Modular wall system
I’m planning a high performance modular build in zone 4A for my home. GC wants to have the modular company build 2×8 flash and batt walls with mineral wool. I think a better wall would be 2×6 with mineral wool batt and 3″ of Comfortboard 80 on the exterior with furring. My argument is that his wall ignores thermal bridging. He counters with a suggestion to add an inch of Comfortboard 80 to the outside of a 2×8 wall. Everything on the outside of the wall would be done on site either way. His argument is that the window and door detailing would be simpler/less work/faster and presumably less expensive with his wall vs my wall. A 2×8 wall would be more expensive than 2×6 from the modular factory (don’t know how much, yet) but 3″ of exterior insulation will be more expensive than 1″ (again, I don’t know by how much). So, which is the better choice from a performance and cost, if you know it, standpoint?
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Brian,
The screen name field is not used for display name on posts. The First Name and Last Name fields are used for display. I'm guessing this is a bug in the programming, but it's been that way since before the most recent site redesign.
You are correct to assume that some exterior insulation will generally create a greater total wall R-value that cavity insulation alone. As you pointed out, it will minimize thermal bridging. It's important to make sure that you have at least the minimal thickness of foam needed in your area to keep the wall sheathing warm in the winter and it is important to plan for your wall to be able to dry in one or both directions.
Your builder is also correct, adding exterior foam complicates window and door flashing and other details. Not that there aren't reliable methods and materials--there are--but it is labor intensive work and therefore, expensive. You can have someone energy model the performance of the two options you are considering to know exactly which will perform better, but unless you are doing the work yourself, the costs are going to be up to your builder.
Here are some articles that you may find helpful.
https://www.greenbuildingadvisor.com/article/thermal-bridging
You https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/article/why-flash-and-batt-makes-sense
https://www.greenbuildingadvisor.com/article/installing-windows-in-a-foam-sheathed-wall
I happen to be on your side. In CZ4, a 2x6 wall with 2" taped rigid foam on the outside is my standard go to wall assembly. You can simply install the window outside the foam, with or without rainscreens, depending on the claddings you are using.
Using rigid foam outside of a flash and batt wall assembly, will have a potentially higher chance to trap moisture at your wood sheathing, becoming more difficult to dry.... and NO, his wall is not "simpler/less work/faster and presumably less expensive".
Thank you, gentlemen.