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Yet another exterior insulation idea

pnwbuilder | Posted in General Questions on

I am building a 1000 sq. ft. house in marine zone 4 and am trying to find an economical way to get more exterior insulation than the minimum required by code. In my area the minimum would be R7.5 over R21 in 2×6 structural wall. I can get the minimum met with 2″ EPS or 8lb mineral wool, but I was contemplating a different approach to get R30 in the exterior insulation for just a bit more money.

Here is what I have in mind. Cut EPS into 7.25″x9.5″x8′ boards. Use glue or nylon clips to temporarily fix them over every other stud in the sheathed and air sealed 2×6 16 O.C. structural wall. This will create the same cavity width as regular 24″ O.C. 2×8 framing. Fill the space between the EPS with less expensive low density mineral wool (Comfortbat R30), install Tyvek WRB over the whole wall and then install 2×4 battens over the EPS boards with 11″ screws. Lap siding will go on top of that. The wall will be fairly vapor open since mineral wool will cover about 70% of the wall, with relatively narrow strips of EPS in between. Tyvek will prevent wind washing and will place WRB pretty far from the sheathing. Maintaining all battens in the same plane becomes very easy too as long as the structural wall is build strait.

Using big box prices to estimate the costs, this approach would save me about $3K compared to two layers of 3″ Comfortboard 80 for similar R value, or will cost about the same as using one 3″ layer of Comfortboard 80 but will provide R30 insulation vs R12.

I would greatly appreciate your opinions on this approach.

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds extraordinarily fussy but not impossible. Certainly not worth it for a professional -- but if you are a hobbyist or owner-builder, there is nothing wrong with experimenting.

    I predict you'll come up with some unexpected hurdles that will be frustrating, though I don't know yet know what those hurdles will be. One hurdle, certainly, will be getting those 2x4 furring strips (the ones you are installing with 11-inch screws) to end up co-planar.

    Make sure that you have a plan for mounting and flashing your windows before you follow this path too far.

  2. canada_deck | | #2

    How do you plan to detail the bottom of this section?

  3. brendanalbano | | #3

    I can't find a detail online, but I recall noticing this project using a similar system of alternating strips of foam with mineral between them as I walked by it while it was under construction:

    So it might not be as economically unviable as it initially seems, depending on the specific contexts of the project.

    It may be a bit of a long shot, but emailing Green Hammer and asking about the technique might get you some tips. I don't think their exterior insulation was as thick as what you are proposing, but the details still should be somewhat similar.

    You may also be able to download the permit drawings here:

  4. pnwbuilder | | #4

    Hi Martin,

    Thank you for your prompt reply. I'll definitely be spending more "fun" time figuring out flashing details, but hopefully they
    will be similar to the "all EPS" approach. Over all though I am not sure if all this is worth the cost savings.

    I'll probably be doing this work myself so in general I chose material cost savings over labor costs, but to a reasonable extent. I can get the EPS pre-cut into 7.25"x9.5" pieces by the manufacturer at no additional cost, so the only extra work will be the installation of the EPS. Mineral wool installation, on the other hand, should go faster with this approach since it will be compression fitted between the EPS. As far as getting the 2x4 furring strips co-planar goes, I actually thought that installing them over EPS will make things easier due to higher compression strength of EPS vs mineral wool , but if the wall is not build flat, you make a great point that adjustments will be more difficult to make.

    Assuming all other things being equal, do you see any problems with this design purely from the insulation efficiency perspective? The only thing that I am worried about in this respect is that I may end up with gaps between EPS and mineral wool, but that shouldn't be any worse than in the case of a single layer mineral wool installation.

    Thanks again for your input.

  5. pnwbuilder | | #5

    @Canada_Deck very good question :) I haven't completely figured that out yet. I would definitely need to wrap the bottom of EPS/MW with a bug screen. I am thinking of using 12" wide bug screen and attach ~2" of it to the bottom plate, install insulation and then wrap the screen around the bottom and up on the exterior side and attach it to the EPS with some plastic staples to keep it in place until the furring strips are installed. Then fill the gaps between the furring strips with rain screen vent like this one

    1. Expert Member
      MALCOLM TAYLOR | | #9


      As martin alluded to, we tend to evaluate building assemblies for their general viability, and sometimes solutions that fail that test work fine for DIY-ing. It sounds like a nightmare to me - trying to consistently hit studs through that much foam, figuring out how to attach it at corners, blocking for vent and electrical penetrations, etc. - but you may be a lot more patient than I am.

      Do spend some time thinking about protecting the foam with something more robust than bug-screen. That's a lot of work to have ruined by insects or rodents that love EPS.

      Edit: Don't you end up with the strapping @ 32" oc? That's a long way between fasteners for almost any siding material.

      1. pnwbuilder | | #18


        Those are very good points. I definitely have carpenter ants running around.

        I probably could change stud spacing from 16oc to 24oc and install foam over every stud to meet siding installation guidelines. That would create a cavity width appropriate for mineral wool used in 16oc construction. But overall I am leaning towards dumping this idea and just going with one 3" layer of mineral wool. My heat load calculations show that going from R12 on exterior wall to R30 (adjusted for effective R values) would decrease heat load by only 5% or $30/year at current prices for electrical heating. Doesn't seem to be worth the troubles.

  6. pnwbuilder | | #6

    @Brendan Albano Thank you for the links! Glad to hear that they actually have build something similar. I'll follow up with them and see if they would be willing to share their design details.

  7. canada_deck | | #7

    Do you think you will ever sell this house?
    This will result in window sills that are meaningfully deeper, correct?

    My gut feeling is that there may be other areas to get experimental that could provide a larger return on your investment of time.

  8. pnwbuilder | | #8

    @Canada_Deck Well, anything can be sold as long as the price is right :) On a serious note though, I am not sure what you are alluding to. A DIY construction is always a concern for potential buyers, but a deep window sill in itself is nothing new. Passive house folks are building 12" and thicker walls all the time, Larsen truss designs and double stud wall designs would result in a deep window sills too. Some people actually like the extra space on the window sill.

  9. canada_deck | | #10

    @Vlad Shpurik - I'm just making a guess that you will have an easier time selling the house if it uses a standard construction techniques (rigid external insulation) vs something that a house inspector has never seen before.

    Where is your air barrier in the proposed wall section? Is it at the WRB on the very outside?

  10. user-36575 | | #11

    @Canada_Deck - I had to laugh at your question. You're concerned about being able to sell a house in the future because of its wall construction? Every green building website in existence has being crying for ages that no buyers or real estate agents or appraisers ever see anything other than countertop material and $/ft^2.
    Your other questions are good and relevant, but that one struck me as funny. Maybe it's just me on a Friday night.

    1. Expert Member
      MALCOLM TAYLOR | | #13


      Short of taking a chainsaw to it, I'm at a loss for how anyone would ever know what the wall stack-up was.

    2. canada_deck | | #16

      I agree it may be a little far-fetched. People love granite countertops and don't generally care or know if the house is sheathed with OSB or plywood.

      On the other hand, I'm in British Columbia. After the leaky condo crisis, we tend to take wall construction reasonably seriously out here. When buying a place, it's typical for an inspector and even realtor to note whether the wall is "rain screened" or not.

      Maybe I'm having a hard time picturing this wall but I think there are going to be some unique parts that raise questions for the inspectors if the house ever goes for sale. Details at the bottom of the wall and the windows may be particularly telling.

  11. user-5574861 | | #12


    Most lap siding manufacturers provide approved assemblies of exterior insulation up to 4”. After that they require you to get an engineer to approve the assembly so add that to your cost. I am currently installing 3” of Rockwool using 7” fasteners in zone 4 (see attached) and I am glad it is no more than that and it really does not need to me more than that. Additionally, I don’t like the aesthetics of super deep walls, but everyone has their own personal preference.

    1. Expert Member
      MALCOLM TAYLOR | | #14


      Looks good!

    2. pnwbuilder | | #19

      Thanks for the picture and info regarding 4" being maximum for lap siding. I somehow missed that part when I was going though the hardie plank installation document. As I've mentioned in my earlier reply to Malcolm I am probably going to dump this idea and stick with 3" Comfortboard 80. Energy savings in zone 4 are not that great to justify dealing with the issues related to walls that thick. Do you mind sharing the details at the bottom of the wall. I've seen others using perforated stainless steel z flashing attached to the sheathing and going over the concrete wall and another J flashing covering the bottom of the insulation, but I am having hard time sourcing those.

      1. user-5574861 | | #22


        I think you are making the right decision to go with the 3" Comfortboard 80. I will a detailed response sometime over the weekend to share my details and what I have learned so far to hopefully help you and anyone else looking to do this.

  12. user-5574861 | | #15

    Thanks Malcolm!

  13. Expert Member
    BILL WICHERS | | #17

    I think this would be doable, but I think it will be a labor nightmare. Lots of cutting, lots of steps. I agree this would not be practical in production, but could work as a DIY job. I think in the end you’ll wish you just spent the money to do it the conventional way.

    I’d not use EPS in this application. EPS is probably the most fussy to get a clean cut with of the three main types of rigid foam board. I’d use XPS or polyiso here since it’s much easier to make a clean cut with either of those two types.


    1. pnwbuilder | | #20

      Cutting with mechanical saws could be a mess, especially with low density EPS. I've used hot wire jig for cutting 2lb (type IX) EPS for under slab insulation, and it went pretty well, but it was fairly slow going.

  14. Deleted | | #21


  15. user-5574861 | | #23


    We followed Rockwool's published detail for the bottom of the wall. They show flashing at the intersection of the sheathing and foundation wall that is taped to the sheathing. We went 4" up the wall, then had to come out about 4" so we would be proud of the finished stone that will eventually be placed on the foundation wall, but shy of the skirt board so you won't see it. We didn't use a perforated flashing, although I had previously thought about using something like a copper bug screen. Before this, I also taped the top of the foundation wall and Zip together using Siga's Fentrim tape. I have an 8" skirt board above the flashing that you can see in the picture I posted before and at the top of it we used Azek's drip edge to help kick out the water that flows down the siding. I don't have a good picture of what is behind the skirt board at the moment, but I will take one tomorrow and post it.

    1. pnwbuilder | | #24

      Thank you for the follow up!

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