1. Q5 HL 2008 U-Value
It is proposed to replace the single glazing in a dwelling house with double glazing.
(a) Using the following data, calculate the U-value of the:
(i) single glazing;
(ii) standard double glazing.
Glass: single glazing thickness 5 mm
Glass: double glazing thickness 4 mm
Space between panes width 12 mm
Thermal data of glazing:
Conductivity of glass (k) W/m °C
Resistance of space between panes (R) m2 °C/W
Resistance of internal surface (R) m2 °C/W
Resistance of external surface (R) m2 °C/W

2. Formulae: r = 1/k R=T x r U =1/Rt
Material
Conductivity
(k)
Resistivity (r)
1/k
Thickness (m)
Resistance (R)
Single glazing
Int surface
Ext surface
Double glazing
space
1.02
0.9803
0.005
Rt =
.008
0.012
0.0049
0.122
0.08
0.2069
0.17
0.122
0.08
0.3798
Formulae: r = 1/k R=T x r U =1/Rt
Single Glazing; U = 1 / Rt = 1 / = 4.83 W/m2/°C.
Double Glazing; U = 1 / Rt = 1 / = 2.63 W/ m2/°C.
Hence, the double glazing is almost twice as effective as the single glazing.

3. Q5 (b)
A choice is to be made between the following types of double glazing:
• standard double glazing;
• low-emissivity (low-e) double glazing.
Using the U-values obtained at (a) above and the following data, calculate the cost of the heat
lost annually through each of the following:
• single glazing;
• low-e double glazing.
U-value of low-e double glazing: 1.1 W/m2 °C
Area of glazing: 25 m2
Average internal temperature: 18 °C
Average external temperature: 5 °C
Heating period: 11 hours per day for 40 weeks per annum
Cost of oil: 80 cent per litre
Calorific value of oil: kj per litre
1000 Watts: 1kj per second.

4. Cost of Heat Lost per year through; (i) Single Glazed Window
Heat Loss: = U-Value x Area x Temp. Diff
Temperature difference= 18 – 5 = 13
4.83 x 25 x 13 = Joules / sec
Annual heating period: = 60 x 60 x 11 x 7 x 40 = secs
(40 weeks a year, 7 days a week, 11 hrs a day, 60 mins an hour, 60 secs a minute)
Therefore, x = joules (divide by 1000) = Kj
Note: Calorific Value of 1 Litre oil = 37350kj
Therefore, / = 466 litres of oil
1 Litre Oil costs -= 80c;
Cost per annum: 466 x 0.80 = €372.80

5. (ii) Double Glazed Window
4.83 =
2.63 = X
Cross multiply:
4.83X = 2.63 x
4.83X =
X = / 4.83
X= €202.99
(iii) Low-e Double Glazed Window
4.83 =
1.1 = X
Cross multiply:
4.83X = x 1.1
4.83X =
X = / 4.83
X = €84.90

6. (c) Using the information obtained at (b) above, recommend a preferred glazing type and give two reasons to support your recommendation.
Low-e Double Glazing window type recommended
Comparison of costs associated with heat lost for each window type for one year: (i) S.G. €372.80; (ii) D.G. €202.99; (iii) Low-e D.G. €84.90
• Four times more efficient than single glazing.
• Two and a half times more efficient than double glazing.
• Costs of low-e double glazed window type recouped over a shorter time (5-8 years)
• Price of home heating oil is predicted to rise.