How effective is Low-E glass in windows and doors

Question: What is the annual savings if I upgrade from Clear glass to Low-e glass on the 5610 Series Single Hung window.

Assumption: Average temperature differential between outside and inside =20°F

“U Value” is actually an engineering number that Mechanical Enginneers study in a class called “Heat
Transfer”, and, yes, it is a class Mechanical engineering geeks have to take.

Anywhich-a-way-how, “U Value” has units defined as: BTU/ (hr ∆°F ft²)
So if you know:

  • The temperature difference between the inside and the outside of the house you know: ∆°F (assume Outside is 90°F, inside is 70°F, then ∆°F=20∆°F)
  • The hours that you are concerned with. (assume we want to know the heat loss over a 24 hour period: hr=24)
  • The square footage of the window: ft² (assume a 3’0 x 5’0 window ft²=15ft²)
  • U factor of the window (366 low-e U=0.30, for our 5610 sh, and the U=0.48 for clear glass)

THEN:
Total heat through a 3’0 x 5’0 5610 sh/hour with Low-e glass on a 90°F outside temp day and an inside temp of 70°F is :
{0.30 BTU/(hr ∆°F ft²}*(24hours)*(20∆°F)*(15ft²)=2160 BTU

By definition there are 3413 BTU/kilowatt-hour.
So:
2160 BTU= .632 Kilowatt-hours (KWH)
If:
The Nashville electric power company charges $0.094/ KWH

Then:
The cost of the energy going through a 3’0 x 5’0 5610 sh/hour with Low-e glass on
a 90°F outside temp day and an inside temp of 70°F per day is:

.632*$.094= $0.06/Day or $21.90 per year (low-e) (assuming 20∆°F)

Using the same logic but now plugging in a U-value for clear glass the energy
cost for the same window per year is:
the cost of the energy going through a 3’0 x 5’0 5610 sh/hour with Clear glass on a 90°F
outside temp day and an inside temp of 70°F per day is:
{0.48 BTU/(hr ∆°F ft²}*(24hours)*(20∆°F)*(15ft²)=3456 BTU
3456 BTU/3413(BTU/KWH)= 1.0125KWH
1.0125*$.094=$.094/Day or $34.68 per year (clear) (assuming 20∆°F)
This is a $12.78 savings per year, or a 37%* savings per year for Low-e glass.
Assuming 15 windows per house there is a $191.70 savings per year for
Low-e over clear glass.

*We are not claiming a 37% savings on your utility bills, we are saying that Low-e windows are 37% more efficient than clear glass if the above assumptions hold true.  If the rest of the house is losing massive amounts of heat due to poor insulation, the consumer would be far better off to address those issues before replacing windows. Put another way, if the house is poorly insulated, you will not save 37% on your utility bills by installing Low-E glass replacement windows.

Here is a link to a PDF of the analysis: Payback analysis


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One Response to How effective is Low-E glass in windows and doors

  1. John Strauss says:

    Good info on low-e glass and low-e storm windows

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