Appendix 5C: Superwindows: ten benefits from one expenditure

Superwindows are probably the single most important technology for making possible very energy-efficient, cost-effective, and delightful buildings. Their special, invisibly thin high-tech coatings let visible light through but reflect infrared heat rays, while their inert heavy-gas filling, typically krypton, make them block the flow of heat and noise twice as well as air filling does. These features cost extra: in small retail quantities, a cold-climate superwindow with more than four times the insulating value of clear double glazing1 costs about 15–20% extra in competitive markets. Fancier versions can cost even more. So how can superwindows make a building less expensive to build? The key to their economic advantage is that they provide ten distinct benefits from one energy-saving investment, and several of those benefits by themselves are big enough to pay the superwindow's extra cost.

Most energy engineers traditionally choose windows based only on two technical factors: their insulating value, and what fraction of the sun's total energy they let through. 2 Just as with insulation thickness, if you ask a homebuilder or a traditional mechanical engineer how well your windows should insulate, you'll typically be told that nothing beyond double glazing is cost-effective: the extra cost of a better-insulating window wouldn't save enough energy over time to pay for itself. But that's the same fallacy as evaluating the insulation level for a house based only on energy savings without also counting capital-cost savings. In fact, it's much worse. Here's a more complete list of the main benefits from using a superwindow, geared to both residential and commercial construction:
  1. It saves heating energy, cooling energy, and air-handling and pumping energy to deliver the heat and coolth inside the building (fan and pump energy vary as the cube of flow).
  2. It controls radiant temperature, reflecting your body heat back at you like a transparent blanket in the winter and blocking radiant heat from beating in on you in the summer. Most mechanical engineers ignore radiant temperature because ordinary thermostats don't measure it, but our bodies certainly feel it: if the radiant temperature is the same in all directions, the body perceives the average of the air and radiant temperatures. Superwindows therefore provide better year-round radiant comfort, so you don't need to have so much hot or cold air, respectively, blown at you to help you feel comfortable.3 In practice, good superwindows let you set the thermostat several degrees lower in the winter and higher in the summer, at least for people toward the outside of the floorplan; yet they provide the same comfort. Those thermostat resets significantly increase the energy savings calculated under #1.
  3. Superwindows let you make heating, cooling, and air-handling equipment smaller, simpler, or even unnecessary, thus saving capital cost as well as operating cost. Eliminated HVAC systems are worth close to $3,000 a ton in avoided capital costs. Superwindows often make the difference that makes completely passive cooling sufficient even in hot climates.
  4. Less cold air needs to be delivered to the space for cooling if superwindows are coupled with more efficient lights, dimmed by daylighting through the same superwindows, and ideally also with more efficient office equipment. In commercial building renovations4, the ducts can therefore be smaller: they lose more size because of smaller cooling loads than they gain due to reduce friction and save fanpower as noted earlier. Moreover, the ducts can be not rectangular but round, greatly reducing their cost.5 Smaller, round ducts also take up less ceiling height, with important implications described later. Smaller ducts also mean less floorspace wasted on floor-to-floor vertical ductwork. Mechanical rooms also get smaller and quieter, leaving more usable space to rent. Space efficiency increases by several percentage points. This may be worth even more than the downsized or eliminated HVAC system (#3).
  5. The superwindow blocks ultraviolet rays very well, reducing fading of furnishings, finishes, and art.
  6. It blocks noise better, reducing tension for occupants and making it possible to use more difficult sites, such as near major highways.
  7. The inner lite of glass always stays warm and dry, so there's less risk of condensation that could damage the sash and surrounding drywall.
  8. Because of the greater radiant comfort in winter, and because the warm inner lite doesn't shed cold air to lap around your ankles, perimeter zone heating (the little radiators normally put under exterior windows) may no longer be needed—even in climates like Stockholm or Calgary. The elimination of this capital cost immediately pays for the extra cost of the superwindow. A valuable side-effect is that about 5% of perimeter offices' floorspace is freed up: you no longer need to worry about blocking the radiator, and can rearrange the office with greater flexibility.
  9. Superwindows don't trade off optical against thermal performance, so they make it much easier to let daylighting in and control where it goes, distributing it plentifully and evenly but without glare. Daylighting can save a great deal of lighting energy and capacity, hence cooling and air-handling capacity that would otherwise have to take away the heat of the lights.
  10. By independently controlling heat and light flow on each side of the building through "tuning" of the glazing's spectral response, superwindows make the building's behavior far more passive, greatly reducing or even eliminating the need for costly and maintenance-intensive control systems.
These improvements in thermal, acoustic, and visual comfort are the prime causes of the 6–16% improvements in labor productivity commonly found in well-designed green buildings (Chapter 5/Building Blocks)—a value roughly 6–16 times greater than the entire energy bill. Even ignoring that entirely, and counting only the direct engineering-economic benefits, it is clear that the key to inexpensive buildings is often to specify more expensive windows.

This isn't just theory. Adding superwindows and some other measures as part of a whole-system design enabled architect Teresa Coady to make an 84,000-square-foot British Columbia office tower twice as energy-efficient as its previously built twin, despite increasing its glazed area to 60% and bringing in 50% more fresh air. The fourfold smaller chiller—50 tons rather than the twin tower's 200—more than paid for the extra cost of the glazing and other improvements, resulting in a slightly reduced total construction cost.6

Likewise, in a 127,800-square-foot laboratory wing at the University of British Columbia, completed in 1995, architect Terry Williams used superwindows, daylighting, and other whole-system design elements to eliminate mechanical cooling altogether, substituting only a passive technique—automated "night flush" ventilation.7

1 Such as Hurd's "Insol8," Southwall's and Alpen's "Superglass," and equivalents from Pella and others.

2 The latter is measured by their "shading coefficient," which is proportional to the solar energy transmitted rather than, as one might suppose, to how much energy is blocked.

3 Today's best superwindows can admit light while rejecting heat to about the maximum extent that's theoretically possible: their visible transmittance can be slightly more than twice their shading coefficient.

4 This is because new commercial construction should generally use the underfloor displacement ventilation described in Appendix 5E. With that approach, reduced duct size is not valuable because there are almost no ducts.

5 Because round ducts enclose more volume with less metal and provide greater bursting strength against high air pressure, they use about 70% less metal, and ducts are bought by the pound. The labor costs of installing the ductwork go down even more: one worker can handle a section of duct manyfold longer, and the joints go together faster and better with much less leakage. Duct and piping together total about half the installed cost of complete HVAC systems in large buildings.

6 Rocky Mountain Institute 1998: Green Development: Integrating Ecology and Real Estate (by Wilson, A., Seal-Uncapher, J.L., McManigal, L., Lovins, L.H., Cureton, M., & Browning, W.), John Wiley & Sons, New York NY at p.43. Accompanied by a CD-ROM of 100 case-studies, Green Developments, available (as is the book) from Rocky Mountain Institute (1739 Snowmass Creek Rd., Snowmass CO 81654, 970/927-3851, FAX -4510, ) under publication numbers D97-11 and D97-12 respectively.

7 Id., p. 50.

To make comments or report problems with this site, please contact

© All rights reserved. Published by Rocky Mountain Institute.
2317 Snowmass Creek Road  |  Snowmass, CO 81654-9199  |  Ph: 970.927.3851


Powered by Intrcomm Technology's SMC