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HEALTHY HOME DESIGN: Better Indoor Air Quality Using Better Air Barriers

Updated: Apr 17, 2023

photo of an abandoned wood shed in a country field by a meadow of blooming dandelions

How safe is the air in your home? Not very, and you may not realize how the indoor pollutants are getting in.

People who are planning to remodel their house or build a new one need to consider the quality of the air in their home. A typical house – small or big, modest or luxury, old or new - has hundreds or thousands of tiny holes and cracks in the building envelope. You don’t generally notice these leaks in the summer when you open the windows to let in fresh air, but during a late autumn windstorm you might physically feel drafts at the floor or near windows.

All year long though, those countless openings let into your home chemical pollution from vehicles and buildings in urban and suburban areas and biological contaminants in rural areas. During a major wildfire, everyone can smell and even see some smoke indoors regardless of where they live.

During the winter, warm, moist indoor air escapes through these tiny openings; and during summer heat waves, hot, humid outdoor air enters through them. Both scenarios lead to the formation of black mold in the walls which will rot the house framing and threaten the family’s health.

Your home’s heating system wasn’t designed to keep up with the new, longer severe cold weather events already facing us. A typical residential air-conditioning system also isn’t powerful enough to counteract new, record-breaking heat waves. Even an "energy efficient" house with good mechanical ventilation systems that leaks heated and cooled air through the building envelope results in a home environment that is uncomfortable because it’s too warm or too cold.

I go into much greater detail about these concepts in my other article, "HEALTHY HOME DESIGN: How To Improve Your Home's Indoor Air Quality To Improve Your Health".

An increasing number of homes are being built with far more airtight building envelopes in pursuit of energy efficiency to reduce energy costs. No more drafts. Eliminate a common source of black mold, other indoor air pollutants, and wood rot. Your home can be healthier and more comfortable - warmer in the winter and cooler in the summer just by cutting down the air leakage by 80-90%. Enjoy the added benefits of far less dusting and lower energy bills.

Here’s your home’s ventilation system in a nutshell: the bathroom exhaust fans, kitchen exhaust fan, and dryer exhaust move air from inside to outside. This pressure drop sucks in unfiltered outdoor air through hundreds of tiny cracks, gaps, and pinholes, and outdoor pollutants become indoor air pollutants.

In the winter, heated air expands and escapes through these openings. In the summer, hot humid outdoor air is drawn in through the same openings. In both cases, the plentiful water vapour in the warm air condenses somewhere inside the wall, and then black mold releases its spores into the air.

photo of fibreglass insulation in wood studs exposed to show growth of black mold

Moving air can carry large amounts of heat. Air leakage can therefore be the cause of a significant amount of your total heat loss in the winter and can cripple your air conditioning system’s effectiveness in the summer.

Simplistically speaking, the roof and exterior walls of a house need a continuous airtight layer – an air barrier - primarily to stop the movement of moisture-laden air. It’s almost always plastic sheeting that sits just behind your drywall (below left), but it is not well sealed at joints, at penetrations, and around openings like windows.

There are two basic locations for the air barrier in walls.

Interior Air Barrier

photo of pink fibreglass batt insulation in wood studs with polyethylene sheet applied and red tuck tape sealing around electrical receptacle

The clear plastic sheets (polyethylene) or new “smart” vapour retarders that sit behind the drywall are very airtight materials. The problem is all the joints and holes poked in them during construction, so the solution is to install it more carefully. Seams, edges, and the joints around pipes, duct, conduit, light switches, and electrical boxes must be perfectly sealed.

As you can guess, sealing all these locations perfectly takes considerable time, skill, and therefore money. The problems are hidden behind drywall, consequences of poor work won’t show up until years later, and the resulting house still meets building codes, so some sloppiness is commonplace.

Exterior Air Barrier

photo of black building sheathing paper secured with many uncoated staples

Since there are so many things installed in the exterior walls, the arguably simpler location for the air barrier is on the outside of the plywood. After nearly a century of using a black, water-repellent paper, houses now use a white, paper-like sheet called sheathing membrane. It’s quite airtight itself, but hundreds of staples typically used to hold it in place are just hundreds of holes through which air and water leak. Joints, seams, and other penetrations are also not poorly sealed.

photo of Vancouver Convention Centre in British Columbia showing haze from wildfires

In addition to blocking warm, moist air, an air barrier will stop most outdoor air pollution. The air pollution levels don't even have to be visibly high (think Beijing or Los Angeles). Tires and brake pads on every vehicle – even EVs – wear away, and some of the resulting brake and tire dust is drawn into your house along with other outdoor pollutants. Recently, wildfire smoke has become a common concern. Even with the windows closed, enough smoke may enter the home to cause health issues after only a few days of exposure.

collage photo of colourized scanning electromicrographs of various microorganisms

Many gaps and holes are large enough for airborne pollen and microscopic insects to invade the home. Airborne viruses and other disease-causing pathogens are small enough to be drawn easily through the countless miniscule holes in your walls and roof.

Introducing the vapour-permeable weather-resistive barrier (WRB). It’s installed on the outside of the plywood either as a self-adhesive sheet or as a liquid that dries to a rubber-like coating (see the photos below).

closeup photo of various synthetic weather resistive barrier membranes (Henry Blueskin, AirOutshield, Delta Fassade,)

First, being exterior air barriers, they avoid problems with electrical outlets and light switches. Second, these fancy, colourful sheets eliminate staple holes, and the continuity of a liquid-applied product is a perfect seal against outdoor pollution or other contaminants, resulting in more airtight homes.

closeup photo of cured liquid-applied weather resistive barrier

These sheet and liquid WRB products are typical in higher-end commercial buildings, and most homebuilders have little or no experience using them to build airtight houses. They must be installed correctly in fair weather by skilled applicators to be effective. The products do cost a little more than typical sheathing membrane, but achieving very airtight walls is less labour-intensive (less costly) than with a typical sheathing membrane.

The result of the improved airtightness is a more comfortable home, cleaner indoor air, and better protected walls that extend your house’s lifespan. You should be confident in the safety of your home's indoor air quality and the integrity of the shelter it provides, so talk to your builder about using a superior WRB or book a free Diagnostic Session with me.

photo of elderly couple cuddling and resting on sofa


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