Hemp & Lime
Hemp & Lime is a lightweight construction material that can be used for walls, insulation of roofs and floors and as part of timber-framed buildings. It also can provide very good thermal and acoustic performance, and offers a genuinely low-carbon contribution to sustainable construction. Hemp-based solid wall construction is breathable and is able to absorb and omit moisture, leading to much healthier buildings. The small dwelling used the technique of casting hemp & lime around a centre-wall timber stud frame, to form a monolithic construction (ie containing various materials but not separate layers). The hemp & lime casting-work was filmed during its construction in November 2008 (see below)
Objectives of the Hemp Cottage project
In addition to providing a dwelling the objectives were:
- To demonstrate how natural and low-impact materials can be used to create energy efficient buildings
- To learn more about the use of hemp crete (hemp & lime) composites walling with timber frame construction
-To understand the built-ability issues for both builders and designers: considering both crucial sequence and detail-design aspects
- To find out how easy or difficult it would be to specify other compatible natural and low-impact materials
- To create an air-tight but "breathable" building envelope that can be tested on completion, to discover how successful we'll have been in achieving good energy efficiency standards.
The Publication "Hemp Lime Construction" includes information on building construction techniques used to date. The form selected for this Hemp Cottage is based on the detail for hemp & lime cast around a site-constructed stud wall. On this project the primary structure is a separate timber post & beam frame, in locally sourced Douglas fir. The monolithic hemp wall is tied to the primary structure, but does not carry the roof loads. As the video illustrates the walls are cast using temporary shutters, each approx 700mm high, allowing "tamping" by hand of the hemp & lime mix. The mix then allowed to firm-up before the shuttering is removed. The heads and reveals of openings are made using permanent shuttering boards. The monolithic wall shall be rendered externally with a compatible lime-based render.
Conclusions to date
It has been relatively easy for the contractors to master the various build-ability issues, through bespoke-shuttering is time consuming. There are alternative techniques such as spraying that could be more cost-effective on a larger building. Prefabricated shuttering can also be used.
It has been difficult to source many of the other materials and products that we have wanted to use, as they are not available in Northern Ireland.
To date we are confident that we can achieve our objectives - and we will testing the buildings's performance.