Thursday, January 28, 2010

The delicate balance of airflow

I've been thinking a lot about attic airflow lately. In the warm, humid climate that I live in, having appropriate airflow through the attic is important in at least two ways. First, it is important because it allows cooler outside air to automatically replace the hot air in the attic. Secondly, it is important because it reduces the chance of condensation, the enemy of all homeowners. How does it do this? By allowing the air to find its way out of the attic as it cools. This particular point may we worth delving into in detail, because many a roll of insulation and many a roof has been lost to the perils of unexpected condensation.

Critical Features of Condensation

Two main principles drive condensation. The first is hopefully known to everyone who attended elementary school: hot air rises. Or more accurately, air that is warmer than the surrounding air is also less dense than the surrounding air, so it is buoyed upwards. This principle explains why we don't hold our hands over candles, why all the nice hot air in your house rests against the ceiling, and why hot air balloons work.
The second is known to anyone who has taken a passing interest in meteorology: warm air can hold more moisture in it than cold air. This little phenomenon explains why cold fronts are often led by a line of rainstorms, how fog forms, and why clouds tend to dump all their moisture when they reach the front of a mountain range. This last one is quite fascinating to me: the surface winds sweep up the mountainside, bringing warm lowland air higher and higher. As the air gets higher (via orographic lifting) it naturally loses pressure and therefore temperature (due to a process known as adiabatic cooling). Once the air gets high enough that it can no longer hold the moisture that it carried before with no problem when it was warmer, the moisture condenses out of the air and falls to the ground as rain.

Now, given these two main principles, we can apply them to attics and see why they can create moisture problems. One side of effect of human activities such as breathing, cooking, cleaning, showering, and so on carried on inside a house is the creation of warm moist air. Thanks to the principle of warm air rising, the warm air we create in our houses moves upwards. Since things like cracks, gaps, and seams exist in most houses, the warm moist air finds its way into the attic.
Once the air is there, if there is not sufficient ventilation to move it out and replace it with new outside air, the moist humid air will stay in the attic, encouraging bad things like mold growth. Furthermore, if the attic cools down quickly like it would on a nice clear fall or winter night, that trapped warm moist air would become trapped cool moist air. If the air cools enough, thanks to our second principle above, it will no longer be able to hold the moisture that it held when it was warm and the water will come out of the air and "rain" (condense) in the attic. Two factors determine the severity of this "indoor rain event". The first is the amount of moisture that was put into the air to begin with, and this can be reduced by good habits like running the vent fan when showering. The second is how far the air cools: the cooler it gets, the more water will be squeezed out of the air. The way to ameliorate this is to have enough ventilation so that as much of the air as possible can escape before it cools down enough to drop its moisture.
If these "indoor rain events" occur often enough in the attic, you will begin to see water damage to your house.

Seasonal Balance

So if condensation is bad and ventilation can defeat it, why don't we just get as much ventilation in place as possible? Why don't we just cut vents in all 4 attic walls, stick screens over them, and be done with it! The mindful reader will recall that the title of this post implies that there is a balance to be achieved here. That balance, like most balances one can achieve, depends on one's place in life. Or more correctly, the place that one lives.
If you live in an area that is almost never cooler than you want the interior of your house, putting in as much attic ventilation as humanly possible is probably the way to go.
If you live in an area like I do, where most of the year is spent with the outside world a lot hotter than I want my house to be, but a good 3 months is also spent with the outside world a lot colder than I want my house to be, I want enough ventilation that I get some good cooling in the summer and good protection from condensation year-round, but I don't want so much ventilation that my attic loses all it's heat-trapping potential for the winter. This is particularly important in my house because a lot of water pipes run through my attic, and an attic that freezes solid in my few freezing winter nights would freeze and burst water pipes over my living space.
I would guess that if you live somewhere that's cooler outside most of the year than you want your house, and rarely much hotter than you want your house, you would want just enough ventilation to avoid condensation problems, but not more than that.

Flow Balance

So have just discussed one axis of balance for our ventilation. There is another axis as well: the amount of space dedicated to air leaving your attic (net free area outflow) should be balanced as closely as possible by the amount of space dedicated to air entering your attic (net free area inflow). Why does this matter? In fact, why do you need any intake at all? Isn't the point just to get that hot air out?
If you've ever tried to pour milk out of a gallon jug, you might have noticed that it doesn't work well if you just tip the jug over so far that the milk just slops out of the jug in series of bursts, interrupted by gasps when you can hear the air rushing back into the jug, almost as if it were taking a breath before dispensing another sloppy burst of milk. Why is it so difficult for the milk to get out? What's slowing it down?
The answer, of course, is lack of replacement air. When a certain volume of milk leaves the jug, it leaves a vaccuum, or more accurately, the air inside the jug is left at lower pressure than the air outside. This causes the air outside to force its way in, interrupting the smooth flow of milk out of the jug, and reducing the speed at which the jug can be emptied. In fact, the best way to really move that milk out of the jug is to punch a hole in the bottom just before we turn it over, to allow the air to come in from the top as the milk empties out the bottom.
Now: if we turn the whole thing upside down, replace the jug with our attic, the milk with hot air, and the opening in the jug with attic roof vents, the analogy makes perfect sense. Bear with me as I explain.
If you just had roof vents in your attic, and did not have any soffit vents, gable vents, or other air intakes, your attic would be just like that milk jug turned upside down before we punched a hole in the bottom. The air would slosh out of those vents as it heated up and tried to leave, but that would create a lower air pressure region inside the attic. This would tend to pull air back in the vents (the only openings), dramatically slowing the airflow out of the attic, just as the inrushing air slowed the milk from leaving the jug.
The solution, of course, is to punch a hole in the bottom of the attic just as we punched a hole in the bottom of the milk jug. That is what soffit vents and other air intakes are: a way to get air to "backfill" for the hot air that leaves through the vents in the top of your attic. The fact that the backfilling air can come in through a different path than the air leaving means that you can get a nice, uninterrupted flow of ventilation through your attic. Plus, well designed intakes will pull replacement air from cool, shady spaces like the underside of the eaves.
Now you might begin to get a feel why the outflow area should equal the intake area. If the intake vents can't supply enough air, the air to make up the difference will try to come through the outflow vents, and the more it does that, the more you mess up your nice smooth outflow, and the less the attic air will be able to escape.
Indeed, attic airflow is a delicate balance.

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