Lime Ash Floor

Q I am interested in learning about installing a lime ash floor on the upper story of my strawbale house in Vermont, USA. would the lime ash be durable with in-floor heat (hot water with oxygen barrier pex tubing)?

A It is very important to have the right mixture and consistency for the lime-ash and in particular to allow it to harden and dry out naturally before any heat is applied. This should be left for at least one month and ideally longer. Some would say that ‘lime-ash’ is often a misnomer for old floors, which were often made of gypsum not lime. Hydraulic lime would be safest for the binder, otherwise you need to be very sure of the pozzolanic qualities of any ash, brickdust etc used, as carbonation will only take place in the outer surfaces of the slab.

I have successfully carried out repairs to a lime ash floor using NHL 3.5 with brick dust, sharp sand and crushed charcoal added to replicate the appearance, but not the mix, of the original. Do you intend it to be left uncovered? If so, you can create a very attractive finish with the traditional mix (including quartz). We have treated ours with hard oil and wax which really brings out the colour and material variation. If you are happy with materials which are currently more easily available, you may want to use LECA (light expanded clay aggregate) in a layer under the heating pipes. This is very light (which will help with any weight issues) and also has good insulation qualities, so will direct heat upwards. We laid the mix onto reed, but straw may be more appropriate for you as you are using straw bales.