Hemp Concrete Walls (R30 + Fireproof) – You Wont Believe How They Built This House!

Hemp Concrete Walls (R30 + Fireproof) – You Won’t Believe How They Built This House!

Foreword by Ian Thompson, Editor

Imagine a building material that not only provides superior insulation but also plays a positive role in the fight against climate change. This is the story of hempcrete, a carbon-negative material that absorbs more CO2 during its lifecycle than it emits. It’s a compelling tale for any new house builder looking to leave a lighter footprint on our planet.

As you embark on the journey of building a new home with hempcrete, you’ll find that it offers more than just environmental benefits. The home you build will be a haven of comfort, thanks to hempcrete’s excellent insulating properties, reducing the need for artificial heating and cooling. As the seasons change outside, the indoor climate remains stable and comfortable, potentially saving significant amounts on energy bills.

The story doesn’t stop there. In a hempcrete home, the air is cleaner, as hempcrete naturally filters out pollutants and regulates humidity levels. There’s an added feeling of security too. Hempcrete is naturally fire-resistant and unappealing to pests. And it’s durable, standing up to mould and humidity, reducing maintenance over the years.

But every story has its challenges, and the hempcrete tale is no exception. The journey to a finished hempcrete home is often much longer, as the material requires more drying time compared to traditional concrete. This means construction timelines can be extended, increasing labour costs along the way.

Importantly, a hempcrete house will still require a backbone of another material, typically wood or steel. This requirement for additional structural support increases the complexity and cost of the project.

Then there’s the challenge of sourcing the necessary materials. Depending on your location, finding hemp and lime can be like embarking on a quest, possibly increasing material costs and adding another layer of complexity to the logistics.

Lastly, like any hero in a new land, hempcrete faces the challenge of unfamiliarity. Many builders and inspectors are yet to learn its unique properties, which can lead to uncertainties in the construction and approval processes, and we know how troublesome dealing with inexperienced building consents and building inspectors can be.

So, the hempcrete story is one of balance. It’s about weighing the compelling benefits of sustainability, energy efficiency, and health against the challenges of extended construction timelines, added structural requirements, cost, and unfamiliarity in the industry. It’s a tale of innovation and potential, with the next chapters yet to be written by those bold enough to embrace this sustainable building material.

Over to Matt.

Hemp Concrete Walls (R30 + Fireproof) – You Won’t Believe How They Built This House!

Hemp construction! Hemp has been used in various forms of construction dating back to the Romans, in bridges, sails, ropes, and now more recently in residential construction. Reintroduced first in Europe, Hemp construction takes form as a low weight, high insulating, vapor permeable wall component. Placed usually in one of three ways: cast in place, sprayed, or by block. Hempcrete utilizes the core of hemp plants in a woodchip like form as the aggregate, combined with natural binders, and water to achieve the finished product. We were fortunate to visit a site in central Texas where they are using a hempcrete in the wall assembly, to surround a roughly 400 year old Japanese timber frame structure. Huge thanks to Mattie Mead, who founded Hempitecture, an Idaho based company bringing hempcrete and hemp wool insulation to the American market, for taking time to show us this project. Hemp has gotten a bad wrap over the last century thanks to its medicinal cousin. But from the sound of it, we should be seeing more and more applications of hemp here in the states, as it is a bomber agricultural commodity with so many applications.

Project – Central Texas

Mattie Mead – Hempitecture

Chad Burnel – EIM inc. – Builder

Axel Vervoodt – Architect

Mell Lawrence – Local Architect

Seth Willison – Timber Frame Designer

Video Transcript:

Guys, this building behind me is absolutely beautiful. But what we’re here to talk about today is this crazy wall section. It’s like fluffy marijuana concrete. It’s actually called hempcrete, and you’re going to see the entire process, guys, from mixing it to forming it to pouring on today’s episode. This is really going to be a fun one. Today’s build show is all about hemp concrete. Let’s get going.

Guys, I’m coming to you from a beautiful hill country job site here where we’ve got a building that is absolutely drop-dead gorgeous under construction. Japanese-inspired architecture, and in fact, the timber frame, the skeleton in here, is timber framing from Japan. The wood itself is three or four hundred years old. We’re actually not sure how old it is. But let’s first start talking about this right here, which I think is super interesting. This is hempcrete, and I actually have the main man here, Maddie Matty with Hemp Attention. You’re the contractor on the hempcrete, but you’re also kind of the North American hempcrete guy, right? And you’re actually selling these materials to other builders who are building with it. Tell me about this. What is this?

Yes, what we’re looking at here is a hempcrete wall system. It is a combination of industrial hemp, the wooden core of it, and a specialized blend of limestone. Let me break that down. This is the bag of the hemp, and that’s what this looks like. It’s kind of like wood fiber, like I was about to make OSB or particle board or something, right? It looks just like wood chips, Matt. And then this is a, for lack of a better term, a cement, right? A binder. It is a binder, and it has a component of natural cement and limestone in it.

Okay, and you’re mixing that in a big industrial mixer on the job site, maybe two yards at a time, and then pouring it into a form to get this. It almost looks like rammed earth or some other type of built-up product, is that right?

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Yeah, that’s right. And really, the first thing we do when we mix these together is we add water. We mix it with water. It’s a very controlled amount of water. We bring that over from the mixing station to our wall, and we tamp it in forms, which is why it has that rammed earth aesthetic.

But this is not a structure wall, right? There’s actually some structure embedded in this wall, is that right?

That’s right. As of now, it’s treated as a non-structural wall infill material that’s also highly insulating. So this is our insulation, this is our sheetrock. It’s multiple materials in one.

Wow, okay. So this particular wall section here, Maddie, we talked about this earlier, this is like 10 inches thick, right? Inches thick, and in the center of that is a 2×4 framing, which is the, for lack of a better term, the bones for the building. It’s the structure. And then the hempcrete that you’ve made on-site, that you kind of mixed on-site, is the muscles and the fat and all the other tissue that makes up the body of the building. And then, will this be left exposed on the outside?

This will not be left exposed. It’s going to get covered with a vapor-permeable lime-based plaster, usually in two to three coats, and that’s going to give this product a long-lasting life. Okay, so almost like a stucco finish, right? Not a non-painted stucco-looking finish on the outside.

Lime plaster can be used inside or outside. One cool thing about this architecture: you’ve got overhangs everywhere. Um, but what are the big properties? What do people like about this hempcrete? What compels us to use this?

Well, first off, from a building perspective, it’s really insulating, about R-2.5 to R-3 per inch depending on your mixture.

Nice, that’s pretty good. So we’ve got an R-30 wall right here. We’ve got an R-30 wall. That’s a big deal. The second thing is an environmental reason. Industrial hemp, when it’s growing and harvested on a two to three-month cycle, absorbs carbon dioxide from the atmosphere and stores it into the hemp stock. We combine it with our binder, and a reaction occurs that also sequesters carbon dioxide from the air and stores it in the wall. So we’ve got a carbon-sequestering, thick insulated wall.

Isn’t there some fire benefits to this as well?

That’s right. The fire benefits of hempcrete are that it’s 100% fireproof. It will not burn. We could take a torch to this. We could try all day long, and it will not catch on fire.

Dang, that’s pretty cool. There are a lot of benefits to using this.

No, I made kind of the not inflammatory, but the bold statement that this was a fluffy marijuana wall. It’s not really marijuana. For those of you who are going to comment about that, let’s preempt that. What’s the relationship between industrial hemp and marijuana?

To really simplify it, industrial hemp is sort of like the sober cousin of THC-producing cannabis. It is used as an agricultural commodity. It’s been used since the beginning of time to make ropes, sails that brought explorers to discover the new world. It’s used in all sorts of products and primarily industrial applications.

Are there companies today that are using industrial hemp in fabrication or manufacturing that you can think of?

Interestingly, in Europe, a lot of automotive companies are using hemp fiber in automotive panels. There are now companies in the United States that are making hemp wood replacements for conventional wood-based products. Even hemp bioplastics are becoming a new tangible idea. Wow, very interesting.

So this hemp, this industrial hemp, it’s a plant that grows in a field. You’re stripping the outside, and that core, that stock that, as I said earlier, kind of feels like a wood chip almost, is what we’re talking about here. This really has nothing to do with marijuana, except that it’s a cousin plant, so to speak. But there’s nothing here you can smoke or get high on or any of that kind of stuff. This is the sober cousin.

I like that term. Right, Maddie. Let’s transition to actually showing people how you form, and for lack of a better term, pour this. Okay, so we’ve got a fresh batch of hempcrete made right here. Let’s take a look at what the form boards look like.

Now, if you look at the outside of the finished wall, you can see there’s kind of a strata going on, almost like a rammed earth project, and this has some similarities. This is the form board right here. They’re basically doing this in two-foot lifts or two-foot sections. So we’ve got a poured section down below. Here, you’re going to notice the electrical conduits. We’ve already run that. The electrician’s been here. You’re also going to notice that the structure is inside the middle of the wall.

Now, most of the walls are framed just with two-bys and, in fact, two-by-fours on a lot of the walls. But this particular section of the wall is a shear wall section, and the engineer has designed a moment frame. This is basically just a U-shaped piece of tube steel that’s been welded together and bolted to the foundation, and that’s going to keep the shear value, the building from racking.

Now, what’s interesting about how they do the form boards, I think, is these form boards need to be released later, and so they want the minimum amount of attachment that’s going to hold it while it’s being formed but also be able to remove it without a lot of damage. Now they’re using kind of an interesting product. They’re using Fasten Master’s Timberlok screws. If you’re not familiar with these, we use these a lot in construction. You’re going to be able to replace a lag screw with this, but what these are doing is just giving us a temporary hold.

You’ll notice they’re putting a fender washer on the outside so they can get some good bite on this OSB plywood. This is three-quarters inch thick, so it’s nice and strong. And then to make sure it’s spaced off correctly, they’ve got a piece of PVC pipe here that’s cut the correct distance. I believe these walls are somewhere around 10 inches thick, although this one, I think, might be slightly thicker. And they’re running it from this post right here, which is a structural and also beautiful post that they’re going to express. And then on the outside here, this is going to be a door, so they just have a piece of OSB as the form there as well.

Now, the other thing that’s interesting is this won’t grab onto smooth steel. What I’m talking about is the hempcrete. So what the engineer did was they’ve got these bolts that are actually welded on as a tab, and that’s going to give that hempcrete something to actually bite onto.

Now, currently, the hempcrete is not given shear value for this project. The engineer is not relying on that for shear. But I think that in time, as we use this more in America, probably engineers are going to get more comfortable with it. They’ll be able to assign some shear value because as you touch and feel it.

It’s very hard, but it’s not as hard as a cement or concrete wall, let’s say. But it’s got a lot of, um, it feels very stiff. There seems to be a lot of mass on it. And then when these form boards get replaced eventually, on the inside and the outside, they’re going to put a lime-based plaster on here so that what you’re seeing here with the strata isn’t actually the finished look. Maddie, walk us through what you’re actually doing, putting the hempcrete into the forms.

Yeah, so we’ve got our hempcrete in the buckets, and what we need to do is first, we’re going to dump some in, but it’s a controlled amount. We don’t want to overpour. This is a very controlled process where we build in layers. So what I’m going to do is I’m going to put this bucket on the edge of the form board, carefully dump some in here. That’s step number one. Step number two is we rake it out level because we want to build up in even layers. After we rake it out level, we then grab our tamping tools. We take the long side of the tamping tool, looks like a little homemade tool right there, it’s a totally homemade nice man. And we take these, we put them right on the edge of the form board. We’re packing really firmly along the edge of the form board, but we’re leaving it looser in the wall. Okay, and you’re not like putting body weight on it. It’s more like a tamp. Yeah, it’s more of a tamp. You know, this is kind of a marathon. It’s, you don’t want to do something that’s so labor-intensive and, you know, you’re pounding it too hard. This is more of just, you’re stamping the edge.

So when we move through this, what you’ll see is a stamped edge along the form board, and it’s looser in the middle. Awesome, let’s do it, man. Show us what to do. And so you’ve got Sean on the outside and you on the inside so that you’re kind of even. Yeah, we’re working together on this wall because we have these steel posts, the shear posts in the way. A lot of times we’ll fully sheet one side and work from the other side. That’s kind of a more efficient way of doing it, but this is a little bit too far of a reach. So we’ve now got this material in the wall. I’m going to rake it out, spread it evenly. And you’re looking to go about two inches or so, is that what you told me? Two inches is about where we want to be. And you’re just using your hands to kind of level it out. There’s no special, uh, love it. It doesn’t need to be perfect. It doesn’t need to be perfectly level. Um, and in the lower portion of the form board, we can just stick this in and use this as a rake. So I’m going to start down here. We have a removable form board, so we want to have a good tamp edge. So I take the backside of my tool here, really press that in. And then I like to use the long side of the tool. I’m just going to work right along the edge of the form board. You can really start to see that stamped edge coming out. It’s one of the things that you notice when you walk up to these walls. Maddie, they’re so crisp, they’re really flat. Your formwork’s really, really nice. My assumption would be, however good or flat your formwork is, ultimately means however good and flat the wall is going to be, right? Absolutely, absolutely. Formboarding is really key to having a good, high-quality finished product. Yeah, after I tamp the edge there, and you can really see that stamped edge, sometimes I’ll just take my hand and just gently tap that. Yeah, you’re hand-tamping. Hand-tamping, and that’s the process. Right, then you’re ready for the next lift. Yeah, this definitely takes some time, doesn’t it? It sure does. If you’re not bothered with the random YouTube crew, how long would it take for you to do a two-foot form board section like this? Normally, for you and Deshaun, depending on the run, maybe 30 minutes, okay? If we’re really cruising, you know, 20 minutes. It’s not bad. What really determines how long this takes is how much is in the wall. If we have electrical conduit and things that we’re working on, we really need to be conscious of that, so we’re working around it really carefully. Makes sense. So now you can see that when I dump this in, it’s kind of gathered to the back. What I want to do is just rake, pull it down the wall, okay, leveling it out. And level being a relative term. Yeah, it doesn’t need to be perfect level.

And I’m also tamping pretty firmly over here because this green form board is temporary. It’ll be pulled off, and we’ll actually see the hemcrete return into our window buck, right here, actually our door buck, I should say. And there’s no release agent on these forms, right? You didn’t have to put wax or anything else like you would if it was a concrete form, correct? We can just pull the screws off, and this form will come right off.

People often ask how long until you’re ready to move the forms. You can remove the forms as quickly as you can form it. Generally, we like to always stack another form board on top. You started level, you’re going to continue exactly. We just continue leveling up the wall, and this is basically just a half rip of plywood, right? It’s a two-foot sheet ripped in half. Yeah, so we’re using three-quarter inch OSB, ripping it red in half. But on sides that are less difficult to work around, we’ll just take four by eight sheets. We don’t even need to rip them down, and we’ll just go all the way up those walls over there. We’re completely formed on the inside, so we were all on the inside, and so those big long runs, you’re able to get a four-foot section in there. As long as you can reach down in the middle of the form, absolutely. Cool.

Now, what are those white wood boards in there doing? That was a joke. That was a joke.

Guys, so cool to see how Maddie and the crew from Happy Texture formed and poured this. It is a beautiful cake that you baked here, my friend. But the rest of the architecture here is pretty stunning as well. What’s going on with this house? Give us the backstory here, Maddie.

The backstory is that this is a minka house, a post and beam from Japan, about 350 maybe 400 years old, shipped over in parts and pieces and reassembled by a team of Japanese carpenters. Damn, these guys didn’t have any electric saws. They used mallets and hand saws. So all the timber we’re seeing inside, that’s like 400-year-old Japanese timber that some Japanese carpenters came in and assembled here on this Texas site.

That’s right. Now it’s got a new life. Wow. Now, who’s the designer here?

The designer is Axel Vervoordt, a Belgian-based designer, and he worked in collaboration with Mood Architecture, a firm out of Belgium, and then also a local architect, Mel Lawrence Architects, here in Austin, Texas. Mel’s a fantastic architect. And then who’s the builder here on site?

The builder is Chad Burnell with Earth in Motion. Gotcha.

Man, beautiful. Are these cedar that you’re seeing on this porch? That’s, we see that all the time. In fact, we’re among cedar trees. All those trees just to the left are all cedars. Did you tell me earlier those are all locally sourced? They were all sourced from the property that we’re standing on right now?

They went out and found trees with the right caliper size. They had an industrial pressure washer pressure washed the bark off of them, so it is all from the property. So all those posts and all the rafters are straight from this ranch that’s right here. Man, that’s really cool. What a story.

Tell me about a couple of details as I’m walking around this house. The first thing I thought was, how do you install a window and door on this? You’re probably not going to take a screw into the hemcrete, right? That’s right. That’s right, yeah. So with our frame being inset to the wall, or on center as we call it, we’re going to use a flangeless-style window that’s going to be basically set into the window frame, backer rod, and sealed around it. And the lime plaster is going to return right into the window, so it’ll be a real clean, nice-looking aesthetic.

Gotcha, that makes sense. So you’re going to use, like, a Schuco European window, flangeless, and we’re going to screw right through that jamb into that 2×4, which is right in the center. Which, by the way, means we’ve eliminated all our thermal bridging. This is going to be a really high-performance house. It also has some mass to it as well, so that thermal mass is going to want to stay the same temperature as well as have a bunch of air pockets in there, which are making that high R-value for the walls.

How do you insulate the roof on this project, Matty? This roof is rather unique. There are two layers of our product called hemp wool. It’s a fiber bat insulation made from plant fiber, from industrial hemp. Okay, 92% of the overall content of it is plant fiber. There are two layers of that to reduce thermal bridging, which is our R-40 right there, and there are two layers of Gutex, which is a wood fiber board from Germany.

I saw that when I was in Germany at the Bau Show. That’s not that’s totally almost unheard of here in America, but pretty common when I travel through Switzerland and Germany. It’s an amazing product. And yeah, on top of that, you’ve got this gorgeous cedar shingle roof, too, or cedar shake roof, I should say. I mean, it’s going to be an incredible architecture when it’s all done.

Tell me about what else you can do from hemp, right? Because you mentioned you’ve got hemp insulation that’s more like a traditional bat insulation. Are there other ways that you could use hempcrete? Yeah, so really the easiest way to incorporate plant-based building materials into your home is probably using hemp wool. But there are other products. We can spray-apply hempcrete. We source hempcrete building blocks from other countries that are making them on a large scale, so we have these different options here in the United States, so we can now start using them.

Alright, Maddie, you’ve got a couple of mock-ups on-site. Let’s go check out those mock-ups. All right, Maddie, so you’ve got a couple of mock-ups that you built before construction started. Let’s talk through these three. What is this first mock-up? This first mock-up is using hempcrete building blocks. They’re mortared together. This is actually a two-foot-thick wall assembly. Damn, a lot of R-values. And how big is this block that we’re seeing here that’s made out of hemp? The block is about two feet by one foot. And what are you using to mortar them together? We’re actually using a hempcrete binder, the same stuff that we’re using in our cast-in-place recipe. We’re using it to mortar the two blocks together.

Gotcha. And what is this that we’re seeing here as the final finish? This kind of stucco-looking product? Yeah, so this is a breathable lime-based plaster that’s usually applied in two to three layers. So this is a weather-resistant coating that will last for the life of the building. Wow, beautiful and non-painted. It’s all color integral, I’m assuming, right? Yeah, actually, you incorporate a little bit of pigment into it, and so you can see we do have some kind of color swatches on this wall where we’ve got different pigments incorporated into that final coat so that it’s tinted however the client desires.

Now, this is not what you built with, though. This is just an option. And this mock-up is actually pretty similar to what you built with, or is the same wall assembly as what you used, right? This is very much so what we are doing right next door in the Minka. Yep, so we’ve got a 2×4 right in the center, you can see in front and behind that we’ve got a couple of inches of hempcrete. So those 2x4s are buried in the wall, so to speak. But because that hempcrete is, for, I hate the term breathable, but it’s the term that we use, it’s vapor-open, meaning if it gets wet, it can dry to the inside or the outside, which really means that you could use this in any climate zone. That’s right, that’s right. And one of the amazing things about this is that this has been sitting outside, unprotected, uncovered for over a year. Oh, wow. And it’s not breaking down. It’s… You would think that there might be some degradation of the fiber or maybe even decay, right, of the hemp, and that’s not… That’s not happening. And what we’re also not seeing, which is huge, is no mold. Oh, that’s right. And it’s sitting out, too, and there’s no mold and there’s no decay. Why is that? That’s a really good question, Matt. And the short answer to kind of a long question is the fiber itself, the core, is petrified. It is essentially made inert by the process of being mixed together with our binder, which has a high alkalinity, making it resistant to mold. And that organic aggregate becomes a lot less like plant matter and a lot more like a stone. And that’s also why you guys are wearing PPE, right? You’re not touching that with your bare hands. You’ve got long sleeves on because that lime, if that gets on your skin, is really going to irritate you. It could burn your skin, right? That’s right. And the other very critical element of that equation is, you know, we’re always making sure we have face protection on. We’re protecting our lungs, our nose, mouth while we’re mixing hempcrete.

What’s the last mock-up? So this last mock-up is a little bit of a different sort of style where we have a frame shift to one side. It’s using a different kind of binder and hemp. These mock-ups did kind of play with different types of binder methods, but this was primarily done just to see what hempcrete would look like flashing out to studs. Because we do have some partition walls inside where we’re just infilling in between the partition walls as opposed to burying that frame on center to our wall. But if you wanted to vary the way you did this on the outside, your full hempcrete that has plaster on it, right, and that could be your exterior finish. And on the inside, if you didn’t want that, you could hang drywall, you could do other wood finishes or whatever and have your studs to the inside of the building, which also would be a good assembly. And this is kind of interesting look, you’ve got a little mold growing on your wood, but there’s not that growth on the hempcrete as well.

You know, this does bring up one interesting question. I saw in the building all the way around that your bottom plate, that 2×4 treated bottom plate, is wrapped with a, you know, a vapor barrier, for lack of a better term, a plastic asphaltic flashing. Tell me about that. Why is that? Yeah, that’s a really good question, Matt. What we don’t want to have happen is moisture to migrate up through the concrete into the hempcrete wall. So it’s essentially our damp proof course that’s going to be providing a barrier between our concrete and the hempcrete above, right? So you wouldn’t just pour a slab and then go hempcrete right above that. You always want something to stop that rising damp. That’s right, and be a capillary break as well. And then also, that bottom plate’s giving you the structure to land on. And then I think you told me earlier where you’re seeing that protector wrap on the outside of this other building, you’re gonna put a lath over top of that, and then the plaster will just cover that. You won’t see that. It’ll be invisible.

Maddie, impressive, man. Your crew, top-notch. The architecture here, beautiful. The builder’s doing a great job. So fun to meet you, man. How can people get a hold of you if they’re interested in any of these products or doing hempcrete on their project? The first place to start is to visit our website, www.hempitecture.com. We’ve got a lot of information, a lot of resources there. We’ve got YouTube videos where people can learn and see, “Is this right for me? Is this something that I want to explore?” And feel free to send us a message through our website. We’d be happy to get back to you and learn more about your project and how we can help you out. Yeah, and also, I’ll tell you guys, Maddie’s got an awesome Instagram feed with some gorgeous pictures of this job site that I stalked. That’s one of the reasons why I got a chance to get out here. Just to clarify, this is not a sponsored video. We’ve not exchanged any money. I just thought this was really cool. So guys, go check out Maddie’s site. If you’re not currently a subscriber to the Build Show, though, we publish every Tuesday and every Friday. And oh, by the way, on Build Show Network, we have five new videos a week, so go check that out as well. Follow me on Twitter, Instagram. Otherwise, we’ll see you next time on the Build Show.

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