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Rather than scrapping what's there, enhance and detail it to improve performance

节能改造总是充满挑战。他们是a delicate balance between what to keep, what to eliminate, and what to add. That’s what we were facing on this recent project. The existing structure had 2×4 walls 16 in. o.c. and a turn-down concrete slab foundation. We stripped the building down to the studs by removing the exterior fiber-board sheathing and interior gypsum board. With naked 2×4 walls and an exposed slab, we started the retrofit.

The first step was to insulate the slab, which was positioned significantly above exterior grade. We spec’d 2-in.多异尿素刚性刚性绝缘. Because the existing slab is coupled to the ground temperature, which is most likely cooler than the desired conditioned temperature of the living space, there is a constant thermal drive for the room to attempt to warm the ground below. The polyiso provides resistance to that thermal drive.

The continuous nature of the polyiso insulation nearly eliminates thermal bridging across the floor assembly. (Note that the level of insulation here is appropriate for a moderate climate. For colder climates, 4 to 5 in. of rigid foam would be advisable.) On top of the 2 in. of rigid insulation, we added two layers of 3/4-in. Advantech floor sheathing. The second layer was laid perpendicular to the first layer with the joints offset by 2 ft., and the two layers were glued and screwed together to form a 1-1/2-in. “floating floor” assembly. (The Advantech allows for any type of finished flooring—hardwood, tile, carpet, etc.)


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  1. James Howison||#1

    Any thoughts on the trade off between adding the insulation over the slab vs continuing it down the edge of the slab?

    1. Alexandra_Baczek||#3

      Continuing it down the edge of the slab doesn't solve for heat loss to the ground.

  2. 查理·沙利文(Charlie Sullivan)||#2

    This looks like a good solution all around and I really appreciate the level of detail provided. I'm curious about two things.

    One question is the 2x4 blocking at the base of the drywall. It seems like that would add some minor thermal bridging at a corner that is a bit of a thermal weak point anyway, and I'm not sure what function it serves. Is it to anchor netting for blowing in cellulose? Or to facilitate detailing the drywall as a secondary air barrier?

    我的另一个问题是,我觉得有些人avoid polyiso in direct contact with a slab for fear of moisture wicking into it. Is that more of a concern in a basement and less of a concern with a slab that is at or above grade? Or is it a case-by-case decision based on whether it seems like there have been moisture problems in the past?

    1. Alexandra_Baczek||#4



  3. Expert Member
    马尔科姆·泰勒(Malcolm Taylor)||#5


    A question recently came up ab0ut rain-screens in the comment section of your last blog. Do you worry much about venting the top of rain-screen cavities?

  4. Thomas Pippin||#6

    Interested in any feedback.

    1. 查理·沙利文(Charlie Sullivan)||#7

      Your detail has a lot more thermal bridging than at the corner than the one in the article, through concrete which is much more conductive than wood. The results will be cold baseboards, increased heating energy consumption, and possible mold problems in those corners. Raising the floor more, similar to what's in the article would be the best solution, but if you can't do that, some exterior insulation would help.

      Also, conventional Foamular XPS has huge climate impact, way beyond the other materials that people worry about when they get into worrying about embodied carbon. If you must use XPS, Owens-Corning now has an "NGX" version with only 10% of the impact of their conventional one and that can be special ordered anywhere in North America. Graphite-loaded EPS or polyiso are even better, but no matter what, don't specify conventional XPS.

      1. Thomas Pippin||#12

        Hey Charlie, thanks for the input. Just saw this now.... Agree with all you've said. Thanks!

  5. Charles Campbell||#8

    “(Advantech允许任何类型的成品地板 - hardwood,瓷砖,地毯等。OSB现在被批准了吗?

  6. tslagle21||#9


  7. Charles Campbell||#10

    "To deal with the exposed edge of rigid insulation, we applied a 2x continuous treated wood block." Why can't the edge be exposed?

    1. Expert Member
      马尔科姆·泰勒(Malcolm Taylor)||#11




  8. 劳伦斯·马丁||#13


    Regarding raising the floor with insulation and Advantech, did any windows now become close enough to the floor causing you to have to replace glass with tempered glass? I presume this may be more of an issue in colder climates where the insulation would be thicker. I forget if it's 18" or 24" from the floor.


  9. 大卫·巴格(David Baerg)||#14

    In a colder climate, would you be concerned about frost penetrating under the slab and causing it to heave?

  10. Jibu J||#15

    - If I applied a WARB and spray foamed from crawl space with closed-cell foam and then did Aerobarrier from the interior to the existing flooring (T&G boards) could I get an improvement in airtightness that is comparable to replacing the subfloor with Advantech or some other subfloor?
    - Regarding the wall exterior sheathing - what kind of disadvantage is there to keeping existing sheathing (uncertain on type - could be T&G or plywood sheets), applying a fluid applied WARB, and doing Aerobarrier? Will this setup help cover the airtightness effectiveness gap, if any, relative to replacing the exterior sheathing completely?
    上面的想法是减少我从项目中产生的浪费 - 假设现有的地板和外部护套状况良好,我的想法是,我继续使用现有的和使用现有的系统来改进这些系统,而不是生成太多。从基本上好的材料中浪费。这个想法有缺陷吗?

    A few more questions:
    - For the rain screening, how do you do ventilation at the top for the air gap on such a retrofit?
    - What is the function of the acoustic sealant? is it only on the bottom sill or also the vertical 2*4s?
    - Can you use 2” Rockwook instead of the poly-iso?
    - You show the 5” screw to secure the furring through everything into the framing, but none are shown for the insulation - what are you specifying for this? Does it go directly into framing or is supported by the sheathing?
    - 您显示1*3条毛茸茸的条而不是1*2条毛茸茸的条 - 是否有较大条带有任何性能优势,还是为了更轻松地安装?

  11. MSmuts||#16

    Thanks for the article Alexandra. I found it when I was searching for an answer to my question regarding top of slab insulation in a remodel in Taos NM (Zone 5B).
    I am using BEOpt V2.8.0.0 to model a semi deep energy retrofit of a home my wife and I recently purchased. I was quite shocked to find when I added insulation to the top of the slab that it dramatically increased my loads not met (Hrs/year) with the same sized equipment (24,000Btuh minisplit for a single story 1200sf house). BEOpt doesn’t have a way to add “insulation” to the top of the slab so I used the Foundation Floors>Carpet input which allows you to specify the R-value of the carpet pad.
    In the attached graph:
    Point 4 is 4ft R-20 vertical insulation on the exterior of the slab (no top of slab insulation)
    Point 5 is 4ft R-20 vertical insulation on the exterior of the slab and 100% top of slab R-10 insulation
    I assume the increased loads not met have something to do with de-coupling the slab (thermal mass) from the interior space but I’d love for someone to give me a sanity check that I am modeling this correctly and, if so, does it make sense to insulate the top of the slab at all?
    Is using the carpet input as a way to model top of slab insulation a reasonable approach?
    I ran a second case with just 40% top of slab insulation (approximately the same area that would be covered by 4ft of insulation around the inside perimeter of the house. While the model doesn’t know what the exact location of this insulation is the Loads Not Met numbers were lower but still significant and followed the same pattern of the higher the R-value of the top of slab insulation the higher the Loads Not Met hours.

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