Should I use boards, such as anti-racking (‘racking’) boards, as part of a hempcrete wall?

You should not use boards that are not vapour-permeable (or not sufficiently vapour-permeable) such as OSB or ply except as temporary shuttering boards – used to shutter wet-cast hempcrete. There are specialist vapour-permeable racking boards, such as Econic. If possible, it is better to add boards after wet-cast hempcrete has fully dried, as even vapour-permeable boards will significantly slow down the drying process.