Earthen floors

A great way to incorporate mass into a home for passive solar design is with an earthen floor. If designed in the correct manner it will absorb the low winter suns heat during the day and radiate it out at night. With suitable detailing the opposite effect can be achieved in summer.

They are composed of clay and sand and sometimes chopped straw (or other plant fibre) and are sealed with a natural oil and wax finish. If the right sand-to-clay-to-fibre content is reached and control or concealment measures are used, cracking should not be a problem. Test batches are required to get the ratio right.

Earthen floors are covered in the Earth Building Standards.

Points to consider

● 125-200 thick cob mix (can also use mortared adobe bricks)
● Similar buildup to concrete slab with capillary break, sand blinding and DPM
● Natural pumice insulation can be used, stabilised with clay slip (use old carpet beneath to protect DPM)
● If colour is desired, white china clay (also know as kaolin) can be used for the finish coat with a natural mineral based pigment
● Clay alis (clay paint) may be applied over earth floor to conceal cracks
● Shrinkage control joints can be incorporated into the floor at 1.2 m - 1.5 m centres, door thresholds, internal corners and other areas where the floor narrows quickly to control cracking. They can be filled with an earthen grout once the floor is dry. Alternatively cracks can be allowed to form naturally and filled later (perhaps with an alternate colour to create a marbled look)
● Seal with cold-pressed boiled linseed oil (which is actually boiled in the traditional manner not cut with high VOC drying agents) thinned with citrus solvent, true pine resin turpentine or mineral spirits, or use tung oil in hotter or more humid regions
● Can be buffed and waxed if desired for the final finish
● It is risky to install hydronic radiant tubing due to the potential for the earth to crack and damage the pipes
● Not suitable for wet areas
● Not suitable beneath point loads and high traffic areas. Set stones or tiles into these areas as appropriate
● May take several weeks to dry

Further reading

● Sukita Reay Crimmel and James Thomson. Earthen Floors. New Society Publishers, 2014.
● Jacob Racusin and Ace McArleton. The Natural Building Companion. Chelsea Green Publishing, 2012.
● Joseph F. Kennedy, Michael G. Smith and Catherine Wanek, Editors. The Art of Natural Building Second Edition. New Society Publishers, 2015.