Traditionally, the most thermally efficient buildings contained as few windows as possible. Today, many buildings contain almost 100 percent glass but at what cost? In order to achieve the valued daylighting and views associated with glass, there is a compromise. Glass is inherently more expensive and is thermally inferior to walls.

The thermal component of the building envelope should supply an unbroken thermal barrier to maximize energy efficiency. LEED provides for 19 energy optimization credits and three credits are available for daylighting and views. These three LEED items account for:


With sustainable standards calling for “energy optimization” and “daylighting and views,” how do we address this balance?

R-value is measured by the thermal resistance of a surface calculating heat loss by conduction, convection and radiation. The higher the R-Value, the better the insulation. The thermal conductivity of a good insulated wall would typically be around R-20 hr.ft^2°F /Btu whereas a high performance low-E window would be about an R-3.5. Single glazing (R-1) has poor thermal performance and is appropriate only for applications where interior and exterior temperatures do not vary to a large extent. The vast majority of architectural glazing for storefront windows, curtain walls and skylights consists of insulated glazing units. IGU selection is extremely important in the outcome of the building's thermal performance.

Heat gain and loss also takes place due to thermal bridging through the framing. Glazing systems typically quote center of glass U-value (U-Value is the inverse of R-Value), which is always better than the total system value because heat transfer through the framing is not taken into account.  In addition to heat transfer through the framing itself, there is also a decrease in the R-value 2 inches around the perimeter of the glass lite due to framing. Aluminum has a very high thermal conductivity. Often an element of low thermal conductivity such as rubber or plastic is integrated to decrease the flow of heat through the framing. Needless to say, a high quality framing system with particular attention paid to thermal bridging is imperative in reducing heat loss or gain of the complete system.

Integrating daylighting and thermal performance is not always a cost-prohibitive exercise. When we run informal polls with architects and ask why they do not implement triple and quadruple glazed units with argon or krypton, the resounding response is that the return on investment versus performance delta does not make economic sense. Furthermore without clear daylighting benefits the case becomes harder.

The Centre E&P Senechal Center proves that when it comes to daylighting, you can achieve both form and function on a budget with products like Solera. The building is unique because of the reduced energy costs they have achieved by improving thermal performance. The lights can be run at 50 percent or 100 percent in order to use the energy saving benefits of Solera. In fact, according to the project architect , Monica Adair of Murdock & Boyd, "Once Solera was in place, the use of lights were significantly reduced during construction."

Solera T (R-5) was considered for the project for its daylight diffusing properties and thermal insulation. The translucent glazing units were placed at the level of the walking track, which surrounds the rink, and also around the fitness gym, which opens onto the arena. They wanted to keep their views. They did so by making a pattern between translucent and the vision glass with translucent along the eastern side and up to the north side of the building. This glass band was critical from the very beginning of the concept design. “Long band became the language of the building,” Adair says. “They liked the silhouettes behind the Solera that kept the centre looking animated."  

Murdock & Boyd were motivated by the manufacturer’s complimentary daylighting analysis. The radiance study and simulations identified the potential pitfalls of using clear glass. It also assisted in fine-tuning the appropriate visual light transmittance so that the space had optimal balance between light and diffusion. “All we had to do was define our daylighting goals and submit our existing drawings to them,” Adair says. “They provided us with a daylighting study and recommendations. Instead of simply providing numeric light readings, the radiance study allowed us to visually appreciate the effects of light distribution in some of our spaces.”

Now, a new paradigm in light diffusing glazings is emerging. If low-E has been marketed as “High-Performance Glazing,” then a new breed of “Super-High Performance” translucent glazing has emerged.

There no longer needs to be much of a trade off between using glass and building envelope thermal performance. New Products such as Solera+Nanogel aerogel are entering the market with insulation values up to a center of glass R-17 to R-20. The introduction of aerogel into various architectural products and, now, in glass make it the highest thermally insulated glass units in the world. This is near to the value of an insulated solid wall, yet it still admits full spectrum natural daylight into a building.

In addition to the obvious benefits of being clad in glass, the translucent can be glazed into the same framing system the architect wishes to use for vision glazing areas of the project. In terms of design consideration, daylighting, thermal performance and cost can now be symbiotic factors instead of challenges.