Passivhaus Myths Explained
1. You can’t open the windows - Myth
This is always the first query we get. All Passivhaus buildings have windows that open, and you can open them when you want to. There is nothing in the Passivhaus standard that states that they must be kept shut.
Opening them in the summer at night is often the best way to reduce heat build-up. In winter, if the windows are open, the building will lose heat, and this is no different from other non-Passivhaus buildings.
In a Passivhaus the ventilation system keeps the internal air fresh, and there is less
need to open windows to remove stuffy air.
2. A Passivhaus overheats in the summer – Myth
Completed Passivhaus buildings maintain relatively stable internal temperatures, even in warm summers.
A certified Passivhaus will keep internal temperatures at no more than 25°C for 90% of the year. External shading and carefully designed windows ensure that this target is reached. In our designs we will carry out additional checks to ensure that overheating is not a problem.
Like non-Passivhaus well insulated buildings, the opening of windows in summer may be counter-intuitive – with draught-proof triple glazed windows, keeping them shut in summer during the day may be the best way to keep the heat out. Opening them in the evening will allow cooler air through. A good ventilation system can also include automatic controls to switch off the heat recovery in summer.
Passivhaus care home in Germany with windows open.
3. The air in a Passivhaus is unhealthy - Myth
The Passivhaus approach requires a scientifically proven amount of fresh air to be continuously provided. This air is supplied by mechanical ventilation with heat recovery (MVHR).
A MVHR system runs continuously with fresh air, gently moving it into living spaces whilst extracting stale air from kitchens and bathrooms. The stale air is not recycled. The heat recovery process ensures that heat from the extracted air warms up the fresh air, saving energy and ensuring comfort.
In cold climates in winter, and in under-occupied buildings, there may be a tendency for the air to become dry. This shows that the risks of condensation are being minimised. The dryness of the air can be reduced by occupying the building, allowing clothes to dry inside, and by having houseplants. Some MVHR systems can also recover some moisture from the outgoing stale air.
4. The ventilation system costs a lot to run, and it is noisy - Myth
The MVHR ventilation system is not an air conditioning system.
Good quality MVHR systems required for Passivhaus certification will run at lower air speeds than air conditioning and more quietly. Air will move gently through the building from living spaces into bathrooms and kitchens before being expelled.
Around 90% of the warmth in the expelled air will be transferred to the incoming air in a Passivhaus certified MVHR system. The efficiency of these systems is such that the energy (in electricity) to run the fans can be less than a tenth of the energy (in warmth) recovered from the expelled stale air.
5. You have to live differently in a Passivhaus - Maybe
Surveys of people who live in Passivhaus dwellings show that they are very satisfied. However some aspects of living in a Passivhaus may surprise you.
A Passivhaus dwelling usually does not have a tumble drier, as they use a lot of energy and the waste heat is not recycled. Clothes in a Passivhaus are therefore left to dry by airing.
If energy hungry equipment is left on, the house will get warm. A typical clothes iron will be rated at 2 to 3 kW. The heating demand at -10 degrees C outside for a 100 sq m Passivhaus is 10W x 100 sq m =1kW. Therefore leaving the iron on will heat the house!
If you always turn the heating up when the forecast says it is going to be cold, doing this in a Passivhaus may mean that you get too warm in winter as a Passivhaus will take a long time to cool down in a cold snap.