Design strategy for low e windows with effective insulation

Whether you want to save your wallet, or save your planet; effective insulation of structures is essential. It turns out that windows are much more interesting and challenging design problem than I appreciated. This is a 2 part story, if you hate serials, the whole story is at www.impattern.com.

As a warm blooded species, our body temperature (37C/98F) determines the temperature that we feel most comfortable, we are most comfortable in an environment that is just cool enough to remove the heat we generate internally. The heat and air conditioning are used to keep the structure at a constant temperature, around 68F/20C. When the structure is hotter that the environment then energy flows out of the structure, and when structure is colder and energy flows into the structure. The bigger the temperature difference, the greater the energy flow. The energy is being exchanged by convection from the air and radiation.

The schematic below illustrates the three uses cases; day time hot climate, day time cold climate and night time. The sketch shows the complete range of energy transfer in the vertical axis; from UV radiation at the top, through visible and near IR down to mid IR, with thermal convection at the bottom. The primary source of radiation is the sun which acts as a hot black body source of photons. Our eyes have evolved with to be sensitive in the “visible” range of 300-700 nm, right at the maximum in solar output. The window must be transparent on the visible to be useful !

The case that most people relate to is the desire to reduce air conditioning during the day in hot climates illustrated at the top of the schematic. In hot climates, the goal is to reduce solar near IR that passes through the window, which is accomplished using spectrally selective coating such as thin silver layers. Only 12 out of 140 US cities have a annual average greater than comfort. Moving down to lower energies, the earth and all its structures also act as a black body that re-radiates in the mid IR centered on 10 um. Window glass absorbs in the mid IR, so mid IR is transferred through glass by absorption and re-radiation. Re-radiation is reduced using coatings that have low “emissivity” of black body radiation. Finally, thermal convection is reduced by using multiple pane windows with a thin layer of static air that acts as a thermal insulator.

The second use case, is daytime solar heating in cold climates. In this case, the goal is to make the window transparent to the solar near IR so as to heat the structure during the day, while minimizing thermal losses in the mid IR and thermal convection from the warm structure. The vast majority of US cities, 128 out of 140, have average annual temperatures less than comfort, also true for majority of industrialized economies. The cold climate case is the most important application in terms of commercial market and planetary impact.

The third use case, occurs at night where the structure is much warmer that the environment and the goal is to reduce both re-radiated heat loss in the mid IR, and thermal convection. For cold climates, this means that the temperature difference from comfort at night is larger than day, so insulation against night time convection and re-radiation loss is more important than solar heating.
The optimal performance of windows designed using commercially available glass is also shown in the schematic.

Schematic of energy transfer in hot and cold climates, and at night. The optimal window performance in both climates based on windows designed using selection criteria applied to the IGDB data base of window glass.

Engineering performance for the 4 different energy bands can be summarized as;
Multiple pane windows for low thermal conductivity
Low emissivity coatings for reduced mid IR re-radiation
Multilayer optical coatings that either maximize or minimize solar near IR radiation, while maximizing visible transparency.

Next time, I will show how analytical models can be used to create design rules for glass selection and window designs resulting in improved windows. The whole story is available at www.impattern.com.