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Synthetic roof underlayment (low perm material) as Stucco WRB?

Sal_123 | Posted in Energy Efficiency and Durability on

Given the history of prior stucco failures and what we have learned, would it not be prudent to use a WRB with an exceedingly LOW perm rating with a wall assembly designed to dry to the interior? I am planning stucco on a residential structure in Zone 5 (despite the barrage of don’t do its, it fits the architecture of the home). Thus have been stopping into jobs randomly where stucco is being installed and find most contractors apply two WRBs under stucco. I’ve seen a wide permutation of choices. Tyvek on the plywood and #15 felt over Tyvek, 2 layers of #15 felt, Plastic stucco screen systems and Tyvek or #15 felt and so forth. These WRBs are essentially protecting the wall from moisture. The fact the stucco absorbs water (or water enters into the wall otherwise) the WRBs are supposed to stop water entry. Particularly the heat driven moisture that wet stucco releases when the sun hits it. So why the use of #15 tar impregnated felt underlayment and not a lower perm barrier?
The use of a product with an exceedingly low perm rating makes more sense to me. The plastic used in stucco underlayment systems are low perm in addition they have channels to allow moisture. Why not use synthetic roof underlayment as the WRB? I used Triflex XT from Grace under the roof and the permeability rating is listed as 0.04 by the manufacturer. The stuff was a pleasure to work with, strong, lays flat, weather resistant and installed easily. Wouldn’t it be a good idea to install the same directly under the metal lathe? On my project, Tyvek went up after the walls were framed a while ago and has been beaten up by ladders and the weather. 1.5″ of XPS, caulked at all edges and seams taped went up over that. The stucco contractor says he will install a layer of #15 felt under the lathe. I appreciate the 1.5″ XPS is itself a low perm WRB, but for a little more why not change the #15 felt at 6 perms to a 0.04 perm WRB? Thanks for any constructive comments of questions.

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  1. Sal_123 | | #1

    btw, cementitious stucco, not EIFS

  2. Expert Member
    ARMANDO COBO | | #2

    My experiences come from CZ 3-5 in the South and SW. For cementitious stucco, the Code requires 2 WRB. I like to use 1”-2”polyiso rigid foam on the outside of the wall to overcome thermal bridging, which is a closed cell foam, and if taped properly, can be a WRB. Then we use StuccoWrap to cover the foam. All this is done by the framer, and since the stucco guys may take a week or longer to start, we don’t want to leave the foam exposed to the elements for too long.
    Once the stucco sub comes by, they like to use felt under the lath, and usually is because price. Using synthetic underlayment can work, buy it is quite a bit pricier in our market.

  3. GBA Editor
    Martin Holladay | | #3

    You don't want a wrong-side vapor barrier in Zone 5 unless you have decided to install a continuous layer of exterior rigid foam.

    For more on exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing

    If you aren't using exterior rigid foam, your WRBs should be vapor-permeable.

    For more on stucco, see To Install Stucco Right, Include an Air Gap.

  4. JackofAllTrades777 | | #4

    I would seriously reconsider using cementitious stucco in a Zone 5 climate. You definitely want a synthetic stucco system that will not crack due to freeze/thaw issues.

    I'm in a Zone 4 climate and cement based stucco can and will crack and spall due to cement based stucco's retaining water/moisture. At night when it gets cold the water in the thin coat of stucco freezes and causes cracks in the stucco walls. Over a few years it looks like hell and must be re-plastered. Zone 3 and lower are usually safe from freeze/thaw issues so cement based stucco systems are more common and safer to use. Zone 4 and higher should use a synthetic based stucco system.

    I went with a synthetic based stucco (StuccoMax) but it requires an EPS backing. It is 100% waterproof and will not crack during freezing spells since it does not contain any cement based products. I would definitely not use a cement based stucco in a Zone 5 climate. Too risky for cracking.

  5. Tyler_LeClear_Vachta | | #5

    A wise contributor on this forum once stated "physics trumps chemistry" and this a case where you are trying to overcome the physics of moisture intrusion with a purely chemical response (a 'better' WRB). Creating an air gap makes physics work in your favor and manages your risk of moisture problems. If you have a continuous air gap for drainage and drying there is not chance for hydrostatic pressure to build up in your wall. Walls - including yours - will have thousands of penetrations from fasterners etc but with an air gap those holes won't leak.

    Joe Lstiburek discussed the perms question in a recent article Inward Drive, Outward Drying that you should look at.

    As far as creating the airgap, there are a few tried and true approaches. One is installing a rainscreen drainage plane such as Sure Cavity. Make sure that the moisture has a path to exit at terminations with a weep screed as well.

  6. Sal_123 | | #6

    Thanks Guys for the input! GBA has and continues to been an excellent resource. I have been a supporter on and off over the years and encourage everyone to be so as well.
    Tyler you are correct. I am familiar with the MTI product and was looking to cut a corner rationalizing the low perm of a WRB will negate the moisture infiltration into the wall. I agree a moisture drainage plane negates the hydrostatic pressure rather than just blocking it. Physics wins over chemistry in this case. This is essentially the conclusion of Martin’s references and result of years of case studies, install stucco with a drainage plane. In a perfect world where cost is not a factor, an impermeable plane (if using exterior XPS) with overlying drainage plane is the ideal solution. Stop the moisture and show it a way out. At $1.28 a square foot for MTI vs the 11 cents/sq ft for Grace Triflex XT, there is a HUGE difference. I think it was Joseph Lstiburek who postulated using two #15 felt under the lathe would serve to create a drainage plane. The exterior #15 layer will warp and ondulate along the wall due to the moisture in the scratch coat, in essence creating a moisture drainage plane. This has the appeal to save the $1.28 a sq ft. Even a layer of low perm WRB, at 11 cents a sq ft, with a second exterior “warp” layer of #15 felt at 4 cents a sq ft, the 15 cents a sq ft is a bargain. With moisture venting via the path of least path resistance, one can theorize the moisture should vent and not seek to penetrate. You guys think this theory holds water? (bad pun intended)
    As for penetrating fasteners, I am seeking to limit their entry exclusively into the studs and mid stud nailing strips. (I know this sounds crazy) I do not want nails or screws sticking into the bays. In winter months I forsee their conductance of 10 degree F* outside temps into the 40 degree bays acting as condensation rods, dripping a constant stream of water into the pink fluffy stuff for a few months out of the year. The interior insulation bays are sealed to the plywood and studs with sealant to minimize air infiltration. 5/8” Plywood has exterior taped seams and all plywood joints are sealed with silicone.
    As for the stucco conversation, yes I know. That’s another thread and dissertation in itself. I see many homes in this area with stucco without spalling and crumbling. Service cracks is a different story. Contractors in the area, are using fibers in the scratch coat and are adding another layer of fiberglass mesh to the exterior layer of stucco. Essentially it becomes a four layer stucco system. Some are calling it a crack shield. Add the location of the home away from highways or high winds, a 40" soffit overhang, it should work. I see EIFS on strip malls, it looks plasticky and homogenous. Thanks for your input and added comments.

  7. GBA Editor
    Martin Holladay | | #7

    Of course you need two layers of WRB. That is standard for stucco. To install just one layer would be madness.

    But two layers of asphalt felt is not a ventilated rainscreen gap.

    Look, it's your house. Everyone here is telling you that stucco needs a gap -- but if you want to roll the dice, the decision is yours.

  8. Tyler_LeClear_Vachta | | #8

    I get the trying to "save a nickel" piece. I've even flown on Spirit airlines once to do so. A home seems like a more important, long-term investment that you want to do right the first time.

    With moisture venting via the path of least path resistance, one can theorize the moisture should vent and not seek to penetrate. You guys think this theory holds water? (bad pun intended)
    - I'm a fan of the pun, but just to be clear, two layers of WRB won't vent. Drainage can happen in a very small gap, but ventilation doesn't. In a perfect world, WRBs would also be perfect barrier systems.

    Even a layer of low perm WRB, at 11 cents a sq ft, with a second exterior “warp” layer of #15 felt at 4 cents a sq ft, the 15 cents a sq ft is a bargain.
    - Check that Inward Drive, Outward Drying article by Lstiburek that I mentioned earlier "The wrb needs to be vapor open enough to allow the plywood or OSB to dry into the ventilated air space. Turns out that 10 to 20 perms is a pretty darn good value for the permeance of “the layer”." Using plywood instead of OSB is a positive from the moisture management view, but you don't want to put a vapor barrier on top of it. The two layers of grade D building paper are indeed a traditional approach that predates Lstiburek, but they are less forgiving. Careful on the #15 felt - it's a roofing product that won't carry a warranty on walls. See Martin's All About Weather Resistive Barriers for more.

  9. Jon_R | | #9

    As Lstiburek writes "Stucco needs a drainage gap of between ¼ inch and 3/8 inch that is vented. Can’t go narrower than that “where it rains” unless we are over a non absorptive sheathing such as XPS."

  10. Sal_123 | | #10

    OK, I get it. Air gap is a must and two layers of #15 felt do not an air gap constitute. Thanks for keeping me honest. It is not just a nickel, if it were a nickel, the MTI would be here already. However lets keep in mind in this case we are over a NON-ABSORPTIVE layer. Thus looking at perm rating of the WRB is a non-issue, agree? The plywood is drying to the interior in this wall assembly. Once it was covered with 1.5" of rigid XPS (caulked and taped seams) the plywood is no longer draining to the exterior. That's said, as per Lstiburek "Stucco needs a drainage gap of between 1/4" and 3/8" that is vented. Can't go narrower than that "where it rains" unless we are over non absorptive sheathing such as XPS". We ARE over XPS in this scenario, can I ask you to qualify what constitutes an air gap over a non-absorptive wall? Does Stuccowrap or Drainwrap, (wrinkled Tyvek, 36 perms) qualify as air gap material? Benjamin Obdykes "Hydrogap" 16 perms, rubber dimpled moisture resistant house wrap qualify? I looked over the products in Martin's article, 3 to 6 mm of rigid material usually with a polypropylene layer to keep mortar from blocking the channels or screen material seems to be an air gap. Is this the same air gap needed over non-absorptive XPS vs the air gap needed over plywood or OSB sheathing? 3mm suffices vs 6mm or 10mm screen or corrugated panel with a poly mesh over it or just wrinkled Tyvek?
    Thanks for the discussion

  11. GBA Editor
    Martin Holladay | | #11

    I'll admit that your decision to install exterior XPS affects the location of the gap, and is worth discussing. I didn't mean to be dismissive of that fact.

    Your assembly now resembles an EIFS assembly (except with cement-based stucco instead of synthetic stucco). Thousands of EIFS-clad homes developed moisture problems in the 1990s; the solution was so-called "water-managed EIFS." In most water-managed EIFS installations, the drainage gap is designed to be located between the WRB and the rigid foam. See the illustration below of a typical water-managed EIFS assembly.

    Remember, without this drainage gap, EIFS installations were often disastrous.


  12. Tyler_LeClear_Vachta | | #12

    I somehow overlooked the layer of XPS, but I want to clarify your wall assembly. You've got studs sealed to plywood, Tyvek, 1.5" XPS caulked and taped, (insert component here), lath, 3-coat stucco. And it's already installed up to the XPS?

  13. Sal_123 | | #13

    Hey Tyler, the planned wall assembly is 2x6 studs, 5/8” plywood sheathing (the studs are sealed with polyurethane sealant from the inside bays to the plywood), the plywood seams are caulked with silicone and exterior plywood seams taped, Tyvek, #15 felt, 1.5” XPS rigid continuous insulation, all seams caulked and taped. I am in the process of considering the low perm WRB exterior to the XPS and a layer of Keene Driwall, 6mm ventilation plane atop that, finally lathe with attached #15 felt, scratch coat, either stone or stucco final exterior. See Joe Lstiburek’s “Perfect wall”, figure 9

    If it sounds redundant, it is. So many warnings and recommendations AGAINST stucco, I want a 2 belts and 2 suspenders approach. My stucco contractor says overkill to the nth degree. That suits me. Part of my research for this wall assembly was visiting random jobs installing cementitious stucco. All had 2 WRBs minimum but not all had a ventilation plane. NONE had exterior XPS.
    My understanding is once you go with the 1.5” XPS, the wall assembly does not dry to the exterior, thus the wall MUST dry to the interior. I’ve accounted for that, planning to use well installed pink fluffy stuff and permeable paint on the sheetrock.
    I see the schematic posted by Martin and feel the ventilation plane undermines the R-value of the XPS and all the effort to make the assembly air-tight. I do not understand adding a 3/8” ventilation plane UNDER the XPS (relative to the exterior). I see the value of the ventilation plane under the stucco (or brick or stone, basically any absorptive cladding system) to combat moisture infiltration, as in Joe L's fig 9. Thus, when wet stucco gets baked by hot sun, the immense release of moisture in the form of water molecules that seek to infiltrate the wall shall exit via the path of least resistance, a ventilation plane immediately under it. The impermeable barrier under the ventilation plane (XPS or low perm WRB, basically the same concept) stop the H2O before infiltrating the wall. The way I understand it, once you opt for exterior XPS, the ideal 10 to 20 perm WRB no longer applies. I see it as make the wall as low perm as possible under the absorptive siding to stop moisture infiltration and vent it accordingly. You agree?
    Thanks for any insight.

  14. Sal_123 | | #14

    btw- keep in mind most EIFS are installed over EPS, considered permeable relative to XPS.

  15. GBA Editor
    Martin Holladay | | #15

    Coating your studs with polyurethane seems a little nuts to me -- but it's your house. I think that your plan will work, and there is nothing wrong with a low-permeance WRB adjacent to the XPS layer. That said, the permeance of the housewrap is irrelevant, because 1.5 inch of XPS is already a low-permeance barrier to inward solar vapor drive.

    If you really want a low-permeance housewrap, then (for warranty purposes) I advise you to choose an actual housewrap -- in other words, a low-permeance WRB designed for walls -- rather than roofing underlayment. An example is Henry Blueskin SA.

  16. Sal_123 | | #16

    I explained it poorly. Where the plywood sheathing meets the 2x6 studs inside the insulation bays, is a bead of polyurethane sealant to stop or minimize air infiltration or circulation. So from the interior, all the bays are caulked. Not studs coated in polyurethane. The low-permeance WRB came to mind since the XPS strapping, 1" x 3" every 16" o.c. will be exposed to moisture. Not sure what the effect of wet-dry cycle over years to untreated wood will have and again adding an impermeable barrier at 10 cents a sq ft to protect the wall seemed like good, cheap insurance.

  17. GBA Editor
    Martin Holladay | | #17

    Sorry for misunderstanding the "polyurethane" reference. Thanks for clearing up my misunderstanding (and for editing your earlier comment to make everything clear).

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