At first thought, building a house out of hemp sounds a bit preposterous. Crazy. Like something a pot smoking hippie on Maui would concoct, a DIY duct tape project on instructables.com. But not only is it possible, it’s very impressive.
|A cottage built of hemp. The first hemp building on Maui and in the state of Hawaii.|
My first exposure to hemp as a building material was at a Hawaii Farmers Union United (HFUU) potluck. At the meeting, we passed around a grayish square block of hempcrete. The hempcrete looked porous and felt lightweight yet had some substance.
HFUU has been very active in lobbying Hawaii’s legislature to legalize the growing of industrial hemp. In August 2014, HFUU organized a Hemp is Hope workshop, inviting county officials and the public to learn about the value of hemp as an agricultural product. I had heard of hemp clothing before, but hemp for houses was a new frontier.
What is hempcrete?
Hemp fibers and stalks + lime plaster to simulate concrete.
Hemp architecture didn’t sound weirder than any of the other alternative construction I had encountered: straw bale houses, cob, or earth ships made using old tires.
Still yet, I had to see it to believe it. Last year during Hemp History Week, the HFUU, IHempHI, and the Maui Hemp Institute for Research and Innovation organized tours to the Hemp Ohana in Sugar Beach, Kihei.
|Architect George Rixey in front of the Hemp Ohana.|
Many lots on Maui are zoned for two buildings: a main house and an ohana. In Hawaii, an “ohana” (pronounced “oh-hah-nah” with the last syllable being stressed), is a smaller dwelling, intended for parents or other relatives, while the main house is for the nuclear family. Nowadays, landlords often rent the ohana to anyone with enough money or rent the main house and live in the ohana. Typically, the owner will build the ohana first (and learn from their mistakes!) and live in it while constructing the bigger house.
For retired NBA coach Don Nelson and his wife Joy, who hired architect George Rixey to design and build a hemp house, it made sense to start with the ohana.
|Rixey mixes hempcrete, hemp stalks + lime binder.|
But the County of Maui is not known for rubber stamping building plans. Some owners find it frustrating getting their permits approved. (What a surprise!) Since the International Building Code is used on Maui, Rixey had to demonstrate to the County that hempcrete would be structurally sound. They gave him two years to experiment with the materials. Also, they would check the frame and construction.
|Photos clockwise from upper left: George Rixey, the square form, the mixer (I think it's a mixer), and a hempcrete sample.|
According to Rixey, the benefits of hempcrete include:
- Termite resistance
- Water resistance (even if not fully hardened) – if you hit it with a hose, it can tolerate some water.
- Mildew resistance
- Fire resistance – you can take a blow torch to it
- Excellent insulation properties – R20 insulation at 8 inches. In one Chicago office, the walls are made of hemp, and they don’t require any heating.
- Strength – the hemp cells hold lime and bind to themselves, without a need for rebar or anything to hold them together.
- No off-gassing – so it’s healthier for the inhabitants
|Inside the hemp ohana.|
Wow, you had me at termite resistance. Anyone who lives in Hawaii long enough, knows the dreaded T word is TERMITES. Plus, hemp is better for the environment.
Hemp as a building material sounds almost too good to be true.
The main downside is that hemp is more expensive to build with than traditional methods. Rixey estimates that for a regular house, like a 700 square foot ohana, using hemp increased the cost of building by 15%. For a larger sized hemp house, with more square feet, Rixey estimates using hemp would increase the cost of building by less than 15% - since the percentage would diminish relative to the increase in square feet.
Part of the cost is that hemp is illegal to grow in most of the United States. So hemp has to be imported from elsewhere. Rixey said one can buy hemp most anywhere in the world except the US. The hemp he used came from Canada. (Those Canadians, they always want to live on Maui, so this is a way of sneaking a bit of Canada into our buildings.)
|Inside the hemp cottage.|
So how does one make hempcrete and what advice does Rixey have?
According to Rixey, one has to figure out one’s mix, but the general ratio is:
1 bag of lime to 1 bag of hemp stalks to 1 bucket of water
“You want very little moisture content in the mixture. You want to see the moisture but not have it sitting on the surface. One gets harder concrete when there’s less water. You want a dry consistency that feels barely moist when you touch it with your fingers. Even though lime is natural when it's that fine it can go into your pores and irritate you.”
The hempcrete mixture is packed into forms against boards of magnesium oxide. This is much harder than drywall. The walls do not need any rebar as the hemp binds itself. The binder is in all the cells. The hemp cells have little pockets that hold the lime.
Tips on compaction and packing it in:
One aims for a certain compaction but not too much, not too hard like a rock, and not too little or it will crumble. Pack the hempcrete with a certain density but not so it loses air. Fill it halfway or maybe 6-8 inches, then use a special tool to compact it.
Rixey suggests that when working with different people, there can be variation or inconsistency in packing, so one has to keep an eye on everyone’s work. The hempcrete will hold its shape within 30 minutes. One has to compact the mixture in shifts because if one packs too high, it won't compact the bottom. One wants to avoid having a helper who is overzealous and packing too much.
|Close up of the texture of the outside, exposed hempcrete walls at the Hemp Ohana. You can see the horizontal lines formed by new layers of hempcrete.|
What about rain or bad weather?
Rixey said they kept forms on for three days. He could feel dampness but no visible moisture. It could take rain. Even after a couple of hours, the hempcrete will hold its shape. But the building site is in Sugar Beach in Kihei, which is usually sunny and dry.
If it rained the first hour, this would not be good for hempcrete. In Haiku (an area of Maui known for wet weather, like Seattle and Portland), Rixey suggests leaving the forms up for maybe two weeks unless the construction is covered with tarps. In Kihei, he left the forms up for three days.
Also, in a wet area, one would worry about getting it wet in the beginning, before it’s fully hardened. In a place like Haiku, Rixey says to get the roof up first. Hempcrete will keep hardening for 6 months before reaching its optimal strength. It should harden for at least 3 months before receiving a finish like stucco or plaster.
What about wall thickness?
Rixey wanted the walls to be 8 inches thick. If the walls are thinner than that, there is not enough to hold it together. In UK, the walls usually go to 16 inches.
|A blurry photo of HFUU president Vince Mina holding some hempcrete.|
What about construction equipment?
In addition to standard tools, Rixey used a lot of paddle mixers and a pounder to pack in the hempcrete, and lots of trowels.
What about the structure inside or under the hempcrete?
Rixey said he had to install the electrical system before filling with hempcrete. His question was, “How can I build with the least amount of stuff on the outside walls?” He used soft flexible conduit which increased the cost or one could do a router afterwards but that would be labor intensive.
There's a wood frame under the hempcrete but he used metal cross bracing for lateral strength. For lateral bracing on the inside he selected magnesium oxide panel board which is harder and lighter than drywall. The inside form is this and two foot shifts on the outside. It's breathable and has the same structural strength as 1/2 inch plywood or lateral bracing. Rixey’s son is a structural engineer and did the research.
He thinks they didn't need cross bracing. For the next hemp project, the main house, he does not plan on using cross bracing. The big house will use concrete piers in the middle of the building for adequate shears for hurricanes.
What if holes are discovered in the hempcrete?
For small pukas (a puka – “pooh-kah” is a hole in Hawaiian) less than 1 inch, Rixey suggests just using stucco to fill in. For bigger holes, one can use hempcrete.
|Materials that Rixey used included Baumit multicontact, Epoca 800.|
What about sealing the hempcrete or paint afterwards?
The building we saw was exposed hempcrete which looked and felt like a hard sponge. Rixey says one can seal it with natural hydrated lime plaster and paint with a natural stucco paint.
Where can one get natural based lime products on Maui?
Rixey recommends RME building supply and says one can match the color. Kevin is particularly helpful there.
What other resources does Rixey recommend?
Artel Inc. for the finish, Epoca 800.
www.tradical.co.uk for a lime paint finish.
George Rixey was generous with his time, answering numerous questions about hemp and demonstrating making a hempcrete block. I even got my hands dirty packing some hempcrete into a block form. We walked inside and outside the cottage and got to touch exposed hemp walls.
Outside, we sampled hemp food products provided by Steve Rose, another enthusiastic hemp volunteer and director of the Maui Hemp Institute for Research and Innovation.
A year after visiting the Hemp Ohana, the Hemp House – the big house – has also been completed with a big celebration and fundraising dinner for the Maui Hemp Research Institute. Two “Hempsters of the Year” were awarded: Denise Key who is indeed a “key” volunteer for the hemp movement in Hawaii and spearheaded IHempHI and "Organic Cowboy" Doug Fine who recently published a book about hemp.
|Maui Hemp Institute for Research and Innovation Director Steve Rose for Hemp History Week last year.|
I didn’t attend this year’s Hemp History Week fundraising dinner. But I feel part of the celebration and part of hemp history, by visiting the hemp ohana last year and submitting my testimony to legalize industrial hemp cultivation in Hawaii the last two years. This year, there is really something to celebrate because SB2659, the hemp bill, passed! It was so close last year, but one key lawmaker held it up.
It’s gratifying that legislation has passed. Change can happen, even in a state that is more leisurely than most. I would have added George Rixey as a Hempster of the Year because it takes diligence and planning to build a house out of a nontraditional material, AND to get it approved by the County of Maui. As Maui Time puts it, “Hemp, Hemp, Hooray!”
|The Hemp History Week celebration in 2015 on Maui at Sugar Beach.|