Energy Savings of Insulated Glazing Units

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Energy Savings of Insulated Glazing Units

• The Cost Effectiveness of Double-Pane Windows
• Shaun Jeffs

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Problem Statement
ARE DOUBLE PANE WINDOWS WORTH IT?

• Insulated glazing units (IGU) use a gas-filled
  cavity to increase the insulative properties of
  the window.
• IGUs are very expensive.
• It is claimed that IGUs save enough energy to
  eventually pay for themselves.
• Will changing to double pane windows in a house
  save an appreciable amount on energy bills?
• My sister installed IGUs in her house in January
  2008. Is the price she paid for them justified?

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Definitions

• Insulated Glazing Unit (IGU) a fenestration
  cell that utilizes multiple panes of glass to
  improve insulation
• Fenestration an opening in a building structure
  (window)
• U-Factor a measure of the insulative properties
  of a window. It is measured in Btu/hrft2F.
  It represents the total thermal resistance of the
  IGU
• Decatherm (Dth) 1,000,000 Btu

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So, Whats the Difference?

• Single pane of glass
• Glazing (coating) can be applied
• Higher U-factor means lower insulating properties

• Two panes of glass
• Gas-filled cavity usually filled with inert gas
  (typically Argon)
• Lower U-factor means increased insulating
  properties

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Knowns

• Total Window Area
• ATOT 257.4 ft2
• Double-Pane Area
• After installation
• ATOT 200.4ft2
• Total price of new windows
• 7,850
• U-Factor of OLD windows
• 0.98
• U-factor of NEW windows
• 0.32

• Average outdoor temperature in January in Salt
  Lake
• 20.1 F
• Average indoor temperature in January
• 64.7 F
• Average Price of Natural gas
• 8.06 /Dth
• Average gas usage during January-February
• 0.5 Dth/Day 20,833 Btu/hr

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Problem Set-Up

• Two components to heat loss in the home heat
  through windows, and heat lost through rest of
  home.
• I will use the Jan Feb gas bill (with IGUs) and
  the equation for qWindows to solve for qOther.
  Then I will solve for qTotal for the single-pane
  windows.

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Assumptions

• No convective or radiative heat transfer through
  windows (no wind).
• Heat transfer through the rest of the house,
  qOther, remains constant.
• The average temperatures for Dec Jan (single
  pane windows) are the same as Jan Feb (double
  pane windows).
• Natural gas prices remain constant.
• Temperatures taken as AVERAGE temperatures.

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Results
WITH NEW WINDOWS
OLD WINDOWS
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Conclusions

• Based on these assumptions, the house will save
  on average 34.20 per month.
• Based on a 7,850 purchase price, it would take
  19 years to pay off the windows.
• The assumptions made produce inaccurate results,
  but give a good idea of the magnitude of the
  energy savings.
• If all factors were taken into account
  (convective and radiative heat tran), I predict
  that it would take at least 10 years to pay off
  the windows.