An Introduction to Pressure Diagnostics

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An Introduction to Pressure Diagnostics

Fans, wind, and the stack effect cause some areas of your home to be pressurized and others to be depressurized

Posted on Aug 18 2017 by Martin Holladay

When it comes to understanding heating systems, most of us are comfortable with the basics. To warm up your house on a cold day, you need a source of heat in your living room — say, a wood stove or a radiator. To keep the heat in your house, you need insulation.

That’s the way most builders understood heating from 1935 to 1980. Somewhere around 1980, however, building scientists began to realize that the old picture was imperfect.

We now know what was missing from the old picture: an understanding of air pressure. A house is a dynamic system, and that system is greatly affected by air pressures inside and outside the home. Forces affecting the performance of the home include wind, the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season., and many different types of fans. These forces operate in buildings, and all buildings include leaks in their thermal envelopes. Moreover, most homes have ducts with leaky seams. Finally, many of these ducts are partially indoors and partially outdoors.

If you put all these factors together, you realize that you can’t understand home performance without understanding the effects of air pressure.

What happens when the furnace comes on?

Building scientists use a variety of techniques to measure air leakage through a home’s thermal envelope. Air leaks faster on cold days than mild days (because the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. is stronger when the outside temperature is cold), and it also leaks faster on windy days than on still days. No surprises there.

Let's say that technicians are studying a typical American home. They are measuring the air leakage rate (the infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. rate through the home's thermal envelope) while the furnace is off. They get a reading. Suddenly, the furnace turns on, and the technicians notice the air leakage rate has suddenly increased dramatically — for example, by a factor of two or even four. What just happened?

The answer to the question is complicated.

Air pressure basics

Let’s start by establishing a few important concepts.

Every house needs an air barrier. The air barrier should surround all of the home’s conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. , and should have as few leaks as possible.

For air to flow into or out of your house, you need a hole and a difference in pressure. The larger the hole, and the greater the pressure difference, the greater the rate of air flow.

Differences in pressure are caused by both natural and artificial forces. Natural forces include wind and the stack effect; artificial forces include all sorts of fans.

The stack effect causes the upper levels of a home to be pressurized with respect the the outdoors. Lower sections of a home are depressurized with respect to the outdoors. The cooler air that drops to the lower levels of a house is denser than the warmer air that rises to the top of the house, and these differences in density lead to the stratification of air in different temperature layers. The middle region between the low-pressure area and the high-pressure area is called the neutral pressure plane.

We measure differences in air pressure with a manometer. The unit of measurement is either inches of water column (i.w.c.) or pascals.

The tool used to measure the rate of air leakage through a home's thermal envelope (and to locate envelope leaks) is a blower door. For more information, see Blower Door Basics.

The tool used to measure duct leakage is a Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage.. For more information, see Duct Leakage Testing.

The furnace and its ducts are supposed to be indoors

Ideally, your home has a thermal envelope that is very close to airtight. If you have a forced-air heating and cooling system, then your ductwork (ideally) should be located inside your home’s thermal envelope. (For more on this topic, see Keeping Ducts Indoors.)

Under these circumstances, operating your furnace shouldn’t affect air leakage through your home’s thermal envelope. After all, the forced-air system is simply pulling air from one indoor location and delivering it to another indoor location.

In the real world, however, ideal houses are extremely rare. Most homes have leaky thermal envelopes. Homes with forced-air systems often have ducts that are located in vented unconditioned attics. These ducts often have leaky seams.

Suddenly, the pressure dynamics got a lot more complicated. Once the 1,200-cfm furnace fan comes on, it’s hard to know where the air is coming from and where the air is going.

Leaky attic ducts change the pressure dynamics of the entire house

What happens if a supply duct in your attic has a significant leak? The return air system may be pulling 800 cfm of return air from the various return-air grilles in the house, but the supply air system is only delivering 650 cfm of supply air, because 150 cfm of supply air is escaping through leaks in the attic.

When this happens, the house ends up depressurized with respect to the outdoors. When the furnace fan is operating, the furnace is helping to pull in extra outdoor air through all of the home’s cracks. That’s one way that operating a furnace can increase the home’s air leakage rate.

On the other hand, what happens if a return air duct in your attic has a significant leak? The supply air system may be sending 800 cfm of supply air to your home’s registers, but the return air system is only pulling 650 cfm of return air from the home’s grilles, because the duct system is pulling 150 cfm of return air from the attic.

When this happens, the house ends up pressurized with respect to the outdoors. When the furnace fan is operating, the furnace is forcing conditioned indoor air outdoors, through cracks in the thermal envelope.

Pressure diagnostics

An experienced home performance contractor understands pressure diagnostics. For those interested in learning more about pressure diagnostics, I recommend a textbook called by John Krigger and Chris Dorsi. Below, I'll provide three extended quotes from Krigger and Dorsi’s book to give three examples of how a good diagnostician works.

One trick uses interior doors:
“Technicians sometimes use interior doors to test parts of a home’s air barrier. Three common methods, used during a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas., can give you a rough idea of which areas of the building are leaky and which are tight.
1. Compare rooms to one another by closing the door to a 1-inch-wide opening and feeling how much air is coming from each room.
2. Close interior doors one by one and measure the pressure difference between the home’s main body and that room. The greater the manometer’s negative reading during depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home., the more leakage through that room.
3. Open and close doors to rooms and intermediate zones, noting the effect on blower door airflow (cfm50).”

Another trick uses the basement door or attic hatch:
“Opening basement doors, crawl space hatches, or attic hatches can give an indication of whether the floors or ceilings (between living spaces and these intermediate zones) are effective air barriers. If the blower door shows little airflow difference between an open or closed basement door, the foundation wall, not the floor, is the tightest barrier. If the blower door measures a large airflow difference between open-door and closed-door tests, the floor is the tightest air barrier.”

Sometimes it’s useful to poke a hole in the ceiling:
“To test the airtightness of a home’s ceiling, depressurize the building to 50 pascals, and then insert a manometer hose through a small hole in the ceiling so the hose’s tip reaches above the insulation. Below are three possible pressure readings with a probably explanation for each.

  • 50 pascals: The attic has the same pressure as the indoor/outdoor pressure difference, indicating the ceiling is the tightest air barrier, the attic is well-connected to the outdoors, and the roof is not acting as an air barrier at all.
  • 25 pascals: The ceiling is a partial air barrier, and the roof is a partial air barrier; they are equally airtight or leaky.
  • 5 pascals: The home is almost completely connected to the attic. The roof is the tightest air barrier, and the ceiling is a very leaky secondary air barrier.”

Inside or outside?

What do we learn from the examples above? There are several lessons:

  • With leaky buildings, it’s not always clear where the “inside” ends and the “outside” begins. Some spaces — attics, crawl spaces, mudrooms, and enclosed porches, for example — aren’t really inside or outside. They are half inside and half outside.
  • It’s dangerous to assume that ducts are doing what they’re supposed to do. A supply duct may be designed to carry warm air from a furnace to a supply register, when in fact it’s also delivering warm air to the attic. A return duct may be designed to convey air from the hallway to the furnace, but it may actually be pulling air from your crawl space.
  • Air sealing work and duct sealing work prevent unexpected (or undesirable) air flows. If you seal the envelope leaks, the air that is supposed to stay indoors will be more likely to stay indoors. If you seal the duct leaks, the air that is supposed to be delivered to certain rooms is more likely to be delivered where it belongs.

Slight pressurization or depressurization may be useful

In some situations — for example, when the non-smoking occupants on one side of a duplex are irritated by tobacco smells from the smoking occupants of the other side of the duplex — it make be helpful to tweak a home's ventilation system so that a home is slightly pressurized or depressurized. Those who are irritated by odors from their neighbors would probably rather live in a slightly pressurized apartment than a slightly depressurized apartment — especially if the supply ventilation system is design to pull in fresh outdoor air.

Martin Holladay’s previous blog: “Stair Design Basics.”

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  1. Fine Homebuilding

Aug 18, 2017 4:53 PM ET

Nice article. Maybe add
by Jon R

Nice article. Maybe add something about summer/AC stack effect (small but present) and the effect of pressure and the resulting flow on partition moisture.

Aug 21, 2017 11:21 AM ET

Edited Aug 21, 2017 11:28 AM ET.

Response to Jon R
by Martin Holladay

As you probably know, in an air conditioned building, the stack effect reverses during the summertime. That means (at least in theory) that hot outdoor air will enter a house through ceiling cracks, while cold indoor air will escape through cracks near the lowest part of the house.

To some extent, this will happen as the theory proposes, at lease on windless days. In practice, however, this stack effect is weak -- since the summertime delta-T is smaller than the wintertime delta-T -- and is therefore easily ovewhelmed by the effect of wind and various indoor fans.

The basic lesson is that sealing cracks (improving the tightness of the home's thermal envelope) has summertime benefits as well as wintertime benefits.

I think that you are also making the point that interior partitions are often connected with basements and attics through hidden air pathways. That's true. This fact results in several phenomena, including:

(a) During the winter, cold attic air can fall down an open stud bay, freezing plumbing in an interior partition.

(b) Air leaks through an electrical outlet located on an interior partition can pull warm indoor air into the stud bay of a partition, and the warm air can escape into the attic through the crack between the partition drywall and the partition top plate.

(c) If humid attic air enters a partition stud bay during the summer, the partition drywall (which is cooled by the air conditioning system) can take on moisture.

Aug 21, 2017 12:56 PM ET

Also, the "wrong" pressure
by Jon R

Also, the "wrong" pressure will add moisture to walls and ceilings. The right pressure can eliminate moisture problems due to air leaks (which always exist). In some remediation cases, adjusting the pressure is more effective and more cost effective than reducing the leaks.

Even with all ducts inside, it's easy for a forced air furnace to cause wrong pressure . For example, a closed door bedroom with the supply mostly closed and the return open.

A differential manometer is a useful tool (even without a blower door).

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