Spray Foam Insulation: How Tight is too Tight?




Spray foam contractors look for random air leaks are likely to bring moist, stale, or contaminated air into structures versus the fresh air distributed by proper ventilation. Sources of random air infiltration include gaps, cracks, attics, walls, crawl spaces, basements and garages. Homes with attached garages allow unhealthy chemicals to infiltrate the home. The average person may not be keenly aware that this is happening. Mold spores and mildew that build up from moisture-laden areas, such as basements, crawl spaces, and attics, can easily spread throughout a residential or commercial structure. Mold and mildew contribute to allergy, asthma, and other upper respiratory symptoms. Air containing impurities can become distributed throughout the residential or commercial structure by chance, in uncontrolled and unpredictable ways. Proper ventilation allows control over how fresh air gets evenly distributed throughout the structure.

Another issue of concern is the presence of combustible appliances without proper ventilation. If a residential or commercial structure is sealed too tightly, without ventilation, open-combustion appliances, such as water heaters, cannot operate safely. Open-combustible appliances must be able to draw in air fresh outside air in order to replace the air that carries the by-products of combustion. Structures that are too tightly sealed, without fresh-air ventilation, can cause open-combustion appliance to back draft due to negative pressure. For example, back drafting can result in flames burning the outside of a water heater and cause a potentially dangerous situation. Interior structures need to breathe. There are three main types of ventilation: Exhaust Ventilation, Supply Ventilation and Balanced HRV and ERV Ventilation. Moving air into and out of a structure can also causes changes in air pressure.


Exhaust Ventilation: Installing exhaust fans in kitchens and bathrooms moves air out of a home or building, taking excess moisture with it.

Supply Ventilation: Fresh air can be drawn in to the home by running a duct from the return plenum of the air-conditioner to a gable end or eave of the house. Mechanical ventilation is also an option, but it is not as cost-effective. Commercial buildings are often ventilated by mechanical control devices; however, many commercial buildings use a combination of mechanical and HVAC integration.

HRV and ERV Balanced Ventilation: Heat Recovery Ventilation and Energy Recovery Ventilation Existing and in-coming air can be transferred by a heat-exchanger. These helps moderate temperature differences and reduces the energy impact on the ventilation system.


Interior Pressure


Exhaust Ventilation Negative
Supply  Ventilation Positive
HRV Balanced Ventilation No change in pressure
ERV Balanced Ventilation No change in pressure

It is important to think of a home or building as a system--every part in connected in one way or another. This means that air can become distributed throughout the structure.