Window Selection Part 3: Hopper vs Awning

What to look out for

If your design goal is vision or light, go with a fixed window, but each of these three hinged windows offers a distinct ventilation advantage.

Awning windows, opening toward the bottom can let cool ground air in when installed low on a facade, but be careful, high up on a facade they may be more likely to let hot air, rising up the building slip right in. Of course, there are projects where that ability to let cold air convect out and capture rising hot air could be helpful in some situations.

Hopper windows are great for venting hot air out and when paired with a lower window for bringing in cool air can work great with the natural stack effect. Most hopper windows are manually operated with friction hinges to hold them open. This limits their effectiveness to places where users can easily reach them.

Folding windows fit neatly in the middle. Their unobstructed openings can let a lot of air through right at the height of most users. Unlike hoppers and awnings which are often installed close to the ceiling or floor so that they don’t block view, folding windows can be installed at eye level because, when open, they don’t block view.

Since the sides contribute very little to the ventilation effectiveness of hopper and awning windows, it’s helpful when making your selection to remember that a short hopper or awning is as effective as a tall version of the same width. The distance the windows open is generally a function of the hardware, not the size of the window. Taller awnings and hoppers are specified because they look better in some situations but that extra height requires stronger construction and doesn’t produce much more useful ventilation.

Deep Dive

It’s pretty obvious that a window which opens upward will let in more rain than a window which opens downward, but what about when they're closed? For closed windows, we have to look at the design pressure that the window was tested to. The DP, or Design Pressure, rating of a window or door is based on laboratory testing to industry standards so that all windows are tested the same way. The metrics set by FGIA and CSA use a standard test measured in psf, or pounds per square foot. The actual testing is done independently of the standards body.

Testing and Standards

Organizations involved:

The FGIA/AAMA/WDMA/CSA 101/I.S.2/A440 test has evolved over time to include the requirements of roughly 4 groups.

• FGIA (Fenestration & Glazing Industry Alliance)

• AAMA (American Architectural Manufacturers Association)

• WDMA (Window & Door Manufacturers Association)

• CSA (Canadian Standards Association)

FGIA is the current industry body setting and administering the standard.

AAMA (American Architectural Manufacturers Association) and WDMA (Window & Door Manufacturers Association) are two trade groups who previously managed similar standard which were merged into the FGIA. Their names were synonymous with window standards for so long that even though they have merged, the acronym remains part of the name of the standard.

Finally, the CSA Group is the Canadian Standards Association responsible for ensuring the quality of products sold in Canada ensuring that the 101/I.S.2/A440 test is applicable in both the United States and Canada.

Components of the test:

The test has three performance components:

• Air

• Water,

• Structural

Understanding test ratings:

The windows water rating is measured at 15% of the tested pressure. So a window with a DP70 rating is tested to 10.5 PSF for water infiltration under pressure.

The DP rating is also used for the structural test, but for this calculation it is test to 150% of the DP rating. So a window with a DP70 rating is tested to 105 PSF for maintaining it’s structural integrity.

Higher DP ratings resist higher loads and can be a shortcut for understanding the strength of a window but it’s important to understand what Design Pressure a particular project is subjected to. Coastal areas are subjected to both higher water and structural loads and require high DP ratings. A high rise in a dry climate may require the higher structural DP rating but not as much the water load. An low rise inland project not subject to high wind loads may find that a window optimized for strength has less efficient thermal performance and costs more. Consult local codes for wind driven design pressures and your structural engineer for the structural load which can vary greatly between different parts of a building.

Air infiltration through a window is separate from the design pressure. The standard test for air infiltration uses a consistent 1.56 psf load which simulates a 25mph wind.

Understanding class ratings:

Together with this test windows are also subjected to multiple other standards for quality and durability which together add up to a class rating. The classes are R, LC, CW, and AW. These classes started out as benchmarks for different types of buildings, for example an R window used to refer to a residential window which was expected to see less wear and tear than a CW which stood for commercial window. There was no standard for school or hospitals or hotels so the sometimes confusing descriptive names were dropped and now the letters simply indicate different levels of durability. R windows are intended for the least amount of wear and AW represents the highest durability standard.

All of the standards a window is tested against to earth a performance rating together with the results of a windows DP testing are reported as the performance class and performance rating, for example Class AW, Grade 70.

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