The B rating of CSA A440 also defines a window’s ability to resist wind-driven rain, but it is not a measure of how the installed window with perform in the wall assembly.
Like the A rating for air leakage, the B rating is determined according to the results of laboratory testing. Table 2 shows the standards for B ratings.
Water is sprayed against the outside of the window at a known rate for at least 15 minutes, and an air-pressure difference is applied across the window, in an attempt to approximate wind and rain conditions.
The test method is described in the ASTM E547 Standard. The window is rated according to the highest pressure at which water ingress does NOT occur.
Note that “water ingress” can have several meanings. Under the pass–fail criterion of the ASTM E1105 procedure, water ingress is defined as any water that passes beyond the innermost projection of the test specimen, not including interior trim and hardware. Water ingress in CSA A440 means that:
water has penetrated the window assembly and wetted room surfaces OR
water has passed through the window into the wall below the sill OR
water remains trapped in the window after the test pressure is released.
Clearly, the A440 criteria are more stringent than the E1105, and the designer should be clear on which criteria are more important in the performance of the window assembly.
As with the A rating, a window must pass the B1 level to meet the minimum requirements of A440 (and therefore, the Building Code requirements). The A440 Standard includes a User’s Guide — called CSA A440.1 — that gives some design assistance in selecting an appropriate B rating according to the location and building height. The specifics of the design process are best obtained by reviewing the A440.1 Standard. For tips on what to do with the design value once it is obtained, see the section entitled “Specifying ABC Ratings.”
Reviewed by Jan Luistermans