Low-e Windows Built Using The Design Rules

By Michael P. C. Watts
Last week I identified the four energy bands that determined the effectiveness of windows: visible, solar near IR, re-radiated mid IR and thermal convection. Each energy band requires its own mitigation strategy, dictated by the available materials. This week I will describe how to create low e windows using design rules for each energy band. The whole story is available at www.impattern.com

The first step is to recognize how the materials properties constrain the design, shown below. The different energy bands are shown vertically, with the black body spectrum and optical properties of materials across the top. The thermal properties are shown across the bottom.

Materials engineering the optimal solutions. At top left, the black body emissions of sun and re-radiation from earth. In the middle, the transmission of soda lime glass. At top right, the transmission spectrum of the optimal coatings for cold climates (IGDB#6261), hot climates (IGDB#1506), and emission spectrum of Ag films. At the bottom, thermal convection illustrated by phonon vibrations and the reduction of thermal conductivity with atomic weight for the noble gases.

At wavelengths greater than 3 um (mid infra-red), soda lime window glass is no longer transparent, as shown in the optical properties of glass shown above. As a result, the emissivity of the glass in the mid IR controls radiant energy transfer. The re-radiation energy transfer for a multiple-pane window is determined by the resistance of a series of paired transfers from an emitting surface to an absorbing surface through a transparent air gap. Emissivity is primarily a function of coating conductivity. A window requires the coating to be visibly transparent, which limits the choices. Materials based on tin oxide such fluorine doped tin oxide (SnO2:F) are used an transparent conductors for displays and have an emissivity of 0.25. Very thin (15 nm) silver (Ag) is partially transparent and used in “partially silvered mirrors” with a very low emissivity of 0.02, as shown in optical properties of silver also shown above.

At shorter wavelengths, window glass has excellent transmission, as shown in the glass transmission spectrum above. In cold climates, the solar IR acts to warm up the structure saving heating bills. In hot climates, solar IR adds to heat that must be offset by air conditioning. Therefore the goal is to find coatings that include thin silver layers and function as multilayer filters that either reflect or transmit solar near IR, depending on climate.

There are hundreds of coated “solar glasses” available today, and suppliers do not disclose the materials they use. The Department of Energy has developed the International Glass Data Base (IGDB) and programs (Window and Optic6) to calculate window performance. These programs are the industry standard for reporting window performance to customers.

An optimal window for hot climates requires a high visible transmission with low solar transmission. A search of the IGDB identified the coated glass with the lowest solar to visible transmission, which was combined into a triple pane, with two coated outside panes, a clear glass mid pane, and filled with Krypton. The transmission of the optimal glass is shown in the silver coating spectrum as shown above , with a narrow band pass in the visible and very low transmission in the near solar IR.

An optimal window for cold climates requires a glass with high solar transmission and low emissivity. In the IGDB, the glasses with the highest solar transmission had moderate emissivity. There was one optimal multilayer coated plastic film with moderate solar transmission and low emissivity, shown above.

The next question is how best to assemble the glasses ? The majority of energy transfer in and out of a structure occurs by convection from air to the window at 7 Watts/m2 C, and by re-radiation in the mid IR at 5.5 Watts/m2 C. The thermal conductance “U”, measures the combined convection and re-radiation loss, with units in the US of Btu/h ft2 F. Convection and re-radiation are similar in magnitude, so both losses must be mitigated to produce a low “U” window.

Convective heat transport through a multiple pane window includes a series of steps; transfer between air and glass, conduction through glass, transfer between glass and fill, conduction through fill, etc. Each step acts as a series resistance (“R”= 1/U) to heat transport. The results of an analytical model of heat transport are shown below for an optimal triple pane window with a low conductivity gas (Krypton) as the fill between the panes. The high atomic weight noble gases are preferable as a gas fill as shown above.

Window engineering for convection and re-radiation of an optimal triple pane window, showing silver coatings and Kr fill as the biggest contributors to window insulation.

The re-radiation resistance is also shown above. Each pair of emission and absorption events creates a series resistance to mid IR energy flow. Thus example is for a triple pane window with two moderate emissivity outer panes, the multilayer plastic film as the mid-pane, and Krypton fill had the optimal window properties for cold climates

The complete performance of the optimal hot and cold windows were shown in the previous blog . There is a 2x difference in Solar Heat Gain Coefficient (SHGC) for optimal hot and cold climate windows. The insulation is usually ranked by “R” values (= 1/U), and R= 7-10 for the optimal windows. This is a big improvement over conventional double pane windows with R=2, it is still less than good quality walls at R=20.

Financial impact
The financial impact of the improved window depends on the extremes of the local climate. In Texas, the annual average is 68F and the quarterly seasonal swing relative to comfort at 70F is +_ 15F. The typical energy loss for a modern, well insulated, double glazed house is 1KW, and the monthly AC or heating bill is around $100 a month. In Fargo ND, the annual average temperature is 40F and the quarterly seasonal swing relative to comfort at 70F is +0, -60. For the winter quarter, the heating bill in North Dakota will be roughly 4x the bill in Texas, averaged over the year, the ND bill will be 2x .

Windows cost around $250, low e windows cost 10-15% more. With energy savings of 50%, the payback in Texas on a new house installation with 20 windows is around one year, and on replacing an existing set of windows around 10 years, half the time in North Dakota. Improved windows are best made as a new house construction decision.

—Mike Watts has been patterning since 1 um was the critical barrier—in other words, for a long time. He has a consulting shingle at www.impattern.com.