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.
I had no idea that rainscreens where a thing you could get for your roof. It is cool that it helps match the pressure of the rain. Having that gap in between does seem like ti would be useful if there were every any leak as well.
Cheaper rain screen assembly for a vaulted or cathederal vented roof than shown, is to place the wrb membrane over the batt or rolled fluffy insulation but below the ventilation gap and also below the outer sheathing or deck. This assembly reduces the use of more expensive foam or foam board, gives a deeper ventilation space than a standard rainscreen assembly, and allows a drainage plane that carry off either bulk or condensation water. The perm rating of the wrb membrane would need to be open enough to allow drying without significant stopping water migration long enough to condense on the warm side. This assembly also takes the place of a ventilaion chute and acts as a air barrier so the fiberglass or mineral wool hold heat longer to enable greater insulation efficency. This assembly would work best with an interior vapor retarder, such a so called smart membrane or caulked and painted dry wall. This would not take the place of the roof underlayment but would work with it in catching any bulk water that got past it due to aging, poor flashing or penetrarions such as nails or screws. The soffit detail would need flashing so that water could drain from soffit vents without damaging paint or building materials.
I don’t remember having a discussion with anyone else but me who talks about water draining out of soffit vents. However, water does drain out of soffit vents from time to time. An example would be when the outdoor temperature and relative humidity changes rapidly, like it sometimes does here in Montana. I once recommended that a friend of mine use foil backed drywall to encourage cavity draining without damaging the drywall. The Cathedral ceiling was already on the blueprints and he wouldn’t do anything with that. His contractor thought my idea was crazy, but a few months after they finished the project, my friend called me and reported excitedly that with there was water flowing out of the soffit vents in a number of places. I believe that the weather conditions were freezing then thawing with very humid air venting the space. I guess it was kind of dumb to allow condensation to drain through fiberglass insulation and flow against the foil backed drywall, but I looked at the blueprint and couldn’t think of a better idea. I should go walk on that roof and see if there’s any soft spots. One time many decades ago, I walked on a cathedral ceiling roof and stepped through the wood shingles and sheathing knocking out a recessed can fixture below. I wonder how many cathedral ceilings experience that type of moisture problem? Thanks for the post. Obviously you and I have done a lot of thinking along the same lines. It’s not a perfect world yet, is it!
Yes, useful for a leak and provides ventilation for drying. Thanks for the comment.