Make-up or compensating air is outside air, which is introduced into the building to replace air that is exhausted from the building through ventilation or combustion processes.
Newer energy efficient, “tight” or “green” homes are well air-sealed so outside air will have more trouble finding a way in to replace exhausted air, as opposed to older draftier homes. So what happens is your house depressurizes. Hood vents can't push out air, because there is no air in your home to replace.
For this reason, there is a new code dictating air replenishment for newly built homes. Basically, exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute need to be provided with make-up air at a rate approximately equal to the exhaust air rate.
It is not a very difficult implement. You simply need air to replace the exhaust by installing a damper that is in-line or in the duct itself and is wired to a Universal pressure regulator. The regulator opens and closes the damper depending on how much air is exhausted. The wall cap simply lets in air.
Supplying make-up air is a rather simple and inexpensive process. Broan manufactures three simple, inexpensive tools to satisfy the code. The cost is less than $300. Have a look below.
Make-up Air Damper Tool by Broan
Check out the Range Hood Make-Up Air Specifier on the Broan website which offers code compliant Make-Up air solutions for domestic range hoods in dwelling units.