A thermally insulated and air tight house needs ventilation to get out the vapor, that is produced by breathing, showering, washing etc.

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You can ventilate by opening the windows on a regular basis. Usually that doesn't work all to well and it is expensive. Through your opened window you'll loose the energy you want to keep inside your house.

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A controlled home ventilation (CHV) provides you with fresh air 24/7 and a device for recovering the energy will get more than 90% of the energy of the air extracted out of your rooms and gives it back into the air, that is flown into your house.