Why seal electric outlets and switches

Posted Tuesday, February 4, 2014 in Sustainable Maine

Why seal electric outlets and switches

by Paul Kando

The blower door test conducted by a professional energy auditor makes all leaks in the heated envelope blow inward.  Unlike outbound leaks we generally don’t feel, this incoming air-flow can be easily detected.  And the amount of air blowing out through the fan is equal to the amount filtering in through leaks. It is thus possible to determine how much air is leaking in and out of a house, then find and mark where those leaks are, so they can be fixed later.

One place you will surely find these little tags is on electrical outlets and switches – prime mechanisms that convey not just warm air, but also the moisture carried by that air into wall cavities. Under typical conditions in a heated house, as much as a pint of water can enter a wall in vapor form, through a mere one square millimeter (mm2) hole. The openings in a duplex  electrical outlet add up to 114 mm2, meaning that as much as 28 gallons of water could enter a wall through every duplex outlet every day. How much water actually finds is way into the wall cavity obviously depends on how much moisture is available for any escaping heated air to absorb in the house. My point is that electrical outlets throughout your house provide a path for about as much air-absorbed moisture to enter your walls as there is available in your house – not a very comforting thought.

Outlets and switches provide a path for leaking air because they are installed in junction boxes that have pre-drilled holes for the convenience of electricians installing them. Most exterior walls have fiberglass insulation, which is basically a heavy duty air filter that allows air movement into and through the wall cavity. Walls between rooms, on the other hand, are seldom sufficiently sealed off  from the cold attic above. They provide a convenient path for warm, moist air to rise to the attic, where the moisture condenses on the first available surface colder than the dew point.  Furthermore a convective loop may establish itself within the empty wall cavity. As the air adjacent to the wallboard or plaster bordering the heated rooms on either side is continually heated (and therefore rises), it is replaced by cold attic air sinking down the middle of the wall cavity, to be warmed as the cycle is repeated. This convective cycle will cool your house as long as you are heating it.

As air is heated, it expands. This turns  a heated house into somewhat of a hot air balloon with pin-pricks in it. As the less dense, expanding warm air rises, each little “pin prick” in the upper part of a heated house becomes a warm air leak to the outdoors. The lost warm air is continually replaced by cold outdoor air entering through leaks in the lower part of the house, including the basement,  unless the basement ceiling is well sealed.

One consequence of this air exchange with the cold outdoors is dry indoor air. As the heating system warms the air, it absorbs all the moisture it can find in the house, and takes it along as it leaks out. Meanwhile the cold  outdoor air filtering in contains too little moisture to replace what has been lost. This is why a leaky house is a house with dry air looking for moisture.  It may only find it in your nostrils or on  your skin, making you feel dry and like getting a cold.  A humidifier is no solution. Air sealing is. In fact, the more moisture your humidifier adds to the indoor air, the more will be there to leak out – and condense along the way.

Condensation reliably occurs on the first cold surface warm, humid air encounters . At temperatures at or below the dew point,  air can no longer hold on to its moisture content. One such surface is the underside of the roof deck. Another is the inner surface of the boards or plywood to which the exterior wall  siding is attached. Water condensing here, inside the building structure, can cause mold,  mildew, and rotting of wooden building elements  – studs, plates, boards, plywood – exposed to the condensing moisture. Furthermore, moist wood attracts carpenter ants, to help destroy the structure faster.

Confused?  Let me refer you to the mantra that reappears time and again in these energy-rumblings: Heat, air, and moisture move together. To improve our energy efficiency without causing damage we must control all three. Old timers say “houses must breathe” and they are right: a tightly sealed house accumulates more moisture.  The trouble is that if a house “breathes” it also loses most of its warm air along with its moisture. So, letting the house “breathe” is an option only for those willing to waste energy – and money. The real solution: (1) insulate to control heat loss; (2) air-seal and add an air/vapor barrier to minimize air leaks; and (3) install heat recovery ventilation to remove  excess moisture.

I will take on these themes in upcoming  columns. 

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