Hemp in construction - The Hempcrete Topic: Designing and Building with Hemp-lime - page 43

They take longer to dry out - anything from a week to a month or more, depending on
the substrate, the thickness applied and the weather conditions.
If you have built a hempcrete wall, unless it is a very thin one, then you already have
enough insulation without adding it in the plaster, so at Hemp-LimeConstruct we have lim-
ited experience of these plasters and have not used any of the proprietary products avail-
able. We have, however, mixed our own lime-hemp plasters, with both coarse and fine
hemp, when working on heritage buildings or when dubbing out uneven masonry walls.
The extra structure the hemp provides enables a thicker coat of plaster to be applied, and
we have found it an effective solution, and the plasters to be a pleasure to work with.
In the products currently on the market, different grades of hemp fibre are used to create
plasters described as either coarse, medium or fine. Quite a variety is available from the
different manufacturers, although most lime-hemp plasters are supplied as a ready-mixed
'wet' product made with air lime. Womersley's supply a plaster that is mixed from quick
lime and hemp with pozzolanic additives to ensure that it sets throughout the depth of the
plaster; perhaps because of this, the maximum depth of coat advised is higher than for
some of the other lime-hemp plasters on the market. 4 See Chapter 3 for more information
about different types of lime.
Hemp-fibre quilt insulation
The natural, breathable quilt insulation made from hemp bast fibres provides a sustainable
alternative to conventional glass wool or mineral wool insulation products, such as is used
for loft insulation. Various companies in the UK (see Resources ) currently produce quilt-
type insulation materials made from natural fibres: usually sheep's wool, wood, flax or
hemp fibre. These offer a healthier and safer alternative to conventional synthetic quilt in-
sulations, which are often made from irritant or toxic materials.
Natural-fibre insulations have an equivalent or better thermal performance than conven-
tional quilt insulations, with a typical thermal conductivity of around 0.04W/mK and a
typical U-value of 0.16 at a thickness of 250mm. (Thermal conductivity is a measure of
a material's ability to conduct heat. A material's U-value describes the ease with which
it allows heat to pass through it - this varies with its thickness.) There is also evidence
that natural-fibre insulations remain effective over a longer period of time than synthetic
products. 5