Building assemblies can get wet

Building scientists assume that some rainwater leaks through the cladding or roofing and that water vapor migrates through insulated building assemblies, sometimes condensing within those cavities. These two phenomena both can dampen building assemblies.

What are rainscreens?

Builders often provide a ³/₄-inch air space between siding and the water resistive barrier (WRB) as a drainage plane, also called a rainscreen. If this space has screened openings top and bottom, we call the assembly a ventilated rain screen or pressure-equalized rain screen. This type of rain screen provides excellent drainage and drying potential for the wall cladding, roofing, and structural sheathing. These durability benefits require builders to install screening at top and bottom of a wall or roof to allow airflow while excluding insects.

Rainscreens performance

A ventilated rainscreen functions as both a drainage plane and ventilation. The ventilated rain screen also reduces both air pressure and water pressure of wind-driven rain against the siding and WRB.
Research by John Straub indicates that the permeability of the water resistive barrier and the airflow through the rainscreen determine the rate of drying. His research showed that a WRB with a perm rating of 50 significantly outperformed a WRB with a perm rating of about 5.

With absorbent and permeable cladding like wood and stone, the sun can drive moisture from absorbed rain water through the cladding into the structural sheathing. A ventilated rainscreen interrupts this solar driven moisture and allows moisture out of the wall assembly.
Asphalt shingles, wood shingles, and masonry tiles all absorb rainwater. A rain screen below the roofing allows room for drainage of leaked rain water and drying for the shingles and structural sheathing. The more permeable the roof underlayment (WRB) the better and insulated roof cavity can dry to the exterior.

Saturn’s recommendation

Persistently wet cladding, roofing, or sheathing isn’t durable and can deteriorate over the life of a building. Saturn recommends that all heavily insulated roofs and walls have ventilated rainscreens to promote drainage and drying, except in dry climates.
 

Building assemblies can get wet

Building scientists assume that some rainwater leaks through the cladding or roofing and that water vapor migrates through insulated building assemblies, sometimes condensing within those cavities. These two phenomena both can dampen building assemblies.

 What are rainscreens?

Builders often provide a ³/₄-inch air space between siding and the water resistive barrier (WRB) as a drainage plane, also called a rainscreen. If this space has screened openings top and bottom, we call the assembly a ventilated rain screen or pressure-equalized rain screen. This type of rain screen provides excellent drainage and drying potential for the wall cladding, roofing, and structural sheathing. These durability benefits require builders to install screening at top and bottom of a wall or roof to allow airflow while excluding insects.

 Rainscreens performance

A ventilated rainscreen functions as both a drainage plane and ventilation. The ventilated rain screen also reduces both air pressure and water pressure of wind-driven rain against the siding and WRB.
Research by John Straub indicates that the permeability of the water resistive barrier and the airflow through the rainscreen determine the rate of drying. His research showed that a WRB with a perm rating of 50 significantly outperformed a WRB with a perm rating of about 5.

With absorbent and permeable cladding like wood and stone, the sun can drive moisture from absorbed rain water through the cladding into the structural sheathing. A ventilated rainscreen interrupts this solar driven moisture and allows moisture out of the wall assembly.
Asphalt shingles, wood shingles, and masonry tiles all absorb rainwater. A rain screen below the roofing allows room for drainage of leaked rain water and drying for the shingles and structural sheathing. The more permeable the roof underlayment (WRB) the better and insulated roof cavity can dry to the exterior.

Saturn’s recommendation

Persistently wet cladding, roofing, or sheathing isn’t durable and can deteriorate over the life of a building. Saturn recommends that all heavily insulated roofs and walls have ventilated rainscreens to promote drainage and drying, except in dry climates.

 What are rainscreens?

Builders often provide a ³/₄-inch air space between siding and the water resistive barrier (WRB) as a drainage plane, also called a rainscreen. If this space has screened openings top and bottom, we call the assembly a ventilated rain screen or pressure-equalized rain screen. This type of rain screen provides excellent drainage and drying potential for the wall cladding, roofing, and structural sheathing. These durability benefits require builders to install screening at top and bottom of a wall or roof to allow airflow while excluding insects.