Whenever the surface temperature of any part of the window is low enough, the humidity in the room air will condense on the cold surface. If condensation continues long enough, a significant amount of water can build up. This can lead to damage of interior finishes, mold growth and—if the water drains into the wall below the window—perhaps damage to the wall assembly. While many factors affect the formation of condensation (for example, presence of drapes, air circulation in the room, interior humidity levels) the window itself contributes to the problem if the surfaces are too cold. Therefore, it is useful to be able to rate a given window design for its potential to allow condensation.
The CSA A440 Standard describes a method of rating a window’s condensation potential. A window is tested in a hot-box chamber, its surface temperatures are measured at specified locations on the window, and a weighted average interior surface temperature is determined. The “Temperature Factor” is then calculated according to:
TF = (Ts–To) ÷ (Ti–To) x 100 <1> where Ti and To are the indoor and outdoor air temperatures in °C, and Ts is the average room-side surface temperature measured in the test.
For the standardized conditions of Ti = 21°C and To = –18°C, then TF = [(Ts +18) ÷ 39] x 100 <2> The TF is non-dimensional, and represents the interior surface temperature relative to the interior and exterior air temperatures.
TF = 0 implies Ts = –18°C, which is the same as having no window at all (because the interior surface temperature is the same as the outdoor temperature). TF = 100 means that Ts is +21°C, the same as the room-side air temperature. This window would be a theoretical perfect insulator, and the best possible rating. Thus, TF ranges between 0 and 100, with a typical value for a clear double-glazed window with a metal frame being about TF40.
TF is assumed to be independent of Ti and To, so that given ambient conditions of Ti and To, one could use Equation <1> to calculate Ts from a known TF.
For example, given a design condition of To = -9°C for Vancouver and Ti = 20°C, a TF of 40 implies that Ts = 2.6°C. This by itself is not enough information to predict condensation potential. You must also know the relative humidity or dew point temperature of the room air. A relative humidity of 30 per cent (reasonable indoor conditions for Vancouver in winter) gives a dew point temperature of approximately 11°C. The estimated surface temperature Ts = 2.6°C is well below the dew point value, so one would expect a significant amount of condensation to form on this window.
There is no minimum TF requirement, and the procedure is not a mandatory part of the CSA A440 Standard or the Building Code. The A440 user’s guide provides some guidance on the use of this parameter, and little more can be added in this article.
Regulatory considerations
Some performance parameters are dictated by local building codes. These generally relate to life and safety concerns, such as fire or unauthorized entry. In some jurisdictions, impact resistance is an important consideration. In Florida’s Dade County, windows must pass an impactresistance test that involves firing a length of dimensional lumber from spring-loaded cannon at the window assembly, to simulate the condition of wind-driven debris in a hurricane. Windows with tempered or laminated glass can pass this test, but usu ally steel shutters are required. Although this may not be of direct interest to Canadian readers, the point is that designers should be familiar with local code requirements.
The CSA A440 Standard contains a method to rate windows against forced entry. The test is relatively simple, but it does not guarantee prevention of unauthorized entry (after all, most standard windows are made of glass, which is not that difficult to break). Still, due diligence requires that designers should consider this aspect of window performance.
Windows can also be rated for fire resistance.
Although most vinyl-framed windows would not pass the fire test, they are suitable for use in sprinklered buildings. Again, it is incumbent on the designer to be familiar with local code requirements.
Several safety tests are included in the A440 Standard, including safety drop, blocked operation, sash strength and stiffness, screen strength, ease of operation and sash pull-off. These tests are intended “to ensure that an installed window will perform to a reasonable degree of satisfaction under conditions of normal use and to a limited degree of abuse or malfunction.” Windows that do not use the materials, preservatives, coatings and so on specified in the standard do not conform to A440 and do not meet Building Code requirements. Therefore, besides meeting the performance criteria (minimum of A1, B1, C1 and several criteria related to blocked operation, ease of operation and so on) a window must meet prescriptive criteria (materials, minimum component thicknesses, glazing clearances and so on) to conform to local codes. Window designers and specifiers should be familiar with all aspects of the criteria listed in the A440 Standard.
User concerns
All the design parameters this article considers are user concerns. Some less technical aspects of window performance tend to be foremost for users, and these are briefly discussed here. Reviewed by Moishe Alexander.
