Two Mechanisms of Heat Transfer in the Home

Traditional windows are an energy weak spot in a house’s design. The energy and heat we generate to warm our homes simply flows out through ordinary windows in winter and flows in through these same windows in summer.

The use of energy efficient glass acts as a plug. We can essentially control our physical comfort through the right window selection.

Conducted Heat
– Thermal Insulation

Design Thermal Insulation
  • Windows represent ~8% of the surface area of a typical residential building
  • Yet their very low resistance to conducted heat flow makes them a ‘thermal wound’ in the building fabric
  • Heat flow takes the path of least resistance, hence ordinary windows allow a disproportionate loss of energy through conduction (~50% in an otherwise well insulated home)
  • An ordinary window (alum framed, clear glass) can conduct around 6.5 W/m2.K (its U-value)
  • So if there’s a 10˚ temperature differential between the inside and outside, then that’s 65W of energy loss for each m2 of window (typically around 40m2 to 60m2 per house)

Radiant Heat
– Absorption, Reflection & Shading

Design Absorption, Reflection & Shading
  • Generally speaking, windows (and other glazed areas) are the only entry point for radiant heat into a home
  • Responsible for around 90% of unwanted heat gain in otherwise well insulated homes
  • In cold places radiant solar heat gain is desirable — ‘passive heat gain’ reduces need for artificial space heating
  • In hot places this is undesirable
  • In mixed climates a median level of solar heat should be struck
  • The theoretical maximum heat load from the sun is around 750W/m2. Windows don’t see all of this due to the angle of incidence and shading from eaves etc
  • In tropical climates, radiant solar heat gain equals or surpasses conductive heat gain (during hours of sunshine)