Brooklyn brownstones are being built with tighter and tighter envelopes, which means there are no leaky cracks to help ventilate the building.

This is a problem, sort of. Gone is the wasted heat. But you need to make up for that air and the best way is good design and mechanical ventilation.

ASHRAE is the group that keeps track of how much a house needs in terms of heating, cooling and general ventilation. Their standard 62.2 is the benchmark of acceptable indoor air quality in low rise buildings.

They suggest the following to find how much air circulation your house needs. It is calculated in cubic feet per minute (cfm) of air change.

Multiply square footage of the house by 0.01
Then multiply number of bedrooms plus 1 by 7.5 (7.5cfm is what one person needs)
Add these together.

For example for the Brooklyn Green Show House we have two unites with a total of 3000 square feet and 7 bedrooms. That would look like this:

(square footage x 0.01) + (bedrooms plus one x 7.5) = cfm
(3000 x 0.01) + (8 x 7.5) = cfm
30 + 60 = 90

This brooklyn green building (i.e. very tight envelope) needs mechanical ventilation of 90 cubic feet per minute to keep the air fresh. That is 5400 cfm per hour, which is almost two total air changes per hour.

That might sound like a lot but a leaky house can do a lot more than that. The good thing about doing it mechanically is that you can use an energy recovery ventilator and recover the heat or cold that is passing out of the house. This reduces energy costs that would be wasted on a leaky house.