To place or not to place (double glass windows)?

The executive board of a company is considering replacing the office’s single glazed windows to double glazed windows. According to their financial officer, the inherent fixed costs after the transition (maintenance, insurance, etc.) are €2000,- higher than in the current situation.

The company uses natural gas to heat its building, which costs 11 eurocents (100 cents = 1 euro) per 10 megajoule (10⁷) ; a year counts 2 . 10⁷ heating seconds and, on average, it is 10 °C warmer inside the office building than it is outside.

The exterior heat transfer coefficients inside and outside are both 20 W/m²K. A single glass pane is 3 mm thick (λ_glass=1 W/mK) and in between the two glass panes there is 5 mm of stagnant air (λ_air=0.025 W/mK). Because the air in this gap is stagnant, the convective heat transport is negligible there.