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Hemp / Lime

Re centre Bradford

Hempcrete is now an established building material in the UK. This is the re:centre at the University of Bradford

Hemp lime walls are a relatively new construction method to the UK although they have been around in mainland Europe for a couple of decades. Hempcrete, as it is generically known is a mixture of lime and hemp shiv (chopped hemp straw after the fibres have been removed) in a ratio of about 1:5 . It is usually cast into a structural timber frame with an external render or rain screen, although it can also be obtained as blocks, similar to breeze blocks. Hemp lime can also be sprayed or trowelled and has been used to good effect as a replacement for wattle and daub in historic building repairs.

The outstanding advantage of hemp lime is its low embodied energy at about 1.06 MJ/kg. Hemp is an extremely fast growing crop requiring little or no chemical applications and lime is a binder which uses a fraction of the manufacturing energy compared with cement. Other points in its favour are the ease of use (although basic shuttering does have to be employed), a fairly low conductivity value of about 0.12 W/m²K.  (although there are emerging issues about using ‘raw’ U values – particularly in temperate climates – see Decrement delay) and a good level of air tightness when constructed properly.

So far there is little information in English on the subject of hemp lime. Probably the best place to learn more is the excellent book ‘Hemp lime construction‘ by Rachel Bevan and Tom Woolley.  There is a detailed write up on a hemp lime house recently completed by Rachel Bevan and Tom Woolley in Northern Ireland. Very few companies have so far got involved with production, supply and construction of hemp lime although it seems to be growing fast. Tradical supply Hemcrete (a mixture of hemp and lime/cement binder) for wall insulation around timber frame and also insulation for roofs and floor screeds.

There is an excellent on line publication called An Investigation of Hemp and Lime as a Building Material by John O’Dowd & David Quinn at the department of Civil Engineering, University College, Dublin. They recommend further research into a weaker mix of 6:1 and 7:1 hemp:lime, in order to increase the insulation value.

Although Hemcrete does not have the same insulation values as, for example, mineral fibre, there is a detailed explanation here about why it achieves better overall results due to the moisture content in the thickness of the wall.

There are several new hemp-based products currently coming on the market such as Isohemp

See also the LILI article on hemp/lime

Building regulations

Structure

The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)

Site preparation and resistance to contaminants

This section,  Part C, covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc. There is an abridged version of the Approved Document specially for houses.

 

Books

The Hempcrete Book. Designing and building with hemp-lime

2 comments to Hemp / Lime

  • admasu

    I got the same problem i was trying to source it i got no luck, and i want to know what kind of lime i could mix it with. please help.

  • elizabeth

    Excellent article – I am trying to acquire hemp straw to do some lime insulation but am having trouble sourcing it – can you help please
    Many thanks

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