Paul Baldwin

Thanks John and Jay C for your insights.

We actually live on the coast in the south east of Australia which has a temperate climate. As a bit of background, the dwelling is load bearing strawbale rendered with mud/chaff and finished with sand/lime.

You have both raised issues which are relevant to my situation. With the moisture, I have noticed that after periods of consistent rain, parts of the floor darken and actually soften. So despite concrete footings 2 feet wide and 2 feet deep, the moisture is still making its way up to floor level. This is just in random patches, not widespread. Since moving here I have improved the drainage around the building and weather has been somewhat drier so no more soft patches.

The mould on the other hand occurs wherever the surface is covered with mats, furniture or under anything that sits directly on the floor for more than a few days. I have also noticed that the floor is still somewhat dry where the mould is growing. There appears to be no correlation between the previous moist patches and where the mould occurs.

I never considered that the problem maybe related to the cob floor mix used or finishing. The floor was actually just a mix of clay and straw only, which was done by hand in a bath tub. There were no other additives such as cow/pig blood or dung. The initial sealing was done with pure flax oil and then successive coats of flax oil and turpentine. No additives such as salts or mould inhibitor were added during the sealing process. The final wax finish was a mixture of bees wax and turpentine. One thing was that whilst the literature recommended boiled flax oil I could not always get this and often used raw flax oil.

Seems like the issue needs to be addressed from both a moisture and finish perspective. I figure the best way to minimise the moisture problem is to have the best possible drainage around the building supported by the dehumidifier inside. Will also do a test area using Jay C's suggestion of borax wash with mould inhibitor additive. Not sure how this will work over the waterproof wax finish but will give it a go and report back.

Thanks so much to you both for sharing your knowledge.

Regards,
Paul

I have a cob (clay/straw) floor around 4" thick which was laid about 10 years ago. It was sealed with hot boiled linseed oil and then finished with bees wax. When the house was being used as a weekender, I noticed mould developing on the floor under the mats and furniture. I figured it was moisture coming up from the subsoil as I neglected to put a plastic membrane down before laying the floor. It didn't worry us in the early years but now that we live here full time I am concerned about the impact of the mould from a health perspective. We recently re-waxed the floor and the mould problem seems to have gotten worse. Digging it up would be a last resort as it is really a work of art which took weeks to lay. I thought about sealing it with some water proof sealant but not sure this would work? Any ideas?