Exterior Insulation, Increased U-Factor, Smart Vapor Barriers
Hi all,
I am building a house in Connecticut (Zone 5) for me and my family. It is a single-story, 1800sq.ft. ranch with a walk-out basement. This house is for my personal family so I am willing to spend more than a typical spec house, but the budget has to stay reasonable.
At the end last year our building code changed and now requires more insulation. After talking with my building inspector, he informed me that I can avoid the exterior insulation by framing 2×8 walls, using closed cell foam, or increasing the U-value/possibly shrinking my window and plugging my information into ResCheck.
I would like to avoid 2×8 walls to avoid encroaching into the interior of the house and closed cell spray foam would be very expensive and the off-gassing concerns me. I am not opposed to increasing the U-value on my window, but the more I read on exterior insulation, it doesn’t seem too bad. I’ve been looking into Halo’s GPS Exterra and would do 2 inches to avoid any cold condensing surfaces on the back of the sheathing. I plan on using a combination of vinyl lap and vinyl board & batten siding.
My question is would my money be better spent on:
1. Exterior insulation for the thermal break with Anderson 200 series windows
or
2. Traditional 2×6 framing, zip sheathing air sealed/flashed correctly, and high efficiency windows.
I’d also like to get everyone’s opinion of Halo’s Exterra GPS exterior insulation. I live in Connecticut so I am a little concerned I wouldn’t be able to dry to the exterior but Halo claims their Exterra product is vapor open up to 2 inches even though its appears to have a facer on it. I am open to other suggestions on exterior insulation, but I really like the R-5 per inch, GPS provides a permanent long-term R-value and won’t deteriorate over time like XPS and ISO, and GPS is 100% recyclable and ozone layer-friendly — no CFCs or HCFCs.
My last question is Siga Majrex 200 worth the money? I know there are a lot of smart vapor barriers on the market, but I don’t have any experience with them. I’ve only used poly as a vapor barrier in past builds. If you have suggestions for other smart vapor barriers that are just as good as Siga’s Marjex and more economical, I would like to hear them.
Thank you in advance. The GBA community has already taught me so much.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I like exterior continuous insulation. If you stay at about 2" or less, exterior details aren't that bad. When get up over 2", that's when you're in "thick insulation" territory, and things get a little more complicated. Over 3" I would say is when things start to get tricky. Do be sure to stick to the receommended ratios of exterior to interior insulation for your climate zone though to avoid moisture issues in the walls.
I would not use spray foam in walls -- you really don't gain much using it there, but it does add a lot of cost. For me, the ideal wall assembly is mineral wool in the stud bays and exterior polyiso over the exterior sheathing. Add a rain screen for some extra resilience for the entire assembly, and a smart vapor retarder on the interior to help avoid moisture issues in the stud bays. That makes for a pretty robust wall assembly with good performance that is relatively easy to build and uses standard materials without any special skills being needed on the part of the builders.
GPS is basically beefed up EPS. You get a little more performance in terms of R per inch for minimal additional cost. I can't speak to any particular manufacturer's variations on GPS, but I don't have any problem with the product in general. I usually use polyiso for more R per inch, and I typically use either reclaimed material or "factory seconds" that are often sold by the same people who sell reclaimed material.
I don't worry much about recylability of GPS. I doubt in practice much of this insulation is likely to be recycled if the structure is torn down in the future. This is one of those things that would be a positive if it was ever actually done, but I have my doubts it will be. Most of the stuff from a residential structure just goes in the dumpster at demolition time, including a lot of material that could be reused. This is unfortunate, but it's reality. The manufacturing impact is a more practical difference between the products, since that has essentially already happened, so you DID get that benefit with GPS compared to something else. As I understand it though, most of thoe blowing agents used in the manufacture of rigid foam, especially in the case of EPS, are recovered at the factory though, so that limits the impact of the manufacturing processes for those materials.
Bill
Thanks Bill.
Should I be concerned about the loss of R value with polyiso? From what I have read, polyiso is a poor choice if your winter temperatures dip below 50°F or 10°C. Source below.
https://www.ecohome.net/guides/2254/polyisocyanurate-foam-is-a-hot-new-building-product-learn-where-and-where-not-to-use-it/
Hi Jeff. I live in a primarily cold climate in Colorado, IECC Climate Zone 5 Dry. I specified XPS over polyiso for all of my rigid insulation on my new home build because of the de-rating of polyiso in cold vs. XPS. It's a very valid concern. I was able to source my XPS as factory seconds via Insulation Depot and paid less than 1/2 of retail because of this.
To facilitate ease of construction, I spec'd. 1.5" XPS attached to the structural sheathing with plasti-capped screws. 2x2 purlins were spaced 24" o.c. and the XPS went between these. Tyvek house wrap went over the assembly and the windows were placed over the whole thing (called "outies" here on GBA).
Here is a 30-second timelapse video I created with some captions that might be helpful. https://www.youtube.com/watch?v=xO1bd8DQaak
I am planning on placing a Siga interior air barrier on the face of the studs inside after I have performed all of the electrical, plumbing, and insulation inside. I believe it is worth the investment to make a VERY tight home.
Hope this helps!
I did 3" of XPS exterior insulation in my house addition and renovation. I installed everything outside of the sheathing myself and found it to be very simple to detail and build. I installed the windows as "outside" and doors as "innies". I used info at http://cchrc.org/remote-walls/. You should check the install specs for your proposed sidings to make sure they can be installed over strapping.
Thanks for the feedback everyone!