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air & moisture barrier retrofit for post & beam structure

Brad964C4 | Posted in General Questions on

I’m working on a project for a client who has a timber frame/post & beam garage that was built a few years ago. The garage was sided with 1×8 T & G vertical pine directly over the frame, so no bldg. felt, house wrap or sheathing exists.

He would like to finish the second floor which has 2 gable end walls, with 4′ high knee walls on both sides. I was planning on insulating the exterior walls with 2″ of closed cell foam & 5″ of open cell foam. While closed cell foam acts as an air barrier, the bigger concern is the lack of felt or house wrap under the siding to act as a weather barrier/drainage plane.

is there any option other than removing the siding in order to install house wrap over plywood sheathing & then reinstall the siding? Maybe leave the siding in place to act as the sheathing, then install house wrap & new siding?

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  1. Expert Member
    Dana Dorsett | | #1

    Applying foam directly to the back of t & g siding would likely cause the siding to warp & split from uneven drying.

    Installing some sort of moisture tolerant sheathing with at least 1/4" clearance to the siding can work (half inch is better) with the foam applied to the other side of the sheathing. The sheathing isn't structural, so something like 1/2" asphalted fiberboard with asphalt only on the exterior side, with the foam applied to the interior would work. Installing ripped down half inch plywood or 1x furring to the interior side of the siding as spacer and tacking the fiberboard to the furring can work.

    It'll probably also need to be supported on the interior side by some 2x lumber secured to the framing too. What might make more sense is to install the spacers on the interior of siding and install the fiberboard on a non-structural 2x3 or 2x2 studwall that's assembled on the floor with good fastener spacing on the fiberboard-to-studs, then tipped up and shoehorned into place before fastening it to the structural timbers.

    Five inches of open cell foam is at LEAST as air tight as 2" of closed cell, and would allow better drying in both directions than closed cell foam. Half inch fiberboard is good for R1, and that leaves you ~6.5" for the open cell foam (~R24), which even if thermally bridged by studs the full thickness would meet code minimum performance in most US climates. But with 7" to work with and fact that you're not building a structural wall allows you to avoid most of the thermal bridging. Even a full 7" of half pound open cell uses less polymer than 2" of closed cell, and uses water instead of HFC245fa (an extremely powerful greenhouse gas) as the blowing agent.

    Where is this garage located? In humid-summer US climate zones 1A- 3A you may want something more vapor retardent than fiberboard sheathing on the exterior if the space is going to be air conditioned to some low temperature, depending on what the interior finish wall would be.

  2. Brad964C4 | | #2


    This in 5. That info should have been included in the original post. We must use a combination of open cell & closed cell foam in the walls to meet the energy code, as we only have 7" to work with at the roof (which is framed with 4x8 timbers).

    HVAC....will most likely be using a mini-split to heat & cool the space.

  3. Expert Member
    Dana Dorsett | | #3

    In post #2:

    "We must use a combination of open cell & closed cell foam in the walls to meet the energy code, as we only have 7" to work with at the roof (which is framed with 4x8 timbers)."

    But in the original post:

    "I was planning on insulating the exterior walls with 2" of closed cell foam & 5" of open cell foam. "

    That doesn't read like it's a roof(?).

    But the roof & walls need not be insulated in the same way. A thermally broken 6" or more of open cell foam will be fine for the walls. Code min is 2x6/R20 (-5.5" of open cell foam), after all, and that's with thermal bridging of the framing with a 25% framing fraction. With post beam the framing fraction will be pretty low compared to a structural studwall with more & deeper studs.

    The roof needs at least 1" of vent space below the roof decking (is that also t & g, or is it something else?) not the 1/4"-3/4" I was suggesting for the walls. That leaves only 6" for insulation, installing cardboard baffles/chutes for maintaining the vent space. That's not enough to get to a code-min R49 with any commercially available product.

    But you can probably get to IRC 2015 code min on a U-factor basis (< U0.026) with 5.5" of an HFO blown R7/inch foam (R38-ish at center cavity) such as Demilec HEATLOK XT HIGH LIFT or LaPolla FOAM-LOK 2000-4G, depending on the spacing of those 4 x 8 roof beams and the thickness & type of roof decking. The framing fraction is probably below the typical 7% for roofs, and the "whole assembly R" only needs to hit R38.5, factoring in the R-value of all material layers & air films, and the thermal bridging of the framing fraction.

    If you want to go unvented, 7" of HFO blown closed cell foam does pretty much hit R49.

    The HFO1234ze (Honeywell Solstice) used has very low global warming potential, making these products far greener than the HFC245fa blown foam (the industry standard). It's still a lot of polymer, but unless you're going to be re-roofing the place and can install a few inches of polyiso up top with fiber snugged up to the roof deck below it's really the only practical way to get there.

    If exterior roof insulation is a possiblitity, you need at least 40% of the total R to go up top. With 7" of space for the fiber you'll get about R26 if cellulose R29-R30 if blown fiberglass, so count on 4" of roofing polyiso. Derate the polyiso to R5/inch for dew point control in your climate (the 40% minimum) but used the labeled R5.5 for code-compliance on an R-value basis. With a continuous R20+ above the roof deck and R25+ in under the roof deck it's pretty much guaranteed to make it on a U-factor basis if anybody questions it. There are several reclaimed foam companies operating in southern New England, and reclaimed roofing polyiso is dirt-cheap, typically ~$25/sheet for 4' x 8' x 3.5" goods (labeled R20), half that for 2", $20/sheet for 3". Prices vary by vendor, condition, and seasonal demand, but there's a lot of it out there.

    The two biggest operators are Nationwide Foam in Framingham, MA and Green Insulation Group in Worcester, MA but there are others.

    [edited to add]

    In CT the summertime outdoor dew points rarely stray higher than the mid-70sF, and average in the mid- 60sF during the peak humidity months. Unless the owner is constantly cooling the place to well below 70F in summer, using highly permeable asphalted fiberboard sheathing as the weather resistant sheathing for the walls will be just fine.

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