The glass and glazing industry understandably prefers the window-to-wall ratio (WWR) in buildings to be more window and less wall—or more specifically, more glazing and less opaque non-glass material.
That was the topic of discussion during a recent webinar hosted by ComEd Energy Efficiency Program and Seventhwave. It covered the broader conversation of the window-to-wall ratio but focused in heavily on the multifamily high-rise segment.
Architect Allison McSherry of Klein & Hoffman gave a detailed rundown of the current energy code requirements architects face when designing a building envelope, as well as the available compliance pathways. Architect Brett Bridgeland of Seventhwave then talked about putting building envelope design decisions in context—particularly regarding daylighting and views.
Bridgeland said that while daylighting is a key driver in maximizing a building’s glazing area, in many cases, “less is more.”
He demonstrated modeling examples in which a building with a “strategic” 40-percent WWR had a higher Useful Daylight Index compared to one with an 85-percent ratio. He also showed a graph demonstrating trade-offs between natural light and electric lighting, noting there is an optimum transition point around the 20- to 30-percent WWR.
“The point here is that in most cases, designing the window-to-wall ratio for the sake of saving electric lighting probably isn’t a valid strategy, and that will become even more so as electric lighting and LED lighting is becoming more efficient on its own,” he said.
“Of course we know we want great views out of our home and office,” said Bridgeland. “…It’s a more complex optimization problem than just maximizing the amount of glass in the space.”
He showed two rendering examples—one featuring windows that span from side-to-side and floor-to-ceiling vs. another with limited glazing in certain areas (see image to right). He said in a building such as the first example (on right), people often have their blinds closed due to privacy, glare or thermal comfort. The second example shows how strategic optimization of glazing can maximize views, because “Windows that don’t have their shades closed provide more views.”
That design strategy includes limiting glazing in private areas such as bedrooms while prioritizing it in living rooms and more “active” spaces.
Bridgeland added that a glazing area with a high window-to-wall ratio will require glass with a high solar heat gain coefficient, which he said can mean a darker glass. “So if we’re talking about maximizing views and making it a great energetic day-lit space, it’s not just the amount of glass, it’s also the properties of that glass,” he said.
Another solution to consider is the use of external shading devices, which he said may improve comfort on certain sides of a building but can have limitations on others depending on orientation. He added that they provide a “minor reduction of annual energy cost” and can “adversely impact passive heating.”
Ultimately, he said the key is to be deliberate about the design.
“In the case of multifamily residential, concentrate the expanses of glass in the living spaces where they will be best used, and where people are awake and occupying the space,” he said. “And be more strategic in the sleeping spaces where you’re probably going to have the blinds closed anyways.”
McSherry added that in her line of work, she is often solving issues with current buildings and is observing their building envelopes from both the outside and inside.
“Pretty much all the ones that have floor-to-ceiling glass, bedrooms always have the shades drawn,” she said. “You never have the bed against the window because you’re getting either thermal gains or losses … it limits the layout you can have with your furniture. The same thing in the living room. Unless you have a really large unit, you can only lay out your furniture set a certain way. And again, we see a lot of shades drawn due to glare.”
She noted that this also applies in an office setting. “Anything that is below your worktop surface, you’re not going to take advantage of [in terms of view],” she said.
In answering a participant question at the end of the webinar, Bridgeland conceded that views—or lack thereof—can impact property values, and considerations for office buildings are different than multifamily. “Now you’ve got to start thinking about, ‘what’s the interior planning grid, and what’s the module for how the space is going to be used?’ One strategy there is raising the sill height—do you need to have vision glazing at your feet? But absolutely, views are important to property values.”