Properties of glass, including typical U values, are listed in the table at

http://www.viridianglass.com/products/downloads/Viridian%20Glass%20Performance%20Data%20-%20Energy.pdf

For standard glass a typical heat lost through the glass is U = 6.7 watts per square metre per degreeC.

For low E glass U = 3.1W/sqm per degC

For double glazing using standard glass U = 2.7W/sqm/degC

For double glazing including low E glass U = 1.9W/sqm/degC

If the average outside temperature is 7degC when the inside temperature is 22degC the temperature difference across the glass is (22 - 7) = 15degC.

With a 15degC temperature difference across standard glass the heat loss outwards is 6.7 x 15 = 100 Watts per square metre.

For 20sqm of glass the heat loss is 100 x 20 = 2,000Watts = 2kiloWatts.

In one hour the energy loss is 2kiloWatthours.

If the room is kept at 22degC for 24 hours the energy loss is 2 x 24 = 48kWhr.

At 20cent/kWhr the energy cost is 48 x 0.20 = $9.60 per day.

For double glazing using standard glass U = 2.7W/sqm/degC so the daily energy cost becomes $9.60 x (2.7/6.7) = $3.90/day a saving of $9.60 - $3.90 = $5.70/day.

If heating is for 200days the saving is 200 x 5.7 = $1140/yr.

A cost of $5500 for double glazing would be repaid in 5500/1140 = 5years.

A reverse-cycle air conditioner provides 3kWhr of heat from 1kWhr of electricity.

Using a reverse cycle air conditioner the heat energy costs 20 / 3 = 7cent/(kWhr of heat), the saving drops to 1140/3 = $380yr and it takes 15years to pay back $5500.

It is interesting to compare the heat loss through a wall with the heat loss through glass.

For a timber frame wall constructed with gyprock inside, 100mm of fibreglass insulation, foil, a 50mm reflective airspace and weatherboard outside the total thermal resistance, R= 3.23 so U = 1/R = 0.3 watt/sqm per degC which is less than 1/20 (5%) of the loss through standard glass.

Posted Wednesday 27 Apr 2011 @ 9:15:12 pm from IP

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