Air & Water Tightness and the Building Code

So far in the Master Class, we’ve been discussing thermal performance and how to navigate it in the face of code restrictions. Now we’re moving on to our next lesson, covering two more important aspects of window performance – air and water tightness.

What does the BC Building Code say on window air tightness?

Much like wind load, the airtightness of windows is not explicitly referenced in the Building Code - though it is in the North American Fenestration Standard, which is itself referenced in the code. The Building Code says there is actually no minimum threshold for a window to meet air tightness to be code complaint other than a generic reference to buildings needing to restrict air movement through exterior assemblies, so the specific air tightness limit is generally driven by the specifier’s expectations on performance.

What does the Building Code say on the water tightness of a window?

Water tightness is very similar to wind loads and air tightness. The code references the NAFS standard and Canadian supplement for how to determine the minimum requirement for a window rating – so all windows and doors used on buildings must meet the requirements of NAFS.

The NAFS standard will factor in building height, location, and exposure to determine the water tightness required for a window. It’s worth noting that some conflict can occur on the subject of exposure. In the NAFS standard, you can select open versus rough terrain – this can become a bit subjective. As a result, the Building Code only requires windows to meet rough terrain exposure, and the desire to use open terrain can be a design consideration made by the owner.

There is only one qualifying item in the BC Building Code for water tightness, and it is related to side hinged swing doors. A door that is considered protected (which it defines as the same rule for a window/door to need a head flashing) only needs to meet what is referred to as a Limited Water rating. This is where water is applied using a spray rack with no pressure differential and only applies to swing doors, not sliding patio doors.

So that’s what NAFS and the BC Building Code have to say on these matters – but what about the Step Code? Join us next time to find out and further grow your window and code knowledge!

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