The term “reverse vapour drive” is often used in describing how moisture enters a wall. What does this mean and how is building paper affected by this?
Reverse Vapour Drive is the process which occurs when water vapour is driven into a wall from the outside. An example of this is when stucco is wet from an overnight rainstorm and is then warmed by the sun during the following day. The warm moist stucco will create a high vapour pressure at its surface, which will liberate vapour from both its outer face and its inner face. The vapour leaving the inner face will condense on colder surfaces inside the wall. It will condense on the outer face of the sheathing membrane firstly, and then proceed further into the wall and condense on the wood sheathing.
