Build it Tight, Ventilate Right.

The building envelope is a total system of construction materials and design components that control the temperature, movement of air, and moisture both into and out of the building. A building's insulation, air barrier and vapor barrier all need to work together to achieve a more stable, comfortable and healthier indoor environment. Many new materials and design practices are being implemented to extend the sustainable service life of buildings and homes.

Most builders and design professionals are not familiar with modern materials and progressive building science techniques that can virtually eliminate all of these problems that force the traditional, less effective requirement for ventilation in these building spaces. 
Builders and design professionals will also make the argument that your home needs to breathe.  Well, they are absolutely correct.  But why rely on cracks, gaps and holes in your building for passive ventilation, when you can build your home tight, healthy and energy efficient, and let the mechanical ventilation systems due the job properly.

Spray foam insulation can still provide benefits far greater than traditional materials such as fiberglass and cellulose, regardless of whether you decide to ventilate these spaces or not.  By using spray polyurethane foam insulation you can increase your home's energy performance, structural integrity and air quality.