01 · Heat through the wall

On the Hottest Afternoon, Which Bedroom Stays Cool?

Two identical bedrooms, with one difference: the west wall. One room has an LGS wall; the other has the wall you pick, in clay brick, AAC block or precast concrete. Move the clock, set the air-con temperature and schedule, and see how much heat reaches each room.

03 · Assumptions and sources

• Wall section and charts above: a hot April day in Bangkok, air 29–40 °C, dew point 25 °C, afternoon sun on the west wall peaking at 600 W/m², a mid-coloured outer face.
• A 4 × 4 m bedroom, 3 m high, with a concrete floor, the same in both houses except the 12 m² west wall. The air-con holds the room at 55% humidity.
• LGS wall: 16 mm Nichiha, 15 mm cavity, 9 mm Glasroc X, Scottsdale C90 studs of 0.95 mm steel at 600 mm with 75 mm insulation (40 kg/m³), 13 mm DuraLine and 12.5 mm Habito. The heat the steel studs carry comes from a 2D finite element model of the real section: they cut the insulation layer's R-value to 0.615 of its value. The model includes the cavity air: sun-warmed air rising at about 0.2 m/s (assumed) carries heat out.
• The LGS wall's outer panel runs hotter than a brick face (56 against 51 °C at 17:00): the insulation keeps the heat outside, where the cavity air carries it off, and the room stays cool.
• Wall sections: a 2D finite element model of each wall's plan section, seen from above, 120 mm along the wall, checked against the ISO 10211 test cases and the CalculiX solver, and reduced to 14–15 shapes that run in this page within 0.03 °C of the full model. The AAC blocks' 3 mm joints every 600 mm and the precast panels' DB12 bars every 200 mm are typical sizes (assumed). In the pictures the concrete is plain, 2.0 W/m·K to ISO 10456, with its bars drawn; the heat figures use 2.3 W/m·K, with the steel mixed in.
• Walls compared, every thickness including render: clay brick 100 mm (15 render + 70 brick + 15 render), double clay brick 170 mm (15 + 70 + 70 + 15), AAC block 100 mm (12.5 + 75 + 12.5) and 140 mm (7.5 + 125 + 7.5), and precast concrete 100 and 140 mm, painted with no render.
• Materials: clay brick 1,600 kg/m³, λ 0.70 W/m·K (assumed); render 1,800 kg/m³, 1.0 W/m·K; AAC grade G2 type 0.5 to TIS 1505-2541 (dry density 410–500 kg/m³), 0.13 W/m·K, the top of the 0.089–0.132 range Thai makers quote (dry values, which rise in damp air); precast concrete 2,300 kg/m³, 2.3 W/m·K, ISO 10456 for concrete with 1 % steel.
• Air leaking through the wall (assumed): every rendered masonry wall, clay brick or AAC, 0.6 m³/h per m²; precast concrete, which leaks only at its panel joints, 0.3; LGS with taped Glasroc X joints 0.1.
• R-values in the table follow ISO 6946, through every layer of the wall. For LGS the figure is through the insulation between the studs; counting the steel studs, the whole wall is about 1.8 (2D model).
• Colours show temperature: near-white is 26–28 °C, normal room temperature, neither warm nor cool; blue is cooler, yellow to red is warmer, and deep red is 40 °C and above. Each band spans 2 °C, and the thin lines between bands are isotherms.
• "Feels like" is the US National Weather Service heat index, from the room's humidity and the air temperature combined with the heat the wall radiates, for someone about a metre from the west wall, where it fills about a quarter of the view. In the middle of the room the difference between walls is about half as big.
• Monthly bill: Bangkok's average weather for each month from NASA POWER, 2001–2020 (13.75° N, 100.50° E); air-con on the schedule set above, at EER 12; THB 5 a unit; solar 4.08 kWh a day per kWp, 550 W panels.
• On all day or on a timer: the model runs the same schedule for two days before the day shown. By then the walls and floor follow the same cycle every day, as if you had used the air-con this way for weeks. So if a wall is still hot while the air-con is on, it is not because the air-con has only just come on.
• Air-con a wall needs: the most heat and moisture the air-con removes in an hour for the bedroom's 12 m² west wall, air-con on all day; a real air-con must also cover the roof, windows, people and appliances. Wall cool-down: air-con off 08:00–18:00, on at 18:00, timed until the room face is within 2 °C of the setting.
• The bedroom has a 12,000 BTU/h air-con (assumed; pick another size above). While it is off, the room warms with the heat through the wall and the air leaking in; once it is back on, it runs at full power until the room is back at the setting. "Off All Day" starts from a room the air-con kept at the setting until midnight.
• Every figure comes from our model, not from a test, and still needs an engineer's sign-off.