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Community and Q&A

Air Barrier Location

Dan C | Posted in Energy Efficiency and Durability on

I’m in the process of beginning our new home in the next few weeks. We’re located up in Newfoundland, Canada, just around St. John’s. We’re planning on incorporating a double stud wall to increase the R-value of the home. The plan is, from outside to in, : R8 Comfortboard, Tyvek, Plywood,, 2×6 wall with fiberglass batt, 6mm poly, and 2×4 wall with fiberglass batt which will contain all of the electrical and plumbing.
Since the 6mm poly will be the main air barrier in the home, If I were to insulate the 2×4 wall on the inside of the air barrier, am I creating a high potential for moisture to come into contact with the fiberglass batt and is it worth the risk? Or am I better off just leaving it without insulation to increase the durability of the wall?

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Replies

  1. Charlie Sullivan | | #1

    You'll be fine with insulation in the interior wall. The possible problem would be if it was extremely cold outside and moderately humid inside, but your minimum temperatures aren't very severe. The other situation of concern would be hot and humid outside with the inside cool and air conditioned. There could be condensation against the outside surface of the poly if the inside was cool. Your summers are mild enough that that is unlikely to be a problem, but insulating the inner wall to protect the poly from the very cool inside temperatures would be a good thing.

    If you have any lingering worry, a cheap way to reduce the potential for problems would be to use membrain instead of poly but I don't see any problem with poly if you use the insulation on inner wall.

  2. Malcolm Taylor | | #2

    David,
    Charlie is right.
    Unless the Newfoundland code has been modified from the NBC, it requires that the "Vapour barrier shall be installed sufficiently close to the warm side of the insulation to prevent condensation at design conditions". The default interpretation given to this by most building inspectors is that 2/3 of all insulation must be outside the poly - and in your assembly that is the case.

    One small point: Is there a gap between your inner and outer walls? I don't see any allowance for insulation between the two.

  3. Dan C | | #3

    Thanks Charlie and Malcom. Glad to hear I'm on the right track. We'll be avoiding the use of air conditioning all together; there's not really a need for it here.
    There was no gap left between the inner and outer walls, no. I've thought about installing the comfortboard in there instead of on the exterior but it will make it harder to install the poly. I'm going to keep the air barrier as continuous as possible with minimal penetrations. I like the idea of dense pack cellulose with the spaced walls but i haven't been able to find a local contractor to install it.

  4. Charlie Sullivan | | #4

    I recommend that the 2x6 and 2x4 studs be staggered, rather than aligned, in order to reduce thermal bridging. Is that your plan?

  5. Steve M | | #5

    hi Dan,

    I'm looking to do a double stud wall as well, and in the city of St. John's. Is your house in the city, or in one of the surrounding communities, because I'd be surprised the building inspectors would allow you to build that. According to the NBC, the way I read it, in our climate zone only 20% of the insulation can be inboard of the vapour barrier, which assuming your wall is comprised of the R8 comfortboard + R20 FG + R12 FG = R40 * 20% = R8 allowed inside of your VB.

  6. DanC1 | | #6

    Looks like I missed charlie's recommendation from way back in august. I did build with the 2x6 and 2x4 walls staggered, both walls were 24 o.c. I used as little framing as possible in the inside wall as it had no structural requirements.
    As for your question Steve, your correct in guessing that I'm located outside of St. John's in an area that doesn't require any inspections that would cover the building envelope. I believe my wall has met NBCC requirements though. To sum up NBCC, it says that the vapor barrier is fine part ways through the insulation as long as the temperature of the vapor barrier doesn't reach dew point of the interior air. So with R19 + R8 outboard, the chances of the exterior temperature having an affect on the poly reaching dew point should be minimized.
    In saying that, I don't have any official data that I could use to convince a building inspector that this wall is OK, and I was unsure myself so that's why I turned to GBA and other sources for some advice. As long as I watch the RH and the interior temp everything should be good.
    Just out of curiosity, where did the building inspector find that the 20% of the insulation has to be inboard in NBCC?

  7. DanC1 | | #7

    Steve, I had a little rake through the NBC and I can't find anything that makes this wall assembly non-conforming. The closest thing to the 20% rule you are using is 9.25.5.2 but this is referring to the position of low permanence sheet and panel-type materials and the ratio to insulation inboard and outboard of the innermost surface. It says that structures in St. John's need a ratio of .20, but either way polyethylene isn't a sheet or panel-type material so it doesn't apply.
    This wall assembly should be fine with the building inspectors.

  8. Steve M | | #8

    hi Dan, sorry for the late reply. I don't have access to the code, so can't really evaluate what it says. What I can tell you is I had submitted plans for a double stud wall, 2x4 outer, 3.5" insulation space, and 2x4 inner wall, with the poly on the outer face of the inner wall, so using R14 mineral wool, there would be R28 outside the poly and R14 inside. The response , as you suspected, references 9.25.5.2, and in my letter from the inspectors, states that not more than 20% of the total thermal resistance shall be placed on the warm side of the vapour barrier.

    It seems to me though based on your response that the inspectors were correct in refusing my proposed wall layout. That poly is certainly a low permanence sheet product, and I had more than 20% inboard of it.

  9. Dan C | | #9

    Hi steve, no worries buddy. Sorry to hear that your wall assembly got turned down.
    I don't want to dive too far down the rabbit hole that is NBC here on GBA, but in my interpretation, section 9.25.5.2 has been included in the code book to ensure that there is enough insulation installed outboard of the wall sheathing to prevent condensation from forming in the wall cavity when the outboard insulation is also considered a vapor barrier. This doesn't apply to your wall assembly but I mentioned it above because this was the closest thing I could see to the 20% your inspector mentioned.
    The appendix of the code book actually lays out an example of how this calculation should be preformed and the poly is completely ignored, because for purposes of this section, poly is listed as a membrane-type material, whereas 9.25.5.2 and the ratio of .2 apply to sheet and panel-type materials only, like cladmate insulation
    I believe Malcolm had mentioned in another tread you had started that building inspectors usually use a 1/3 to 2/3 rule of thumb when determining the amounts of insulation inboard and outboard. I guess it's possible that your inspector may be using 20% instead.
    This is how i'm reading the code book anyhow, and I could be wrong. But it's also possible that the inspector may interpreting it wrong as well. There's quite a bit of info in NBC explaining all of this, way to much to go into here. If you like, you can reply with your e-mail and I can either scan the pages in or I could borrow you a copy of one of my books for you to have a look through, I believe they are still over 300$ to buy.
    Either way, I wouldn't give up on trying to build a more efficient home, especially with electricity rates heading up soon

  10. Malcolm Taylor | | #10

    Dan and Steve,

    Don't you find it completely ridiculous that we don't have free access to the laws we are expected to follow? I paid over $300 for a hard copy of the latest edition of the code and a year later they mailed out almost six hundred pages of errata and changes. Nuts.

  11. Dan C | | #11

    It certainly would help streamline the process of trying to be code compliant.

  12. Steve M | | #12

    Dan

    If its not too much trouble, that would be great. It's s m c 1 9 0 0 n l at gmail - no spaces btw.

    If you go to http://www.cwc.ca, they developed a wall calculator that shows the percentage of exterior to interior insulation and the requirements in various cities, with st. John's as one of the cities. 20% is the value used for us, but other cities show 33%, so maybe that's where the 1/3 rule came from. The calculator was developed by a panel of experts from various companies and organizations.

    I'm awaiting my permit for the revised wall as you've seen in the other post. Hopefully I hear back this week.

    Malcolm
    I absolutely agree, part 9 anyway should be available free online.

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