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Can polyethylene on top of your rainscreen prevent inward solar vapour drive?

user-1083543 | Posted in Energy Efficiency and Durability on

I am working on a wall design for a super efficient home (near Passive House standard). The double stud wall with cellulose insulation, and a fibreboard exterior sheathing (no polyethylene vapour barrier on the interior) will- in places- have a stone veneer as the exterior cladding. Because we live on the west coast of British Columbia (similar climate to Seattle), and because some of the walls are south and west facing, I am concerned about inward solar vapour drive.

So, I am wondering the following: In such a wall assembly, where one has a brick or stone veneer attached to either cement board or hard-panel, which is itself placed over a vented rainscreen gap, could all inward solar vapour drive be prevented by placing a layer of 6 mil poly ON TOP of their rain screen directly behind their cement board backing for the stone veneer? I am wondering if the stone or cement board itself might over time be compromised by losing its ability to dry inwards into the rainscreen cavity, but do want to retain the high vapour permeability of the framed wall itself, hence the fibreboard sheathing. Just wondering if anybody has any experience with stone/cementboard/hardi-panel being moisture saturated for too long, or if this poly on top of the rainscreen may in fact be a good preventative measure for inward vapour drive. Thanks in advance for any thoughts and advice!

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  1. GBA Editor
    Martin Holladay | | #1

    Don't do it. You definitely want the cement board to be able to dry from the back side. Polyethylene stops drying.

    The best approach is to use rigid foam sheathing. The sheathing is installed on the interior side of the rainscreen gap -- either directly over the plywood or OSB wall sheathing, or installed instead of plywood or OSB wall sheathing (in which case you need to install T-profile steel strapping to brace the walls).

  2. user-1137156 | | #2

    Take a look at Greenguard DC14. DC14 applied between the fiberboard and cement board/masonry will effectively block the inward vapor drive from wet masonry's being heated by the sun.

  3. davidmeiland | | #3

    I agree, don't do it. CDX sheathing, exterior foam, WRB, a 1x4 rainscreen is all you need. I like the WRB outside the foam because I like putting the windows there, but you need to determine how to do it for your project. You need to nail the flashing details, especially at the top and bottom of stone/brick, around windows, roof/wall, etc.

  4. user-1083543 | | #4

    Hi guys. Thanks for your strong responses! Is there any concern with the foam (especially xps) from keeping the framed wall from drying outwards enough? It seems like it may be adding a bit of a vapour barrier on the cold side of the wall, which is why whenever we do outsulation on our homes for clients, we use the Roxul Comfortboard instead of foam. I understand the balance of preventing moisture from the stone getting into the wall, but am still concerned that on the other side of the coin it will prevent it from letting moisture out of the framed wall. EPS is more vapour permeable than XPS, but then run the risk of letting too much moisture from the stone in. It seems like a tough balance. Any suggestion on which foam would strike the safest balance? And while I have read that using fibreboard behind stone is generally considered a no-no, would the foam alleviate this concern, or would you all stand by NOT using exterior fibreboard on any areas where stone will be the cladding?

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Is there any concern with the foam (especially XPS) from keeping the framed wall from drying outwards enough?"

    A. No -- as long as you follow the guidelines in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    For more information on inward solar vapor drive, including several suggestions for ways to build wall assemblies that are safe, see When Sunshine Drives Moisture Into Walls.

  6. user-1137156 | | #6

    With the DC14 you have a drainage path and some venting on the inside of the impermeable layer. Because of this you do NOT have to use any more foam.

  7. user-1083543 | | #7

    Martin- I appreciate the links to your articles (the latter of which opened my eyes to all of this and got me asking this question in the first place!) . My hesitation, or reluctance to use foam on the exterior of my wall, is that I plan on having a wall this is in the order of a true R38 (+/- 12" thick with dense-pack cellulose). So, to keep that exterior sheathing (fibreboard) warm would require a HUGE amount of rigid foam. Which is why I started considered a way of blocking the moisture from the stone from moving inwards to the framed wall to begin with.

    Jerry- Thanks for the recommendation of the Greenguard product. The British Columbia Building Code requires at least 10mm of free-draining air space for any rainscreen cavity to be considered effective at allowing water to drain and dry behind cladding. The Greenguard DC14 is only listed at 1/4" total thickness, so the "airspace" would likely only be about 1/8", or 2mm. That doesn't seem like with would be enough to effectively provide much drying for the stone or backing cementboard in a wet, coastal environment. It seems like a great solution to the problem I'm up against, but I would feel a lot more comfortable with an actual air space on the product of at least 5/16". Anybody have thoughts?

    This seems like a tricky problem. Perhaps, in accordance with Martin's article about solar vapour drive, combining a few strategies such as having a 3/4" ventilated rainscreen cavity, using building felt instead of a house wrap, and not having any kind of vapour retarded on the interior side of my framed walls would all work together to stave of any problems. Does that sound safe, or would people still assert that an exterior rigid foam would be necessary?

  8. GBA Editor
    Martin Holladay | | #8

    If you are really concerned about inward solar vapor drive -- and I'm not convinced that you need to be -- you might want to consider using Delta-Dry. More about Delta-Dry here: All About Water-Resistive Barriers.

  9. user-1137156 | | #9

    To meet the local code I'd use the DC14 with furring strips on the outside, between the cement board and the DC14. This would give TWO drainage paths. The path between the fiberboard and DC14 would be rather small, yet still function while the outer path could be as large as needed. I'd try to use 3/8" lath as the furring which would add 3/8" to the, admittedly small gap provided by the DC14. BTW the DC14, if joints are properly lapped and taped, eliminates any need for additional WRB materials.

  10. user-1083543 | | #10

    Thanks guys. The Delta-Dry product for stone looks like exactly what I'd be looking for. We use Delta Drain on our basements, and didn't know they had a similar product for above grade walls. In looking at their site, I also came across their Delta Fassade product, for open joint cladding, which solves another problem that I was wondering about how to detail for another part of our house! Thanks for getting me on the right track Martin!

    Jerry- your suggestion makes me wonder if perhaps just skipping the DC14 altogether and placing poly between two layers of rainscreen might be a less expensive way of achieving what the Deltra-Dry product would be doing. Although, even with 10mil poly, I still suspect that it would get brittle and break down over time, becoming ineffective. (We've all opened up 15 year old walls in our houses and found poly that just absolutely flakes apart as soon as it is brushed against!).

    Thanks to both of you for your insight; your experience and wisdom is much appreciated in this age where all the older mentors are out of the field, leaving us younger carpenters to scratch our heads and try to reinvent all kinds of wheels on our own!

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