Unvented rainscreen?
Great website. Thank you! I am building a home on my farm in CA (zone 4B). 2×6 construction on top of a 4-inch concrete slab on pandeck over an ICF daylight basement. Incorporating passive solar concepts, going off grid with a 4400 watt array. Heating with a woodstove in our wet and mild winters (very rarely gets below 20F). Hoping a tight envelope, R-25 walls and an R-36 roof, thermal mass in the slab, and opening all the windows during our generally cool-ish nights will keep us comfortable in our very hot and dry summers (regularly over 100F during the day).
I have a few questions for the community:
1) My engineer wanted the stick framed walls bearing on the concrete core of the ICF walls. So the outside face of the 2×6 studs are 2.25 inches in from the outside face of the foam on the basement walls. My plan to add 2.25 inches to my stud walls is: sheet the walls with OSB (.5 inch), wrap with Tyvek, then attach 1 inch of foil faced eps foam, and then screw 1×4 furring strips through the foam into the studs. Then the stick framed wall face will be in the same plane as the above grade portion of the ICF wall and the stucco guy can attach his paper backed lath continuously and we’ll have a nice smooth wall (the below grade portion of the ICF wall will have bituthene waterproofing and the above grade portion will have Tyvek). Generally, I know that rainscreen assemblies are vented top and bottom, but as long as I lap my wall WRB over the WRB on the ICF walls will this assembly present any problems since that .75 inch dead air space is unvented?
2) I know the Tyvek might be redundant, but I have some, and I don’t fully trust the taped foam. If the Tyvec is the WRB, do I need to attach the windows to the OSB or could I still attach the windows outside the foam and flash to the foam with bituthene tape?
3) I am worried about cooling in the summer, since AC is out of the question on our home power system. I ended up putting radiant tubes in the slab because it was cheap and easy. While I kind of doubt I will ever hook up a boiler for heat, I am wondering about using the tubes for cooling the slab in the summer with 50deg well water….. has anybody tried that?
4) I have the option to purchase some ultra high performance windows at a deep discount (UF.15). Still, they are over 2 times the price of the entry level windows I was planning on going with (UF .3). We have a very tight budget and I know money is going to run out at some point…. are windows the right place to bust the budget or could I get a better bang for the buck somewhere else? I am thinking about going with the pricy low U value, high SHGC windows, for the 3 large south facing windows and budget on all the rest.
Thanks for reading!
Galen
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Replies
Galen,
Many questions. I'll tackle a few.
1. The lack of ventilation openings in the rainscreen is not ideal. I would recommend that you detail this like a brick veneer wall. That means that you need flashing at the bottom of your wall studs, directing any water drops to the exterior of your home. Copper or stainless-steel flashing would be the best choices. Then you need to install weep holes at the flashing level. The flashing has to be above grade.
2. Talk to your stucco contractor to be sure that he or she is comfortable installing paper-backed lath on furring strips. Some contractors are comfortable doing this; others, not.
3. If you have a submersible well in your pump, it will drain your batteries fast if you run the pump for more than a few minutes at the time. I don't think you'll have enough electricity to circulate well water through your slab. Pumps are a killer; every off-grid homeowner hates them.
Thanks for the feedback Martin. I talked to my stucco guy, and he will do the lath over furring strips. However, nobody seems to be building with rainscreens around here (we are pretty dry) and he recommended just forgoing the furring strips and attaching the lath straight through the foam and using a decorative thing to blend the two walls together. That system seems less robust than the idea I described before.... unless the unvented dead air gap introduces problems that you wouldn't have without an air gap. Aesthetically, I just want a monolithic look....and can't think of a way to do that while venting the bottom of the rainscreen
Galen,
In a dry climate, you may be fine. This is a judgment call. At least you have foam behind the stucco. That will go a long ways toward keeping you out of trouble.
Stucco is a problematic siding, because it can trap water and is very slow to dry. I hope your system works for you.