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Best wall system for Vermont?

calinder | Posted in Energy Efficiency and Durability on

I am building a modular home in Chittenden County Vermont and have been debating what type of wall system and insulation combination will be best. There are three wall systems under consideration, but I am open to alternatives.

1) 2×8 framing 16″ on center with Roxul cavity insulation (R30) and Thermalstar sheathing w/1.5″ EPS foam (R7.5) for total R-value of 37.5.

2) 2×6 framing 16″ on center with Roxul cavity insulation (R24) and 2″ polyiso rigid foam (R13) for a total R-value of 37.

3) 10″ double stud 2×4 wall, BIBS cavity insulation and Zip sheathing for total R-value of 39.

Any advice on which system would be best is quite welcome.

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  1. GBA Editor
    Martin Holladay | | #1

    If those are the only three options, I choose Option (3). This option would be much improved if (a) you chose a sheathing other than OSB, and (b) you included a ventilated rainscreen gap between the sheathing and the siding.

    Option (1) violates the minimum R-value requirements for exterior foam, as explained in these two articles:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

    Option (2) looks OK on its face, but it is tripped up by the fact that polyiso performs poorly at cold temperatures. For more information, see Cold-Weather Performance of Polyisocyanurate.

    I urge you to read this article: How to Design a Wall.

  2. Expert Member
    Dana Dorsett | | #2

    And stop comparing center-cavity R, which is not an apples-to-apples metric (not even close!) The thermal bridging of the framing in different configuration matters (a LOT!)

    To get a handle on that, take this document as your bedtime story for the next week or three, until you fully understand the merits & shortcomings of different approaches:

    Take special note of Table 3, and make sure you understand the column headings. There are detailed discussions of the constructabilty, cost & performance aspect of each of the assemblies being compared, beginning on page 28 (p 32 in PDF pagination.)

    Edited to add-

    The 10" double studwall is significantly higher performance than the other options- it'll be well north of R30 whole-wall, whereas the other two will be well shy of that mark, and have higher moisture susceptibility (unless you swap the rigid foam for rock wool, which isn't a vapor retarder/barrier.)

    Even if the polyiso performed as-labeled, with 16" o.c. 2x6 studs you won't do better than R27-R28-ish. Properly derated for climate it's under R25.

    The 2x8 /R30 16" o.c. with R7.5 foam assembly ends up in the ~R26-ish whole wall range, R27-ish when uprated for the higher performance of EPS with colder temps.

  3. calinder | | #3

    Would wall 2 become a good option with 3" of rigid foam? Exterior mineral wool is probably not going to work given that it would not travel well. Would XPS perform better than the polyiso given the cold climate?

    I've read through most of the articles posted, but cutting through the contradictory systems is difficult. I am trying to keep it as simple as possible while still following tried and true best practices.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Would wall #2 become a good option with 3 inches of rigid foam?"

    A. Yes.

    Q. "Would XPS perform better than the polyiso given the cold climate?"

    A. Yes, but green builders try to avoid using XPS (because XPS is manufactured with a blowing agent that has a high global warming potential). Either EPS (usually about R-4 per inch) or polyiso (call it something like R-4.5 or R-5 per inch, depending on whether you are conservative or not) is a better choice than XPS.

    Q. "Cutting through the contradictory systems is difficult."

    A. Here at GBA, we try our best not to be contradictory. If you want to read just one article, this is the article to focus on: How to Design a Wall.

  5. Expert Member
    Dana Dorsett | | #5

    Using graphite loaded EPS instead of XPS retains the up-rating characteristics of polystyrene sheathing in cold climates without the extra climate damage and long term performance degradation you get with XPS. The R/inch of graphite loaded EPS is comparable to XPS.

    Wall #2 works from a sheathing dew point perspective with 3" of 1.5lb Type-II EPS, but doesn't approach the thermal performance of wall #3 until there's 4" of EPS.

    With graphite loaded EPS #2's thermal performance is roughly equivalent with the 10" double studwall at 3.5" of foam.

    If you dropped back to 24" o.c. spacing for the studs (and maybe includes some AF corners, and dropped to single top-plates rather than doubles) it would be roughly equivalent with 3" of graphite loaded EPS.

    One vendor of graphite EPS:

    If thicker foam cost is an issue (isn't it always? :-) ), a greener approach is to use reclaimed or factory-seconds EPS or XPS. With reclaimed XPS, for design purposes assume it's the same R4.2/inch of EPS, since that's where it will be after all the blowing agent has dissipated. There is at least one foam reclaimer in Bennington VT that has also has a yard in Barre. It seems they handle predominantly polyiso. But at 4" thickness on a 2x6 wall the average temp through polyiso would be warm enough on average that it would equal or exceed 4" EPS on seasonal average performance. I've never dealt with that outfit directly but they regularly show up here:

    There are multiple vendors in MA carrying a wider range of reclaimed and factory seconds goods. The place 2 miles from my house recently quoted someone $12 for 4x8 sheets of 2.5" EPS (R10.5), which is pretty typical for "looks like new" stuff. I've seen 3" polyiso in the $12-15 range too, but it's usually a bit more.

    At some quantity and price point it's worth renting a box van and spending the day driving to/from central MA, if you can't get it locally.

    ...or paying them to bring it to your site: (<<<these folks will ship anywhere in the lower 48, for a price, but you can also pick up in Framingham MA during their weekday business hours.)

  6. charlie_sullivan | | #6

    I agree with Martin. I also note that I've run into an insulation contractor in New Hampshire who uses the term BIBS for cellulose, even though BIBS is a trademark for a proprietary blown fiberglass system. So you might check on which they really mean, although I don't think the difference matters all that much--more important is the quality of the air sealing and the insulation installation.

    Martin has given you plenty of good reading, so there's not much need to say more here until you have questions about it, but one comment is that option 2 could be rescued by using mineral wool on the exterior instead of polyiso. Its R-value isn't any higher, but it wouldn't create the same moisture issue because the sheathing can then dry to the exterior.

  7. Robert Opaluch | | #7

    The graphite loaded EPS sounds really cool (no pun intended). How does the price compare to EPS or XPS? Literature says cheaper than XPS but seems like a special order item. Google search only comes up with your Q&A article and one other outdated 2013 article.

    FWIW I've been thinking about double stud wall [email protected]"o.c., insulated with Roxul rockwool, ply sheathing, with EPS foam outboard (R-value half of rockwool R-value), vertical furring strips 24" o.c. for rainscreen. 2x4 top plates and 3/4" ply 2nd top plate that spans both walls (for firestop, structural and other reasons). Air barrier could be at plywood with Zip tape. R-value would exceed Craig's chosen target. But for Burlington VT area, design temp around -7F and 8,269 or so heating degree days? Personally I'd rather put a couple thousand more into much better insulated walls, among other things. Save on heating system and utility bills.

  8. Reid Baldwin | | #8


    I used graphite loaded EPS on the exterior of my house. I paid $19 per 4'x8'x2" sheet not counting shipping. It would have been a few bucks less per sheet without the facer. If I had it to do again, I would not have used the facers. I don't think they added value. We used a Tyvek layer over the top of the foam as the WRB.

    That is more per R than regular EPS but less than XPS or Polyiso. If you don't have a problem increasing thickness, regular EPS would be a better value. If you are concerned about thickness, graphite loaded EPS is a good choice.

  9. Chaubenee | | #9

    The wall I created is 2x6 24"oc with AF corners, 7/16 OSB with 3M taped seams, lots of air sealing, dense pack cellulose, Drainwrap, 2.5" reclaimed eps foam board, with taped seams, and a few cans of spray foam to fill any voids or depressions, 1x4 furring strips, innie windows, and lots of attention to detail. We are using 6" screws which run about 50 cents each mind you. So it is not cheap but it is close to being a conventional system that is efficient to build. Using 2x8 seems like madness. If I could do one thing to improve my wall, I would of used 1/2 " polyiso under the 2.5" eps and stagger the seams. IF I could snatch a super super deal on the needed sheets of the stuff. And that is the only thing I would do differently if I had a little more money and a little more time. Both are hard to come by lately!

  10. Chaubenee | | #10

    Ps, but that being said, on the first floor we have a bump in where the house goes from 28' deep to 24' deep which forms a single story great room. In that little 2' wall bump in on the front and back of the house the bump in is under a deep porch overhang. Inside the wall is a big jack to hold up a 24' long steel beam. So effectively that little wall section on the front and back has little room for fill insulation. So I doubled the exterior foam there to 5" EPS to reduce that bridging and I bought some 8" long screws just for that small area. Not a large area of square footage but it makes me feel good!

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