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Potential wall design feedback from professional

kcubed | Posted in General Questions on

I will be building a house in climate zone 7B (dry) in Colorado at 9300 ft. I am wanting a “pretty good house.”  average low temperature for Dec, Jan, Feb is -2F.  Average high temperature for Dec, Jan, Feb is 35F.  I have read Lstiburek’s book on building in cold climates and learned a great deal.  Through my reading and google etc… I keep coming back to this design from outside to inside:

James hardie siding
1×4 rainscreen screwed with heco topix screws
3″ ComfortBoard (outtie triple pane vinyl windows using thermalbuck)
dorken delta vent s
1/2 plywood sheathing over 6″ studs 16″oc
2″ closed cell foam
fiberglass batt (flash and batt system)
latex paint

I landed on the closed cell foam between the studs for two reasons.  If I understand this correctly this will change the position of the first condensing surface to be on the sheetrock side of the foam.  based on temperature of sheathing calculations in Lstiburek’s book this should keep me above the dew point “most” of the time.  The second reason is I believe this would eliminate the requirement for an interior vapor barrier if I understand the code correctly?

I am not a builder, contractor, engineer etc… just a guy that can read a book and get a membership to GBA.  Would some smart people that have experience give me their opinions of this design?  If you need any other information that I  left out please let me know.

Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    Your design will work. It won't have condensation issues.

    1. kcubed | | #2

      thank you for your reply. no condensation issues are great! do you see any other concerns recommendations?

  2. Expert Member
    Akos | | #3


    Getting 3" mineral wool insulation flat is not an easy job. I would check that your builder is comfortable in doing it.

    Going with 4" rigid (I think even 3" is enough) foam on the outside and skipping the interior sprayfoam would get you a wall that performs better and way cheaper and easier to build.

    1. brendanalbano | | #5

      If Curtis wanted to eliminate the closed-cell spray-foam, they'd need to get 45% of their insulation outboard of the sheathing according to

      3.5" of foam at min. R-5/in and an R21 batt gives you a ratio of 17.5/38.5 = 45%.

      Foams that hit at least R-5/in would be polyiso or the r-5/in graphite infused EPS. (Standard foam note: XPS is bad for the environment, don't use it unless its recycled.)

      Hitting the ratio with just mineral wool would require going a little thicker. 4" mineral wool at R-4/in + an R-21 batt gets you 16/37 = 43% which is pretty close. Switch to an R-19 batt and you get 16/35 = 45%

      1. Expert Member
        Akos | | #7

        There seems to be a misconception that walls with insufficient foam are "unsafe".

        A wall with some foam will always preform better then a wall with no foam, so it is always better to get some foam on there even if you don't hit the full ratio.

        For example in around here (zone 5 and 6) there are a lot of houses built with 2x6+1" foam. If these walls did not work, we would hear a lot more about it.

        Bottom line, put a much insulation on the outside as you can, if you can't quite hit the BSI numbers, don't worry about. Worry about air sealing the assembly, this makes a bigger difference in durability.

        1. Jon_R | | #11

          > "A wall with some foam will always preform better then a wall with no foam"
          Not in the case of 1/2" foil faced foam. The foil will typically do more harm than the foam will do good.

          The moisture safety of "insufficient" foam is highly dependent on the permeability of the foam and sheathing (ie, outward drying ability). I wouldn't use less than recommended foam when exterior perms are less than .1. At > 1 perm (wet), I'd use any amount of foam. Both the IBC and Lstiburek provide support for this.

          Once can be less conservative and have it work, but moisture safety is reduced. For example, poly + 1" XPS has a low drying rate after bulk wetting. Add a lateral diffusion layer and good interior side air sealing and it might be good enough.

          1. Expert Member
            Akos | | #12

            I don't know, 1/2" foam is not far off from 1" (R2.5 vs R5). With a 2x6 wall, if using 20F outside 70F inside, the sheathing is around ~30F with 1" and ~26F with 1/2" . Not that far off.

            For condensation control neither should work as they are both well bellow 40F. At least the 1" works for sure around here (with interior vapor barrier). I doubt that 1/2" would be that much worse.

            The BSI folks say that 1" ridig+2x6 with interior vapor barrier is fine even in zone 7 ( There the foam is not enough for condensation control and even has the double vapor barrier sandwich but still works.

  3. Expert Member
    BILL WICHERS | | #4

    +1 for thicker exterior foam and no spray foam.

    It’s pretty much impossible to get the sprayfoam applied so that it leaves a flat surface. There is going to be some rounding on the edges and some overspray, all of which will make it very difficult to get batts installed properly.

    I’d go with thicker exterior foam and completely fill the wall cavities with mineral wool batts. This should give you the same performance, and may even be a little cheaper.


  4. user-723121 | | #6


    I have linked this study before but it does apply to your part of the country. Give the double wall some serious consideration. In a dry climate like CO, performance should be high.

    At that altitude your infiltration heat loss calculation will be much different than at sea level, I would guess .012 vs .018 per cubic foot per degree F. Denver uses .015 I believe for altitude 5280.

    1. kcubed | | #9

      thank you. I will consider this and talk to the architect.

  5. onslow | | #8


    As a fellow Colorado resident at 8,000' and educated by GBA as well, I would like to ask if you have experienced vinyl windows in this intensely sunny environment. I have experience with (admittedly builder level) vinyl at 6,000' and 12 years out, the results are not pretty. They may or may not even have argon in them by this point if ever. When spec-ing my final window choices for our new house, I was shocked to find that (in 2014) Marvin and many other manufacturers would not ship windows with argon above altitudes ranging from 4800' to 6,000'. Most strange given the Denver market and mile high fame.

    Once again I will shamelessly plug for the Alpen windows I installed. The fiberglass frames are much stiffer than the vinyl of my experience and the sealing is phenomenal. Thermally fixed are U-.15 and operable U-.19. The little bags to allow shipping with argon to high altitudes may seem odd, but they work. The fear mongering on the internet about the film layer in the middle are essentially ancient history from a different company about 30 years ago now. The Serious Window mess is also past history. The internet never lets go, so research with that in mind. They are local and you won't have to wait for a boat to arrive in six months.

    The dark paint color we chose has been a bit problematic, but the sun here is brutal. Dark vinyl colors will likely chalk quickly here if my white/yellow ones are any indication. I have heard they get wicked hot, too. The thermal cycling the windows experience over the typical day here is well beyond my midwest living years. The expansion stress between frame and glass unit was a key concern of mine and the fiberglass framing data indicating a better expansion rate match between the two seems to be born out.

    If you are new to the environment or altitude, wind will become a very real factor to deal with regardless of window company. Most of the year it averages 3-5 mph in our area. Higher up it can gain significantly depending on siting. It is possible to see gusts up to 35 for extended periods come spring and we have had some that felt like 60 this winter. Main relevance is to not select a large number of operable windows thinking that you want breezes to waft through the house. Wafting to wind tunnel is more likely. And the dust is terrible here in a dry alpine environment. Do note that in some localities on the front range you need to meet near hurricane levels to meet wind codes. This may require tempered glass that can be hard to order.

    I was warned about the wind situation in time by a friend who built nearby a few years ahead of me. He advised I drop several awning windows I had planned. Egress windows, of course, must be available and for practical purposes that means casement. The Alpen open widely and seal very tightly which is very nice for noise reduction.

    We skipped any AC option based on the 30-40 degree temperature swings that are normal here during the warm months. One window open at the top level and one on each side of the house at ground level pulls the days heat out with stack venting. All the drywall is 5/8" for thermal mass which cools to 62 or so overnight. Since we have high R value walls and ceiling, the days heat and sun load on walls and windows has not pushed us above 72 (yet). Dry heat is real and actually 76 or higher is not bothersome in other homes I have been to in the area.

    For my own design reasons, I opted to put all LOW SHGC windows. The predicted gains during the winter were not likely to balance the losses, something this very gray winter would further guarantee. Just remember the potential gain in the summer months is quite a bit more than winter and infrared bounce from the ground can defeat overhangs. Plan patios carefully.

    On a separate note about your wall choice. I will try to find and post a picture of my roofs during light snow fall. The 10" inch screws we used to anchor the 8+ inch nailbase insulation reveal themselves quite nicely. Much discussion on GBA over the years about how significant this is, so maybe it will be a FYI only post.

    Lastly, if you are on the front range, you will probably be able to find builders that will work with your design choices. If you are on the west slope or mountains between, start looking and interviewing soon as possible. It is quite likely you will hear "we always do it this way" more than you will like.

    1. kcubed | | #14

      I really appreciate your thoughtful response and energy put into your reply! I am taking your advice on fiberglass windows. thank you!

  6. rockies63 | | #10

    I was reading an installation document about mineral wool exterior insulation and they said that Roxul recommends using a product like Cascadia Clips whenever the insulation panel is 3" or thicker. This will solve the problem of the insulation compressing when you install the furring strips and cladding.

  7. Expert Member
    Peter Yost | | #13

    HI Curtis -

    I just want to be sure that everyone remembers two things about first condensing surface temperatures and dewpoint calculations:

    1. Staying above dewpoint per the calculations does not mean you will never see any condensation on the first condensing surface; if it gets cold enough, you will. It's just that it will not be sustained or pose a problem in the long run.

    2. All dewpoint calculations need to assume an interior temperature and relative humidity; the code uses an interior air temperature of 68 F (20 C) but does not specify an RH. Dewpoint requires both and so what you assume or run your wintertime RH at has an impact on the calculation and just how warm the first condensing surface needs to be to stay above dewpoint.


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