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Building a 1,900 square foot one-level house, slab on grade

user-7031471 | Posted in Energy Efficiency and Durability on

Building a 1900 sq. ft. one-level house, slab on grade. Plan is to insulate with closed-cell foam. With high-quality glazing (casement windows with triple layer weatherstripping) should I expect to pass a blower door test for western Massachusetts?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name?

    Passing a blower door test does not depend on the materials; it depends on the care exercised by the builder.

    The builder has to pay attention to transitions and penetrations. Potential leak areas include the gap between the wall bottom plates and the slab, around windows and doors, at seams between panel materials (for example, sheathing panels), at ceiling electrical boxes, and at penetrations (hose bibs, exterior light fixtures, the attic access hatch).

    In other words, a house insulated with spray foam can still be leaky.

    For more information, see these two articles:

    Blower Door Basics

    Questions and Answers About Air Barriers

  2. Expert Member
    Dana Dorsett | | #2

    Why closed cell foam? Closed cell foam is one of the least-green insulation materials in common use, using twice the polymer per R of half-pound open cell foam, and is usually/often blown with HFC245fa, an extremely powerful greenhouse gas (~1000x CO2 @ 100 years.)

    Worse still, when installed between studs it's buying you at best a 10% improvement over filling it with open cell foam (which is blown with water, using 1/4 the total amount of polymer.) The thermal bridging of the R1.2/inch stud timber over the typical ~25% framing fraction robs it of it's potential performance.


    On a 2x6 framing adding even 1.5" of exterior polyiso on an R20 open cell studwall comes in at over 1.5x the performance of a 2x6/R35-ish closed cell insulated stud wall. In Massachusetts there are multiple sources of reclaimed foam from demolition at less than 1/3 the cost of virgin stock foam, and factory seconds (usually cosmetic blemishis, such as dinged facers, etc) foam at about half the cost of factory perfect goods. This takes a large chunk out of the cost of the insulating sheathing approach.

    The two largest vendors of reclaimed foam are Nationwide Foam in Framingham, and Green Insulation Group in Worcester, but there are others.

    If using exterior foam you'll need at least 1.5" if polyiso, or 2" if polystyrene (XPS or EPS) for dew point control on R20 cavity fill in a 2x6 stud bay. That keeps the sheathing warm enough to not accumulate excessive moisture over the winter, and allows one to use plain old interior latex paint as the interior side vapor retarder.

  3. user-7031471 | | #3

    Thanks for your thoughtful responses! My name isLukas Schwartz, the project is in Great Barrington. I appreciate your thoughts on closed cel performance and drawbacks. I did try to win the architect over on doing a rigid foam exterior blanket, but was not successful. I am simply trying to carry out the plan I have been given in a thoughtful way. The architect wants to spray 2” if closed cel and fill the rest with batts. So wondering if the foam conscientiously installed will be enough without a smart membrane on the interior.

  4. user-7031471 | | #4

    Also wondering what is the best sill seal that won’t break the bank?

  5. Expert Member
    Dana Dorsett | | #5

    Depends on what you mean by "...will be enough...".

    In a 2x6 cavity 2" of closed cell foam (R12-R14), and 3.5" of high density batt (R15) has a HUGE amount of dew point margin at the foam/fiber boundary, so there is no need for smart vapor retarders.

    But it's not the be-all-end all of AIR sealing- it's merely "...a good start...". Nothing you put in the stud bays cavities will seal the seams in the doubled-up framing, or between the framing & subfloor, etc., all of which must still be attended to.

    The structural sheathing has very little capacity to dry toward the interior through 2" of closed cell foam, and for good resilience the stackup exterior to the sheathing has to have good exterior drying capacity, such as back vented siding, and all of the flashing/drain-plane details have to be done right.

    If you MUST use closed cell foam, shop around for a water blown foam such as ...

    Empire/Aloha Energy's 2lb foam:


    Icynene's MD-R-210:

    or HFO1234ze blown foam, such as...

    Demilec Heatlok HFO High Lift Lift:


    LaPolla's FOAM-LOK 2000 4G:

    While it's still a massive amount of polymer per R the total impact is but a fraction of any HFC245fa blown foam.

  6. charlie_sullivan | | #6

    Is the architect working for you or are you working for the architect? If the architect if working for you, you might need to remind her of that arrangement.

    If, for some reason, you must used closed cell foam, specify one of the new "HFO" foams such as Lapolla 4G or Demilec "Heatlok HFO High Lift" to avoid the global warming impact Dana mentioned; although it won't avoid the other problems he mentioned.

  7. Expert Member

    Hey, hey now Charlie. The relative merits of the two approaches in this case aside, there are any number of good reasons architects don't simply use the building assemblies clients want just because they are paying them.

  8. GBA Editor
    Martin Holladay | | #8

    The best sill seal is either an EPDM gasket from Conservation Technology or ProtectoWrap Triple Guard Energy Sill Sealer. It's also possible to seal this crack on the exterior with one of the high-perforance European tapes like Tescon Vana.

    If your budget is tight, use closed-cell foam sill seal from your local lumberyard, and then caulk the joint on the interior and exterior with polyurethane or silicone caulk after the roof has been installed.

  9. user-7031471 | | #9

    great and helpful responses! thanks. martin- i was prompted to join this forum by listening to your great interview on the fhb podcast. Fantastic resource for the caring builder.
    Charlie- I am working for the homeowner - the homeowner's son is the architect. There is some flexibility in the design, though exterior foam blanket seems to be off the table. He is suggesting (as per Dana's comments) we opt for filling the cavities with open cel ("bio"foam). Another complicating factor is that the wall structural elements will be 6x6 posts and beams (covered by drywall) - which is all the more reason for a continuous blanket of foam board...any other angles I'm not thinking of?
    Also- thinking if it is all open cel foam, I need a smart membrane on the walls and ceiling, no?

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