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Bob Clayton – Hempcrete House, Tarpon Springs, FL. Summer 2012.

<Excerpts from Bob Clayton’s website>
We are building the first hempcrete house in Florida and one of a very few in America. Follow along as work progresses. We broke ground in mid-April and anticipate completion by November.

There are many hempcrete structures in Europe, but few in America. There are buildings in the south of France, but we found little information to compare with Florida’s climate with hurricanes and high humidity. We will be trying every feature of hempcrete houses we can.

Hempcrete is famous for breathability. People say it is very comfortable and healthy. The natural materials reduce the effects of allergies and illness from chemicals. No formaldehyde, latex, urethanes or VOCs. Certainly something we are looking forward to enjoying, but these are subjective issues too difficult to measure.

Our house attempts to be “hempcrete for the common man”.  Tarpon Springs enjoys a charming old world style with its strong ethnic Greek roots. We want to show that we can build a 21st century house that fits the traditional qualities of the city. Strong and solid with enduring value. The lime washes used to paint hempcrete give it an Old World charm.

The Site

A 50×100 residential lot in the center of town is the site for our house. It will be 1560SF, three bedrooms, two baths with a two-car garage. The site is near the Sponge Docks and Riverwalk tourist area, a block from thePinellas Trail, a 40-mile bicycle trail that circumnavigates the county.

The site is urban so the good insulating properties of hempcrete will be important. One advantage of the urban zoning is you can use more of the lot. Urban zoning is greener than suburban sprawl. You are within walking distance of city services. The City encourages closer buildings with intimate attachment to the sidewalks and porches for more neighborly interaction.

Pinellas County experiences hurricanes and Tarpon Springs received a direct hit from a Category 3 hurricane on October 25, 1921 bringing 115 mph winds. Our site is in Flood Zone A about 8 blocks south of the Anclote River and its sponge docks so an extra foot or so of fill will be added to the building footprint to meet FEMA A-zone flood insurance requirements.

Design Goals

Besides testing the use of hempcrete and the new frontier of high insulation that it offers, there were other goals in mind as we designed the house. We wanted to maintain green credentials as best we could, though some compromises were required.

Our walls will be 12″ thick for R30 insulation. The ceiling will also have 12″ of hempcrete insulation, but it is a lighter mix. This adds load to the roof trusses and the garage door header. There are special fabrication details, some of which we just have to learn as we go. It is difficult to go beyond R19 insulation with fibreglass batts. Where it is needed, a double wall technique is often used. So even though hempcrete appears expensive, for the insulation and renewability it provides, it is the more cost effective choice.

Our hope is that we will have a low electric bill thanks to the high insulation and the natural gas. Provision is made for a future solar array since solar prices are dropping. We can’t size the array until we have a year of electrical bills in hand.

Why use Hempcrete?

Hempcrete is carbon-negative. Cement production and use generates too much CO2 – the most of any man-made material. Lime production is a lower temperature process that produces much less CO2. The hemp plant absorbs carbon as it grows. The lime absorbs more CO2 as it mineralizes back to calcium carbonate. The result is a material that consumes more carbon than it produces.


Florida is a coastal state severely affected by sea level rise from global warming along a thousand miles of coastline. Florida spends $4-$5 million per mile to renourish its beaches lost to erosion. Renourishing must be repeated every 10-15 years. Carbon negative construction therefore is extremely important to Florida. Streets in Tarpon Springs are often only 6 feet above sea level as they are in most of the barrier islands, the Keys and many coastal areas of the state. Some global warming extremists anticipate several feet of sea level rise by the year 2100 – essentially the life span of this house.

Hempcrete has good insulating properties and fills voids permanently. Traditional insulating batts leave gaps and often create more as the structure ages. Because of this, an R19 wall with hempcrete insulates better than an R19-rated wall of traditional multi-layer construction, and that should remain true as the building ages. It can also make very thick insulation that loose batts cannot easily do. Our house will be R30 with 12″ thick walls. The lightweight nature allows it to be used as attic insulation too. Air conditioning costs are a big factor in Florida homeowners’ budgets. Any technology that can produce reliable R30 insulation is very valuable to Floridians. R30 homes are just a whole new world. The home building industry has not seen it and neither has the air conditioning industry.

Hempcrete also has good thermal mass. As the sun rises, the walls begin to heat up. As the sun sets, they release the heat back into the night sky. Thermal mass gives rise to thermal intertia: the walls cannot transmit heat into the living space until they themselves have heated up. A 12″ wall is large enough that the occupants would see little oscillation in internal temperature. With no air conditioning or heating, the room fluctuates little from the mean temperature throughout the day. Unfortunately in Florida, the summer mean is about 85° F. So cooling is still required, but it will cycle on less. Often we only get one or two months a year when A/C is not needed so the thermal mass feature can extend that a few months a year making a significant A/C saving.

Besides thermal insulation, hempcrete offers good acoustic insulation as well. This will be important in busy inner city neighborhoods. We want to test that property. We are leaving the inner walls lightly treated. Often hempcrete walls are “rocked” over with porous magnesia sheetrock. In traditional multi-layer wall construction, the bulk of the noise is blocked by the sheetrock, not the insulating batts. We want to know if rocking the walls is required here for good noise reduction so that is another part of the test. The hard exterior render on hempcrete should help block noise.

High insulation brings on secondary problems: trapping moisture or making good homes for insects and rodents. Blown-in roof insulation is rat heaven and hollow wall construction is home for palmetto bugs (designer cockroaches – a Florida specialty), ant nests (fortunately, fire ants stay in the soil. Bull ants and carpenter ants don’t.), lizard nests, wasp nests and sometimes bee hives. Hempcrete naturally avoids this so it has the potential to make very well insulated buildings with reduced pest problems. This hasn’t been done before and leads to potentially large energy savings in the future. Air conditioning costs are a significant fraction of the building operating costs in Florida. We want the insulation, but we don’t want the bugs and pests.

When the energy crises hit, we sealed our buildings up, but this brought a secondary problem: moisture accumulated and fed mold and mildew. Our internal environment became toxic as materials outgassed chemical vapors. Hempcrete seals well, but it is porous and breathes well. This breathability makes a comfortable house that controls humidity naturally. No condensation hence no mold and mildew, no clammy feeling. Cement block houses in Florida risk mildew growth from damp environments. It is a very serious problem – some houses are condemned for it. Unoccupied houses develop sick house syndrome so you are obliged to run the air conditioner when you are gone. Hempcrete’s ability to breathe and inhibit mold and mildew will be very valuable. Hempcrete is a natural material with no toxic vapors.

The lime in hempcrete is also a good fire retardant. A 6″ hempcrete panel wall section survived 73 minutes in a fire test. It did not support flames, but it charred and eventually lost structural integrity. A 12″ block wall survived 110 minutes. Actually the hempcrete survived. The lime mortar holding the blocks failed. In case of fire, you have time to shower and pack a suitcase. We had no issues with the building department. We submitted fire test reports from England as an example of anticipated performance.

Many of our modern homes can achieve deadly flashover fires in minutes. There was no evidence in the fire test reports of hempcrete causing or supporting flashover. This is largely an issue of your furnishings. As we said earlier, are you going to stuff your natural hempcrete home full of unnatural fabrics, foams and poly fill cushions? So another benefit of a natural life is reduced flashover.

Termites are a serious issue in Florida. Formosan termites, notorious for heavy destruction in New Orleans, have arrived in Florida. By age 35, most houses have dry termites in the roof trusses. Fortunately, they are slow. It is the wet termites coming out of the soil you must fear. Hempcrete does not support termites and by embedding all our framing in hempcrete we reduce the potential for termite damage. The upper portions of our roof trusses are exposed, but wet termites coming up from the soil will have nothing to feed them.

Hempcrete is recyclable. When the house is demolished the material can be ground up and spread on farmers’ fields. Concrete cannot do that. Landfill costs are a significant part of demolition costs. Eventually, landfills will become closed to construction debris. When that happens, construction must be recyclable. This is starting to happen in Europe. Old hempcrete can be used to make new hempcrete. You can recycle up to 10% of old material into your batch of new material. Cement cannot do that.

All of these features indicate hempcrete will be a valuable player in 21st century Florida, but we need to know how it performs in real-world Florida conditions: heat, humidity, tropical storms in an environment rich with mold, mildew and insects.

Hempcrete does not tolerate wet feet, but it tolerates rain well. It sits atop a 12″ cement foundation to allow drainage. When it rains, moisture penetrates and dries out after the rain. A spray test in Scotland showed that after 72 hours, water had penetrated a few inches. Florida gets substantial rains, tropical storms and hurricanes. It should be a good test for hempcrete performance.

Notes on Production


First-time users may approach the form-stuffing process with trepidation, but after stuffing a row 15-20 feet long, you will have resolved most of your fears and concerns. The process is pretty fault-tolerant. There are many tips that can help. First, resist the temptation to tamp the material too hard. The R-value of wood is 1.4 per inch of thickness and that for hempcrete is 2.5. The more you tamp your material, the closer it approximates solid wood and not porous hempcrete. You can turn your R30 wall into R18. You also lose porosity for the breathability, and you consume more material. Just fill the voids and tamp sufficiently to achieve fiber-to-fiber contact so the material bonds to itself. In places like corners you can tamp harder to achieve strength. Secondly, lay your material evenly about 8″ deep before tamping. Less, and you tamp too dense. Resist the temptation to begin tamping before this layer is fully done. Each of these layers leaves a striated appearance and you want them level, not wavy.

Wherever there are horizontal members such as windows, noggins or the top plate, stuffing will be difficult. There is high risk of leaving voids that aren’t revealed until you pull forms. Sometimes you just have to patch those gaps by hand after the form is removed. Hempcrete patches well.

A batch of hempcrete remains workable for over an hour. Longer than that, and it should be recycled into new batches up to 10% of recycled material per batch.

You will use a mortar mixer, not a cement mixer. The blades of the mortar mixer toss the material and give it “loft” About the largest rental mixer you can find is an 8 or 9 cu ft machine. This can only do half a bale at a time which slows you down a lot. Any more, and the motor simply stalls. Wet hemp is heavy and binds between the walls of the tub and the blades. Most rental mixers are heavily encrusted with cement. It helps to bang as much as you can off to avoid loose pieces in your hemp mix. This is harmless, but it can mar your finish if you are looking for a natural finish as we are. Working this way, one man on the mixer can feed a four man crew along the walls.

For measuring purposes, we used 5-gallon buckets, and a 4-1-1 mix: 4 buckets of hemp, 1 bucket of binder and one bucket of water. Mix the water and binder for about a minute then add the hemp and mix another minute or so. One man on the mixer can feed 3-4 people along the wall stuffing hemp pretty well. The mixer was not the critical path to production.

Animal feed stores sell two-handled rubber baskets that are nice for hemp work in 6.5 gallon and 10.5 gallon sizes. Search online. These are good for pouring hemp into the forms. They are also larger than the 5-gallon buckets obtainable at building supply stores, and they take abuse well.

We used Geoplast™ panels from Italy imported through their rep in Canada as our shuttering system. A minor logistic problem there. Allow time for shipping and customs snafus. These panels are expensive, but reusable for many houses. They set up and re-position pretty quickly with their built-in clamps and keep the walls straight both horizontally and vertically. You can screw them directly against the studs with small screws when your studs are exposed as they are along our inner garage wall. When you need to set them off 3.5″ as we do on our thicker walls, you can use quarter inch lag screws. When you withdraw the lag screws, you must push material in the holes. This can require some skill if you are not plastering over the surface defects.

The Geoplast panels were expensive so we never had enough, but they proved good for establishing a straight line for lining up additional plywood. Wherever there are penetrations, you must cut holes in the forms which cannot be repaired, but the process tolerates holes in the shuttering. You can tape over holes in the plastic forms with packaging tape.

Form setting is the critical pacing item. All work stops while you change forms for another row. All of your wall crew should be experienced form setters with sufficent screw guns and spare batteries. You do not want stuffing to wait while batteries are charging. You can pull shutters in a few hours. Hemp will stick a little bit so it helps to slide your form before you pull it. The plastic forms could be cleaned before setting them again.

Lime is caustic, and the mixture is very hard on hands and exposed skin. Use heavy duty neoprene coated work gloves. Do not use disposable vinyl/latex gloves. The work is too hard and destroys them quickly. The mixer man should have a face mask. The loose powder is hard on lungs. Workers along the wall handling wet mix do not need face masks. That said, the next problem is getting workers to wear them. They don’t want gloves and masks when it is 95° and 75% humidity.

Maintaining consistent crews is often difficult. Laborers in the cement trades tend to be here today and gone tomorrow. This makes it hard to maintain consistent quality from day to day so errors like voids or poorly incorporated material start popping up. You can drop things in the wall and bury them in hemp resulting in surprises when you pull forms. Most of the time you can gouge out mistakes and patch material in the voids. However, they will be obvious patches if you are leaving the surface natural as we are on the interior.

Horizontal plastic conduit runs in the thinner garage walls proved to be a problem. It was hard to stuff well and left voids. The hemp did not bond to the plastic conduit and came away in chunks when we pulled forms. Hemp needs a couple inches to get good bonds with itself. Thin sections just crumble away. In most cases you want a minimum of 2″ unless you are hiding it with something like cabinets. Within a 12″ wall, this would not have been such a problem, but the risk of voids always exists with any horizontal members when you are stuffing vertically.

You need to design out problems like this before you start stuffing. You want to identify voids and ask yourself if they have to be there. Vertical voids can be stuffed. Horizontal voids are always difficult.

“Lightweight” is a relative term. Hempcrete is far lighter than cement but far heavier than traditional loose insulation used for attic fill. Hempcrete is very heavy until it dries. If you intend to use it as attic fill, expect to use temporary supports until it dries. Drying is not curing. Drying just removes water. Curing is the process of absorbing carbon dioxide to mineralize the lime. Drying takes some few days depending on your climate. Curing takes 45 days. This wet weight issue affects your truss design. If you attempt to move too fast, you can overload your trusses. You might need truss supports as well as ceiling panel supports. All of this support material can be a very significant cost factor in your construction budget.

You must protect the finished work from rain during curing and you must moisten the surface with a gentle spray mist as it cures so it dries evenly. We found a bulk tarp supplier online. We hung tarps from the rafters. We could roll them up to work and drop them quickly as the skies darkened with rain.


Cured hempcrete can be shaped with power tools and hand tools: recipricating saws, abrasive disks, sureform rasps, belt sanders et cetera. This allows you to touch up shapes and mistakes you want to fix. It is best to leave them until fully cured before working the surface. It does wear down blades and many tools must be treated as expendable.


American Lime Technology American distributor of Tradical Hemcrete fiber and binder. Purveyor of USHG lime mortars and washes. Source of our lime paints and interior stucco mix.

Geoproducts Corp. North American distributor of plastic Geopanel shuttering (Geoplast, Italy).