I know a lot of Architects, but that doesn’t make me one.
Still, I spend quite a bit of time thinking about glass and glazing in buildings (just like I’m sure my architect friends spend lots of time thinking about pumps and fans…)
Glazing can take many forms in a building application: curtain wall, operable windows, storefront, skylights, etc. And while there are myriad performance properties that would be essential to consider from a building occupant, owner or architectural perspective, the ones called out specifically in energy codes and standards are u-value (u-factor) and solar heat gain coefficient (SHGC).
Essential Glazing Factors
U-value represents the thermal transmittance, or heat transmission through a unit area of material or construction assembly and the boundary air films, induced by unit temperature difference between the environments on each side, measured in Btu/hr-sq ft-°F. (If you think these units look familiar, flip them over and you have R-value!)
SHGC is the ratio of the solar heat gain entering the space through the fenestration area to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and absorbed solar radiation, which is then transferred into space. The fenestration area (at least as far as energy codes are concerned) refers to the total area measured using the rough opening and including the glazing, sash, and frame. So when the standard calls out an assembly maximum, it’s not just center-of-glass values we need to take into consideration. (More on that to come in a future post).
Oftentimes, decisions regarding building glazing can contribute upwards of 40% of the required cooling load of a building. I hate to use a ‘typical’ load example, but something like the graphic below isn’t totally unrealistic, assuming a ‘reasonable’ window-to-wall ratio (another topic, for another day).
In buildings where energy-efficiency is critical, the detailed specification of high performance envelope components is becoming more and more critical. It’s no longer enough to specify ‘low-e glass with thermally-broken framing’ – because this leaves a big space for interpretation, which has widely varying implications on both cost and energy, for the lifetime of the building. (This is also true for insulation specifications). Just like MEP systems, the building envelope should be moving more towards a performance-based-specification.
Glazing and Cooling Loads
Therefore, specific window performance information (u-factor, solar heat gain coefficient, framing assembly details) are becoming more important aspects of project construction documents. And there are many high performance window design strategies available on the market today to maximize efficiency while maintaining visual characteristics.
Essentially a window’s energy efficiency is dependent on its framing, glass and operation.
Frames can conduct heat, and since they’re oftentimes metal this heat gain can be significant, so aluminum frames should always have a thermal break (an insulating plastic strip between the inside and outside of the frame and sash) to reduce heat loss and frame u-factor. Composite, wood, fiberglass and vinyl are also available options (but typically relegated to residential use). They all have pluses and minuses with regard to cost, maintenance and longevity as well.
Operation is another aspect that’s often a moot point in commercial buildings since glazing is usually fixed (non-operable). Some operations (awning, casement, and hopper) have lower air leakage rates than others (double hung or sliding), which should be taken into consideration for operable windows.
Finally, the big variable, glass type… there is no ‘one size fits all’ glass for every application. Final selections for the same building might even differ by orientation or location in the wall assembly. Factors like orientation, climate, and building design all play their part here. And since glazing technologies can be combined, there are many, many combinations available.
There’s a very nice article in Architect Magazine that walks through the ‘window innovations’ of the last 50+ years, starting with double-glazing and low-e coatings to thermochromatic glass and integrated thin film photovoltaics.
And as time marches on, so will our options. It’s just important to know both what you’re asking for, and what those answers may mean.