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Below-grade insulation

Yoav Sharoni | Posted in Green Products and Materials on

Hello,  renovating a 2×4 construction frame house.  We wrapped envelope with  Intello X membrane as well as 60mm Gutex insulation.  The foundation wall, cinder block,  is 4′ in ground with several inches exposed above grade.
Looking to insulate below grade, trying to avoid XPS foam and leaning towards using 2 layers   of 1-1/2″ Roxul ComfortBoard 80 instead, as it seems to be available in north of the NYC areat.  Has anyone have experience using the ComfortBoard 80 below grade?  Any suggestion how to transition from the 3″ of ComfortBoard to the Gutex?  Roxul  ComfortBoard cannot be left exposed above ground.

Thank you


P.S  we are in zone 4A,  (Hudson Valley)

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  1. Expert Member
    Dana Dorsett | | #1

    Is the insulation going on the exterior of the foundation or the interior?

    >"Looking to insulate below grade, trying to avoid XPS foam and leaning towards using 2 layers of 1-1/2″ Roxul ComfortBoard 80 instead"

    Have you considered using EPS (on the exterior) or polyisocyanurate ( on the interior)? Either of those are much lower impact than XPS, and are widely available on the reclaimed materials markets. Reclaimed foam is a greener option than virgin stock insulation of any type and far cheaper than rigid rock wool (or virgin stock foam). Many reclaimers advertise in venues like this:

    1. Yoav Sharoni | | #2

      Thank you for the suggestion of using reclaimed material. Looking for outside wall insulation.
      I will follow those leads.
      Is it wasteful to insulate exterior below grade wall? Will we increase the radon penetration problem in this area by insulating wall from outside? Thank you

      1. Expert Member
        Dana Dorsett | | #3

        Insulation has no effect on radon. Air tightness has a much bigger effect.

        The more air-tight you make the foundation, the less soil-gas penetration. Sealing all cracks in the wall & slab with a concrete formula polyurethane caulk is highly recommended, including the seam where the slab meets the foundation wall.

        Whether it is "wasteful" to insulate the exterior of the foundation depends. In new construction when it can be done most cheaply, having the thermal mass of the foundation walls inside the insulation, on the conditioned space side of the assembly stabilizes room temperatures, with a modest improvement in energy efficiency. But if this is a retrofit it's far easier to insulate on the interior side.

        With insulation on the exterior of the framed wall having the foundation also insulated on the exterior helps thermally break the thermal bridge at the foundation sill. It will need Z-flashing where at the transition from Gutex to foundation-insulation to direct bulk water out to the exterior of the foundation insulation, which re-introduces some thermal bridging, but it's still an improvement. The exposed EPS on the exterior can be finished with any EIFS type material for protection against sunlight other damage. In zone 4A roughing up the surface and applying 1/2" of stucco scratch coat will usually work just fine. Stucco scratch coats are similar to cementicious EIFS such as Quikrete Foam Coating, but more widely available. In colder climates stucco scratch coat may suffer more freeze/thaw spalling, whereas a purpose-made EIFS would not.

        Insulating from the interior can be just as effective (or more) if one also insulates the foundation sill and band joist, and is often easier. But that requires an interior side thermal barrier against ignition to meet fire safety aspects of the code. (Half inch wallboard works.)

        Code minimum for foundation insulation in zone 4A is a continuous R10 down to either the slab or the footing. That could be either 2.5" of EPS on the exterior, or 2" of polyiso on the interior. Digging down to the footing on the exterior can be quite a bit of extra work, but as long as the exterior insulation extends to at least 2 feet below grade the majority of the energy savings will still be there.

  2. Yoav Sharoni | | #4

    Thankyou very much Dana for the attention and information!

  3. GBA Editor
    Peter Yost | | #5

    Hi Yoav -

    As far as I know, there has only been one study on indoor air quality that included the relationship between the airtightness of a building and radon levels in existing homes.

    "Weatherization and Indoor Air Quality: Measured Impacts in Single-Family Homes Under the Weatherization Program" (Sept. 2014).

    Unfortunately, the relationship is uncertain; radon levels sometimes go UP and sometimes DOWN with increasing air tightness of the building. Certainly sealing foundation elements, particularly cold joints, is essential, but after working on your basement, a good thing to re-test.


    1. Expert Member
      Dana Dorsett | | #6

      >"...the relationship is uncertain; radon levels sometimes go UP and sometimes DOWN with increasing air tightness of the building. "

      Air tightness of the BUILDING is a different parameter than the air tightness of the FOUNDATION.

      I don't think there is any question that making the foundation more gas-tight can do anything but reduce the radon levels, or at very least makes slab depressurization abatement measures more effective. Sealing the foundation and installing air tight lids over all sumps & other foundation penetrations (both walls & floors) is the very first mitigation task taken up by radon abatement contractors. Sometimes sealing the foundation and installing balanced ventilation in the basement is sufficient, carrying a lower energy use penalty than sub-slab depressurization.

  4. Yoav Sharoni | | #7

    Thankyou Peter and Dana for sharing knowledge of what is possible in mitigating radon penetration.
    We are digging around perimeter of the house and will try to apply my understanding of your suggestions towards sealing gaps in foundation. My desire was to insulate below grade, without using XPS foam, not only for thermal purpose.
    House faces river, which amplifies the loud club music from across the Hudson, causing low vibration to shake house into early hours of morning.
    Search for recycled EXP foam, suggested, did not yield any successful results. One vendor told me that they mostly reprocess (re-fired) EPS foam into blocks to be re used in other industrial applications.
    I wish I knew of a membrane that can be applied to below grade foundation wall to create a better seal of the parted cinder blocks.

    Thankyou again

    1. Expert Member
      Zephyr7 | | #8

      You can reduce the amount of sound that makes it through those walls by using 5/8” drywall on the interior instead of 1/2”. Avoid using “ultralight” drywall if you’re trying to reduce sound penetration because it’s the mass of the wall that’s helping you stop sound. Mineral wool in the wall will also help.

      If you’re stripping the walls down to the studs on the inside, consider two layers of 5/8” drywall with green glue in between and hang that on resilient channel. That will give you about the maximum practical sound reduction. If you’re not going down to the studs, installing a layer of 5/8” drywall over the existing drywall with green glue in between will also provide a noticeable reduction in sound levels.

      If you’ll be replacing your windows, ask about getting laminated glass to cut down on sound. Next best is to have the two panes (for a double pane window) made of different glass thicknesses, like one pane 1/4” and the other 1/8”. This will also help cut down on sound transmission.

      If you use mineral wool in the stud cavities, and the extra 5/8” drywall and green glue inside, you’ll get a very noticeable reduction in sound penetration through the walls. Sound reducing windows will help too. It’s nice to be able to go inside and escape the noise. I live about 1/3 mile from a railroad mainline so I’ve been through this myself in my own house.


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