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 screenthis is a (usually thin) outer cladding on a wall which prevents rain, snow, etc getting at the structure of the wall behind. see more on 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 energythe total amount of energy it takes to make a material (or a building). See more on 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 delayThis relates to the lag time that insulation itself takes to heat up or cool down. It introduces a delay into the effect of the insulation. This can help level out peaks and troughs of temperature. See the section on Decrement Delay) and a good level of air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard 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 LILILow Impact Living Initiative. LILI is a network of great organisations with lots of ways to help you change your life and the world for the greener article on hemp/lime
The Building RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place) 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 SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)
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 remediationthe term applied to the method of dealing with pollutants and contaminants in the ground. The Building Regulations cover this in detail. See info about Approved Document C – Site preparation and resistance to contaminants and moisture 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.