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Community and Q&A

Hemp Wool & Lime

Desert_Sasquatch | Posted in Green Products and Materials on

So I’m going to build my own home–or have contractors do it for me.  I’ve begun thinking about using hempwool for insulation.  Compared to the other wall systems I’m looking at (hempcrete, AAC, pummicecrete) it could save me tens of thousands of dollars simply because it is much much easier to install. 

However it does lack a few of the strengths of actual hempcrete: 

1) Mold Resistance:  I’m not really clear how mold resistant hemp wool is.  Does anyone know?  I was under the impression that hemp hurd had a particularly high silica content and I am not clear what part of the hemp plant hemp wool is made out of and if it has a sufficiently high silica content to impede mold growth.  Also of course lime impedes mold growth as well–though I’m unclear what happens when the lime reverts to a neutral pH.  

2) Rodent Resistance:  I’m told that mice don’t love hemp, but I’ve seen the mice around here do a lot of things that people say mice would prefer not to do.  Additionally, I have to assume that the lime in hempcrete deters mice, especially when the pH is alkaline (again, I’m unsure what happens once the lime reverts to a neutral pH, I’d be curious to hear what folks think).

3) Lifespan:  I’m not really clear what the anticipated lifespan of hemp hurd is, but I have to imagine that coating hemp in lime improves it, barring a water leak that washes the lime away.

So it has me wondering if I could maybe get the best of both worlds by dunking hempwool in a lime wash bath prior to sticking it in the wall.  (It may require a bit of “squishing” like when you get water into a sponge.  I imagine I’d also wring it out before putting it in the wall to minimize the hit to the insulation value).

Is this something that folks have tried?  Is there an obvious reason not to do this?  It seems like if it was a good idea someone would have done it by now but…  I want to see what folks think.

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  1. Expert Member

    " it could save me tens of thousands of dollars simply because it is much much easier to install"

    This is false. Not trying to rain on your parade, but if you're going to do it yourself, or have someone do it, the quickest is what people have specialized in doing, IE fiberglass, and cellulose.

    I'm all for you doing it, just trying to give you some clear expectations. It'll only be as fast, and cost more.

  2. Desert_Sasquatch | | #2

    Hi Kyle

    I appreciate your gentle attempt to bring me back to earth.

    However I hope you'll allow me to push back just a little bit: Looking at hemp seems like installation would be nearly identical to installation of mineral wool--which is very similar to the installation of fiberglass, with the caveat that you don't have to worry about slumping. Here's a video, if that's helpful: []. So my hope was that the skills would be transferable. They all basically involve taking a somewhat spongy material and sticking it in a wall cavity. The only expertise necessary seems to be cutting the pieces to the right size, and occasionally cutting out a portion to make room for electrical or plumbing. Am I missing something?

    Obviously if there's this extra step where you saturate the hemp wool in lime then that takes extra time, but you know, how long does it take to soak a sponge? Even if we double the man hours required for the job compared to installing fiberglass insulation it's still probably half as expensive as hempcrete or other monolithic wall systems I was looking into.

    1. DC_Contrarian | | #4

      I think you're not thinking about the supply chain implications of using materials that the installer isn't familiar with. A guy who installs fiberglass batt all day every day has supplier who can get him what he needs for every job, usually the same day. If he miscalculates how much material a job is going to take, no big deal, he can get some more dropped off in a couple of hours. If he uses part of a roll on one job it's not a big deal, he can take it to the next job.

      I looked into hemp insulation. The closest distributor was 2,000 miles away from me. I'd have to get it delivered by truck, which would take about a week. If I needed more mid-job I'd have to order another shipment, which would mean a few hundred dollars for shipping and another week. To avoid that I'd probably order 15-20% more than I thought I needed, and at the end of the job I'd have a bunch left over. I might be able to get rid of that on Craigslist but most likely it's going in a dumpster.

      A professional is going to want to get paid a lot more for working under those conditions.

      For design details you absolutely have to have, it's worth special ordering. For insulation there's just no benefit in not using what's locally available.

      1. Desert_Sasquatch | | #6

        These all seem like legitimate points, but +20% for materials is still just.... +7% or something to the overall cost. Compared to some of the other wall systems like hempcrete, or AAC that just moves it from half the price to 5/9ths the price. (Obviously I'm being a bit rough with the math but you get the idea).

        1. DC_Contrarian | | #8

          If you're DIY-ing it maybe. If you're hiring a contractor, he's thinking in terms of days on the site. If he could do the job in a day, but has to come back because of material shortages, that's two days on site and doubling his cost.

          Let me ask you this: What problem are you trying to solve? Most people here feel there's nothing wrong with conventional materials (except maybe XPS and some spray foams).

          1. Desert_Sasquatch | | #12

            Again thank you, that's helpful. I'm obviously new to this.

            Regarding the issue I'm trying to address: Part of the issue--the reason I'm pushing for strange materials--is that I have environmental sensitivities, including to mold. Without going into details, this complicates the whole planning process because I'm not sure how I will respond to a building material over time. And causes me to give weight to sort of nebulous observations like "I've tended to feel better in homes with monolithic walls."

            So I suppose the issue I'm trying to address here is to find a way to achieve something that works like the monolithic walls that I've enjoyed--at least in whatever dimension is useful to my health--while not being so expensive.

        2. Expert Member
          MALCOLM TAYLOR | | #9


          I think people are getting sidetracked from y0ur question because you framed it as a way to save a lot of money by using AAC or hempcrete as comparators. Walls filled with those have completely different attributes than hempwool, whether it's dipped in lime or not.

          1. Desert_Sasquatch | | #13

            I agree :) Alas, I think it's too late to go back. Though perhaps I can take the lack of affirmative response as a sign that no one knows of anyone who has done this.

  3. DC_Contrarian | | #3

    In a house using modern materials built to modern codes, rodents should be excluded and mold shouldn't be a concern.

    1. Desert_Sasquatch | | #7

      I agree with you, but also I also know that things can go wrong.

      But also I admit that my attraction to a somewhat more solid walls (I hesitate to call what I'm proposing "monolithic" but maybe it's halfway to being as monolithic as hempcrete is) is probably not something I can justify all that well. Homes with monolithic walls have tended to feel better to me. Maybe that's because of the hygric buffering capacity of those walls. Maybe it's just that they are less likely to have mold problems (which doesn't contradict what you said about a properly built home not having mold issues). Maybe the thermal mass regulates temperature in a way that I only register subconsciously. Maybe it's all in my head. But walls and wall finishes are the one place I'm willing to pay a bit more without necessarily being able to justify the cost in terms of longevity or energy efficiency vs something like rockwool.

      But as my original post suggests, if I can get these benefits without paying a whole bunch more...well that would be nice.

  4. artisanfarms | | #5

    I think if you're planning on dunking the wool in a bath prior to installation, you'll also need a true drying step beyond "wringing" it out to truly get rid of the water, otherwise you'll be installing a lot of water in each cavity.

    Regarding cost and installation, why Hempwool vs. Rockwool? Rockwool is more reasonably priced, has a high recycled component and is dead nuts simple to install. It is so much easier and quicker to install well than fiberglass in addition to having better rodent resistance, fire, moisture, insulating and sound deadening properties.

    1. Desert_Sasquatch | | #10

      I share your concern about installing too much moisture in a cavity. However hempcrete is installed somewhat wet too--I'd say about as wet as a wrung out sponge. From what I've seen, however, it's left to dry for some time (weeks, in the build I saw) before a finish is put on it. And of course that finish is usually pretty vapor open anyhow.

      I guess I'd have to experiment to know how long it takes to dry or harden, but that's what I've seen...

      You're right about Rockwool. It's there in the back of my mind. I don't love that it's coal fired but that's not my main concern. My primary concern is mice. I had read a thread on this website about it, suggesting that mice might like it just fine:

      Secondary to that, though, is my own subjective preference for more "monolithic" walls. :shrug:

      1. DC_Contrarian | | #11

        You're much better off building the house rodent-proof to begin with rather than trying to rodent-proof the insulation. With modern codes you have to build air-tight, if you put any thought in it's going to be rodent-proof too.

  5. Expert Member
    BILL WICHERS | | #14

    For best mold resistance, you want to use inorganic materials that can’t act as food for the mold. I don’t think hempwool qualifies here, since it’s plant based. Rockwool would be much better, or fiberglass, or any of the rigid foam materials. Mineral wool isn’t going to be any worse for mice than hempwool. I would actually think hempwool would likely be more desirable to mice since it’s a natural product. I have found mineral wool to be less of an issue with mice compared to fiberglass, especially if you use Dow Corning’s mineral wool, which I’ve found is denser and stiffer than some others out there. I agree with DC that you’re really best off sealing the structure against mice regardless of the insulating materials you decide on though.

    If you want to seal the wall materials out of the home, consider a layer of foil faced polyiso on the interior with the seams taped. The foil facer is typically a heavy weight aluminum foil, so it’s a hermetic (nothing gets through it) seal. You’d want to avoid smelly tapes, but I’ve found the foil Nashua 324A tape to not really smell after application, so it’s probably a pretty safe bet here.


    1. Desert_Sasquatch | | #15

      Thanks Bill, that's helpful to hear about the mice and mineral wool.

      I agree that hempwool itself would likely be even more attractive to mice than, say, fiberglass. My hope, however, was that by coating it in lime it would become much less attractive. But as I said, I truly don't know what happens once the lime settles into limestone and it stops burning their little paws.

      I have concerns about fiberglass regarding moisture intrusion. If everything works like it's supposed to then it's fine. But things don't always work like they're supposed to. Roofs leak, or get damaged in a storm. Caulk fails. Water lines break. And my understanding is that once fiberglass gets wet, it's kind of worthless--it doesn't regain it's former insulation value once it dries, and in some cases the heat loss that results can cause further water damage (for instance by causing a water dam on the roof). Plus fiberglass can slump, leading to the same problems I just mentioned.

      Mineral Wool, of course, fixes pretty much all of those problems. And I may go with it.

      My concern with mice is similar--though I admit possibly overstated. I can seal up the walls perfectly but if a mouse wants in, it can probably chew through whatever wallboard I use. I'll be renting out part of this home and I can imagine some worst-case scenarios where mice get a taste for living in the home and find a way into the wall.

      1. Expert Member
        BILL WICHERS | | #16

        Fiberglass is insulation again after it dries out. The only time you might have problems is if the batt got saturated -- completely soaked -- and pulled out of place in the wall cavity. As long as the fiberglass batt holds it's shape, it will regain it's full R value after it dries out. I do prefer mineral wool myself though, partly because I find it easier to install well.

        If you are really worried about mice, put 1/4" mesh hardware cloth, which is like superman's window screen, in the structure to act as a barrier. Hardware cloth is steel, and it's chew proof.


        1. Desert_Sasquatch | | #18

          Thanks Bill, I'll look into that.

  6. Expert Member


    Back to your question! You left out a few details that we should have asked earlier. About where are you located? Advice on here is very climate specific, and we’ll be able to discuss more in the right frame.

    Personally, for the options you listed, I think AAC would be the best way to go if you’re into a more monolithic like approach. It’s light, masons won’t shy away from working with it, has pretty good properties if you’re not in a seismic / tornado / hurricane zone, and is becoming more available all the time.

    An AAC cavity wall with insulation, like you see in all the britt’s brick laying videos, can make for a nice structure, and shouldn’t be any more bothersome to build than a normal brick house.

    1. Desert_Sasquatch | | #19

      I'm in climate zone 6B. Cold, but generally dry.

      I did consider AAC but decided against it because it couldn't deliver the R value I was hoping for (R-25 to R-30 or so). However I haven't run across an AAC cavity wall, that does sound interesting. Any chance you could link me to an example of what you're talking about? Is it just using hollow AAC blocks, sort of like cinder blocks? Or is it just two sets of AAC walls with a gap between for insulation?

      1. Expert Member
        KYLE WINSTON BENTLEY | | #20

        I would think of it in an alternative way.

        AAC block is a light weight structural alternative to ordinary CMU. The fact that it has insulation properties is a nice bonus over CMU. Use insulation for insulation, not AAC. From inside to outside, without any interior finish, You’d have what is probably an 8 x 8 x 24” block wall, air/water barrier, insulation, air gap, and brick or other finish. Like this image, but with AAC block instead of CMU.

        Its got plenty of strength, don’t let the
        Relatively low compressive psi value scare you off when you compare it to concrete block. There’s a ton of area there, compared to just the web/face area on a CMU.

        The bonus is that you can use a relatively low density insulation here, if you use a self supporting facade like brick. That usually means it’s cheaper.

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