The outer layer of the hemp stalk is called bast fiber which has been used for centuries to make ropes, clothing, canvas and sails and now Hemp wool insulation batts. Let’s look at how sheep wool insulation is made. This pendulum swings back and forth to make multiple wool layers. The layers are bonded with a mechanically driven needle punch which stitches them together.
Hemp wool is sold by the pallet in various depths and widths. A 2” deep batt has an R value of 7 while a 7.5” deep batt is R28. The R value of sheep wool is exactly the same as hemp wool. A 2” deep batt is R7 and a 5.5” batt is R20. The 5.5” batts will fit in a standard 2×6 stud bay for an exterior wall. 5.5” thick Hemp wool costs $1.95 per sqft while the same sheep wool costs $2.25 per sqft.
Just like most insulating products, hemp and sheep wool operate on the insulating property of trapped air between the hemp and wool fibers. As you can tell, hemp is much stiffer, denser and heavier. It bounces back to its original shape after being compressed. Sheep wool on the other hand is very floppy, malleable, soft, lightweight. But the unique thing about wool is that you’re not only relying on trapped air for insulation, but the wool fibers themselves are good insulators. Each fiber is composed of protein molecules or keratin organized into five follicles.
Hemp wool friction fits into stud bays on walls and ceilings without any additional fasteners. Sheep wool has to be stapled to walls and fastened with a wire mesh or cables to the ceiling. Hemp is very tough and cannot be easily cut with an xacto knife. You have to use a table saw or a miter saw to get a clean cut. Sheep wool fibers are also surprisingly strong. You need to use a pair of sharp scissors or a proper insulation cutter.
Now, for the water repellency test. Hemp absorbed some water, allowed it to pass through to the other side and changed color when wet but it also dried up fairly quickly. It did not lose shape or disintegrate. Sheep wool is very similar to hemp in this aspect. When I poured water over the sample, it did absorb it but did not disintegrate. Sheep wool can also naturally absorb moisture while staying dry and retaining its high insulative properties.
Hemp wool is supposed to be a Class A fire retardant but it caught on fire very easily and released a lot of acrid smoke and fumes. On the other hand, sheep wool performed pretty well when lit. The flame didn’t spread even though it turned black and singed.
They are very environmentally friendly. The hemp plant requires very little water to grow and process, about a quarter of the water needed for cotton. Hemp wool contains no VOCs or volatile organic compounds. Sheep wool is also very eco-friendly. Wool insulation contains the least embodied energy of any insulation available, half that of cellulose and one-sixth that of mineral wool.
They are both excellent eco-friendly alternatives to traditional insulation but they need to be refined. For example, it would be great to use hemp wool as a fire-retardant continuous exterior insulation. Sheep wool either needs to have better dimensional stability or we just use it as loose fill insulation.
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