It is proposed to replace the single glazing in a dwelling house with double glazing - Leaving Cert Construction Studies - Question 5 - 2008

To calculate the U-value of standard double glazing, we first need to determine the overall resistance using the same method:

• Resistance of the internal surface: $R_{internal} = 0.122 \ m^2\degree{C}/W$
• Resistance of glass: $R_{glass} = \frac{0.004}{1.02} \approx 0.0007843 \ m^2\degree{C}/W$
• Resistance of space between panes: $R_{space} = \frac{0.012}{0.170} \approx 0.0706 \ m^2\degree{C}/W$
• Resistance of the external surface: $R_{external} = 0.080 \ m^2\degree{C}/W$
  Calculating total resistance: $R_{DoubleGlazing} = R_{internal} + R_{glass1} + R_{space} + R_{glass2} + R_{external}$
  Thus, $R_{DoubleGlazing} = 0.122 + 0.0007843 + 0.0706 + 0.0007843 + 0.080 = 0.2741 \ m^2\degree{C}/W$
  Calculating the U-value: $U_{DoubleGlazing} = \frac{1}{0.2741} \approx 3.65 \ W/m^2\degree{C}$

Given the U-value of single glazing, the heat loss can be calculated using
$Heat \\ Loss = U \times Area \times Temp. \\ Diff$
Where:

• U-value of single glazing: $4.83 W/m^2\degree{C}$
• Area: $25 m^2$
• Temperature difference: $18 - 5 = 13\degree{C}$
  Substituting values: $Heat \\ Loss = 4.83 \times 25 \times 13 = 1569.75 \text{Watts}$
  To find the annual heat loss, convert it: $Annual \\ Energy = 60 \times 60 \times 11 \times 7 \times 40 \times 1569.75 = 17403588 \text{ kJ per annum}$
  Calculating cost per annum with the oil cost (80 cents per litre): $Cost per annum = \frac{17403588}{37350} \times 0.80 = €372.80$

For double glazing use:

• U-value: $2.63 W/m^2\degree{C}$
  Calculating heat loss: $Heat \\ Loss = 2.63 \times 25 \times 13 = 854.75 \text{Watts}$ Annual energy lost: $Annual \\ Energy = 60 \times 60 \times 11 \times 7 \times 854.75 = 9477468 kJ \\ per annum$ Calculating cost per annum: $Cost per annum = \frac{9477468}{37350} \times 0.80 = €202.99$

For low-e double glazing, use:

• U-value: $1.1 W/m^2\degree{C}$
  Calculating heat loss: $Heat \\ Loss = 1.1 \times 25 \times 13 = 357.5 \text{Watts}$
  Annual energy lost: $Annual \\ Energy = 60 \times 60 \times 11 \times 7 \times 357.5 = 3963960 kJ \\ per annum$
  Calculating cost per annum: $Cost per annum = \frac{3963960}{37350} \times 0.80 = €84.90$

Based on the calculations:

• The single glazing incurs the highest cost of €372.80 annually.
• The double glazing is more economical at €202.99 per year.
• The low-e double glazing offers the best efficiency with a cost of only €84.90 annually. Thus, the recommended glazing type is low-e double glazing because:

1. It results in significantly reduced heating costs compared to both single and standard double glazing.
2. It enhances energy efficiency and offers better insulation, which can lead to long-term savings on energy bills.