Building science catch-22!
Happy_Lambo
| Posted in Green Building Techniques on
Alright building science nerds,
I am currently in the midst of what I thought was a re-siding project on the south facing exterior wall of my house in midcoast Maine. Turns out I am in a bit of a building science catch-22. The real limiting factor is that I have limited depth (due to the shallow roof overhang- see image).
Underneath the eastern white cedar siding (which was cupping and cracking in places) there was no rain screen. There was however 1″ polyiso foil backed rigid foam insulation (hence the warping in the direct sun with no way for the shingles to dry to the back). The previous builder had used the foam as the WRB as well, by taping the seams, since there was no barrier found underneath the foam insulation.
So here is my dilemma, with limited depth, is there a way to maintain the R-value of the exterior insulation, but also put in a barrier and provide a rain screen for the backing of the cedar shingles? I would love to either keep the rigid foam since it is a sunk cost and sunk CO2 cost as well (although it is proving difficult to remove successfully enough to salvage it), or switch to a more carbon neutral insulating board, like the TimberHP product produced here in Maine! Originally I was planning on just doing 1x furring strips for the rain screen, vertically screwed to the studs, with horizontal strips on top for attaching the shingles (also limits the number of intrusions into a potential new barrier) but there just isn’t that much depth to do this. Is there a fairly slim and effective rain screen that can support cedar shingles (I know there is the Benjamin Obdyke Slicker product, but I am not crazy about if for several reasons: cost, plastic, effectiveness/compressibility, and the fact that I am already over an inch away from the sheathing with my staples as is)?
If you can’t keep the insulation while still providing space for a rain screen, what is more important when it comes to building science, the insulation or the rain screen? I fear that without that air space, any new shingles I put on will quickly warp and cup same as these, but if I sacrifice insulation space to make an air gap, I am losing efficiency in my wall assembly and potentially increasing the likelihood of the dew point moving inside the WRB…
Any thoughts you could provide would be appreciated and any product suggestions would be welcome!
Thanks
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Replies
Happy_lambo,
There is no magic answer. Given the restraints - the lack of overhang, and your understandable reluctance to mesh shingle backers over the foam, it's a straightforward choice between the increased longevity of the cladding against a diminishing of the walls efficiency. One thing you don't have to worry about is "the likelihood of the dew point moving inside the WRB". That's true of all wood framed walls without exterior foam. They do fine as long as they have a good drying path to the outside, and are well air-sealed.
One complication is that shingles need two layers of strapping - a vertical one to form a drainage plane, and a second horizontal one to fasten the sheathing to. Given that a rain-screen gap may be off the table as an option.
Why don’t you just increase your overhang by adding some furring/ thicker stacked sub fascia at the eaves
I don't know if it would be available in your area but it might be worth looking into Ventgrid. It's a plastic mesh but it's much more rigid. It's 1/2" thick.
Have you looked at a dimpled house wrap like Hydrogap? It gives back drainage without noticeably increasing the thickness.