I have also seen a building tightness limit (BTL) standard of 15 cubic feet per minute per occupant, which translates to 0.15 exchanges per hour for my situation. Just open … If this house has an air exchange rate of once per hour, we add 190 W/° C, nearly doubling the energy required for warmth (or significantly lowering the temperature inside if you, like me, don't habitually heat your home ).