We can never totally stop air movement in and out of homes. Not only are our attempts to seal homes never perfect, materials change over time and sometimes create new gaps where there were none to begin with.  Some homes are more forgiving than others because of the choice of materials used.

In this first picture we can see the very common black staining that happens around the edge of the carpet where air is finding its way in and out of the living space at the wall bottom plate.  The carpeting is merely acting as a filter.

In this next picture the black staining on the soffit around this beam of a home, with a flat roof, is where air is leaking in and out of the building.

I have long held that since it is going to be difficult to completely stop air movement, why not do our best to at least torture the heck out of it as it moves?  In other words make it difficult for air to flow.  We can do this by choosing really good quality insulation like cellulose fiber insulation–it gets into every little nook and cranny making a very good barrier to air movement.  We can caulk the connection of bottom plates and sub-floor.  We can use weather-stripping on doors and windows and access hatches. We can glue drywall to studs and top & bottom plates. We can seal around pipes and ductwork that penetrate the building thermal envelope.

If we can stop or significantly slow down air movement something else happens in the process.  One of the major ways moisture moves through the building envelope is in moving air.  If we can stop air movement we can eliminate or greatly reduce moisture movement as well.

Fiberglass insulation is notoriously bad at stopping air movement–and thus moisture movement.  In fact, in the early days (early 1970’s) of making homes more energy efficient, one of the main reasons we went to such great lengths to install plastic vapor barriers behind the drywall was because fiberglass insulation could not do the job adequately.  Everyone has experienced the cold drafts that can occur around electrical outlets of homes insulated with fiberglass insulation.  This does not happen with cellulose fiber insulation.

In my opinion stopping air movement is perhaps the single most important quality of insulation. Its “R-value” is a moot point if it can’t stop air movement.  How much money are we really saving by insulating our homes if we cause structural damage by moisture related to air movement?

Your home inspector will often be on the lookout for signs of air infiltration/exfiltration during the inspection.  An important thing to keep in mind is that air moves both in and out of homes depending on atmospheric conditions.  At times your house can be pressurized and at other times depressurized–close to “neutral” is ideal but not always easy to manage.

For example, if you turn on all of your exhaust fans, where will air come into the home that is being displaced by the exhaust fans? 

Well, if it can’t “easily” find a way in, a negative pressure in the home will be created and the fans will actually not move any air–won’t do their job.

As an example of this effect, my sweetie and I had checked into a little motel a few years ago and experienced this first hand.  The motel was brand new and very well built.  The motel had a little kitchenette and when my sweetie decided to make toast in the morning, she did her typical method of making toast–which is to have the smoke alarm tell here when it is done.  Well, NOT REALLY wanting the entire building’s alarms to go off, I thought it more prudent to turn on the kitchen exhaust fan and get rid of the smoke before the alarms went off.  I turned on the fan and the smoke just sat there like a dense motionless fog.  It occurred to me that perhaps if I opened a window it would help and immediately the fan became functional and pulled all the smoke out of the unit and the alarms never went off.

In most homes, air does find a way in-somewhere, but fans will be much more effective if they can obtain air easily.  Air will find its way into the home around poorly sealed doors and windows, around the bottom of walls, around electrical outlets, around crawl space access covers, down the chimney or even back in through un-dampered exhaust fans.

Another important factor to remember is that we want air to come into the home–we just want control over how, where, when and how much.

Energy efficiency is compromised when we lose control.

Take this picture of a heating system supply-air duct in a crawl space.  This is a great example of how inadequate fiberglass insulation is as an air barrier, as well as a good demonstration of air movement at a location where we don’t want to see air movement.

All duct connections should be air-tight and on my planet ductwork would not be allowed to be installed outside the building’s thermal envelope.  If the connections are not air-tight, when the heating system is operating, warm air will be pushed into the crawl space–right through the insulation.  When the system is not running, these leaking ducts will become a place for air to enter the home when the home is under negative pressure (as when exhaust fans are operating or due to the home’s stack effect).  As you can see from the black band around the ductwork, the yellow fiberglass insulation is acting like a “filter” as air moves through it.

These are just a few of the “visual” indications of poor control of air movement in the home.

By Charles Buell, Real Estate Inspections in Seattle

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