Low-E Glass for Cooling Loads

Mark Meshulam is an expert witness and consultant for glass, windows and building facades.

The previous post focused on how Low-E glass reduces the loss of room-side heat by reflecting long-wavelength infrared radiation back into the room. This would be typical of a Chicago-based dude who hasn’t seen much sun since last September.

Nonetheless, your Chicago Window Expert recognizes that there are folks out there much smarter than he, who live in climates basking in sunlight, where beautiful bronzed people roam, where bikini budgets exceed that of sweaters, where tanning oil is consumed the way we Northerners consume heating gas. Such people should be flogged! But I’m not jealous.

As my friend Solomon from LA once said to me, smiling a goofy grin as we bicycled along Santa Monica beach among throngs of burnished pagans, “Nothing of importance was ever accomplished in a warm climate”. Take that!

To show my equanimity toward our sun-baked brethren, today I will focus on the use of Low-E glass in controlling cooling loads. When I refer to cooling loads, I mean the exterior heat and sunlight which warm our buildings and cause us to spend lots of money running the air conditioning. Obviously our sun worshippers in the South pay dearly for their sunlight, but even we in the Northlands suffer many a hot and sticky day in the summer and our AC units run furiously.

A lesser known fact is that in our Northern downtown office buildings, cooling loads are much more of a cost than are heating loads. This is due to the way one of my favorite ratios, the surface area to volume ratio, plays out as buildings get bigger.

One of the biggest factors for the ability of any object (including living creatures) to dissipate its heat is the ratio of that object’s surface area to its volume.

Geometry question: what geometrical shape has the lowest surface area in relation to its volume? Answer: a sphere. As buildings, or people approach a spheroid shape, or have a larger footprint, they lose surface area as a percentage of their volume.

Surface area is needed to radiate heat away from the object. the less area, the harder it is to dissipate heat. A close to home example:

Gomer Pyle’s famous backhanded compliment to a Rubenesque lovely, “For a fat girl you sure don’t sweat much!”

Large buildings are the fat girls and boys of construction. Even in cold weather, they have trouble dissipating heat. The fact that we insulate more now than ever before exacerbates the heat dissipation issue. Throw in the fact that office buildings run lots of heat-producing equipment and house gaggles of warm-bodied junior executives, and you have a formula for a big cooling cost.

A well chosen Low-E glass will contribute to the solution by reflecting away heat energy before it can enter the building, and by possibly allowing interior radiant heat to escape.

Manufacturers of Low-E glass and coatings have developed products which are contoured for warmer climates or larger buildings. One of the tricks here is to create the product that also allows good light transmission and does not look like a mirror.

Many cities – take Dallas for example – have struggled with solar heat gain for decades. For decades their response was to build buildings with great expanses of highly reflective curtainwalls, giving the skyline a unique glistening appearance. But there were two downsides: adjacent buildings were blasted by the reflections, and interior views were dimmed by thick glass coatings and colorations.

A more current trend is to work toward balancing the loss of visible light with the need to reflect heat, while also getting away from the mirrored look. Each year more is accomplished in these areas. Major players in this field in the USA market are PPG, Guardian, Viracon. Next week, we will talk with all of them and hear not only their solutions, but where we are all going with energy and windows in the future.

Are you loaded with cooling costs?