With various newspaper articles recently on the subject of well-insulated homes overheating in hot weather, I thought it was time to reflect on how my superinsulated home responds to high outside temperatures.
The short answer for my house is "no". We've had some very hot weather during July and August 2013 but inside the house I've not seen a thermometer show higher than 25 degrees C, and usually less than that.
This is not a big surprise. Passive House design means that overheating must be considered, and the temperature should generally remain below 25 degrees C.
The well-insulated roof and walls help to keep heat out of course, and we can add to this effect by closing curtains as the sun moves round the house if we wish. However, our secret weapon is the thermal mass - the solid brick walls, covered internally with a dense render before the plaster skim, soak up heat when the air temperature is high, and release it in the evening when the air cools.
This is not a big surprise. Passive House design means that overheating must be considered, and the temperature should generally remain below 25 degrees C.
The well-insulated roof and walls help to keep heat out of course, and we can add to this effect by closing curtains as the sun moves round the house if we wish. However, our secret weapon is the thermal mass - the solid brick walls, covered internally with a dense render before the plaster skim, soak up heat when the air temperature is high, and release it in the evening when the air cools.
We can maximise this effect in the evening by opening patio doors downstairs to bring cool air in, and opening the attic windows to allow warm air to rise and escape (drawing more cool air in). This leaves the house at a cooler temperature for the night and into the next day.
Even if we keep the windows shut to keep the hot air out, the ventilation system with heat recovery does bring warm air in from the outside. However, when outgoing air is cooler than the incoming air, the heat recovery unit will to help cool the air brought in from the outside until indoor and outside air reach the same temperature.
In contrast, many modern cavity wall insulated houses are not designed with overheating in mind. All sorts of elements can contribute to overheating. Large areas of glazing, particularly if in a sloping roof can allow excessive solar gain. Loft rooms are often not that well insulated, turning them into an oven on hot sunny days. Theoretically, the inner skin of brickwork could provide thermal mass to soak up the heat, but it is commonly of lightweight blockwork or is covered in plasterboard (which prevents heat from the air from being absorbed by the brickwork behind it).
So, as usual, the devil is in the detail. A lot can be done to reduce overheating even on a limited budget. Fancy automated external blinds may be the ideal, but careful use of curtains (e.g. leaving them shut before going out for the day) can make a significant difference. Likewise opening windows on the cooler side of the house / at cooler times of day is worth doing, with or without a ventilation system.
Climate scientists are predicting a greater frequency of heatwaves in future years, so it is worth keeping overheating in mind when buying or improving a home...
Even if we keep the windows shut to keep the hot air out, the ventilation system with heat recovery does bring warm air in from the outside. However, when outgoing air is cooler than the incoming air, the heat recovery unit will to help cool the air brought in from the outside until indoor and outside air reach the same temperature.
In contrast, many modern cavity wall insulated houses are not designed with overheating in mind. All sorts of elements can contribute to overheating. Large areas of glazing, particularly if in a sloping roof can allow excessive solar gain. Loft rooms are often not that well insulated, turning them into an oven on hot sunny days. Theoretically, the inner skin of brickwork could provide thermal mass to soak up the heat, but it is commonly of lightweight blockwork or is covered in plasterboard (which prevents heat from the air from being absorbed by the brickwork behind it).
So, as usual, the devil is in the detail. A lot can be done to reduce overheating even on a limited budget. Fancy automated external blinds may be the ideal, but careful use of curtains (e.g. leaving them shut before going out for the day) can make a significant difference. Likewise opening windows on the cooler side of the house / at cooler times of day is worth doing, with or without a ventilation system.
Climate scientists are predicting a greater frequency of heatwaves in future years, so it is worth keeping overheating in mind when buying or improving a home...
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