choosing drain plain material or no drain plain at all
Michael Lind
| Posted in GBA Pro Help on
I have a problem about a decision.
houston tx, 16 foot tall wall 2×6 studs with Dense pack cellulose or open cell SPF, 1/2 plywood, ccw-705 P+S vapor closed, (probably should have used vapor open P+S, but too late to change, a drain plane just incase water gets behind foam, then foil faced polyiso taped at seams which will be the WRB, 1×4 strapping rainscreen, then Hardi lap siding. The space will be air conditioned and heated and have a dehumidifier and ERV
my question is choice for the drain plain.
option 1) 1/8″ waterway mesh vented at bottom (will I loose too much R value due to excessive air movement from chimney effect through any air leaks higher up the wall, no guarentee they will all be sealed)
option 2) drain wrap, or hydro gap, more restrictive to air movement.
option 3) forget drain plain, would water trapped between foam foil and P+S be a problem,
yes, I do realize that (In a perfect world) a good tape job on the outside of the foam-foil should prevent water from ever getting back there
I appreciate any help I can get, I have been stewing over this for way too long, but have been unable to get a definitive answer anywhere on GBA/ FHB / JLC
I am posting this on all three sites
Thanks Mike
PS pix shows bottom of wall and roofnado air block for waterway drainage path
Replies
Yes I have read "wrinkled-housewrap-behind-exterior-rigid-foam"
Michael,
Venting just the bottom is fine, although adding a top vent will not degrade the R-value of your walls.
Rain-screen gaps improve any wall, and if you ar going to add one it's worth making it effective - meaning at least a mesh uunderlayment, rather than bumpy house-wrap.
However, as you have an impermeable WRB you aren't going to get the benefit of drying to the outside a gap affords, just the benefit of the cladding drying better, so whether that's worth it in your climate I'm not sure.
Malcolm
thanks for your reply, this is concerning the drainage plain between the foam and the vapor closed P+S. ( Its sole purpose is to allow any water that ever gets behind the foam to have a way to escape ) I have a conventional 3/4 gap rain-screen between the outside of the foam and the hardie siding.
My concern is If I vent the top, would enough air move up the 1/8" gap to have a significant effect on the R value of the wall remember this is the drain plain not the rain screen
Michael,
Ah got it. I don't think a gap is necessary between the foam and WRB, but lets's see what others say.