Understanding Passive ConstructionA passive house is the way we will build in the future and the standard to benchmark all other building standards I was asked recently by a client to explain what was entailed in building a passive house. In the process of explaining it I was struck by the need to simplify what it means to me as much as to my client.
A passive house attempts to remain at a certain temperature throughout the year without the need to use heating. The standard is less than 15KWHm2 year of delivered energy. A house loses and gains heat. The gains are solar and a house can gain heat all year round depending on its orientation and the weather. It also loses heat and this is through convection conduction and radiation. Houses have barriers to heat loss which is its structure. Some parts of its structure are more efficient than others to retain heat.
Let’s take an example of a house with a wall U value of 0.27w/m2K. U value is the rate of heat loss per m2 through a wall when the temperature difference is 1°C This wall will transmit 0.27 watts per m2 if the temperature difference is one °C. So if the wall area of your house is say 200m2 and has a U value of 0.27w/m2K then it loses heat at a rate of 200m2*0.27w which is equal to 54w. Now that is the heat loss at one °C temperature difference. If the temperature you wish to maintain in your home is 20°C and the outside temperature is -10°C then the temperature difference is 30. The heat loss will now be 54w * 30 which is equal to 1620w. Now say we add up the total area of the house and it comes to 400m2. That is the wall areas floor areas and roof areas as well as the windows. We will assume a standard U value of 0.4w/m2k and an air tightness of 0.5ac/h and a thermal bridge value of 0.15w/m2k. The inside required temperature is 20°C and the outside temperature is -5°C. The difference in temperature is 25°C. Ok the overall heat loss is roughly 400m2*0.40w = 160w*25(temperature difference) = 4000w We now add in heat loses from thermal bridging and air leaks. Thermal bridge assuming 0.15w/m2k = 60w*30=1800w Overall heat loss =4000k +1800k= 5800w Allow say 50% heat loss due to air leaks. Then we have 5800w * 50%=8700w So the house will need 8700w of heat to maintain a 25°C temperature. If the house was heated electrically it would need nine one KW heaters. If you maintain this heat for one hour at €0.20per kwh the cost of the heat will be 8.700kwh*€0.20=€1.74 per hour. Now Ireland and the UK have reasonably temperate climates so the above example is a bit extreme but it gives a good idea of heat losses. We haven’t allowed for solar heat gain in the above example. Now the heat loss will rise and fall as the temperature difference increases or decreases. If the outside temperature was the same as inside then the system is in balance (to a degree) and the need for heat is not required. When determining the overall heat requirements solar gain is normally taken into account and the pluses and losses are tallied. Note. (For the above calculations I have used the DEAP formulae for thermal bridging and an assumed loss for air tightness.) Now let’s look at the same house build to a passive design. The overall U value of 0.15w/m2K Thus the 400m2*0.15w=60w The temperature difference is 25°C Heat Loss is equal to 60w*25=1500w The thermal bridge is 0.01 The additional heat loss is 4w*30=120W The air tightness is 0.015 adding 10% to the overall heat loss. So the final heat loss is 1500+120*(10%) =1782w. So on a day with the temperature difference of 25°C the house will require 1782w of energy. The passive house needs only 20% of the heat that the first house needs. Here the difference between both houses widens. The Passive house will always use a heat recovery system to bring fresh air into the house. These are between 90% and 95% efficient. Thus the heat in the house is recycled to maximise heat gain. And heat gain is very important to a passive house. The heat gain from the sun in the form of solar gain and the incidental gains from electrical appliance and human and animal occupancy will help stabilise the heat levels in the house. In brief a passive house will be up to 80% more energy efficient than a standard house built to 2005 regulations. That doesn’t tell the whole story though. To achieve the standard of construction needed in a passive house the whole way we look at building will change and the processes will change. For the house owner the result is simply the best house you can live in. The house is at a temperature that never changes and the air quality is always fresh and the comfort levels are unmatched. The house will be super quiet. Finally the house will be build at no extra cost.It’s a bit like been able to drive a Mercedes for the same price as a Ford Mondeo.
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I hope this page goes someway in explaining the passive way of house building to you.
