Beware the Closed Bedroom Door
It may sound like a tabloid news story, but building science researchers have found that simply closing a bedroom door can create serious safety, comfort, and health problems in a home.
January 01, 2001
Why? Today’s air conditioning and furnace fans move large volumes of air--between 1,000 and 2,000 cubic feet per minute. Pushing around that much air is the equivalent of moving 1,000 to 2,000 14-inch balloons every minute of operation. When you close a bedroom door, you effectively block the air's pathway, and that blockage can lead to big trouble.
Ever notice how you can’t put your mouth over the end of a Coke bottle and blow air into it? If there is no way out, you can’t blow air into a space. A room functions in the same way. Closing a bedroom door reduces the air flow into the room and the air flow through the system.
When the system puts air into a room and it gets trapped, it pressurizes the bedroom. This positive pressure forces the cooled, or conditioned, air out of the house through any opening in the room that the air can find. One simple lesson we learned from building science research is that for every cubic foot of air forced out of a building, a cubic foot must be drawn in from outside to replace it.
So, what happens when air is forced out of a bedroom under pressure? An equal amount of air is drawn into the main body of the home to replace the forced-out air. Depending on the number of doors that are closed, the rate at which hot or cold outside air enters the home goes up by from 300% to 900%. In turn, utility bills go up, comfort goes down, and health problems proliferate.
Where does all of that air come from that the furnace or air conditioner needs to replace? Air always seeks the path of least resistance, so the biggest, smoothest, and straightest holes make the best pathways. The chimney, the flue of the water heater, or the furnace flue are likely passages, as they go straight outside and are very large and smooth for air to slide down. This reverse flow of hundreds of cubic feet of air per minute brings in carbon monoxide (CO), outdoor pollutants, and humidity. If you are lucky, the only noticeable symptom of this reverse flow will be smoke being pumped back into the house from the fireplace. The not-so-lucky could experience CO poisoning, cold drafts, high humidity, or mold!
Mold was the symptom that motivated a preacher and his wife to call me. They had just bought and moved into a home in the nicest section of town. The family was large: the couple, one daughter, and four adopted teenage sons. A few months after getting settled in, the entire second story had bloomed with really impressive mold colonies. Had the previous owner hidden some horrendous house defect at the time of the sale, they wondered? During the initial interview, the wife also mentioned that the CO alarm had gone off a few times. If my brain had been fully engaged at this time, I would have nailed this one quicker, but no one is perfect.
When I went out to the home, I found that the humidity level was more than 60%. Surprisingly, their air conditioning ducts were very tightly sealed--a condition rarer than an honest politician in this country. When I went into the attic, I found that the previous owners had taken the house through the utility energy conservation program, which explained the tightly sealed ducts. I did a blower door test and found the house to be tight, but not too tight, about 0.42 ACH natural.
One suspicious sign I noticed was that there were telltale streaks that I call filter marks around the bedroom doors. I asked, "Do the boys close the bedroom doors often?" "Why, of course they do. They keep them closed most of the day when they are home!" she said. "In fact, we all close our bedroom doors all night for fire protection, like the fire marshals recommend."
OK, no stroke of genius required here. I went upstairs and closed the four bedroom doors. I tested the pressure in the house using a blower door and found that the house pressure went to negative 6.9 Pascals with the doors closed.
Earlier I had noticed that the water heater was in a closet off the kitchen. I went to the water heater and held my hand up to the draft diverter. The rush of wind coming down that flue, instead of up and out of the house, was truly impressive. This condition, known as a back draft, is hazardous, as wind coming down the flues can bring carbon monoxide and other combustion byproducts into the house.
I got the homeowners and brought them to the water heater, explaining what I had found as we walked. They each felt the wind for themselves. I left them there as I went upstairs to open the doors. I called down and asked them to check the draft again. It was OK, no back draft! I closed the bedroom doors and showed them the suction that the house developed in this condition, explaining how it caused humid outdoor air to get sucked into the house and down the flues.
We sat down and reviewed the facts. I got out a chart that allows you to calculate the amount of outdoor air flowing into a home, given the tightness of the house structure and the pressure developed. This home was drawing in almost 1,000 cubic feet of outdoor air per minute with all of the doors closed!
I then explained that, if they would relieve the pressure in the bedrooms, the problems would go away. The CO alarm would stop going off and the humidity in the house would come under control, creating conditions unfavorable to the growth of mold. Of course, they would need to clean up the mold that had already grown. What had caused their problems was the way they were operating the house, not the house itself.
How do we solve these types of pressure problems? We could undercut the door by 14 inches to give the air a big enough escape route, but most homeowners would veto the Dutch-door sized undercut idea. Installing a return duct going back to the air conditioning unit in every room would give the air the pathway it needs, but this solution would be very costly.
Instead, installing transfer grilles or jump ducts that allow for air movement between rooms would be the most reasonable solution. Transfer grilles or ducts give the air a path back to the system that is always open, yet they don't transfer sounds between rooms and can not be seen through. A handy homeowner or a competent air conditioning contractor can install them.
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