We decided to buy 18 mm cement-bonded particle boards (CBPB) as the bottom layer of our floor. Despite these panels containing cement (which increases the carbon footprint) we still choose them over other wooden panels because of their durability. We also considered steel panels but this would turn our floor into a vapor-closed construction, which is something we wanted to avoid. The cement-bonded particle boards can withstand water, mold, insects and rodents but they are still vapor permeable with a µ-value (diffusion resistance factor) of around 50–100.
To install the boards we layed them onto the steel deck while leaving a gap of 5 mm in between the borders. Since these are still fiber boards it is possible that they can increase or decrease in size over time. The space prevents them from working against each other which could cause their shape to distort. These gaps were later filled up with Illbruck SP525 Proflex Seal 25. While it’s eco-friendlier than most sealants, this kit is still a synthetic product. If you have any tips on how to fill up gaps between cement-bonded fiberboards with a more natural alternative, let us know!