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Advice needed to insulate an existing block wall from the exterior

jwyman | Posted in Energy Efficiency and Durability on

The local parish hall has a 40′ x 44′ addition with exterior walls of 8″ concrete block and a bow truss roof/ceiling that was built in the 60’s. On the interior, there is no additional wall build-out or insulation, just exposed block. We are planning on insulating the exterior walls and adding blown in cellulose to the existing fiberglass roof insulation after air sealing all penetrations.

My question has to do with insulating the exterior walls. Since there are rooms, interior finishes, electrical and forced hot water baseboard heat on the interior, we are planning on insulating from the exterior. The plan to date is to add two layers of 2″ XPS rigid insulation, weather barrier, strap vertically and re-side to the existing fascia and soffit which will go from 10″ to 5″ deep. Existing double glazed vinyl windows will require new trim and aluminum flashing. There are a few areas on the building where grade extends above the floor line, and will have to be dealt with separately.

Is the the best way to solve this situation? Is there a need for the weather barrier other than a secondary protection for moisture and insects?

Climate Zone 5, Western Massachusetts

Thanks for your help.

Jon Wyman

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is the the best way to solve this situation?"

    A. Your R-20 walls will be pretty good. Best? Hard to say. A higher R-value is always better than a lower R-value. From an environmental perspective -- read the Pope's latest words on global warming if your parish is a Catholic parish -- EPS is preferable to XPS, because the blowing agent used to manufacture XPS has a very high global warming potential.

    Q. "Is there a need for the weather barrier other than a secondary protection for moisture and insects?"

    A. Every wall needs a water-resistive barrier (WRB). Either the rigid foam (properly detailed) or housewrap can be used as your WRB, but you have to have a WRB. (Your window flashing needs to be integrated with your WRB.) For more information on these issues, see:

    All About Water-Resistive Barriers

    Using Rigid Foam As a Water-Resistive Barrier

  2. Expert Member
    Dana Dorsett | | #2

    Rather than two layers of 2" of XPS, it's greener to use 2" of polyiso next to the concrete wall, with 2" of EPS on the exterior of that to achieve the same thermal performance. (For the areas where you would have to dig out below grade, use only EPS on the below grade part.) XPS is blown with HFC134a (a common automotive AC refrigerant) which is a powerful greenhouse gas (~1400x CO2), whereas polyiso and EPS are blown with pentane (~7x CO2.) As it loses it's blowing agent over the next handful of decades doing it's climate damage, it's performance slowly drops to about R17-R18 before stabilizing. It'll still do about R20 when it's really cold out, but not during th e

    The reason the outer layer of the dual-foam stackup is better as EPS is that polyiso's performance at first rises with falling temps, then takes a nose dive when the average temp through the layer is below 45F( lower than EPS). But EPS gains performance as the temperatures fall. In a western MA location your mean January outdoor temp might be 20F, and with a 70F interior that's a 50F temperature difference a nominal R20 or 2.5F/R. With 2" /R12 polyiso as the inner layer That's then 2.5F/R x R12= 30F below the 70F interior, which 40F on the cold side of the polyiso. And the average temp through the polyiso would be about 55F which is close to it's absolute peak performance of about R7/inch- it's really more like R14 than R7 during colder weather in that stackup. The conductivity curves for different insulation types as they very with mean-temp through the layers looks like this :

    Similiarly, Type-II EPS is rated at R4.2/inch @ 75F mean temp, but R4.5/inch at a mean temp of 40F, and R4.7 at a mean temp of 25F. So at your January mean temp the exterior 2" will be running a bit more than R9. So together, it'll average R23-ish during cold weather, falling to a bit more than R20 or so at the very end of the heating season.

    Use window flashing that extends to the exterior of the foam layer, and use a standard flat housewrap on the exterior side of the foam. The alternative might be a spray-applied WRB at the CMU wall, but with 4" of foam on the exterior you'd have to guarantee a micro-cavity between the foam & CMU to guarantee that the water drains rather than seeping through the CMU toward the inteiror.

  3. JonathanRupp | | #3

    I was thinking of doing something similar on the exterior of my concrete wall (Also in MA/zone 5, wall was insulated in 1970 with just 1" of interior fiberglass insulation), but my wall would have vertical siding / horizontal strapping. Is there any need for plywood (or ZIP) sheathing outside of the foam before the siding? (clearly all the wall strength, air sealing, etc. is from the 10" concrete wall)

    Also what is the best way to make the foam carpenter ant resistant? I've seen recommendations of using PT 2x4s or a metal U shaped flashing

  4. Expert Member

    The chances of making your wall tight enough to keep out carpenter ants is pretty remote. You can buy treated foam or treat it with products like Bor-a Care.
    Carpenter ants still need a path to the outside to forage for food, so treating the foundation below the wall can be effective too.

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