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Community and Q&A

WRB between panel siding/sheathing and studs in dry, mixed climate?

etting | Posted in Green Building Techniques on

I’ll be building a new, small house in central Arizona, the cooler part of Climate Zone 2B, where summer days go from 70 at dawn to 100 or so in the afternoon and winter nights sometimes drop to 20 with days usually reaching 50. It can get noticeably more humid in the summer “monsoon” season, but nothing remotely comparable to the Southeast. I’ll be using SmartSide 4×8 panels as my siding and structural sheathing on 2×6 walls insulated with Roxul batts.

SmartSide requires a vapor-permeable WRB underneath it. After reading lots of articles and discussions here at GBA on WRBs, including using rigid foam insulation as such, my best choice, tight budget considered, seems to be an ordinary WRB like Tyvek housewrap. Moisture accumulation inside my walls does not seem to be much of a concern in my climate, given that I’ll be using airtight drywall with vapor-permeable paint on the interior, but I do wonder whether I should be concerned about the pumping effect I’ve read about with something as flexible as Tyvek attached directly to studs. I’m hoping that the springiness of the Roxul batts will make them press the Tyvek against the inner surface of the SmartSide enough to suppress any pumping motion.

Although I feel as if my plan makes sense, I’ve learned from experience that I’m quite capable of having overlooked something important. Would a WRB other than Tyvek be a better choice?

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  1. Expert Member

    Regular Tyvek tears if you look at it the wrong way. Commercial Tyvek is much stiffer and resistant to damage. It has a lower permeability, but not significantly. Unfortunately it's also more expensive.

  2. etting | | #2

    Thank you, Malcolm. The CommercialWrap looks to be around 50% more expensive and half as vapor-permeable, but twice as tear-resistant, five times as resistant to air penetration, and 33% more resistant to water penetration. It looks to be a good choice if I can get it locally. What do you think of Typar?

  3. GBA Editor
    Martin Holladay | | #3

    If you are looking for a tear-resistant housewrap, you might want to look at four housewraps from Cosella-Dörken Products. In ascending order of price, Cosella-Dörken’s tear-resistant weather-resistive barriers are Vent S, Delta-Foxx, Delta-Maxx, and Fassade S.

    For more information on these WRBs, see New Green Building Products — September 2010.

    I'm not a big fan of houses without sheathing -- it's basically the T1-11 approach. The Z-flashing required at horizontal seams is a giveaway, and these houses look cheap. If I were you, I would install sheathing under the siding.

  4. etting | | #4

    Thank you, Martin. I'll check out those WRBs. Almost all of the houses around here have panel siding, which looks fine to me on the one-story houses that prevail here, or stucco, which is too much trouble and expense for me. Vinyl can't survive the sun here. A few houses have composite or cementitious lap siding.

  5. Expert Member

    Sorry, I haven't used Typar. I'm not sure how tear-proof you need a WRB to be, but my experience is regular Tyvek is below that threshold and I end up ripping it.
    Since we install all our siding on rain screen furring, there isn't much of a difference between having sheathing and not. I just finished a house using Hardi Panels and trim to give a board and batten look, and Hardi Reveal Panels are widely used here for a more modern facade.
    Aesthetic considerations aside, I don't know enough about people generally build down there to comment on your proposed wall. It doesn't seem very robust, in structure or insulation, but maybe that's how things get done in Arizona.

  6. iLikeDirt | | #6

    I live in neighboring New Mexico and this is NOT how I would build. Lightweight framed houses that are not super-insulated have poor performance in the desert and high desert, especially in summer. The large diurnal temperature change will alternately bake you and freeze you, and the sun can and will burn you out of the western part of the house. To offset this, you need something capable of delaying those temperature swings from affecting the interior air; the degree of comfort gained by appropriately adding a lot of insulation or mass is significant. If you want to stay with lightweight framed construction and go with the insulation route, an 8-12" thick double stud wall is sensible and works great around here. If you want to go with the more-mass route--beneficial because termites are a constant problem--ICFs are sensible, and good old fashioned CMU with a thick layer of mineral wool or foam over them on the outside still works great, too.

    As for siding, with our sun, may want to reconsider your choice and go with something made of masonry. I would be wary of any kind of organic composite product. It'll warp, split, check, and become brittle from the sun. It's not a matter of if but when. There's a good reason why stucco is typically the material of choice. If you think it looks ugly or don't like the cracks, maybe look into thin brick or stone veneer.

    Finally, I would recommend paying close attention to orientation and shading! Don't put any windows at all on the western side if you possibly can. Build deep roof overhangs to shade your windows. These factors are highly significant in our climate.

  7. etting | | #7

    Thank you, Nate and Malcolm. I carefully considered CMUs, all kinds of ICFs, and double walls, but concluded that the benefits wouldn't outweigh the strain on my initial budget. I will, however, have 2' overhangs all around and only two very small windows on the west side.

  8. iLikeDirt | | #8

    If you can at all manage it, I strongly recommend exterior-insulated CMUs instead of wood. It's one of the only things you can't change later, and you'll be glad you upgraded. Termites won't eat your walls. You'll never worry about hidden mold in them. The surface temperature of the wall will be lower during the day, making you more comfortable due the diminished radiant heat. Kids won't bash holes in the interior surface by running toys into them. If you fill the cores, creepy-crawlies won't make them home. I live in a lightweight framed house and constantly wish the walls were masonry.

  9. etting | | #9

    Here's what I've learned reading about housewraps all day:

    1. Comparisons can be difficult because each manufacturer publishes results of different tests.

    2. Typar makes a point of publishing results of the same tests as Tyvek, because their housewraps are at least 4 times more tear-resistant than Tyvek's, and many of their other numbers compare quite favorably, but Tyvek publishes studies indicating that the thin membrane Typar depends on for its water and air resistance is easily degraded by abrasion and UV. Typar does not answer this criticism in its literature; therefore, I am inclined to believe that it's valid.

    3. recommends a housewrap with a texture that will provide a path for water to drain downward between the housewrap and the back of panel siding like mine, because water could otherwise get trapped between a flat housewrap and the flat back side of the panel siding. Of the draining housewraps I examined, these seem the best:

    The Delta Foxx Martin recommended,, looks quite good, but it's expensive. It has a few advantages over the less expensive Vent S from the same manufacturer, the most important of which for a sunny climate like mine is much longer allowable sun exposure. The sun exposure limits (from all manufacturers) seem to be based on national average climates; therefore, for intense and constant sun like mine, they would be likely be shorter than stated.

    HydroGap drainable housewrap,, looks promising and affordable, but they don't publish tear resistance.

    TamlynWrap™ Drainable Housewrap,, has good drainage characteristics, but they publish even fewer test results than HydroGap.

    Kimberly-Clark BLOCK-IT,, has very good UV resistance, but they publish the fewest test results of any I've seen.

    Tyvek DrainWrap,, looks fairly good, except for its trapezoidal tear resistance of 7/9, which isn't dramatically better than the regular Tyvek's 6/6.

    Typar just came out with TYPAR Housewrap DW,, which looks as if it has excellent drainage and tear-resistance, but it may have the same vulnerable membrane.

    Most interesting of all is Insultex,, which claims R3 for its 1mm thickness and R6 for its 1.5mm, along with a drainage texture. I find the R-values hard to believe, and Martin commented at that they are impossible, but I don't know exactly what the theoretical limit of R-value per millimeter would be or why. Home Depot sells it at a price that would be great if the product performed as advertised.

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