Window open stack effect (C) Daniel FriedmanAir Movement Direction in Buildings

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Building air movement direction, forces, measurements & effects:

How to become a "house doctor" for building energy & IAQ problems. This article discusses how & why air moves in buildings and explains why sometimes air can move in surprising directions such as warm air moving downwards.

We include an index to research and to additional articles discussing building air movement, the effects of air movement on building environmental testing for contaminants, mold, gases or other indoor air quality measures, as well as the effects of air movement on heat loss or on the transport of odors or other contaminants between building areas.

The photograph at page top illustrates how a strong updraft of air may occur in tall buildings: the tenant, troubled by inability to turn down a heating radiator,leaves this window open all winter. A white tissue we taped to the bottom of the window sill flaps outwards illustrating the direction of air movement: up through the building and out at this window.

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How does Air Move Through a Building? A Quick Look at the Basic Forces of Building Air Movement

Open partition wall (C) Daniel FriedmanAn understanding of air movement in buildings is essential when studying building heat loss, building heating and cooling costs, or when investigating building indoor air quality by making checks for sources of common indoor air contaminants in buildings such as mold or allergens.

In addition, researchers have pointed out that air movement in buildings can impact more than occupant comfort and heating costs, extending even to the rust or deterioration of building components Bundy (1984).

Here we include expanded annotated information about building air movement, its causes, directions, and effects from the US EPA and other expert sources.

We will discuss a range of air movement questions. What cause air to flow up or down in a structure: unexpected air flow directions; What is the effect of air movement on indoor air quality tests & measurements for contaminants, gases, molds, or other particles in indoor air.

What forces that determine the direction of air movement in buildings and how are they measured. Why is it that air movement in buildings can reverse direction? How building air movement transports indoor air pollutants, mold, gases, odors, and heat indoors. Index to articles & research on building air movement indoors.

Article Contents

Contributors to Air Movement & Determinants of Air Movement Direction in Buildings

Moldy ceiling (C) Daniel FriedmanAir flow patterns in buildings are the result of combined forces but are dominated by the chimney effect or by mechanical ventilation. The importance of understanding air movement indoors and its effects on occupant comfort as well as on building heating and cooling costs has been long and widely discussed.

Less well understood and too-often ignored in indoor air quality investigations and tests for gases such as formaldehyde or for airborne mold or other particular contaminants such as asbestos are the effects of air movement in the transport of these contaminants and the effects of changes in air movement on the accuracy of indoor air quality (IAQ) measurements.

My own investigations [DF] have found one to three orders of magnitude variation in the level of small airborne mold spores such as Aspergillus sp. caused simply by walking through a mold-contaminated area or waving a notebook in the air. Turning HVAC system fans on and off or opening or closing windows or doors can produce still higher variability, making some IAQ measurements highly inaccurate.

Below we list the principal sources of and influences on the direction & force indoor air movement in most structures. McGuire (1967) emphasizes that among these effects, the dominant natural influence on building air movement is the chimney effect but all of these forces must be considered and sometimes one or more of these can become the dominant factor in how air is moving indoors.

Air Usually Moves "Up" in Buildings in a Chimney Effect

Window open stack effect (C) Daniel Friedman Generally in buildings warm air rises, carrying gases, smoke, odors, particulates, other indoor air pollutants including ultra-small airborne particles such as asbestos (if present and friable and disturbed) or particularly small mold spores (Aspergillus sp. for example) in the 1u range.

Air Movement may be "Down" in Buildings

But air movement in buildings can be tricky, the natural chimney effect can be overcome, and air may sometimes move in un-expected directions. For example:

Case report: hot air flows down through roof opening into a cooled space

Roof vent hatch over sleeping loft can let hot air flow down and into a building (C) Daniel Friedman

In a New York home I had installed a roof ventilating fan to be used as a hot air exhaust to cool a sleeping loft below a cathedral ceiling that covered a second floor addition on a small home originally built in the 1920's. Though the cathedral ceiling / roof was heavily insulated still the building was uncomfortably warm during very hot weather.

For cosmetic reasons we built a stained wood hatch cover that could be set in place to cover the exhaust fan intake opening when the fan was not in use.

Later the occupants installed a window air-conditioner that was able to cool this entire space and the roof hatch was seldom if ever opened. It remained in place undisturbed for a decade. During building renovations I thought to open the hatch to inspect the roof opening for leaks (there were none).

At the time that the hatch was opened the room's air conditioner had been operating for several hours and the lower area of the room was comfortably cool and dehumidified: it was perhaps 68 F indoors and close to 90 F outside on a hot humid summer day.

But up on the sleeping loft and close to the under-side of the roof it was a different story: warm air was still collecting in this area and it was hot. The cool air blowing into the room from the window-mounted air conditioner could cool the lower portion of the room and even the downstairs but it was not blowing much cool air upwards towards the loft area.

Spiral stairway as air passage (C) Daniel FriedmanWe had already observed that cool air from the comfy second floor flowed downstairs through a spiral staircase opening. When the second floor was cooled the first floor of the home was also being cooled and dehumidified by that down-flow of cool dry air.

At left you can see the spiral stairway between building floors. The opening through the upper floor was a particularly comfortable spot to stand on a hot day as cool conditioned air fell through the opening to the lower floor.

But on opening the roof hatch (above the red arrow in the photograph at above left) I expected the hot humid air in the loft area to flow upwards and out of the roof hatch by natural convection.

After all it was hot in that loft and warm air rises, right? And the warm air was already sitting pressed against the cathedral ceiling in the loft area.

But on removing the roof vent fan hatch and after wiping a decade of smut and dust from my eyes I was surprised to feel a strong down-draft of still hotter humid air from outdoors.

Opening the roof venting hatch over a room (mostly) full of cool air on a hot summer day gave an intake opening that allowed the flow of room air down to the building's lower floor to increase. The down flow of cool air to the lower floor was sufficient to overcome any natural chimney effect. Hot outdoor air flowed into the loft area, actually increasing its temperature.

We could reverse this air flow direction by turning on the roof vent fan mounted above the hatch, but left to natural convection over a cool room, hot air flowed down not up.

The Direction of Air Movement Can Change Significantly & Quickly in Buildings

Whole house fan mounted in a ceiling, viewed from the attic (C) Daniel Friedman

The effects of operation of this whole house fan, when it's turned on, are rather obvious, though the amount of exit vent openings in the attic (soffits, ridge, or gable-end vents) will affect its total air flow rate. But what about when the fan is off during building IAQ measurements.

A building investigator who fails to notice the existence of this fan or fails to check in the attic to see if a winter fan-cover has been left in place (slowing the chimney effects through this opening) or removed (increasing the chimney effects of natural draft ventilation) is, as Dr. Lewis K. Johnson (W&L Univ. 1964) frequently told his students, Asleep at the Switch.

All building conditions & systems that can affect the direction and rate of air movement must be considered when measuring or reporting building IAQ, heat loss or similar study results.

Here are examples of building conditions or changes that can have a great impact on both air movement direction and air movement rates.

Small Openings in Buildings can Move Large Volumes of Air & Pollutants

These photos show a common ceiling return air plenum over commercial offices. In the first or left hand photo, notice that rectangular opening in the distance? The second photo is a close-up showing a big surprise: the ceiling area used as return air plenum for an office suite is wide open to the rest of the building ceilings on the same floor.

Photograph of commercial air conditioning system ceiling plenum with debris Photograph of commercial air conditioning system ceiling plenum with debris

These air pressure differences and air movements in buildings readily move particles, gases, and odors between building areas through both large openings (windows, doors, stairways, elevator shafts, ductwork), and tiny openings (gaps and cracks in framing, openings around electrical wires, pipes, ducts).

In the photo at above right the air passage shutter may have been manually chained up, overriding an automatic closure feature. If this is the case, the modification of an important fire safety feature could be a real fire and smoke-spread hazard in this office building. See BALANCING AIR DUCT FLOW.

Below we illustrate one of the most commonly-encountered subversions of the best efforts of HVAC technicians to adjust and balance the air flow within a building: uncomfortable local office occupants remove suspended ceiling panels to make their own adjustments to local heating or cooling levels.

Photograph of lifted ceiling tile subverting air conditioning system air flow

What surprises those unfamiliar with building air movement and indoor air pollution studies, is that the myriad of tiny openings in buildings, much smaller than the openings shown just above, can in the aggregate permit significant levels of air and particle or contaminant movement between building areas even when there are not large obvious holes or "windows" between the spaces.

Simple Tests Demonstrate Air Movement In Buildings

Smoke test for air movement (C) D FriedmanOur photo (left) shows the author [DF] demonstrating air movement between rooms beneath a bedroom door, using a chemical smoke test gun. That smoke has long been a valid and effective means of visual study of the direction of air movement in buildings is well established.

McGuire (1967), discussing how smoke moves in buildings, pointed out how easily small pressure differentials can cause significant air movement in buildings.

Firemen make high level openings to deliberately induce air flow through lower-level openings in the same structure, a step to improve visibility in the building for firefighter. In discussing how smoke moves in buildings during a fire McGuire points out that

... as far as movement is concerned, smoky and normal atmospheres are virtually indistinguishable. ... The properties of the particulate smoke component differ .. but the concentration will not be sufficient to influence the over-all movement of the atmosphere ... The mechanisms to be discussed are not, therefore, unique to the movement of a smoky atmosphere as distinct from air.

Pressure differentials associated with winds, blowers, fans, and mechanical ventilation systems will contribute to smoke movement; however, temperature differentials and variations are usually more important factors. ...

Expansion only occurs as temperatures rise, but another effect, known colloquially as "chimney effect" proceeds continuously whenever there is any temperature differential between interior and exterior. ... Because some level of temperature differential usually exists between a building and the exterior atmosphere, chimney effect is also responsible for much of the normal air movement in buildings. ...

[Regarding thermally induced movement of air in buildings he continues] ... The displacement of gases by the expansion mechanism can be conveniently discussed on the basis of the universal gas law, which states that for any given mass of gas the product of pressure, P, and volume, V, is proportional to the absolute temperature, T.

... In other words, PV = RT where R is a constant (usually taken to relate specifically to 1 g mole of a gas).

The pressure differentials required to establish substantial [air] flow velocities are very small compared with absolute atmospheric pressure.

... It may be said therefore that the volume of a given mass of gases is proportional to its absolute temperature, since, during all the relevant processes short of explosions, P in the above expression hardly varies.

Articles Related to Building Air Movement & Its Effects on the Building Environment

Photograph of  this overheating and improperly-made aluminum to copper pigtail splice.

Sooty gas burner © Daniel Friedman

Watch out: backdrafting that draws combustion products or sewer gases into a building risks a fatal explosion or carbon monoxide poisoning.

Thermal bypass leak (C) Daniel Friedman

Watch out: The toolbox for studying air movement, heat loss, airborne particle levels, gases or chemicals moving in buildings, is a large one, but tools and gadgets alone do not make a building expert, and worse, they might be used improperly giving misleading and unnecessarily expensive results use to "sell" treatments or "improvements" that might not be the best way to spend on improving building air quality or saving on building energy costs.

Understanding the basics about air movement and appreciating the benefits of an expert visual inspection (see THERMAL TRACKING & HEAT LOSS) can help consumers guard against bad advice or superficial building studies. .

Building Air Movement Research

Loose blower assembly pulley or belt reduces airflow (C) Carson Dunlop AssociatesPhoto at left provided courtesy of Carson Dunlop Associates, a Toronto home inspection & education firm.


Continue reading at PASCAL CALCULATIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AIR FLOW MEASUREMENT CFM, how to make air flow measurements, where to buy air flow measurement equipment.

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