Updated: Nov 17, 2020
research and text contributed by Olivia Page
Hempcrete brick is a composite of lime and the stalk of hemp. Hemp is one of the earliest domesticated plants, with first recordings of its use in Neolithic times and the word “canvas” being derived from “cannabis”. A mature plant grows to a height of 1.5-4m in just a few months. With such fast-growing times and competitiveness, it requires less fertilizer than most industrial crops for example corn.
The inner woody stem ('shiv') broken up to 10-25mm pieces comes from France and the Lime comes from Portugal. They were mixed together to make insulative infill bricks over three months in a local warehouse to the Hempcrete project, Paraíso. The bricks replace hollow cement bricks and in comparison require three times less heat to create than standard limestone concrete.
Hempcrete has superior thermal performance, with a heat transfer coefficient (U-value) of 0.17 m2K/W compared to 0.55 m2K/W of hollow cement blocks, thus five times more insulative.
The project demonstrates that Hempcrete can be used in Porto's wet climate; it is particularly suitable, in historical building restorations, to undo the damage that the moisture retention of Portland cement has caused in many of Porto's unique historical buildings.
In Paraíso there is minimal plaster and render, you get to see the hemp from inside, leaving a stucco-like and toxin-free finish. The construction arrangement is legible and fixable, very accessible as a usable home. For example, you can see where you can and can’t hammer in a nail, or to hang a picture.
The exposed material retains the character of a recently growing thing, which, together with the pine timber that frames it, is a soothing pinkish-grey as well as adding soft acoustics. Although the house is built of organic and relatively light materials, it has some of the sense of mass and substance you get in old masonry construction, with a high thermal mass, giving an interior comfort throughout the year's seasons and daily temperature fluctuations.
There aren't many reasons to suggest Hempcrete can't be adopted on a larger scale, although in-situ casting is a more cost-efficient way of constructing. The challenge is how to scale the techniques up to the extent that they make a significant impact on the emissions of the building industry.