Ensuring Energy Efficiency: Specifying windows and curtain walls for cold climates

Thermal testing
Before the advent of finite element computer thermal modelling tools, the only reliable way to determine thermal performance characteristics of complex frame-glass assemblies was in a guarded hot box test facility constructed per ASTM C1199, Standard Test Method for Measuring the Steady-state Thermal Transmittance of Fenestration Systems Using Hot Box Methods.

The guarded hot box is essentially a large highly insulated room divided into cold and warm sides. (An insulated wall separates the two, and it is into this wall the test specimen is mounted.) On the cold side, a large fan simulates a brisk wind directed at the exterior face of the test specimen. Cold side temperature is held constant at either −18 C (0 F) or −30 C (−22 F).

The warm side is held constant at room temperature by a small electric heater. By measuring the energy necessary to heat the warm side, an indirect measurement of heat loss through the specimen in W (or Btu) per hour is made. As both the insulating walls surrounding the specimen and the warm side’s chamber walls are highly insulated against extraneous heat loss, only small adjustments must be made to these heat loss measurements. In this way, the warm side is ‘guarded’ against heat loss, hence the apparatus’s name.

Thermal transmittance
The energy input measured in the guarded hot box test, approximating test specimen heat loss, is divided by specimen area and the temperature drop air-to-air to yield the ‘normalized’ U-Factor. This measure of air-to-air heat flow per unit time, area, and temperature drop is the reciprocal of the familiar R-value cited for insulation products.

With U-factors, it is a case of the smaller, the better. The measurement is employed with project-specific parameters (e.g. wall area and climatic conditions) to model building performance. For example, mechanical engineers use this data to estimate annual energy consumption for both heating and cooling, and to determine peak loading for sizing boilers and chillers. Local effects and cold spots can impact occupant comfort as well.

It is important to compare ‘overall unit’ U-factors, including not only centre-of-glass area, but also edge-of-glass and frame areas. Overall U-factors range from:

  • 6.8 W/m2·K (1.20 BTU/hr·sf·F) for single glazing in non-thermal barrier frames;
  • 3.4 W/m2·K (0.60 BTU/hr·sf·F) for uncoated insulating glass; to
  • 1.1 W/m2·K (0.20 BTU/hr·sf·F) for state-of-the-art, multi-thermal barrier systems with triple insulating glass.

Condensation
During the guarded hot box test, interior surface temperatures are monitored at a number of locations on the test specimen after steady state conditions are reached. These are used to calculate unit-less ratios, such as Canadian Standards Association (CSA) Temperature Index (I) or the American Architectural Manufacturers’ Association (AAMA) Condensation Resistance Factor (CRF).

CRF is defined by AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, as:

CRF = {[min(FT,GT) − Text]/(Tint − Text)} x 100

Where:
FT = Average frame temperature (adjusted for cold points);
GT = Average glass temperature;
Tint = Interior ambient temperature; and
Text = Exterior ambient temperature.

CC_Oct13_HR-22With most standard products, CRFs range from 29 (for a single-glazed non-thermal aluminum window) to 52 (for standard single-seal insulating glass with aluminum spacer in a thermal barrier frame). The factor reaches as high as 80 or more in state-of-the-art, arctic-performance windows and curtain walls. A higher CRF means greater resistance to condensation.

In Canada, the CSA Temperature Index (I) is used to quantify condensation resistance. It is essentially the ratio of the difference between an average inside surface temperature and the outside air temperature and the difference between the inside air temperature and the outside air temperature.

I = [(Ts − Text)/(Tint − Text)] x 100

Where:
Ts = Average frame temperature;
Tint = Interior ambient temperature at 20 C (68 F); and
Text = Exterior ambient temperature at −30 C (−22 F).

While this calculation is similar to the AAMA CRF, it differs in the outside and inside temperature settings, as shown in Figure 1.

The Temperature Index is dimensionless and expressed as a number between 1 and 100, obtained under standard test conditions as prescribed in CSA A440.2, Energy Performance of Windows and Other Fenestration Systems. A higher ‘I’ means greater resistance to condensation.

There is a third condensation metric, the National Fenestration Ratings Council’s (NFRC) Condensation Resistance (CR). While reported as part of the NFRC Certified Products Directory, CR as determined by finite element modelling and calculation in accordance with NFRC 500, Procedure for Determining Fenestration Product Condensation Resistance Values, has not been widely cited in project specifications to date.

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