Is Hemp in Your Future?

 

This new insulation could eventually find its way into off-site construction. Here, we look the material itself, as well as its thermal value, cost and fire resistance.

• Hemp batts are currently the most user-friendly option, with the most predictable costs. But the techniques being developed for site application could eventually be used by off-site manufacturers.
• Research studies have shown that the thermal performance of hemp is on par with other popular insulation types.
• Fire-retardant treated hemp insulation seems to have a fire resistance that falls between non-combustible materials like rockwool and standard fiberglass batts.

 

Hemp insulation isn’t something many builders or off-site manufacturers have considered, but it will become more of an option in the future. In fact, some types of hemp seem a better fit for factory-built than for site-built housing.

The insulation comes in two broad forms: hemp batts and “hempcrete.” The former is similar in form and function to fiberglass batts; the latter is usually site-applied, although a few companies are making precast hempcrete blocks.

Hempcrete (which, despite its name, is an insulation, not a structural material) is made from hemp hurds (small, chopped-up pieces of the hemp plant’s woody core), a lime binder and water. At the moment, a hempcrete-insulated building typically needs an alternative materials variance for code approval. However, the material was recently approved for use by the 2024 International Residential Code (IRC), so approvals will likely become easier.

 

 

Hempcrete Basics

There are currently two main ways of applying hempcrete on-site. One is to cast it in place, a process in which the wet mixture is hand-rammed between temporary wooden forms. Another is to wet spray it into the forms through a nozzle.

Cast-in-place. Ray Kaderli, who is serving this year as President of the US Hemp Building Association, is building a house in San Antonio, Texas using cast-in-place hempcrete. He says it has “the consistency of oatmeal cookie dough, perhaps with a little too much oatmeal.”

In this approach, wet hempcrete is tamped down by hand between the forms. After the forms are removed, the hempcrete then must cure for four to eight weeks before it can be enclosed.

 

Because a gap is left between the inside faces of the forms and the wall studs, the cured hempcrete will completely encapsulate the studs. This eliminates thermal bridging, which Kaderli says makes hempcrete superior to batts. However, he also believes that batts are a good product. “Just because one thing’s better than another, doesn’t mean the second-place performer isn’t also excellent,” he says.

Spray-applied. One company that installs spray-in-place hempcrete is Americhanvre in Wescosville, Pennsylvania. In this approach, machines mix together hemp hurds, lime binder and water. The resulting mixture is then sprayed through a large nozzle between the forms. Curing time is similar to cast-in-place.

 

 

As with cast-in-place, the forms for sprayed hempcrete can also be placed so that the cured product fully encapsulates the studs. The result, claims company President and Founder Cameron McIntosh, is “a 30% to 60% reduction in heating and cooling costs.” Actual savings will vary with location, architecture (vaulted ceilings, large sliding doors, etc.), residents’ personal preferences, and so on.

Costs for site-applied hempcrete are tough to quantify at this point, because of the special equipment and processes needed. In fact, Kaderli considers it as still in the proof-of-concept stage, with the main market being custom homes. However, he says that various companies are exploring how to use these techniques in a factory setting. Some companies are also considering hempcrete SIPs.

Hemp Batts

At this time, hemp batts are a more suitable product for most builders and off-site manufacturers.

Hemp batts don’t look much different from other types of batt insulation. “They’re a direct one-to-one substitution,” says Kaderli. For instance, if you were renovating a home, you could simply replace the old fiberglass batts with hemp ones.

Cost estimating is also straightforward.

A quick search on Home Depot’s website turned up Owens Corning’s product, Pink Eco Touch Insulation (2 in.  thick, R-value of 6.7 per inch), priced at $1.31 per sq. ft, which is actually more expensive than the 2 in.-thick “HempWool” batt from Ketchum, Idaho-based Hempitecture.

At Lowe’s, Johns Manville’s R13 fiberglass batt insulation (3.5 in. thick, R-value of 3.7 per inch) is priced at $0.72 per sq. ft. — about half the cost of equivalent HempWool ($1.45 per sq. ft.).

However, also at Lowe’s, Owen Corning’s R13 mineral wool batt (3.5 in. thick, R-value of 3.7 per inch) is available for $1.65 per sq. ft — a lot closer to the equivalent HempWool at $1.45 per sq. ft.

These prices may or may not be representative of standard insulation on the market. (They were the result of a few online searches, not an extensive survey.) But they do illustrate that hemp batts need not cost a lot more than conventional batts, and sometimes even cost less.

 

 

Thermal Performance

Other attributes to consider when looking at any type of hemp insulation include thermal performance and fire resistance.

Hemp’s thermal performance is on a par with other popular insulation types. This is shown in the following table, which includes data from a December 2020 article in the Journal of Building Engineering (“An Overview of Factors Influencing Thermal Conductivity of Building Insulation Materials.”)

 

 

A Swedish study published in the Journal of Sustainable Architecture and Civil Engineering in January 2022 came to a similar conclusion. The researchers found that if a house is insulated to the same U-values with hemp, cellulose, or glass wool, its resulting energy use and energy costs are essentially the same.

 

 

Fire Resistance

ASTM E84 is a standardized test for flame spread and smoke development. Results are used by code officials and regulatory bodies.

Hempitecture’s products were tested according to E84 in 2020 and deemed non-flammable, with a flame spread index of zero and a smoke developed index of zero. The flame spread scale is from 0 to 200 and the smoke development scale is from 0 to 450 — with zero being the best possible score on both scales.

Hempitecture’s new hemp batt, HempWool+, is treated with a fire retardant and has a flame spread index of 10 and a smoke developed index of 110.

How does that compare to other batt insulation?

Rockwool, a brand of mineral wool batt, is non-combustible with a flame spread index of zero and a smoke developed index of zero. Owens Corning’s “Pink Fiberglas” product is also designated as non-combustible. However, it’s important to note that the kraft facing on insulation batts will burn.

Owens Corning’s Pink “Next Gen” fiberglass insulation has a flame spread index of about 25 and a smoke developed index of about 50. The same company’s Eco Touch foil-faced fiberglass insulation has a flame spread of 75 and a smoke developed rating of 150.

So, the fire/smoke performance of HempWool+ is better than some commonly available standard batt insulations, although not as good as completely non-combustible mineral wool.

Conclusion

This is just an introduction to hemp. As such, we haven’t touched on other factors that would also need to be taken into consideration when evaluating this material, including acoustic performance, moisture control/regulation, mold and pest resistance, and capacity for reuse and recycling.

However, in terms of both thermal properties and fire resistance, hemp insulation performs similarly to other products on the market. Hemp batts are also easy to use and cost competitive.

We also haven’t discussed the environmental benefits of hemp insulation, but for those who are interested, we have included a box with links to some research.

 

Studies on the Environmental Impact of Hemp Insulation

Energy performance, environmental impact and cost of a range of insulation materials, Renewable and Sustainable Energy Reviews, January 2021. https://www.sciencedirect.com/science/article/pii/S1364032121000472?via%3Dihub

Comparative life cycle assessment of bio-based insulation materials: Environmental and economic performances, GCB Bioenergy: Bioproducts for a Sustainable Bioeconomy, March 2021.         https://tinyurl.com/4tr7c343

Life Cycle Environmental and Cost Analysis of Building Insulated with Hemp Fibre Compared to Alternative Conventional Insulations: A Swedish Case Study, Journal of Sustainable Architecture and Civil Engineering, January 2022.  https://tinyurl.com/5ddcjkwx