June 5, 2020

In 1946, a Norwegian came up with a secondary line of protection from rainwater. Until then, multi-wythe mass walls that had an absorption-storage-redistribution strategy for dealing with water. Each progressive layer of brick allowed only 1-10% of the water to pass though.

When that strategy didn't work, stucco was added to exterior to keep more water out. Yes, that's right-Stucco was applied to keep the rainwater out.

The Norwegian strategy involved an air gap behind the first layer and before the second layer. Canadian research was employed to help decide the size of the gap needed to allow drainage - 3/16", but also an air gap with air flow -1/4-3/8". Canada and Norway share very long, cold, wet winters and allowing the air flow inside this air gap promotes drying and back ventilation of the cladding as well as the drying of the outside rendering of the wall assembly behind the cladding. That's a great thing for durability.

Stucco and brick are reservoir claddings that get charged during rain events and the dissipated when they get hit with sunlight. When the sun hits, and provided you did not paint your stucco and brick with a vapor-closed paint, rainwater gets evaporated out away from the wall, and into the wall assembly. A gap behind the face of the cladding will keep this inward vapor drive from being driven too far into the assembly. Since we no longer build with multiple layers of brick to absorb-store and redistribute this water, the gap becomes more crucial.

Sometimes, a vapor control layer is employed behind the reservoir cladding to prevent solar vapor drive altogether. This assembly works to uncouple the reservoir cladding from the rest of the wall assembly. When coupled to a gap that promotes drying by controlling hydrostatic pressure, providing drainage and back-ventilates the cladding, you have the state of the art rain screens we see today.

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