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Community and Q&A

DensGlass sheathing and rainscreen materials

Domenico Perrella | Posted in Green Products and Materials on

Martin,

Other than not holding siding fasteners, what are the downsides of using Densglass instead of plywood sheathing? A (modest) price difference? Is taped Densglass less of an air barrier than taped plywood?

I’m replacing an unsheathed stucco wall and will probably use plywood sheathing (I want no OSB in my house), but I’m curious about Densglass because I’d like to minimize the use of organic materials that can absorb lots of water or rot. I know densglass means that fasteners for the rainscreen material and sheathing need to be in the studs, which increases hassle and maybe labor costs, but I’m wondering if that’s the only downside.

My wall would be (inside to outside) drywall, 2×4 framing with bays filled with Rockwool (sound is an issue and I’m trying to minimize wood/cellulose), sheathing (plywood, or maybe Densglass Gold), a highly permeable non-perforated WRB, possibly rigid insulation (1.25″ roxul?) , but probably not unless the inspector requires it because the Northern California climate is mild, a rainscreen material like House Slicker or DC14 drainage mat), felt-backed metal lath, 3-coat stucco, and paint.

Do you think DensGlass would be a poor substitute for plywood in that application? Also, what do you think of the DC 14? It has low permeability and the channels seem to be intended to create an air gap on either side of the material. I’m not sure if that’s brilliant or scary.

Thanks for your excellent site.

Nico

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Domenico,
    You may want to read this document: DensGlass Sheathing Technical Guide.

    Q. "Is taped Densglass less of an air barrier than taped plywood?"

    A. According to the document I linked to above, "Per the International Energy Conservation Code (IECC), gypsum sheathing such as DensGlass Sheathing complies with the prescriptive code language for use as a continuous air barrier when the joints and openings are properly sealed."

    Q. "What do you think of the DC 14 [a drainage mat product manufactured by Kingspan]?"

    A. Kingspan DC 14 drainage mat appears to be a knock-off of Delta-Dry. To learn more about the advantages and disadvantages of Delta-Dry, read All About Water-Resistive Barriers. (Scroll down to the paragraph with the "Delta-Dry" heading.)

    -- Martin Holladay

  2. Charlie Sullivan | | #2

    You might also consider the DensElement version of DensGlass that also functions as a WRB when the seams and fasteners are sealed with liquid flashing. If you are putting the effort into sealing the seams as an air barrier, it seems like you might as well get the WRB benefit all in one step.

  3. Domenico Perrella | | #3

    Martin and Charlie,

    Thanks for the information. Delta Dry certainly does sound very similar to the Kingspan DC14 I asked about.
    I'll also look at the Denselement product, although for a project my size, I'll have to go with materials my contractor is comfortable with. They won't be interesTed in tRying something new on a $12-15,000 project.

  4. Tyler LeClear Vachta | | #4

    On the rainscreen front, since you are working with stucco you might want something with built in fabric so that the mortar doesn't block the drainage channels. Sure Cavity is one such product that would fit the bill.

    The photos of the DC14 on Kingspan's site portray something made of a plastic film, but it's actually foam based. It looks like it would compress pretty easily. You want to make sure that whatever product is installed maintains a clear and consistent drainage gap.

  5. Tim R | | #5

    Have you determined if the Densglass will provide the required code level of wall bracing?

  6. Brendan Albano | | #6

    I'm not an expert on this, so grain of salt and all, but the IBC does have provisions for gypsum sheathed shear walls. It looks like the maximum height-width ratio of a shear wall with gypsum is less than for a wood structural panel, but it does seem like you can make a shear wall out of it.

    Curious to hear if anyone has used gypsum sheathing on a residential project and how it went for them! We use it all the time on commercial projects, but that's a whole different animal.

  7. Domenico Perrella | | #7

    Tyler,

    Thanks for the information. I had noticed and was concerned about the fact that the DC14 product was made out of foam. In addition to compression, that makes me wonder about tearing during installation. At this point, I'm pretty sure that I would not want to use that product. I will definitely look into Sure Cavity. The Delta-Dry material Martin mentioned is also made out of sturdier stuff (HDPE)and does come in a version (Delta-Dry Stucco & Stone) that comes with fabric attached to keep stucco out of the air space just like the Sure Cavity. Honestly, I was intending to put felt between the rainscreen material and the stucco anyway. With a non-permeable rainscreen material in there anyway, it may not be absolutely necessary, but I'd prefer to minimize the flow of water into that rainscreen gap, even if it does slow the drying of the stucco.

    Thanks again.

    Nico

  8. Domenico Perrella | | #8

    Tim and Brendan,

    Thanks for sharing your thoughts. I did look at the structural strength specifications for Densglass, but I made no effort to compare them with any code requirements. I am 100% positive that Densglass will increase the shear strength of my currently unsheathed wall, which has survived some fairly substantial earthquakes. I hope my contractor or the permitting folks will let me know if it doesn't meet code requirements.

    I have seen Densglass or Denselement used on many "residential" projects, but those were always fairly large apartment buildings or condo complexes that have more in common with commercial construction than with the single family home construction we're really talking about. I am not sure if I have ever seen it used with wood framing and I have no idea whether the buildings I've seen it used in all have other shear bracing. I'm pretty sure some of the taller ones do.

    Thanks.

    Nico

  9. Tim R | | #9

    Yes you can use gypsum sheathing for racking resistance you just need to understand how it fits into the code. Most older houses with stucco were designed with the stucco as a structural element not just a siding material. The commercial project use is usually for the fire rating aspect.

  10. Domenico Perrella | | #10

    Tim,

    Thanks for the additional information. I've always assumed the stucco provided some real-world resistance to racking, but didn't think it was rated for that purpose. So, if I add sheathing and a rainscreen, I'd guess that the separation of the stucco from the framing would somewhat reduce the structural contribution of the stucco (because the length of the fasteners not embedded in stucco or wood could twist), but I'd probably gain substantially more from adding the sheathing, whether gypsum-based or plywood. And because I don't need to meet the fire code requirements applicable to commercial building, I don't need something like the Densglass. But it would meet the structural code requirements that apply to a single-family home, right?

    My consideration of Densglass may be entirely academic. The contractors who are interested in my small project (about 400 square feet of wall and a couple of windows) probably won't have experience working with it and they're not likely to bid something if they don't know how much labor the project will require. They seem to have plenty of work and won't bid if you're not using their favorite materials, like when I said I wanted a quartz tub surround and it turned out the kitchen and bath contractor only liked tile.

  11. Steve Knapp CZ 3A Georgia | | #11

    Domenico,

    For what it's worth, DensElement installs much more quickly than a typical sheathing plus fluid-applied WRB-AB layer. Installers only have to seal the joints and transitions using FastFlash. If your contractor does commercial projects, he is probably familiar with how these systems go together.

  12. Tim R | | #12

    Densgalss only provides 90 lb per ft (plf) of racking resistance, with a minimum 2:1 height to with ratios. A 1/2" plywood panel nailed 8d 6"oc is 280 plf with a minimum ratio of 3.5:1 height to width ratio & fully blocked.The densglas will be less than 90 plf with other factor in the code) In the newest code documents stucco is not a favored material for shear resistance, it has substantial load factors associated with its use as compared to wood shear walls. The International Residential Code has all the parameters to properly design a residential building. The simple answer may be to replace the stucco with plywood as the structural material to keep the house together when the wind blows or the ground shakes. You would need to learn how to make a modern shear wall.
    It is likely your house was permitted using the conventional constructions provisions of the Building Code, so if you plan to remove and replace structural wall material, that you should know what is acceptable to replace it with. So talk to an engineer, architect or code official to see what are the proper material options.

  13. Domenico Perrella | | #13

    Steve,

    Good points. If my contractor does commercial work, I'll ask him about Denselement. If not, I'll probably be too scared of scaring him off with too many odd questions.

    You'd really be amazed how easy it is to scare off a bathroom guy around here. Any hint that you may get more than one bid scares off some, not wanting tile scares off others, who knows how I scared of some more guys. The only bid I have is $31000 for a 5' by 8' or 10' bathroom, but that includes replacing the failed part of my stucco and bringing my electrical panel into code compliance by turning it around to face into a bedroom instead of a closet, which I don't think the AHJ would require. Without those, it's more reasonable.

  14. Domenico Perrella | | #14

    Tim,
    The difference in strength between densglass and plywood is higher than I'd realized. I'm pretty confident that stucco and Densglass is stronger in shear than how my house was built in 1961 with stucco and no sheathing at all, but I guess I really don't know if it would meet modern codes. So, I'll stick with plywood plus stucco, which I am at least 99% sure satisfies the code, since I see it being used in new construction with no other apparent shear bracing.

  15. Expert Member
    Malcolm Taylor | | #15

    Nico,

    A lot of this really depends on the individual circumstances of your project. Are there seismic or wind loads that need addressing? Does the wall play an important part in resisting shear forces for the house?

    If shear isn't important, and as Tim cautioned it meets code, I'd be inclined to use something other than plywood behind the stucco. Stucco is a really difficult cladding to detail so that moisture doesn't make its way back and wet the sheathing. You want something that doesn't mind being wet for an extended period of time and doesn't rot.

  16. Domenico Perrella | | #16

    Tim,
    I forgot to say thanks. Also, while I'd always known that stucco contributed something to shear strength, until now I'd never known that it was rated and used for that purpose.

    I have no idea if it's accurate, but I just Googled and found a table, apparently derived from the 1997 UBC, that shows that an unblocked 7/8" stucco wall has a shear value of 180 pounds per foot (twice Densglass), which is a smidge less than 5/16" Structural I plywood nailed with 8d Nails at 6" OC (200 pounds per foot), and is a lot less than 15/32" (280).

    I have no idea what strength is required, but this has convinced me to stick with plywood, probably 15/32" unless my contractor tells me otherwise.

    Thanks to all.

  17. Domenico Perrella | | #17

    Malcolm,

    I just noticed your post between my two responses to Tim. Thanks for your advice. My seismic category is D (I live in coastal California and not close enough to to a major fault to rate an E), so shear strength is pretty important. The house was built in 1961 with no shearing other than the stucco and no bracing. Some seismic work was done on the foundation (bolting) but the walls are as they were originally built. It survived the Loma Prieta earthquake and many small ones closer to home, so it has some strength, probably just from the stucco and wallboard and having few long straight stretches of wall. Being a single-story ranch house limits the loading too, but it's never been tested by a large quake close to home. No matter what I sheath this wall with it will be the strongest wall in the house, but I suspect that continuous plywood sheathing with nails every x inches will automatically be approved because of being a standard means of construction that meets standards without an engineer's sign off.

    As for water, it makes me nervous, but with felt, a rainscreen gap, and another layer of felt or housewrap between the stucco and the sheathing, I'd hope not much water would get in and it would have a good chance to evaporate. We only get a moderate amount of rain here, in most years (not this one) and we get almost none for six months and have low humidity when it's not raining. So, it's not the worst climate for stucco.

    I'd love to use something that doesn't rot, but my project doesn't justify hiring an engineer, using alternative bracing, scaring off contractors with atypical materials, etc. So, unless my contractor (who does just about all of his work in seismic category D structures) knows that something like Denselement can be approved without an engineer's stamp, I'm going to have to go with what the AHJ will permit.

  18. Expert Member
    Malcolm Taylor | | #18

    Nico,

    Given the situation, that sounds like a sensible approach.

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