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Advertising supplement provided by Pella Windows

Energy Efficiency

The energy efficiency of the windows has a significant impact on the overall annual operating costs of a building. To obtain the best energy efficiency from a window three factors must be evaluated: thermal performance, condensation resistance and occupant comfort. Additionally, because installation can affect window performance, the manufacturer’s installation instructions should be evaluated and followed for optimum performance.

Wood windows optimize design performance and budget flexibility at the F.M. Kirby Shakespeare Theatre, Drew University, Madison, NJ, by Ford Farewell Mills and Gatsch, Architects.

To compare thermal performance look at the R-values and U-values. The R-value measures resistance of glazing material or fenestration assembly to heat flow. Materials with higher R-values have higher resistance to heat flow and are better insulators. The U-value measures the rate of non-solar heat loss or gain through a material and may be expressed for the glass alone or the entire window including frame and spacer material. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value.

Choice of materials has a bearing on both these factors. For example, in Chapter 30 of the 2001 ASHRAE Fundamentals Handbook, the addition of a thermal break in an aluminum frame improves the U-value from 1.92 to 1.13. An aluminum-clad wood frame further improves the frame U-value from 1.13 to 0.51.

The handbook also indicates that total Unit U-values for the entire window include both the frame and glass. The range for aluminum windows with thermal breaks is from 0.35 (triple glazing with low-E coatings) to 0.64 (double glazing with clear glass).

For aluminum-clad wood windows the range is from 0.25 (triple glazing with low-E coatings) to 0.53 (double glazing with clear glass). The glass is the same so the difference in U-values is a result of changing frame material.

The National Fenestration Rating Council (NFRC) is a reliable source for determining window energy properties and comparing products. NFRC rates window units for U-factor, SHGC, visible light transmittance (VT), and air leakage (AL).

Another factor that affects window thermal performance is air infiltration. Air leakage heat loss and gain occur by infiltration through cracks in the window assembly. This is indicated by an AL rating expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. Select windows with an AL rating of 0.30 cfm/sq ft or less.

Resistance to condensation also is important. Condensation on window frames can damage interior windowsills, finishes and eventually walls and floors. Specifying products and design conditions that regulate surface temperatures and moisture in the air will minimize potential for condensation.

Materials with greater resistance to condensation also provide greater comfort to occupants seated near windows, and occupant comfort has a direct impact on worker productivity.

Wood is considered the best performer from a thermal point of view. It is a natural insulator with excellent resistance to condensation and a high degree of thermal comfort.

Aluminum is a natural conductor and therefore has poor energy efficiency and low resistance to condensation. Aluminum’s R-value is minimal and raises the overall U-factor of a window unit. In cold climates, an aluminum frame can become cold enough to condense moisture or frost on the inside surfaces.

A desirable combination for thermal performance and comfort is an aluminum-clad wood window. This alleviates differences in exterior and interior frame temperatures, offering resistance to condensation, greater thermal comfort and lower annual energy costs than thermally-broken aluminum windows.

The average annual savings from the use of aluminum-clad wood windows is fairly consistent across the United States: 27% to 31% compared to single-glazed aluminum windows without thermal breaks; 6% to 9% compared to thermally-broken aluminum windows.

Vinyl has the same thermal efficiency as wood and in terms of thermal performance vinyl frames are comparable with wood. However, vinyl’s expansion and contraction encourages air and water infiltration which increases maintenance and damage issues.

Fiberglass offers thermal efficiency similar to wood and the lowest expansion and contraction rates of all four materials. Less expansion ensures a consistent bond between sash and glass and reduced energy costs as well as less stress on joints for greater durability.