Subject: BRE2031 – Environmental Science
Tutorial 2 – Thermal Effect in Buildings
1. A cavity wall is constructed as follows: brickwork outer leaf 103 mm, air gap 50 mm, expanded insulation board 60 mm, high performance concrete block inner leaf 100 mm, lightweight plasterboard 10 mm. The relevant values of thermal conductivity, in W/m.K, are: brickwork 0.84, polystyrene (insulation) 0.025, concrete block 0.11, plasterboard 0.18. The standard thermal resistances, in m2.K/W are: outside surface 0.06, inside surface 0.12, air gap 0.18. Calculate the U-value of this wall. (answer for reference: 0.26 W/m2.K)
2. The cavity wall of an existing house has outer and inner brickwork leaves each 105 mm with a 50 mm air gap between them, finished with a 16 mm layer of plaster inside. The relevant values of thermal conductivity, in W/m.K are: brickwork 0.73, plaster 0.46. The standard thermal resistances, in m2.K/W, are: outside surface 0.055, inside surface 0.123, air gap 0.18.
(a) Calculate the U-value of the existing wall. (1.47 W/m2.K)
(b) Calculate the U-value of the wall if the cavity is completely filled with foamed urea formaldehyde (k = 0.026 W/m.K) (0.41 W/m2.K)
3. Compare the U-values of a single-glazed window made up of one sheet of 4mm glass with a double-glazed window made up of two sheets of 4mm glass which have a 5 mm airspace between them. The thermal conductivity of the glass is 1.022 W/m.K. The standard thermal resistances, in m2.K/W, are: outside surface 0.06, inside surface 0.12, airspace 0.11. Determine the proportion of the thermal resistance provided by the glass layers.
4. A blockwork wall measures 5m x 2.8m in overall length and height. The wall contains one window 1400 mm by 800mm and one door 1900mm by 750mm. The U-values, in W/m2.K, are blockwork 0.58, window 5.6, door 3.4. Calculate the average U-value of this wall. (1.27 W/m2.K)
5. A wall panel is to have the following construction: outer metal sheeting, foamed polyurethane board, air-gap, and 15mm of lining board. The relevant values of thermal conductivity, in W/m.K, are polyurethane board 0.025, and lining board 0.16 (the metal sheet is ignored). The standard thermal resistances, in m2.K/W, are: outside surface 0.06, inside surface 0.12, air-gap 0.18. Calculate the minimum thickness of polystyrene needed to give the wall panel a U-value of 0.35 W/m2.K. (60mm)
6. Consider a large plane wall of thickness L=0.4m, thermal conductivity k = 2.3 W / (m . K ) and surface area A=30m2. The left side (surface) of the wall is maintained at a constant temperature of T1 = 90oC while the right side loses heat by convection to the surrounding air (T∞ = 25oC) with aheat transfer coefficient of h = 24W / (m2 . K) . Assuming constant thermal conductivity and no heat generation in the wall, determine the rate of heat transfer through the wall. (reference answer: 9045.9W )
7. One side of a room consists of a window with an area of 2m2, a door with an area of 3m2, and wall with an area of 15m2 (excluding the window and door). The indoor and outdoor air temperatures are 18oC and 0oC, respectively. The rates of heat transmission through the window and the door are found to be 90W/m2 and 54W/m2, respectively. If the rate of heat transmission through the whole construction (including the window, door and the wall) is 882W, determine the U-value of the wall (15m2). (answer: 2W/m2.K)
8. In a summer day, the outdoor air temperature is 35oC while the indoor air temperature is maintained at 25oC. The rate of heat flow from the outdoor air to the indoor air is through a wall with surface area of 20m2 is 400W. If insulation board is added to the wall to reduce the rate of heat flow through the wall to 100W, what minimum thickness of this board is needed? Given that the thermal conductivity of the insulation board is k = 0.1 W / (m . K ) . (reference answer: 0.15m)