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Air barrier questions

barnrestoration | Posted in General Questions on

We are conditioning some space inside of an existing Pennsylvania bank barn from the 1800s.  This is in climate zone 4A, in Southeastern PA.  We’ve already been through the “do this not that” with regards to the building envelope and gotten tons of differing opinions.

We ended up adding 3/4″ furring strips inside of the existing board and batten sheathing/siding to create and air gap so the sheathing could dry to the outside, then used 3″ of XPS caulked, taped and foamed for an air seal, behind and between framing members.  The floors are insulated from the garage space below with 8-9″ of Rockwool and there is a 1x ceiling in that garage space, with 1.5″ flooring above.  Joist bays joints have been can foamed for some attempt at air sealing.  The roof line and upper level ceiling is insulated with 7″ of Demelec Selection 500 Open Cell Spray Foam.  

Here’s the question.  We are planning to use 1×10 tongue and groove boards for the interior walls instead of drywall.  Do we need to apply any kind of vapor retarding house wrap, paper or other air barrier on the inside of the wall and/or ceiling studs before attaching the T&G?  

The foam guy says not necessary for the roof as the avg ~7″ open cell foam will stop 95% of air movement.  Not sure about the walls but in another post Martin Holladay insisted someone else use a layer of 1/2″ drywall before applying T&G.  My thought is probably not necessary in this case as a double barrier situation could be introduced with the XPS blocking most of the air movement.  

One side note is that one of the interior walls that we are treating as an exterior wall, is just a partition wall inside the barn to some unconditioned space.  It was also treated with a continuous layer of XPS before framing out.

So what do you all recommend on in this case?  Any thoughts are appreciated!  


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  1. Expert Member
    Dana Dorsett | | #1

    At 3" XPS is already well into Class-II vapor retardency, and there is no need for an interior side vapor barrier. Whether or not the tape & can foam used on the XPS will hold up over the long term is an open question.

    A layer of gypsum board behind your interior side can be detailed as an air barrier, and even if painted with "vapor barrier latex" it doesn't come even close to being a true vapor barrier. There is no "double barrier" condition for trapping moisture when going that route.

    If the roof is ONLY insulated with 7" of half-pound foam (R25-ish- only about half the IRC code-minimum) and isn't vented between the foam and roof deck it will need some sort of interior side vapor retarder in an occupied residential structure to prevent wintertime moisture accumulation in the roof deck, and summertime diurnal ping/pong moisture migration into and out of the foam as the roof heats up and cools off with daytime sun and night time radiational cooling. Half-perm vapor barrier latex on drywall is good enough, but a "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) or Intello Plus is better.

    When all of the climate damaging HFCs have left the 3" XPS after a few decades it's performance will be about R12.6, which would be sub-code under the IRC even if continuous, and lower performance where thermally bridged by framing. XPS is arguably the least green insulation material in common use, due to the amount of HFC134a used in the blowing agent mix (~1400CO2 @ 100 years), and the polymer per R. Polyisocyanurate has a better polymer/R ratio using a lower impact polymer, and DRAMATICALLY lower impact blowing agents, typically variants of penatane (~7x CO2 @ 100 years.)

    It's almost never "worth it" to install any foam between framing except as a last resort. It would have been higher performance and cheaper to install dense fiber insulation between the framing, and a thinner, continuous (easier to air seal) layer of foam board under your interior-side paneling.

    1. barnrestoration | | #2


      Thank you for the reply. I appreciate your detailed responses and have learned a lot from your insight on other posts as well. In fact I have previously looked for a way to contact you directly without success. Is there a way to do that? I'd love to pick your brain directly.

      Anyway, the XPS foam board for this project was obtained for free. It was left over from a rehab at a local land trust and had been stacked in their basement for years along with some PolyIso boards. I asked and they said to take it. I only used these as the price was right and the damage was already done. The blue boards are stamped as having zero ozone depletion factor by DOW but somehow I doubt that and am aware of the other side effects of its production.

      A little more detail on the install - for the walls, we ran a continuous layer of the 1.5" XPS between posts and beams, then framed the wall studs inside with more rigid 1.5" foam between as a second layer. There is less than 400 square feet of exterior wall space with this setup. There is an additional internal partition wall to unconditioned yet indoor barn space, with a layer of 1.5" continuous rigid blue board applied to the plywood & 1x12 seethed internal wall, and then rock wool will be applied between the 2x4 studs inside of the foam on this internal wall. This wall is also ~400 sq. feet.

      For the roof, I forgot to mention that I used the freely obtained PolyIso boards at 1.5" thick, spaced 2.5" below the metal roof, (1.5" spacer below 5/4" purlins) as a substrate for the 7" of spray foam. The flat section of ceiling below the collar ties is 7" of foam sprayed to the underside of plywood sheathing between 2x8s.

      So, the sloped sections of the roof are indeed vented and have a layer of Polyiso for an additional R9 between the ventilation gap and the open cell spray foam, but it sounds like the flat ceiling section that had spray foam applied directly to the plywood at 7.25" thick, may require a "smart" vapor retarder. Is that right?

      For the walls, are you suggesting nothing at all and to apply the T&G boards directly to the wall studs, or are you saying I need a smart vapor retarder there too? The goal here is no drywall and T&G boards applied to framing to save on additional material cost.

      I'm hoping to be able to apply the T&G boards directly to the framing without any drywall at all. I assume with the gaps in the T&G 1x material, the half-perm vapor barrier paint you suggest for drywall won't make any difference on the wood but is the smart vapor retarder still necessary with the additional information I've provided?


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