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

Air infiltration
Air infiltration through windows and walls is important to total building energy performance, as it not only takes energy to heat or cool infiltrating air, but also may require latent energy to remove undesirable humidity during summer. Air infiltration performance usually is considered separately from other thermal performance characteristics. For consistency and comparability, standardized U-factors and SHGCs assume no air infiltration is present.

The solar spectrum ranges from ultraviolet through various visible wavelengths to infrared. Data courtesy Lawrence Berkeley National Laboratory. Image courtesy Haglund Design Inc.
The solar spectrum ranges from ultraviolet through various visible wavelengths to infrared. Data courtesy Lawrence Berkeley National Laboratory. Image courtesy Haglund Design Inc.

For the purposes of energy modelling, all new fenestration systems are usually assumed to perform comparably with respect to air infiltration. However, the same is not true when replacing old, leaky windows.

Based on the American Society of Heating, Refrigerating, and Air-conditioning Engineers’ ASHRAE Handbook of Fundamentals, air infiltration rates for various types of windows can be assumed to be:

  • 0.5 l/s·m2 at 300 Pa (0.1 cfm/sf at 6.27 psf) for fixed windows or curtain wall;
  • 0.5 l/s·m2 at 300 Pa (0.1 cfm/sf at 6.27 psf) for new AW Class operable windows;1
  • 12.5 l/s·m2 at 75 Pa (2.5 cfm/sf at 1.57 psf) for existing non-weatherstripped hung or sliding windows;
  • 5.0 l/s·m2 at 75 Pa (1.0 cfm/sf at 1.57 psf) for existing weatherstripped hung or sliding windows or non-weatherstripped awning or casement windows; and
  • 2.5 l/s·m2 at 75 Pa (0.5 cfm/sf at 1.57 psf) for existing weatherstripped awning or casement windows.

(Note: 0.5 l/s·m2 at 300 Pa (0.1 cfm/sf at 6.27 psf) is equivalent to 0.2 l/s·m2 at 75 Pa (0.04 cfm/sf at 1.57 psf.)

For calculating energy impacts of air infiltration, tested rates are adjusted for wind velocity at the window face. Site monthly day/night weather data includes both average wind velocity and direction. Only the windows on windward elevations exhibit air infiltration at any one time.

Uncontrolled air infiltration through cracks and voids in the building envelope increases building energy consumption. By putting untimely heating and cooling load into return air, and/or adversely affecting comfort and air quality in the conditioned space, uncontrolled infiltration creates HVAC load. Conversely, ‘controlled ventilation’ usually is defined as the regulated and relatively steady air supply necessary to maintain indoor air quality (IAQ), makeup for equipment exhaust, or balance exfiltration due to positive building pressure.

As long as the exterior average air temperatures for the period in question are known (from meteorological data), it is relatively easy to calculate the energy necessary to raise or lower infiltrating air to the targeted interior temperature. These are called ‘sensible’ energy expenditures.CC_Oct13_HR-28

Hs = c·ρ·QW·(Tin – Tout) W (Btu/hr)

Where:
Hs = Sensible energy to raise/lower air from Tout to Tin in W (Btu/hr);
c = Specific heat of air ~ 512 J/kg·C (0.240 Btu/F·lb) at standard temperature and pressure (STP);
ρ = Standard density of air ~ 1.2 kg/m3 (0.075 lb/cf) at STP;
QW = Air infiltration adjusted for wind at the window face in L/s (cf/hr);
Tin = Interior ambient temperature target; and
Tout = Exterior ambient temperature.

Assuming humidity control is present in the building’s HVAC system, the energy necessary to dehumidify infiltrating air in the summer is called ‘latent’ energy expenditure, because of the need to overcome water’s latent heat of evaporation.

Located in northern New Hampshire, Mount Washington calls itself “home of the world’s worst weather.” With conditions rivaling those of Antarctica, the high thermal performance, operable windows on the Summit Building maintain a comfortable interior in spite of the frigid outside climate. During the few nice days, the windows can be opened for fresh air and natural ventilation. Photo courtesy Wausau Window and Wall Systems
Located in northern New Hampshire, Mount Washington calls itself
“home of the world’s worst weather.” With conditions rivaling those of Antarctica, the high thermal performance, operable windows on the Summit Building maintain a comfortable interior in spite of the frigid outside climate. During the few nice days, the windows can be opened for fresh air and natural ventilation. Photo courtesy Wausau Window and Wall Systems

HL = L·ρ·QW· (Win – Wout) W (Btu/hr)

Where:
HL = Latent energy to add moisture to, or remove moisture from, infiltrating air to go from Wout to Win in Watts (Btu/hr)—summer Win can be derived from 60 per cent relative humidity (RH) maximum for comfort;
L = Latent heat of evaporation H2O ~ 2460 J/g (1060 Btu/lb);
ρ = Standard density of air ~ 1.2 kg/m3 (0.075 lb/cf) at STP;
QW = Air infiltration adjusted for the wind at the window face in L/s (cf/hr);
Win = Interior humidity ratio target in kg (lb) H2O per kg (lb) of dry air; and
Wout = Exterior humidity ratio in kg (lb) H2O per kg (lb) of dry air.

A 12-hour operating schedule is appropriate for summer air infiltration calculations in an office building occupancy.

During daytime hours on cool, sunny days in the swing seasons, the uncontrolled exterior air entering leaky existing windows may provide some temporary natural cooling to offset solar heat gain, especially in buildings equipped with economizers. (For conservatism, one can count only those months’ energy savings due to reduced air infiltration when it is fairly certain the HVAC load would never benefit from air leakage.) When HVAC is not required, operable windows provide a seasonal opportunity for natural ventilation and connection with the outdoors, supporting sustainable design goals.

While not reflected in the aforementioned calculation procedure, excessive air leakage also can adversely impact a building’s stack effect, condensation, thermal comfort, and draftiness.

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