Insulating and Air-Sealing Concrete-Block Wall and Roof
We recently purchased an old concrete block building in Hillsborough, NC (zone 4A) and will be remodeling it for our retirement home. It is currently an empty shell with 10’ tall concrete block walls, concrete floor, and wood truss roof (photo attached). We would like to approach the remodel with “net-zero” energy in mind. Obviously, we want to do this sensibly and as cost efficiently as possible. I have spent many hours reading GBA and BSC articles but still have questions and would like confirmation that we are on right track. As we are in the design phase, we would hugely appreciate your thoughts & experiences on the following:
Walls/Siding: We would like to use exterior insulation to keep thermal mass inside. The exterior will be combination of stucco (or EIFS) and some cement board siding.
2015 IECC required R-8 for mass wall and R-20 for wood walls. Joe Lstiburek in his article “BSI-081: Zeroing In” suggests R-40 walls. Since our walls are “mass wall” (concrete block), it seems like 2” of exterior foam (2 layers of 1”) will meet the IECC requirement. It just doesn’t feel like enough though… your thoughts?
· Does it make sense install 4″ exterior foam boards since I think the maximum I can reasonably attach to the masonry blocks is about 4” due to fasteners?
· Traditional stucco and cement board are heavy. Are there any problems having this weight hanging out 4+” (4” foam + furring strip) from wall since attachment are just some sort of long fastener? Which fastener would be the most economical to buy and easiest to install? Would screws like Tapcon pull out or bend in this application?
· Would EIFS be a better choice for siding since it is lighter weight?
· Has anyone done any cost/durability comparisons between EIFS and traditional stucco? Traditional stucco seems to be more durable and repairable, but I am concerned that the stucco might crack due to movement since it is hanging out 4” (or more) from the block wall.
· If we use traditional stucco or cement board siding, I presume that we will need to attach furring strips on top of the foam to provide a nailing surface. Are 1×4’s sufficient? Will the furring strips need to be treated since they would be in contact with the water absorbing cement board or stucco? Would steel furring strips be a better choice?
· Parts of the block wall exterior have been parged & painted and parts of it is just bare block. What would you recommend for the air/vapor barrier to be applied to this wall exterior?
· Will another water barrier be required on top of the foam, or will the gap created by the furring strips be enough of a rain screen?
Roof: The current roof is asphalt shingles on top of plywood on top of traditional 2’ O.C. 8/12 pitch wood trusses (commons and scissors with 12-16” overhang). Attic is vented with no ceiling nor insulation; the roof is shot. We would like to replace with steel roof. Since half of house will have vaulted ceilings from existing scissor trusses, seems like the right thing to do is to un-vent the roof, seal up the attic, and make it conditioned space. This will be good for the ductwork that will have to be in the attic. 2015 IECC code is R-49 for roof and 3 ach @50 Pa. Joe Lstiburek suggests R-60 roof insulation and 1.5 ach.
· What are your suggestions for the most practical and cost-effective way to achieve this level of insulation and air tightness?
· So far, my current thinking is to put 4” (2 layers of 2”) foam board on top of plywood decking, cover it with water control membrane, attach furring strips on top of that (for roof attachment & ventilation air gap), then install the metal roof. On the inside, spray 2” closed cell foam on bottom of roof and to seal the roof/wall gap. Then get the remaining R-value based on lowest cost through either (1) more spray foam, (2) blown in cellulose, fiberglass or rock wool, or (3) install batts of fiberglass or rock wool. Does this approach seem sensible? Am I missing any other considerations?
· I am relatively indifferent to which approach seems “best”. Should cost then be the only decision factor?
· I am a bit concerned with spray foam due to potential offgas and unknown long term health effects. Also, because of cellulose settling, the initial lower cost may not be real . Am I over thinking this?
· If I do not use spray foam, to improve the sealing for the transition from wall to roof, should I cut off the overhangs (“monopoly” house) and then air seal and insulate the wall and roof together so that the roof & wall seals together and insulation are joined; then build the overhang/lookout above the roof deck? Or would it be more cost effective and simpler to just just seal up the overhang area with spray foam? Again, Am I over thinking this? Can anyone comment based on experience?
This ended up being much longer than anticipated. Thanks in advance for any feedback/help/advice you can provide.
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