Somewhere there is a myth that must start… “In a land far, far away, someone who was really quite important once said “Thou shall not open a Passive House window.” As a Passive House consultant I hear that a lot. The one thing about Myths and Fairytales is that there is a kernel of truth that they are built on, but then they get creative and add all sorts of fantastical problems, or Mythical wizards who have the power to permanently seal your windows and force you to live without air FOREVER!
Frankly, I like the idea that Passive Houses are the fairytale dream that came true, but alas it is good old fashioned science and not sorcery that makes them operate the way they do. The reason behind keeping Passive Houses airtight (the kernel that must fuel the unopenable window myth) is to keep the heat in when you want it so that your home operates energy efficiently. However, if it is the coldest day of the year and you want your windows open – you can certainly open them! Perhaps you have a lot of company and the extra body heat has made your home warmer than you like – the windows are a way to manage your comfort. Even if you just personally want to feel the wind coming through the open window, go ahead and open the window… but remember to close it so that your house can once again regain its equilibrium and seek to operate at the efficiency it was designed for.
In the Summer, opening windows is actually encouraged as a method for allowing excess heat to escape the building. If designers work with the physics of the building and its location, they can even plan for cooling potential by taking advantage of the “stack effect” when placing windows or doors. The stack effect works to draw air out of a building like a chimney. There is a pressure difference between warm and cool air and that pressure can be used to draw cool air in while letting warm air out.
Taking advantage of the characteristics of your project and its location will enable your designer to provide you with a way to naturally ventilate your Passive House. The SummVent worksheet is where the expected amount of natural ventilation is logged for your project. This tells the PHPP how often it can expect the windows to be open, what the expected differences in temperatures inside and outside would be, and what amount of air could be expected to be exchanged using natural ventilation. It also takes into consideration wind velocity during the day. There is so much science dedicated to just sorting out how much cooling would be available by opening windows I am surprised the closed window myth ever got started.
As you can tell from the PHPP manual, quite a lot of consideration must be given to which airflow pattern to use and how to record it on this worksheet. A column for each type of use must be entered – if you leave your windows partially open while away at work, that is one kind of use and a different airflow than if you were to open them more fully when you return home. Nighttime operation will differ from daytime use and there is a difference in the temperature of the air change rate to consider. Refer to the different scenarios presented in the PHPP manual for further details on filling in this worksheet.
Next up – we get into the mechanical side of the Passive House equation!