How to build a house of hemp

When you take the magic in­gre­di­ent out of mar­i­juana, you get a build­ing prod­uct to keep you warm, cosy and very happy.


When you take the magic in­gre­di­ent out of mar­i­juana, you get a build­ing prod­uct to keep you warm, cosy and very happy.

Ahawk banks lazily in the wind over the top of Kāhu Glen. They of­ten float their way over Matt Low and Melissa Burleigh’s block, drift­ing on the winds which gust in off the Taranaki coast­line just a few kilo­me­tres away. Far be­low, Matt and Melissa aren’t too both­ered. They tend to en­joy mostly wind-free days, tucked in be­hind the com­mer­cial shel­ter­belts they in­her­ited when they bought a for­mer fei­joa or­chard.

“We can sit here when it’s windy in a t-shirt and shorts and then go into town and you need a jumper,” says Matt. “The shel­ter makes such a huge dif­fer­ence.”

Their feathered vis­i­tors were the in­spi­ra­tion for nam­ing heir block Kāhu Glen. Kāhu is the Māori word for the hawk (tech­ni­cally the har­rier hawk, Cir­cus gouldi), while ‘glen’ is Scot­tish for val­ley, a nod to Melissa’s her­itage.

In their own wee glen, tucked away from the pry­ing eyes of neigh­bours, is a per­ma­cul­ture-de­signed gar­den and an or­ganic or­chard that pro­duces around nine tonnes of fei­joa a year which the cou­ple have pro­cessed into juice for sale. They can see the fruits of their labour out the curtain-free win­dows of their un­usual home that sits at the heart of their land and which was in­spired by a man named Lance.

“Lance (Palmer) owned a build­ing com­pany,” says Melissa. “He’d done a lot with build­ings and knows

the Build­ing Code in­side out. He did a hemp and so­lar tech­nol­ogy work­shop and he was quite sold on the ideas and he went into re­search­ing it quite a lot.”

It was Lance who con­vinced the cou­ple they could be owner-builders of a hemp house which would be­come only the sec­ond to be given con­sent in NZ.

“He said ‘you’ve got an engi­neer­ing back­ground, you’re prac­ti­cal, you’re more than ca­pa­ble’,” says Matt, who works as a civil en­gi­neer. “With (Lance) we got to share a lot of the cost of im­port­ing the hemp in, we worked to­gether… he’s been a tremen­dous help right through the process.”

When you’re one of the pi­o­neers of a hemp home, you have two jobs: to build your house, but also to con­vince the lo­cal coun­cil that hemp is a great idea. Step up Lance.

“He built a lit­tle pro­to­type, 600mm long and 400-500mm high and had the fram­ing, had it all made up and par­tially plas­tered,” says Matt. “He took it into coun­cil and said ‘this is what I’m do­ing’.

“When he was par­tially through (his build), he got all the lo­cal build­ing in­spec­tors out to his place for a cou­ple of hours, took them all through it to show how it was work­ing, be­cause no-one was ex­pe­ri­enced either from their side.”

A new kind of build­ing tech­nol­ogy is al­ways a chal­lenge for a coun­cil, which has to in­ter­pret the Build­ing Code.

“I think there was a lot of in­ter­nal dis­cus­sion (at coun­cil) about what was the li­a­bil­ity and risk they were tak­ing on?

It was Lance who con­vinced the cou­ple they could be owner-builders of a hemp house.

But they de­cided to give it a go, and they cer­tainly helped out a lot through the process.

“I think the big­gest thing for them was the weather tight­ness. I wrote pages on flash­ing de­tails and ex­ter­nal join­ery, there were quite a few re­vi­sions.”

A stan­dard tim­ber-framed house will have a cav­ity, a small air gap be­tween the frame and the outer cladding, to keep mois­ture out. There is no cav­ity with a thick, solid hemp wall, so Lance and Matt turned to the tech­ni­cal stan­dards for rammed earth homes and fol­lowed many of their spec­i­fi­ca­tions to re­as­sure coun­cil it would work.

“Be­cause it’s new tech­nol­ogy, the coun­cil staff of­ten didn’t re­ally know what they were look­ing for be­cause there’s no pre-line in­spec­tion. The tra­di­tional in­spec­tion list was mod­i­fied a bit to suit! We ended up with one in­spec­tor who saw most of it through to the three-quar­ter mark, and we got him back to do the fi­nal in­spec­tion.”

Hemp may look like the kind of thing you dry and smoke, but it’s a par­tic­u­lar va­ri­ety of Cannabis sativa and it’s miss­ing the magic in­gre­di­ent. In NZ, it must con­tain no or be­low 0.35% tetrahy­dro­cannabi­nol ( THC), which is what gives mar­i­juana its psy­choac­tive ef­fects.

It grows fast in New Zealand, yield­ing around 10 tonnes in just 126 days, but the fi­bre that was the build­ing block of Matt and Melissa’s house came all the way from Europe.

“Lo­cal peo­ple are grow­ing it and we did look at it,” says Melissa. “But they don’t have any of the equip­ment to process it here so it ended up be­ing im­ported from the Nether­lands.”

The hemp stalk has a hard fi­bre on the out­side that needs to be stripped off. The ma­te­rial in their walls is the soft cel­lu­lar cen­tre which is bro­ken down into a very fine chip, then com­pressed into 800cm x 300cm x 400cm bales and wrapped in plas­tic for trans­port.

Once on the build­ing site, Matt and Melissa cre­ated hempcrete. The hemp was mixed with a lime binder (lime and wa­ter) in a pad­dle mixer un­til it was a por­ridge con­sis­tency. Once it holds to­gether in a ball, it’s ready to go.

It was Lance who came up with a method to make things easy for the first-time builders, cre­at­ing a shut­ter sys­tem

that could be eas­ily lifted into place as each wall was cre­ated.

“The house struc­ture is all tim­ber, stan­dard tim­ber walls and fram­ing,” says Matt. “When you’re build­ing you put the two shut­ter boards down either side of it. We had the frame in the mid­dle, then put a shut­ter 100mm on either side so (the wall) was 300mm thick. Then you ba­si­cally empty your hemp mix into wheel­bar­rows and tip it into the shut­ters.”

Once a sec­tion was full, Matt and Melissa used blocks to hand ram it into 80-100mm thick lay­ers. Then they’d lift the shut­ters and re­peat the process to build the wall up.

Or­der­ing the right amount of hemp was tricky, and Lance was on it.

“We didn’t re­ally know our­selves how much we’d need,” says Matt. “But Lance did a lit­tle trial 3m x 3m shed which be­came a chicken shed. He started play­ing with mixes, worked out quan­ti­ties and ra­tios, whether one was too dry and be­cause it was too dry, it was too brit­tle. He worked it all out.”

Their north-fac­ing house fol­lows pas­sive so­lar prin­ci­ples, its insulated con­crete floors gath­er­ing in the sun­light in win­ter.

“It rarely gets be­low 16-17°C even on the cold­est day in win­ter,” says Melissa. “Then you light the fire and it’s up in the 20s re­ally quickly.

Open­ing a win­dow in this house is a Euro­pean af­fair, with each one tilt­ing and tip­ping in­wards, giv­ing a 50-80mm gap so they are still se­cure but there is plenty of cross ven­ti­la­tion, with air eas­ily flow­ing through the house.

“All the win­dows were made of west­ern red cedar,” says Melissa. “It’s a Euro­pean de­sign be­cause we couldn’t have alu­minium next to the hemp/lime mix – it cor­rodes it – and we didn’t want alu­minium any­way. It wasn’t the look what we wanted to achieve.”

The hemp walls are cov­ered with a lime plas­ter in­side and out, ex­cept for one in­door ‘win­dow of truth’ (pic­tured, right) to show vis­i­tors what the hemp looks like.

“It’s quite a breath­able build­ing ma­te­rial, so it reg­u­lates a bit with the sea­sons,” says Matt. “Mois­ture and hu­mid­ity closely con­trol them­selves – you haven’t got a hard cav­ity sys­tem so there’s no sud­den change.”

The outer walls have an adobe fin­ish – “enough so you can hide your im­per­fec­tions as a plas­terer,” says Matt – with a colour mixed into the plas­ter so they didn’t have to paint it. It was then coated with a wa­ter re­pel­lent prod­uct, al­low­ing it to main­tain the abil­ity to ‘breathe’ but rain­wa­ter ‘beads’ on the sur­face and then runs off.

“You cer­tainly can hose it down, you can use a brush that’s at­tached to a hose and scrub it as you go,” says Matt. “It’s hard­ened off and it’s quite a durable ma­te­rial but I think if you sat there with a wa­ter blaster for too long you’d get a hole.”

This very warm house doesn’t cost much to heat other than fire­wood. The monthly power bill is about $90.

“We have a 5kw PV (pho­to­voltaic or so­lar panel) on the roof of the house, but we’re also grid-tied (able to draw power from the grid),” says Melissa. “We looked at bat­ter­ies and go­ing off the grid at the start but we didn’t know where to put the PVS.

“We de­cided to go grid-tied be­cause the tech­nol­ogy just wasn’t there at that stage, and you still have to re­place (bat­ter­ies) ev­ery eight years. You times that by two and the cost of get­ting the ca­ble to the road made more eco­nomic sense.”

They use the so­lar pan­els for power and to heat their hot wa­ter. If there’s any ex­cess, it goes back into the grid.

“Ba­si­cally, we don’t have to pay for hot wa­ter be­cause it’s gen­er­ally done by so­lar.”

Who: Matt Low, Melissa Burleigh-low & son James (4) Where: New Ply­mouth Land: 4ha (10 acres) What: hemp house, per­ma­cul­ture gar­den, com­post­ing toi­let, Biogro-cer­ti­fied or­ganic fei­joa or­chard The hemp plant looks like its close rel­a­tive, cannabis.

CLOCKWISE FROM LEFT: Matt, James and Melissa in the gar­den, stand­ing on the end re­sult of their com­post­ing loo (see thisn­ for more de­tails); the un­usual win­dows tilt rather than open; James spent his first two years liv­ing in a shed with his par­ents while they built their dream home.

Clockwise from top left: James sit­ting in a bale of hemp; mix­ing the hemp, lime and wa­ter to cre­ate hempcrete; the shut­ter sys­tem used old scaf­fold­ing pipe; plac­ing the shut­ters to build the wall; James with a wheel­bar­row of hempcrete ready to go.

Above: the ‘hempcrete’ is a mix of hemp, lime and wa­ter, and is com­pacted into 100mm high lay­ers. Right: the shut­ter sys­tem was cre­ated by builder Lance Palmer to help make cre­at­ing the walls eas­ier.

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