An introduction to lime - The Hempcrete Topic: Designing and Building with Hemp-lime - page 64

Lime and gypsum plasters side by side. The finish of the
cream-coloured lime plaster around the window appears
more textured and warm than the smooth gypsum on the
surrounding walls and ceiling.
Like lime, gypsum plasters are hygroscopic, with a good ability to absorb moisture vapour.
However, unlike lime, which allows moisture to pass in and out of it easily without signi-
ficant damage to the structure of the plaster or mortar (such finishes are designed to be re-
applied eventually), gypsum holds on to any moisture it absorbs. Moisture is locked with-
in the structure of the plaster until it reaches saturation level, at which point the plaster's
structure collapses and it falls off the wall. For this reason, gypsum plasters, usually used
together with non-vapour-permeable paints, were not really a successful replacement for
lime plasters in older buildings. Like cement, they contributed to the building not working
as it was intended to (see Chapter 4, page 58 ) .