seems barely durable enough?
Is there a better way? Absolutely!
Eliminate the exterior foam board, swap out the OSB (highly moisture vulnerable) for either plywood (OK) or sawn lumber (better), and install the shingles over #15 felt.
If you need additional insulation and a thermal break, install that on the inside with cross-hatched framing and dense-pack cellulose (with no vapor barrier).
JP,
The answer to your question is climate-dependent. Your OSB will be at risk of rotting unless you are in Zone 4 or warmer (for a 2x6 wall) or Zone 5 or warmer (for a 2x4 wall).
In colder zones, your exterior foam needs to be thicker to reduce the chance of condensation against the cold OSB.
Martin,
Thanks for your help. I am in zone 7 and am not too worried about the OSB rotting. My more pressing concern is the attachment of the shingles through the foam into the 7/16" OSB.
JP,
1. You can use horizontal 3/16-in plywood strips -- no need to prime them -- as long as your nails are long enough to penetrate the solid sheathing under the foam. I've never done it, but it would probably work.
2. I think you are being too cavalier when you dismiss the advice of building scientists concerning the minimum thickness of exterior foam. If you are in climate zone 7, your foam should have a minimum R-value of R-10 for a 2x4 wall or R-15 for a 2x6 wall. Otherwise your OSB will be cold enough to get wet.
Martin,
Sorry, I am in southern oregon (zone 4).
Thanks for your advice.
J
New spin. I live in zone 4. Building late summer-early fall. I want cedar shingles on the outside, but believe what I've read so far a little complicated. What would be the consequences of the following (inside-out); paint - gypboard - strapping - XPS - insulated cavity (2x6) - OSB - Typar - Cedar shingles. Just a thought....
Marc
Marc,
Your question is unclear. I don't anticipate any particular consequences.
My recommendation:
paint - gypboard - insulated cavity (2x6) - XPS - OSB - Tyvek, taped - 15 lb tar paper - Cedar shingles applied w/ 1" crown 1 1/4" leg galvanized roofing staples, no more than two staples per shingle.
Michael,
Can you explain the reasoning behind the tyvek and and 15# felt?
Also, Interesting to see you recommend the xps under the osb. I just learned of this at an earth advantage class last week. The instructor mentioned additional nailing as the only real requirement to gain shear. Does the code permit this now or would I need an engineer to sign off on it?
Thanks, jpeaton
The reason I'm thinking of the xps inside, is as Jason has stated, I was unsure if I could attach OSB outside of the XPS. With the XPS inside, I can shingle the outside wall the old fashion way, and get more time to properly insulate the inside without the weather playing a factor. It was just a thought....
Marc