In a cold region, large windows may contribute to short-term
overheating in summer, but glazing with a low solar heat gain coefficient must be used with caution because of the energy penalty it causes over the rest of the year.
What can films do?
Heat loss through single pane 1/4" clear glass can be reduced by 30% with a film LE35AMARL that only transmits 31% of visible light, or by 23% with a film LE20SIAR that transmits only 17% of visible light
However conductive losses through aluminium frames and physical transport of heat through air changes are also significant.
What are the potential savings?
Here are some calculations for Edinburgh:
The "U-value" quoted for 1/4" single-pane clear glass is 1.06. The Imperial (BTU/hour foot2 fahrenheit) U-value is equivalent to 5.6782633373 USI (W/m2K), based on SI units of Watts, degrees Kelvin, and metres.
For my office the glazed area is approx 15m2 and the maximum monthly average temperature difference is 17K (20°C internal, external Jan & Feb average 3°C) - so the loss through glass in Jan and Feb is approx 1.5 kW
On an annual basis, average external temperature is 8.5°C so average conductive loss through single pane clear glass is 50W/m2. The potential saving is thus around 15W/m2.
Cost of film including installation was approx £40/m2. Heating costs should be substantially less than £0.05 per kWh - so, even at this high energy price, payback through fuel savings will take around 6 years.