Concept

Build it tight and ventilate it right. A tight house requires mechanical ventilation. It is too tight to rely on wind, pressure differences between inside and outside, or even a few "cracked windows" to provide proper ventilation.

Building standards require 60 CFM continuous mechanical ventilation for a house around 2000 square feet.

There are several strategies to provide this ventilation – exhaust only, exhaust with inlet louvers, and heat and energy recovery systems.

Application

Because we are building a tight, very tight house it must be ventilated. The reason for mechanical ventilation is not just to provide fresh air, but also to eliminate from the house any internal pollutants, excess water vapor and carbon dioxide from people breathing.

Internal pollutants are initially minimized by using low VOC paints, avoiding building materials (chip or particle board, etc.) and manufactured products (kitchen cabinets, furniture, carpeting, etc.) that off-gas, and by storing paints and cleaning supplies in sealed containers or outside of the house.

The garage is also depressurized with a small exhaust fan to prevent any fumes in that area from migrating into the house.

Prevention is not enough and we must mechanically ventilate. I chose to use two small Heat Recovery Systems – one in the up-stair knee wall and one in the basement. The reason for two small units rather than one larger one is primarily to minimize duct runs and also to allow independent operation. For example I may want to continue ventilating the basement in the fall when the upstairs windows are open enough to not require mechanical ventilation. Or I may want to run the up stair unit at a higher rate than the basement unit.

The up stair unit draws air from the kitchen, master bath, and children’s bath and expels it out the west gable end. It then supplies fresh air to each of the three bedrooms.

The basement unit takes air from the utility area (water tanks, batteries and some open cleaning and painting supplies) and expels it out the north rim joist. It then supplies fresh air to the area we use for exercise and storage.

Both units run on timers to get the required 60 CFM x 24 hours a day volume of air exchanged daily.

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