The most important thing to recognize about ABC ratings is that they are not intended to represent field performance.
The CSA A440 Standard is intended to be a consumer guide, to allow purchasers to evaluate one window compared to another under standard test conditions. Consider the example of fuel economy ratings for vehicles: they are done under a standard set of conditions to allow the consumer to compare vehicles for fuel efficiency, but we don’t (or shouldn’t) expect to achieve that level of fuel economy on the road. This is because the fuel economy rating is determined under controlled conditions in a dynamometer testing laboratory, whereas actual driving conditions include hills, turns, stop signs, other drivers, and variable weather (to name a few of many considerations).
Similarly, the rated performance of a window in a laboratory test cannot be used to predict field performance. The test is done with a specific size of window, under controlled laboratory conditions, but the installed window is often a different size and will experience a wide range of weather conditions. The tested window is tightly sealed into the test chamber, but the installed window can leak around its perimeter (between the wall and the window frame). The tested window may also be reinforced at the test size, but not at the installed size (or vice versa), which can produce a large difference in performance. Thus, the rating system is valid for product comparison, but must be used cautiously as a design tool.
There are four important points to consider about ratings and field performance:
1. Field performance is generally not as good as rated performance.
The interface between the wall and the window frame provides additional paths for water and air leakage and the window will not be as completely anchored in the wall as it was in the test chamber (in fact, it must not be anchored as rigidly, as some allowance for movement is required in the field installation). The A440.1 User’s Guide is still appropriate to identify the desired ABC ratings for a given location and building size, but remember that this now defines the required field performance. So, if the intent is to verify field performance, test to the A440.1 design value, but specify a window with a higher rating. Thus, if A440.1 suggests a B3 performance level is needed in the field, conduct an ASTM E1105 test (see Section 3) at 300 Pa to achieve the B3 equivalence, but ask for a window that is rated to the B4 or B5 level, to make sure the desired performance level can be achieved.
2. It is important to understand how the window was tested.
The test size, test conditions, actual measured data (as opposed to the A, B or C rating level), and presence or absence of reinforcement all affect the test result, and this information is only available in the full test report. When evaluating the window rating, a professional designer should request the full A440 report for the window. This is issued by a test laboratory accredited by the Standards Council of Canada and contains all information related to the test specimen. The test results should also contain cross-sectional drawings, which are useful for determining how the installed window was anchored and what seals and reinforcing components were in place during the test. It is important to know whether the window was reinforced if energy performance is a consideration. The U-factor of a vinyl window can be greatly reduced by the presence of steel reinforcing bars in the extruded components. This can lead to a dilemma: the designer must choose between the reinforced product (which has a higher U-factor) and the nonreinforced product, which may not meet the A440 minimum levels. Another consideration is that the window may have been tested as part of a combination. This is permitted in the A440 procedure, but each window in the combination must have been evaluated individually. This means that (for example) a window that is halfcasement operator and half-fixed window may quote ABC ratings, but both the casement and the fixed window must have been evaluated separately to show compliance with the A440 Standard.
3. All windows in Canada have ABC ratings.
All Canadian building codes require windows to meet the minimum A440 criteria of A1 B1 C1. It therefore follows that all windows must have been tested to show that they meet this requirement, before they can be sold. This means that test reports are available for all rated products, and professional designers should insist that this information be provided. If there is no test report, the designer should question the basis for the rated value.
4. All windows are tested under A440 at standard sizes.
The test report will verify the size of window tested, but to comply with the A440 procedure the standard sizes below are used (note: window sizes are always quoted as width x height): Before specifying a requirement for ABC ratings, it is incumbent on the designer to do some homework to make sure that the desired window rating is available for some products. It is not useful to specify A3 B7 C5 (that is, simply choosing the highest level for all parameters) if such a product does not exist.
Moreover, asking for the top ratings without any consideration for cost does not constitute best service to the client, and sends a signal to the contractor that the designer has not taken the time to consider the design requirements. This may have unfortunate consequences when the bids are opened. Reviewed by Jan Luistermans.