Advice – Personal Home – Michigan (Climate 5)
I am approaching 28 and currently in design on my dream house in South East Michigan. The budget is a bit unique, where I don’t mind spending some extra money and have the ability to do some of the work myself to balance cost/performance. I have read countless articles online and found some of the information to be 1- extremely helpful and 2- partially overwhelming. I wanted to bounce a few thoughts to see what may bring the best value and performance in Climate zone 5 of South East Michigan. I did enjoy the “Calculating Minimum Thickness of Rigid Foam Sheathing” but still have a few underlying questions.
– I am looking to find a good wall assembly that makes sense and will last a very very long time. I do like polyiso insulation bc it is rigid & easy to handle, but understand its derated once it gets cold and is vapor closed. Is a vapor closed material a concern in the north if I go thick enough? My largest fear is moisture being trapped somewhere in the cavity etc. but ive also heard zip has the ability to drain etc.
-Is Zip R12 worth the expense with not having to buck windows and worry about funky exterior details, is the material bad? I did note their shear rating but still a bit concerned about constant building movement? obviously I would ensure proper bracing etc. The majority of the building will probably be traditional stone/brick on a brick ledge w/ an air gap which I would help minimize any building movement. Also, slightly concerned for my WRB relying on tape to survive?
-Should I stick to traditional Zip OSB & buck windows & blanket the home in 2” of Polyiso? Note, I could install the insulation myself and balance costs, but seems like when I buy a Zip Panel & 2” Polyiso + Tapes etc. It will probably come to the same cost of Zip R12 with a lot less headache as my framers would install the osb & windows then I would have to come back & install polyiso across the entire house. Is this method really that much better scientifically than Zip R12? The largest difference I see is warm vs cold OSB on the outside? Anything I am missing?
-Im not brand specific, but should I disregard Zip all together and build traditionally with plywood sheathing, vapor open self adhered membrane and 2×6 cavity with open cell?
Thank you for the reply and the discussion!
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Replies
ZIP-R was worth it for my personal build in zone 5. The carpenters that did my house had never used exterior foam before and the zip R was pretty easy for them to handle as it goes up in a traditional fashion. They were much more comfortable with interior jamb extensions rather than exterior bucks. My carpenters also had some similar concerns with the tape. We ended up taping all the seams with the zip tape but then covered everything with tyvek crinkle wrap and then flashed the windows with the dupont flex flash and straight flash. The zip membrane basically became pointless but in the end it still gave a air tight assembly with some drying potential that my carpenters were able to put up well and felt good about. We finished it off with BIBS in a 2x6 wall with MemBrain smart vapor barrier and it has performed really well. I saw on an episode of this old house they used ZIP R but first sheathed the house with OSB for shear strength and to avoid dimpling of the ZIP R where the nails go in. Not sure what the added cost would be but seemed like it had a couple benefits and may be worth considering. We tried hard to reduce chemicals in the house and choose to go foam free, other than some canned foam a few places in the attic. I have yet to complete my blower door but my gut tells me we would not have gained much if we had used foam.
Appreciate the context of an actual user of Zip R12! Hopefully your energy bills are drastically reduced. Just received some samples today from Huber and the polyiso is actually very rigid... im surprised. I would imagine once nailed according to the pattern it would be rock solid.
I would imagine your not seeing any issues etc.
I really like how you went over everything with a crinkle wrap! Great strategy that would definitely give me an extra piece of mind if I see tapes failing 30+ years from now... absolutely something Ill have to consider on this build!
It's the foil facer on polyiso that makes it "vapor closed". If you get polyiso with kraft or fiberglass matt facers it's a vapor open material, somewhat similar to EPS. Foil faced polyiso is used on the exterior of walls all the time with no problems, you just have to use a thick enough layer to avoid condensation issues on the interior side.
I haven't really worked with Zip myself, but as I understand it, the big advantage to it is that it saves a construction step since it's sheathing and WRB in one -- you just have to hang it and tape it. If you use plywood (or OSB) for structural sheathing, you'll need a seperate WRB. If you use foil faced polyiso over the structural sheathing, you can use the foil facer as the WRB when you tape the seams between panels, but many will put a seperate WRB up anyway for "belt and suspenders".
I personally like the "thick exterior rigid foam" wall assemblies, but there are other ways to go. Some like double stud walls. It sounds like you want to go with exterior rigid foam though. There is a reclaimed materials place off I94 on the way to Port Huron that often has polyiso in stock, so they're worth checking as a way to save some money on your project. I can't remember their name off the top of my head though.
Bill
Bill