_This podcast series is excerpted from a two-day class called “Building Science Fundamentals” taught by Dr. Joe Lstiburek and Dr. John Straube, of Building Science Corporation._
For information on attending a live class, go to BuildingScienceSeminars.com
Last week Dr. Joe wrapped up the water management discussion by talking about energy-retrofit work on masonry buildings and the water problems associated with this work. This week, Dr. John begins the discussion of air management.
This two-part podcast series on how air works has been refined into a feature article, called Air leaks Waste Energy and Rot Houses
. It is the new ‘Air Barriers’ section of the Green Building Encyclopedia.
So I’m going to talk about air flow control and what drives airflow, and get to some air-barrier details. And I’m not going to talk much about vapor barriers because air barriers are more important and I know that’s a common issue: the air barrier vs. vapor barrier thing.
Why do we talk about controlling air flow?
Well, there’s a bunch of reasons.
Number one is moisture control.
To prevent condensation of humid air on cold surfaces. A more important reason is actually comfort and health. You want to make sure that the air that you’re breathing in your home, in your office, in your school, in your hotel, is air that’s clean. And the only way you can know that it’s clean is to know where it came from. And if you don’t know where the air comes from, you have problems with air quality. People who say, “I want the walls of my house to breathe,” are really saying, “I want to rely on mistakes made by the plumber and the electrician to provide me fresh air. I don’t want to seal up my house because I want to rely on mistakes to breathe.” And that’s exceptionally dangerous.
Now, we got away with if for many years because our houses were so full of holes, were so leaky, that we always got some air, and we diluted the pollutants that we picked up. But today, as we get tighter homes (for a whole bunch of other good reasons) we can’t rely on breathing through the dead squirrel in the attic to provide us and our families fresh air — or to be able to draw the air in through the garage, over your Suburban, and bring that fresh air into your home. Or to suck the air out of your crawl space, so you can smell like socks in the house.
So we need to be sure that the fresh air is coming from outdoors without passing through our building enclosure. And Joe, when he talks about the indoor air quality issues — many of them are related to poor control of airflow moving through the enclosure which has been damaged by heat, ultraviolet, or moisture exposure. And those are the things that we’re trying to avoid. The biggest reason we want a good air barrier is to have really good air quality. Only with a good air barrier can we know where the air is coming from and have a chance that quality and quantity can be controlled.
Another thing that’s important is energy
In a relatively well insulated building (like many houses are to code — and a lot of green commercial buildings), air leakage becomes a pretty significant part of the total mix. Particularly in colder climates or hot humid climates, air leakage can be responsible for a third or more of the energy transfer across the building enclosure. There are also sound issues. If you’ve got a hole even the size of my finger, in a wall — a partition wall or exterior wall — you’ll reduce your sound transmission class rating by five to ten points.
So in terms of what we’re trying to control: we’re trying to control airflow through the building enclosure, and airflow within the building enclosure, and we’re going to talk about that.
The National Building Code of Canada has required an air barrier for almost twenty years now, but it’s still not a requirement in most states. It’s still not in ASHRAE 90.1 or the IRC — even though we’ve known, from a research point of view, for probably thirty years, that this is a really big deal.
Three forces of airflow
So, first, to understand why air would even move, we’re going to look at the forces that drive air: wind, stack effect, and HVAC. The wind effect means that air leaks through the windward side and is withdrawn on the leeward side. We also have stack effect, meaning hot air rises and cold air falls. And we have combustion and ventilation: all those pieces of mechanical equipment that have fans. Whether they’re a power-vented hot water heater, whether they’re a furnace, or even a chimney. In the old days, fireplaces acted like really big exhaust fans, because two thousand cubic feet per minute of exhaust went out the chimney, and two thousand cubic feet per minute of air came in through the walls. And that provided a guaranteed flow of air through the building.
Today we sometimes use electric heaters or ground-source heat pumps, so we’ve lost that exhaust fan that the chimney used to provide. We use sealed-combustion appliances for indoor air quality, so we don’t have that exhaust air any more. So the remainder is — well, we have the range hood, and the fart fan in the bathroom, as ways for exhausting air. We’re reducing the number of mechanical appliances in houses that are moving air out. So we’ll look at each of these in a bit more detail.
Wind loads: predictable, natural, and variable
The peak wind loads that we put into building codes are actually pretty high. They’re pretty large numbers. But on average the wind pressure is quite a bit lower. On average the pressure is something like ten pascals; on a low rise building, 10 pascals would be considered pretty high. Five is more likely. And in a high-rise building, maybe 40, 50, or 60 pascals. (250 pascals is 5 lbs. per square foot.)
If you have a flat-roofed building, a lot of the roof actually has a negative pressure on it. Air is being sucked up through that roof because of the aerodynamics of the wind passing over the leading edge and causing negative pressures. On a house, because we have a sloping roof, it actually is positive on the windward side — provided the slope is over three or four to twelve — and negative on the leeward side.
But of course the wind is highly complicated. It’s three dimensional, at the very least. Wind is trying to flow around buildings, and so the highest pressure is on the middle sweet spot. And as the wind goes around the corners, it creates some pretty large swirls and negative pressures. And so we often see wind-related structural damage right at these spot,s because that’s where the highest suction pressure occurs.
Now, if I look down on a building in plan view: positive pressure causes air to leak in on the front face, negative on the leeward face, and large suction pressures around the corners. That’s what we should expect under most wind conditions.
Stack effect: slow, steady, and bigger with height
The other big factor that moves air around — our natural factor — is stack effect. The phrase “stack effect” comes from the way chimneys used to work — or stacks — and is basically this: the building produces warm air in the wintertime which is lighter than the cold air around the building. And because of that, the warm bubble of air wants to rise up and out. And in high rise buildings that’s a pretty big deal. The airflow leaves the top of the building draws cold air into the bottom. And if we provide a building that’s 20 or 30 stories high, that’s one hell of a chimney.
And so the pressures that stack effect generates are really significant. So significant, in fact, that when skyscrapers were first developed at the turn of the century, people had to also invent revolving doors — because you couldn’t open the front door because of the stack-effect pressure. The cold air was rushing in with so much pressure that it was difficult to push the exit doors open. Now it requires cold weather and tall buildings before really you notice it. Unlike most other pressures it acts every hour of every day that it’s cold. And those pressures that are dragging in at the bottom are really very significant. So the air is coming in at the bottom, rises up, and gets heated of course as it goes, and then out the top.
What’s really quite interesting is that the people in the lower half of the building get all the air leaking in — they get all the fresh air — and the people in the penthouse apartments get all their burps and farts delivered into the penthouse suite. So the people who pay the most rent get the worst indoor air quality. Now it gets worse if there’s a parking garage below the building, so that all of the exhaust gas emissions from cars and trucks and things that are idling get vacuumed up by the stack effect pressure and delivered to the high-rent customers at the top of the building.
And if you have older buildings — which are actually quite leaky because they have no air barrier — then you end up having some serious comfort issues as the cold air leaks in. The air is leaking in so much that it’s cold in the apartment, so the people turn up the thermostat to get more hot air into the building. And of course the people at the top get all that hot air, right? So that air gets heated up in the lower floors and goes to the top, and the people at the top are saying, “Geez it’s hot!” And they turn the thermostat down. But it’s still hot, so they slide open their windows to try and get cooled off. This of course increases the air leaving, which of course increases the air coming up the bottom floors, so the people at the bottom plug in little heaters… You wind up with this magic merry-go-round of sucking air up the bottom, heating it up, blowing it out the top, and there’s so much airflow in the elevator shafts that you can float! You can just put out your arms and you’ll float in the middle of winter in many of these buildings.
Of course this consumes tremendous amounts of energy. Everyone’s pissed off whether they’re the low level or the high level, and the indoor air quality is completely compromised. And that’s actually the normal scenario. This isn’t the worst-case scenario; this is the normal scenario for high-rise residential buildings in cold weather. In warm climates the reverse can occur, but the temperature differences aren’t as great. I mean it’s rare that you get more than 95 degrees in Miami. The flow and reverse isn’t as bad, but it’s bad enough to be noticeable. The top-floor hotel rooms in the South get more condensation on the vinyl wallpaper than the bottom floors. So sometimes this creates bigger problems at the top, because it’s sucking hot air off the top because of this natural stack effect when the air conditioning is on.
So here’s some numbers: when it’s cold outside: 4 pascals per story of height.
When it’s hot: about 1.5 pascals per story of height.
Man-made forces can overwhelm natural ones
Now, those are the natural forces, due to temperature and due to wind. The mechanical forces due to building design and operation can completely overwhelm this, or be completely insignificant depending on the design and operation because they’re man-made. And so if we blow air into the building we pressurize it. Everywhere. So the pressure will spread through the building. If I have a 10 pascal pressurization it will spread through the whole building and everything will be pressurized. And if I negatively pressurize, everywhere around the building will be negatively pressurized.
One of the most common problems with exhaust fans that are too powerful occurs in high-end houses that have mammoth downdraft range hoods. To have a downdraft range hood, you’ll need a lot of airflow ,right? In order to exhaust all of the steams and the smells of the stove and you need a huge air flow to bring it down and out. So they tend to be rated in the 1,000 cfm and up range — very powerful fans. You turn them on and there’s this big wind up like a Boeing on the runway, and you’ve got to keep the kids away or they’re going to get sucked onto the stove. Well, if you don’t provide makeup air for that large amount of exhaust, the whole house goes massively negative, and you start sucking on the garage, and you’ll pull air sucked backwards through your hot water heater vent stack. It’ll suck air backwards down your fireplace. People have died — people continue to die every year — because of things like this.
With powerful exhaust fans and no makeup air, you start sucking backwards through combustion appliances in your building. And so you have to control these mechanical systems so you don’t get dangerously high negative pressures.
They don’t die that often, but they die often enough that it matters.