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Lime Plaster vs. Gypsum Question

paula_builds | Posted in General Questions on

New Construction, Zone 4, Western Oregon.

I don’t have experience with plastering, but am planning to plaster over the drywall I have installed in my bathroom (and throughout the home, later).  I had been under the impression that lime plaster would give me an advantage against mold and mildew, where gypsum plaster (like drywall) is vulnerable to mold & mildew.

Is this true?  

1) High PH (alkaline) substances are mildew resistant
2) Type S Lime is very alkaline (12 I believe), but
3) Once lime plaster cures, its ph is closer to 8.7, nearly neutral.

Is there any one out there who would still claim (based on experience) that lime plaster is better than gypsum in this regard?

I also like that lime plaster absorbs carbon as it cures.  Any other opinionated folks out there on the gypsum vs. lime debate?  I’d love to hear more thoughts.

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Replies

  1. Expert Member
    Michael Maines | | #1

    It's not direct experience but cured concrete is around the same pH and seems to be alkaline enough to resist microbial growth to some degree. But with your drywall's paper facing so close to the surface, elevated moisture levels could still result in microbial growth. Reducing thermal bridging and controlling interior humidity levels are still important.

    1. Expert Member
      DCcontrarian | | #4

      I know from firsthand experience that mold will grow just fine on cured concrete if the conditions are right.

      1. Expert Member
        Michael Maines | | #5

        And I've seen a lot of concrete that should have mold on it but it doesn't. Thus my qualifier, "to some degree."

  2. kentthompson | | #2

    I have experience with lime plaster, but in a dry climate, so I can't speak to it's properties with regard to mold. It seems like other strategies like dehumidification would be cheaper and better.

    An idea that might appeal to you is to use a lime wash (aka white wash). It's painted on and would have a similar pH as lime plaster. I have no experience with it and don't know how suitable it is for interior walls.

    Keep in mind that large amounts of energy are used to create lime, so the carbon absorbed while it cures is less than the carbon released when it's created.

    1. paula_builds | | #17

      Thanks Kent.

      I did mix up some limewash a while ago and painted it on some random framing. The surprising thing was that it wasn't so white at first but became brighter and more opaque as it dried. Looking forward to using it on some of the exposed wood.

      1. Jemari | | #22

        The product is curing (changing chemically) after you apply it, and this is why you see that change in whiteness. I see limewashes becoming popular again (2023), which is wonderful due to the environmental and durability aspects. The one thing you need to watch out for when product shopping is the stain resistance and washability. These topics are often politely sidestepped, and the difference is substantial compared to the latex and oil based paints we are all accustomed to! Traditional lime wash and plaster is made from slaked quicklime or aged lime putty, and cures by reacting with the CO2 in the air to form limestone. Limewash is just diluted plaster. *But this may not be true of the mix you have.* There are many kinds of 'lime', just like there are many kinds of cement or concrete. Saying "lime" is almost like saying, "flour", because there are many types all sold as "lime". For example a 'hydraulic' lime cures by a process using added binders which are activated by water, and cannot be stored as putty because it starts to cure when moistened. This is completely different from the CO2 curing process for traditional lime wash/plaster I described above. The advantage of hydraulic lime is a powder that can be mixed and applied the same day (vs. requireing a longer curing process, or a boiling slaking process), and it can cure underwater. Hydraulic lime is popular due to the same-day aspect, but because of increased hardness it can become brittle, and is often less breathable. Depending on the final product hardness, hydraulic mixes may not be suitable for use as mortar with brick and stone, as it needs to be softer than the brick/stone for the structure to maintain stability (not crumble). It also may crack a lot on an older home with a creaky foundation. You can make the old-fashioned lime wash by mixing hydrated lime (not the same as hydraulic), and allowing it to cure where it won't freeze for several months. From this aged lime putty you can add pigments, aggregate, and water depending on the recipe to make mortars, plasters and paints. Some recipes also call for binders, fixers, resins and special minerals to get various physical properties and color depth effects. You can layer top coats of waxes, oils and emulsions. Most recipe modifications and topcoats will change hardness and breathability in exchange for flexibility, sheen, water/stain resistance, washability and sheen. As you can see, this is actually a very involved topic with a lot of chemistry. When someone says something about cement or plaster or mortar or limewash or concrete, you have no idea what kind of recipe they were talking about (nor do they), and it is not something you can get quick answers for. To compound things further, temperature, moisture levels, and curing time between coats can have very significant impacts on the final result, even with the SAME recipes. Even the trowels used can change surface chemistry adding color variation. In order to isolate anything you need to understand the chemistry, the detailed process, and the environmental conditions. If you are not ready to geek out with a stack of books and a ton of experimentation, (and terrify or intimidate many construction guys), buy a system from a company that has already worked out all the kinks for you. This is a straight up choice of doing a ton of R&D, or paying for a premium product. If you want to start digging deeply into recipes, a number of historic preservation societies have detailed instruction on pros and cons of historic recipes and modern additives. // I grew up in a home built in the late 70s here in Maine by my father and his friends, and the walls were 100% lime plaster (no drywall!). The walls felt like stone, and every home feels like plastic to me by comparison.

  3. Expert Member
    KYLE WINSTON BENTLEY | | #3

    Also note that the type S (special) plaster you're using, if it's just the normal masonry hydrated lime, has a significant amount of magnesium oxide content. 'True' lime plasters are mostly type N (normal) high calcium oxide, and are somewhat different in nature.

    I think lime would be better here compared to gpysum. Once gypsum gets wet, it falls apart, where as lime is mostly impervious to water by itself, and maintains a high vapor permenance. If it's over drywall, I guess there's some debate as to whether it matters.

    FWIW lime only captures CO2 once, during the curing stage. All those CaO molecules absorb one CO2, to become limestone again, CaCO3. You can nearly take the weight of lime used, and double it, and that's the amount of CO2 sequestered in your wall. Even at that, it's less than a neutral process, since that CO2 was driven off by heat anyways.

    If you can source it, the Carmeuse brand lime and the NHL limes seem to be very popular. I spent an inordinate amount of time one week trying to source pure CaO for a plastering experiment, but had to give in.

  4. Expert Member
    BILL WICHERS | | #6

    You can certainly plaster over drywall, and that used to be done back in the day before drywall was entirely trusted as a wall material. You end up with a sort of extra thick skim coat on the walls. It's a LOT of extra labor though! It takes some skill to get a good finished surface too, and it's much more difficult to do well than just finishing taped drywall.

    I don't think you're likely to see much improvement in mold resistance here, and certainly not enough of an improvement to be worth the time to do the work. I've seen plenty of old wet plaster sites that have mold. The mold will grow not only on the surface of the material, but on any crud that accumulates on that service. I think that thin "crud layer" might act to protect the mold from any natural mold resistant property of the underlying substate material here.

    My folks live in a house that has the old 1/2" drywall plus 1/2" plaster wall construction. They have had mold in some areas that would be considered "problem spots" in the past (areas with higher moisture levels). I can't say if the mold would have been worse with drywall alone, but I can say with certainty that the plaster layer will not PREVENT mold growth, at least not in all cases.

    Bill

    1. Expert Member
      Michael Maines | | #8

      Skim-coat plaster is still popular in parts of the northeast US and for those skilled at it, it's no more work than drywall and they are priced similarly. Plaster makes more of a mess but you can complete an entire room in a day with little to no sanding required, the surface is glass-smooth and uniform, and much harder than drywall or joint compound. Plasterers usually apply two coats, totaling about 1/8", over 1/2" or 5/8" "blueboard," a drywall product with the paper treated for better plaster adhesion.

      I believe plaster is used as a textured surface in other parts of the US but that might be a different product; I don't have experience with textured finishes.

      1. paulmagnuscalabro | | #9

        Michael, the skim-coat plaster you described is what I was familiar with back in New England. I was also used to people painting plastered walls.

        Out west (I'm in Bozeman, Montana) it's a bit different. I can't speak much to the process, but it's much more typical to hang regular drywall, tape it, and coat it with something that primes it for plaster application. The plaster finish is dead-smooth, but there are visible trowel marks etc that catch light differently. It's a really cool-looking dynamic finish, and is not typically painted out here. It's also substantially more money than drywall + paint, so is really reserved for high-end projects.

        This is probably well beyond the scope of paula_builds' original question, but Google "tadelakt." It's this really cool waterproof plaster finish, suitable for use in showers, etc. There's some pretty good info on the New Age Artisans website as well (they're a local plastering company, and some of their work is amazing).

        1. paula_builds | | #16

          Yeah, Tadelakt. I had written it off as being a tricky thing to install - but that it was a good waterproof surface for bathrooms.

          I see a tadelakt course on the Artisans website. Thanks for sharing! I'll check it out. I don't know if tadelakt is what I'm after - but it could be a good option. I'll message them.

          1. Jemari | | #23

            Consider microcement as a Tedelakt alternative. Tadelakt is great, and not necessarily hard to do, just detailed and extremely labor intensive (days of work with a humidifier running in the space; polishing the entire surface with small hard stones...) To me the biggest cons of Tadelakt are 1) needing regular applications of saponified oil to maintain the surface forever (I WILL fail at that!), and 2) supposedly if you fail at applying your saponified oil at the correct and unknown interval at some point down the road, or it is damaged or stained, it is impossible to patch without starting over completely. I don't understand the basis for the starting over item, so I don't know if that is lore or not. Originally Tadelakt uses Marrakech lime, which has a larger variation in granular size of lime particles with the net result of an easier application process and more interesting surface. We jump through a lot of hoops to compress our more homogenous mixture to get the same durabilty and color variation. Some Tadelakt recipes skip these steps to make the process less intimidating (and easier product sales), but the result does not have the depth of figuring that is one of it's hallmarks. It's kind of funny we would wind up with a more labor intensive process than what they are using in North Africa because our materials are more refined. Joke is on the commercialized and relatively wealthy Western world!

  5. paula_builds | | #7

    Thank you all for your replies.

    The carbon absorbed is only offsetting the carbon created in the manufacture of the lime. No need to pat myself on the back for that. Got it.

    Thanks Bill Wilchers for the input on seeing mold on plaster. That's very helpful information. If plaster in my bathroom convinces me that it's too much work for the rest of the house, I may change course, but at this time I want plaster throughout for the aesthetic result as well as the mold resistance (which may not be significant).

    I'm open to (maybe not enthusiastic about) painting over the lime plaster--just in my bathroom--to prevent mold/mildew from entering the plaster body. I intended the plaster to be about 1/4 inch thick, but given my lack of experience the thickness may vary.

    Because one of the advantages of lime plaster is its vapor permeance, covering it with a non-vapor permeable paint seems like a shame. My builder friend wrote:
    "Leaving the wall surfaces unpainted or unsealed in a moist environment will allow molds or fungi to lodge in either porous material. I wouldn’t count on the PH of the matrix to prevent such organics from taking hold.
    Sealing the surface will permit colonies to grow only on the surface, thereby permitting easier cleaning."

    Of course the bathroom has an exhaust fan so that should help as well.

    I did mix up some white wash (a blend of lime, salt, and water) and intend to put a few coats on exposed joists that have mildew on them.

    I like the idea of being able to whitewash over the lime plaster in places where it gets dirty or needs refreshing.

    I don't think whitewash is what my builder friend had in mind for sealing the plaster in the bathroom.

    Is there a sense that I would be wise to somehow seal the plaster in the bathroom since it's a high moisture environment (even with a bath fan on a timer)? Any recommendations as to the type of paint that would accomplish this best?

    My plan at this point is to have unsealed plaster throughout the rest of the home.

    1. Expert Member
      DCcontrarian | | #10

      I'm not sure why you think permeance is an advantage. In heating climates code requires a vapor retarder on the interior of insulated assemblies.

      1. paula_builds | | #15

        You make a good point. I do have the vapor barrier (certainteed membrain) behind the drywall. It's supposed to breathe in warm weather, and not breathe in cold weather...

        I guess I'd heard something along the lines of lime will absorb and release moisture in a way that gypsum plaster will not.

        I don't know if that is a good thing, or what advantage that provides.

    2. Expert Member
      BILL WICHERS | | #11

      You would probably get better results in terms of mold resistance by using a mold killing primer than you would by using plaster over drywall. I've never used a mold killing primer for a regular wall though (I usually use the stuff in problem places like basements, and sometimes attics), but I don't see why it wouldn't work on a regular wall too. You'd be building in a bit of nastiness though (there is some kind of anti mold agent in the "mold killing" primer), but I don't think that's likely to pose much of a concern in practice.

      The biggest downside I can see with the plaster coat is that it takes some skill to get right. I've done a lot of skim coating myself, but not a "thick" old-school plaster layer. I'm pretty sure you'd find there would be a learning curve to the plaster if you go the DIY route here. When I first started doing mud work a loooong time ago (and I hate drywall work in general), it was frustrating because I'd want it to come out perfect, and I'd overwork it. It's better to do two coats than to try to make one coat perfect, but that first coat calls out to your inner OCD and that's where problems start. With a thick layer of plaster, I think there would be a whole lot more room to overwork things and end up putting in way more time than you should. That would be my biggest concern trying to do a project like this.

      Bill

      1. paula_builds | | #14

        Thanks for the comments on letting OCD create a problem (overworking). I will keep that in mind.

  6. jollygreenshortguy | | #12

    I suppose there are quite a few lime plaster manufacturers now. Quite a while back I used Saint Astier lime plasters. They've been around for hundreds of years. They're distributed in the USA by limes.us
    As far as mold, I would focus more on avoiding the conditions that favor it, rather than trying to find materials that don't support it. In other words, make sure to ventilate your bathroom really well, and if you can give it a good south facing window that's a plus. But it might be too late for the window now. It sounds like you've already built it.
    I don't think anything less than glazed porcelain would be a suitable wall finish in a shower enclosure. But for the rest of the bathroom, if you design to reduce the humidity then a lime plaster would be fine.
    I recently did a clay plaster in a living room and just to make it a bit easier to maintain, I rubbed in a couple of coats of beeswax. It came out really lovely and water beads on its surface instead of soaking in. But the beeswax needs to be repeated every 6-12 months.

    1. paula_builds | | #13

      I do have a window; thanks for the perspective on preventing conditions that favor mold.

      Glazed porcelain for the shower, but I wasn't planning to tile the ceiling.

      Beeswax sounds appealing, except for the repeat applications! Might experiment with that a little.

      1. jollygreenshortguy | | #19

        As far as the beeswax, I see it as a sort of "Spring cleaning" ritual, like polishing a piece of furniture. It's just one wall though. I wouldn't do the beeswax everywhere. Strictly speaking it's not necessary.

  7. paula_builds | | #18

    Noticing that my original post mentions lime plaster vs. gypsum in the title.

    If mold/mildew prevention isn't significant with lime, it sounds like it comes down to aesthetics or personal preference which way I go.

    I have a plan to mix up samples of both in a couple weekends to get a feel. So far what I've heard are:

    gypsum
    -dries to a harder surface
    -less prone to cracks
    -cures more quickly (45 minute-1hr)

    lime plaster
    -softer, cures slowly
    -can absorb & release moisture (why is this a good thing?)

    any other things to add to my lists on how these are different?

    My friends (who will help train me) are both recommending gypsum, so I'm sort of open to being persuaded, but I did like the fine homebuilding article that originally had me sold on lime plaster.

    https://www.finehomebuilding.com/project-guides/drywall/hybrid-approach-lime-plaster

    1. Expert Member
      BILL WICHERS | | #20

      If you have friends with experience applying wet plaster materials, I would go with what they like to use. Take advantage of their skill and defer to their expertise.

      Bill

      1. paula_builds | | #21

        Thanks Bill. I did mix up a small amount of the lime/sand mix in a bucket to try, but unless I really don't like how the veneer plaster looks (unlikely), I'll probably go with it since my friends seem to be inclined in that direction.

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