Notice the holes in the frieze blocks? Wind driven rain and attic pressure differentials are all it takes to cause leaks!
The Water Intrusion Problem Explained:
The Owner wondered why water occasionally leaked from the window-heads inside. Rainwater was getting into the attic through the frieze vent holes (see above photo). But how?
While it is possible to spray hose water directly through the frieze vent holes—that does not fully explain what was happening during rainstorms.
Rainwater typically hits buildings on a downward trajectory; therefore, holes that are covered 3:1 (three horizontal cover to one vertical) are generally safe from rainwater intrusion. However, from various angles these vent holes are vulnerable to water intrusion. This house is on a knoll, which increases wind speeds and creates updrafts. An updraft can influence rain to strike the frieze vents on a horizontal. However, water being pushed against the frieze vents does not alone explain how water gets into the holes. This is because the wind against the building creates turbulence, essentially preventing or deflecting water intrusion.
The remaining phenomenon that explains the water intrusion is the fact that the house attic is well ventilated; cross-ventilated per code.
As winds push rain towards the vent holes, the wind blowing past the house creates suction on the leeward side creating a negative pressure difference that pulls water laden air into the attic like a vacuum.
The Fix
ACI Recommends:
Installing round aluminum louver vent covers over the frieze holes. Louvers allow nearly 100% of the hole size for ventilation while they deflect rainwater at an angle that even updraft driven rainwater has a hard time climbing. The louvers look better placed parallel to the frieze, but to be effective the louvers need to be installed level. Alternately, stuffing the holes with reticulated foam allows ventilation, but breaks up water droplets primarily into water vapor as it is sucked into the attic.
