LowE Glass

I got talking to a client about LowE glass today. This was for a retail building in Toronto with some pretty large glazed areas. I was trying to explain the importance of LowE glass especially on the North facing windows.

To clarify the explanation I prepared the following estimation of heat loss. I'm sure there are tools out there that would do this but I thought I'd get a better understanding from doing it from first principles.

Sealed Unit Energy Comparison

Sealed Unit Assembly

	SHGC	VLT	U - W/(m2 ·C)	U – Btu/(h·F·ft2)
Clear/Clear	0.70	78%	2.8	0.47
LowE/Clear	0.43	53%	1.8	0.32
Clear/LowE	0.66	73%	1.8	0.33
Clear/LowE + Argon	0.66	73%	1.6	0.29

SHGC = Solar Heat Gain Coefficient

VLT = Visible Light Transmittance

U = Heat Loss Factor (Winter value used here.)

Energy usage calculation:

Area = 10’x17’ + 3x 5’x8’ = 290 ft² = 27 m²

Heating degree days (Toronto) = 3650 C·day

(temperature difference from 18 deg C inside)

Heat Loss = Area x ΔT x U

= 27 m2 x 3650 C·day x U W/(m² ·C) x 24 h/d x 1 k/1000

= 2365 kW·h x U

Now you won’t actually heat for every degree day. Many days will be warm enough that the natural heating inside (people, machines, sun etc.) will overcome the heat loss. So lets assume that 25% of the heat loss is covered by these gains and 75% by electrical heating. My last bill worked out to 11 c/kWh on average but the actual incremental rate is higher due to graduated pricing. Lets use the 11 c/kWh

	U - W/(m2 ·C)	Heating kWh/year	Cost $/year	Savings/Year
Clear/Clear	2.8	6622	$730
LowE/Clear	1.8	4257	$470	$260
Clear/LowE	1.8	4257	$470	$260
Clear/LowE + Argon	1.6	3784	$420	$310

Sealed unit performance data from the Pilkington website.