How to Insulate Brick and Plaster Walls | Vapor Barrier Needed?

Harry asks,

The outside wall of my house is made of brick and inside, we have plaster attached directly to the brick. Will we need a vapor barrier to keep moisture from coming through brick and plaster wall?

Our Answer

No, actually. Water vapor migrates from warm to cold. Many things we do as a part of daily life generate moisture like taking showers, doing laundry and – breathing – generate moisture which moves through the plaster walls and into the brick where it evaporates outside.

The only time this becomes problem is when we seal off that brick surface (by painting, for example), and therefore prevent it from being able to allow that moisture to evaporate. When that happens, you’ll start to see condensation and mineral deposits forming on the inside of the plaster walls. Old home walls made from brick and plaster work and work well – as long as we don’t mess that up!