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U-VALUE EXPLAINED

GLOBAL ENERGY CRISES

Until fairly recently, individuals and organisations have considered the option to build environmentally and energy friendly buildings an interesting option that is pursued by the wealthy minority. Historically, low energy costs and little/no legislative governance made this view the generally accepted approach.

Recent amendments to building regulations now require architects to consider the energy footprint of each new development. This forces competent persons to consider all opportunities to reduce the wasted energy used to warm and cool down buildings.
Windows and doors present challenges to retain and block unwanted energy movement.

THE U-VALUE

Conserving and managing energy movement has become the challenge of many building designers. Their opportunities and challenges present themselves in paradoxical tandem:

Reduce the power requirement for illuminating internal areas by increasing natural light vs the solar heat gain inside the home that in turn requires cooling

The thermal performance of building materials are measured by considering it’s U-value. This value describes the rate at which heat energy is lost through the material. The lower the value, the less heat energy is lost.

E.g. a clear, single glazed aluminium window has a typical U-value of 5.6, while a clear double-glazed aluminium window has a typical U-value of 2.8. U-value describes the amount of heat lost through one square meter of the material for every degree difference in temperature either side of the material. It is indicated in units of Watts per Meter Squared per Degree Kelvin or W/m²K. (1°C = 1K)

Thermal conductivity (U) measures the heat energy that will be transmitted through 1 m² of material in 1 hour when there is a difference of 1 Kelvin/1°C (across two surfaces of the material). The unit of measure of thermal conductivity (U) is W/m²K.

The thermal conductivity of glass units depends on the type of glass used (e.g. float glass, coated glass, laminated glass), the number of glass sheets in the glass unit, the width of the air space and type of the filler gas and / or glazing film.

Components of U calculations:

Uw (w = window) overall U-value of the window system
Ug (g = glazing) U-value of the glazing
Ψg (g = glass) linear heat transfer coefficient
Uf (f = frame) U-value of the frame

The thermal conductivity Uw relates to the entire window system. This value includes the U-values for the glazing (Ug) and the frame (Uf). The overall value Uw is also influenced by the linear heat transfer coefficient Ψg (g = glazing) and the size of the window.

U-value of window glazing (Ug)

The Ug value is a function of:

  1. The glass type – laminated, coated etc.
  2. The type of gas that fills the intermediate space between the glass sheets – Argon gas lowers the Ug value when compared to air.
  3. The distance between the glass sheets – 12mm to 16mm is the optimum range for the airgap.
  4. The number of sheets – triple glazing has a lower Ug value than double glazing.

U-value of window frame (Uf)

The Uf value for the frame-sash combination is defined by means of measurement or calculation. The area for the calculation of the Uw value is the cross-section of the profile.

Linear heat transfer coefficient (Ψg)

The value Ψg for the edge seal of the glazing unit is a function of the type of material used for the insulated glazing spacer. The standard material with the worst thermal performance is Aluminium. Spacers with improved thermal insulation are referred to as “warm-edge” spacers. These spacers are made of stainless steel or plastic. A larger edge cover of the insulated glazing unit in the sash profile further enhances the Ψ-value of the edge seal.

Examples of Ψ-values:

Aluminium spacer approx. 0.08 W/m²K
“Warm edge” spacer approx. 0.04 W/m²K

U-value of window (Uw)

The U-value contribution ratio of the glass and aluminium frame is influenced by:

  1. The area ratios of the two elements.
  2. Their material thermal conductivity.

For this reason, the window’s U-value worsens as the size decreases. Larger windows feature better Uw values. This is because U-values achieved in glazing (Ug) are better than in the frame material (Uf) and therefore a larger glass area is able to produce a better thermal insulation (Uw) value.

Uw calculation

The following formula is used to determine the window’s thermal conductivity value:

Uw = (Ag x Ug + Af x Uf x Ψg)

(Ag + Af)

Ug = thermal conductivity of the glazing

Uf = thermal conductivity of the frame

Ψg = linear heat transfer coefficient of the insulated glazing edge seal

Ag = glass area

Af = frame area

Aw = Ag + Af

lg = length of inside edge of frame profile

ENERGY MOVEMENT THROUGH GLAZING SYSTEMS

Energy generally recognised as temperature, moves through glazed windows and doors via a combination of ways:

  1. Air leakage – the flow of temperature in or out of a fenestration unit in areas where the unit is inadequately sealed.
  2. Conduction – energy is directly transmitted through a material when there is a difference of temperature across it.
  3. Convection – hot, less dense air moves upwards whilst colder, more dense material moves downward because of gravity. In this process heat is transferred in an uncontrolled manner.
  4. Radiation – infra red heat is transferred through glass.

THE FENESTRATION CHALLENGE

Glass and aluminium are both relatively poor insulators of energy (relatively high U-value). The inherent temptation to maximise opening sizes to capitalize on beautiful views and natural light present challenges to manage energy conservation during both warmer and cooler seasons. For this reason, it has become critical to specify high-quality aluminium windows and doors, glazed with a correctly configured performance glass unit. The system should be designed to have low thermal transmittance with careful consideration of visible light and solar heat gain factors.

In South Africa these design requirements are complicated by significant climate differences experienced between winter and summer seasons, making it difficult to design a system that suits the climate all year round. During summer it is required to allow visible light in, but to limit the amount of radiant infra-red heat (solar heat gain) that would require costly internal cooling. During winter again visible light is required but, in this case, solar heat gain might be encouraged with a low U-value system to keep the heat inside.

Technical consideration of sun penetration angles allows clever utilization of the sun’s energy to maximise it during winter and minimise it during summer.

WHY DESIGN FOR THERMAL EFFICIENCY

  1. Legislation requires you to do so. SANS 204 requires competent persons to conduct energy calculations to establish compliance.
  2. Saving energy saves money – with ever-increasing power costs a properly designed fenestration system pays for itself.
  3. Although a comfortable home is difficult to quantify in currency, the value it adds to the property’s future price is certain.
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