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Community and Q&A

Investing in Increased R-value

Miles007 | Posted in Energy Efficiency and Durability on

Hi, the cabin we’re building is in Skamania County, WA.  Zone 5B.  Please see drawings (attached, and additional posts).  The attic will be a thick layer of loose fill cellulose.  The walls be 2×6 construction, insulated with either R-21 fiberglass batts, or dense-pack cellulose.  Originally, I was going to insulate the floor of the cabin, but now I intend to get approval for an insulated, vented crawlspace.  2″ EPS on the floor of the crawlspace, and the walls of the crawlspace will be a layer of 2″ EPS, with a layer of 1.5″ Polyiso over that.  The rim joist will be insulated with a buildup of rigid foam, held in place with spray foam, as described in Fine Homebuilding and GBA articles.

The footprint is 16′ x 24′, with a possible future addition to create a larger house.

The cabin will have (2) 6′ wide sliders on the South Elevation.  The sliders I spec’d are Marvin Elevate, with a U-factor of .27.  In the book Pretty Good House, I read about the advantages of Lift and Slide doors.  To upgrade to those, the price per slider jumps from a 2020 quote of $2198 each to $22,000 each for the Marvin Ultimate, so I will not be doing that.

To create a thermal break, I’m considering adding rigid foam to the exterior.  Thickness and brand TBD.  With the amount of doors and windows we will have, will the thermal break and increased R-value of the rigid foam be worthwhile?


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  1. Miles007 | | #1


  2. Miles007 | | #2

    Photo - Foundation.

  3. DC_Contrarian | | #3

    The way that the Manual J process calculates heat loss is to calculate the loss through each surface and add them all together. So the loss each surface is independent of the others. Therefore, if adding insulation is cost-effective on a certain surface, it makes no difference what the other surfaces are.

    That's the theory. I would say the big place where theory varies from reality is if there is a lot of air infiltration. As an extreme example, imagine a garage with the door open. It doesn't really matter how well the rest of the building is, all the heat is going to go out that door. I think you'll find that a regular sliding door is going to leak more air than a super-premium one, but I don't think it's going to be enough to negate insulating the rest of the building.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      Can we unpack that a bit? I understand that the heat loss of any assembly doesn't affect any others, but independent of air leakage, surely the relative R-values of assemblies affect the analysis of whether it makes sense to increase one and not another?

      Take a cabin that had windows comprising 75% of the walls surface. If the windows are R-2, and the walls were R-20, how could you decide whether it made sense to add exterior insulation to the walls without taking into account that the overwhelming majority of the heat loss will be through the 75% of the surface covered by windows - no matter how much you improve the wall R-value?

      If you do a cost/benefit analysis that simply looks at whether improvements to the walls will pay for themselves, it neglects that the overall heat loss in the building is disproportionately affected by the poor windows, meaning the improvements to the walls has a negligible affect on improving total heat loss.

      1. DC_Contrarian | | #6

        But the question here isn't to do one improvement vs. another -- the question is whether to do one improvement.

        Let me rephrase your question a different way. Let's say a house has an annual heating cost of $2000. Using your numbers, $1936 of the annual energy loss is through the windows, and $64 is through the walls. Let's say you have the opportunity to do an improvement that reduces the energy loss through the walls in half, to $32. If the cost of that improvement is justified by an income stream of $32 per year, it makes economic sense. The fact that you're still spending $1936 on other heating has no bearing on that decision.

        It may not be very emotionally satisfying to reduce your annual heating bill from $2000 to $1968, but that's not a math question.

        1. Expert Member
          MALCOLM TAYLOR | | #7


          I get it now. Thanks.

        2. Miles007 | | #16

          Thanks to both of you for delving into that.

  4. Expert Member
    Michael Maines | | #4

    I agree with DC that air infiltration is a wild card with exterior sliding doors. Not just for energy loss, but for comfort--drafty doors are just not comfortable to be near in cold weather. But a 10X jump in price would never pay for itself with energy savings, so you have to make a judgement call.

    Lift and slide doors definitely cost more than conventional sliders, but $22K for a 6' slider sounds high, and $2200 for the Elevate sounds low--are you sure those are the numbers you were quoted for those doors? Unfortunately I've known some dealers to charge significantly more than other dealers for the same product, when the product is new or unusual.

    To determine whether the wall insulation upgrade makes sense, in a small house like yours, it will be a tough call. An energy model would help. If you are using XPS (in any of its formulations) it will almost certainly never recover the carbon emissions used to produce it. EPS or polyiso might recover their emissions.

    1. Miles007 | | #17

      Thanks, Michael. The Elevate door is likely a higher cost today. For example, an Elevate picture window quoted in 2020 went up 94% since then.

      And you're right; comfort will be a really big factor. I will investigate some of the options that Jim lists below. Since we hope to add on the rest of the house as shown in elevations, I really want to get the details right on this first phase; the large doors, the exterior insulation, etc.

      Energy Modeling, too. As I was posting my question yesterday, I remembered your book recommending the software. I downloaded BEopt yesterday, and I'm working on figuring out how to use it. I'm not keen on XPS (for the reasons you and others point out), so I am thinking of using EPS or Polyiso.


  5. jimgove30 | | #8

    Miles, for your research, I was recently (this year) quoted $6873 for a double pane Schuco uPVC lift and slide (7-0 x 8-0) from Eastern Architectural Supply (EAS) in Mass. Marvin Essential (std slide) was $6681 from another local supplier. Andersen was $4349 for their FWGD. Due to lead times, I ended up going with Euroline (much closer to you, than me) uPVC "Smart Slide" doors for $5634.

    1. Miles007 | | #18

      Thanks Jim. That is so helpful! I am especially glad to know about Euroline uPVC, and I'm reading through their brochure right now. Thanks for listing all of those different brands and prices,


  6. kyle_r | | #9

    Would you consider replacing the sliding doors with a full lite swinging door/picture window? A similar look with better air sealing.

    1. Expert Member
      MALCOLM TAYLOR | | #10


      Those look really interesting. Who makes them?

      1. Deleted | | #11


      2. kyle_r | | #12

        I got that picture from Logic.

        1. Miles007 | | #19

          Thanks for the link, too. I had been wondering if I should switch to swinging French doors, and I'm so glad to know of this option, too.


          1. Tim_O | | #22


            Denco has tilt turn french doors for around $3000 for a 72"x80". My In-Law's house in Germany has this style door to the patio, and I really like it more than our sliding doors.

          2. Miles007 | | #23

            Hi Tim, thanks for the heads up on that brand. I'll check it out.

  7. greenright | | #13

    Large sliding doors absolutely compromise the wall insulation qualities. I know because I have a lot of them at my house and I have experimented with overinsulating the nearby walls with pretty much zero effect on practical heat load. My advice is to spend as much as possible on quality sliding doors and not overthink the nearby wall insulation.

    My 2 cents.

    1. Miles007 | | #20

      Thanks for your advice on this. Very helpful that it's based on your own real world experience.

  8. Expert Member
    BILL WICHERS | | #14

    >"get approval for an insulated, vented crawlspace"

    You don't want to do that, those are mutually exclusive terms. If you want a VENTED crawlspace, you MUST insulate the underside of the floor for the living space. If you want a SEALED/INSULATED crawlspace, you CANNOT vent it. The vent would cancel out the insulation by letting unconditioned outside air into the insulated space.

    It's usually best to seal and insulate the crawlspace, which brings the crawlspace into the building envelope, making it part of the conditioned space.

    I would use all polyiso on the walls, not the EPS and polyiso mix you suggest. My guess is you are concerned that "polyiso R value drops when it's cold", but polyiso will be more R per inch than EPS even when it's cold, and in a crawl space, the amount of "cold" is less than what it would see if used on the outside of the house. Polyiso is a really good option here. EPS has one advantage on the floor though, and that's that EPS doesn't really care if it gets wet.

    Rergarding the sliding doors, I agree with the other posts about the heat loss in the doors dominating, but you will still gain some reduction in energy loss with better insulation in the walls. Absolute energy loss is reduced regardless of where you put the extra insulation. The problem here is that those doors are likely to lose so much that you won't notice much difference if you beef up the walls. Your doors are approximately R3.7 for comparison purposes, which is a fraction of even a "regular" 2x6 studwall with no exterior rigid foam or any other fancy stuff.

    Have you considered triple pane IGUs for the sliders? Those might be a cheaper option to get some extra insulating value for the doors. Another possibility is Cardinal's i89 interior side coating, which will help cut down on heat loss through the glass.


    1. Miles007 | | #21

      Hi Bill, thanks for correcting the terminology. I should have included this link.
      And I should have referred to it as a Conditioned Crawlspace. I am planning on using the option that uses the bath fan to exhaust the crawlspace air, and pull air from the conditioned space above.

      And that's a good point about Polyiso having more R per inch, even when it's cold.

      I have been interested in triple pane, but I hadn't researched them enough yet. Props to Jim for the links. I will look into the Cardinal coating, too.


  9. Miles007 | | #15

    Hello all. Thanks for the great replies! I will do my best to reply to some of them individually.
    Thanks, Miles

  10. walta100 | | #24

    “I should have referred to it as a Conditioned Crawlspace. I am planning on using the option that uses the bath fan to exhaust the crawlspace air, and pull air from the conditioned space above.”

    What is the appeal of expelling conditioned air from your home with a fan24-7?

    You sound so confident you know from where the replacement air will come from. Seems to me the air is likely to take the path of least resistance and find the leak in the crawlspace wall fill the space with cold outdoor air and not the warm air you imagined.

    I always thought of that line of code as a power vented crawlspace far from conditioned.


    1. Miles007 | | #25

      Thanks, Walta. I feel like Fine Homebuilding is a source we can trust, don't you?

      In the article, they list three different solutions. 1) Provide supply/return air. This would make sense, but on this small project, we are using a ductless mini-split, so there is no duct for us to connect to the crawlspace.
      2) Exhaust air to the exterior. Replacement air enters the crawlspace by way of grilles in the floor or through short ducts and grilles in partition walls, bringing conditioned air into the crawlspace. I was assuming that they were talking about the type of fan that doesn't run constantly, but I could be wrong about that. One that turns on when the humidity reaches a set point. For example,

      3) Add a dehumidifier.

      Of these three solutions, are you saying you would recommend the one with the dehumidifier?


      1. DC_Contrarian | | #26

        All of those are legit, which one is best is going to depend upon your climate and conditions.

        Note that ventilating the crawlspace only results in a conditioned crawlspace if the replacement air is actually coming from conditioned space. If the replacement air is coming from outside then it's not helping. A lot of times in crawlspaces dampness is caused by humid outside air infiltrating. Bringing in more outside air just makes it worse.

        1. Miles007 | | #28

          Thanks. Yes, if I chose the fan option, I would include a grille in the floor.

  11. walta100 | | #27

    "Thanks, Walta. I feel like Fine Homebuilding is a source we can trust, don't you?"

    Seems to me since the code books have 3 prescribed ways to build a crawlspace the author has little choice but to cover all three legal possibilities. I skimmed the article it did not seem to me the author was a for or against any of the three options.

    In my opinion vented and power vented are poor options.

    Consider using a bath fan or 2 to provide enough circulation connect the crawlspace to the main floor or add the dehumidifier to condition the space.


    1. Miles007 | | #29

      OK, thanks for the input on that.

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