Building Science

IAQm offers expertise in building science.

Knowledge about building science is essential for conducting indoor air quality building investigations and developing sound mitigative strategies. For instance, building science can help determine the source of moisture in a building. It can also explain the pathways pollutants take as they move through a building.

The following describes some common building science principals that are relevant to indoor air quality.

A Building is an Interrelated System

A building is more than the sum of its parts. It is a system. The components of the building are interlinked, and a change to one aspect of the building may affect the functioning of other components. As well, the building system itself interacts with the surrounding external environment. The major components of the building system can be grouped into three basic areas:

• the building envelope
• mechanical and ventilation systems
• the occupants, their furnishings, and their activities

Any change in one aspect of a building, either in the way it is constructed or the way it is used, will have an effect on other aspects of the building. For example, making the building envelope more airtight will affect the movement of air through the walls and affect humidity levels. Light sources can contribute significantly to heat levels in the building and the ability of the building’s mechanical systems to regulate temperature. People, pets and plants release moisture into the indoor space and emit and absorb a variety of pollutants.

Forces Exist Solely to Achieve Balance

• water seeks its lowest level
• air pressure tries to equalize itself
• two air temperatures on either side of a barrier try to equalize

A building’s exhaust equipment (exhaust fans, vacuum systems, fireplaces) remove air from the building. This lowers the pressure of the indoor air relative to the outside. This lowering of indoor pressure is referred to as “building depressurization.” The amount of building depressurization that occurs depends on the number and capacity of the exhaust devices in the building, and the ability of the building to supply air through air intake systems and leaks in the building envelope.

By simply turning on a bathroom exhaust fan or a kitchen range hood, the air pressure in a house can be significantly lowered. As the air pressure tries to equalize, combustion gases from fireplaces, furnaces and hot water heaters can be pulled back down their chimneys spilling carbon monoxide and other dangerous gases back into the house. It may also cause soil or sewer gas to enter the house or other outdoor air pollutants to be drawn inside through infiltration.

Heat Flows From Warmer Areas to Colder Areas

Heat flow mechanisms include:

• convection
• radiation
• conduction

Heat transfer by convection can be stopped by physical barriers such as using double panes of glass with a 15mm gap. Heat transfer by radiation can theoretically be stopped by a perfect reflective barrier. Heat transfer by conduction cannot be stopped, regardless of the substance. Even the best thermal insulation can only slow down the rate of heat flow by conduction.

Moisture Flows Through Four Mechanisms

• water is drawn downward due to the effects of gravity
• water can wick upwards or sideways through certain materials through capillary action
• water vapour can move with air flows
• water vapour can diffuse through apparently solid materials, moving from areas of high humidity to areas of lower humidity

Barriers Control the Movement of Pollutants

Air barriers, vapour diffusion retarders, weather barriers, and moisture barriers separate the inside living space from the insulated wall space and the outdoors.

Weather barriers protect the sheathing and insulation of the outside wall from the effects of wind, rain, and snow. They are installed on the outside of the structure, underneath the exterior siding. Although they keep the weather out, weather barriers must permit the diffusion of water vapour from the inside of the wall structure. Weather barriers are made of perforated polyethylene or paper.

Air barriers are designed to reduce the movement of air. It is very important that the air barrier be continuous over the entire building envelope and that no holes or tears are made in it while it is installed or when it is in place. Air barriers can be made of polyethylene, aluminum foil or plywood with sealed joints.

A vapour diffusion retarder or vapour barrier is a membrane material or coating that slows the diffusion of water vapour. It is used to help prevent the occurrence of moisture problems in the structure of a building. Vapour barriers are most commonly made of polyethylene and in cold environments must be installed with at least two-thirds of the insulation value on the outside of the retarder to avoid problems with condensation.

A moisture barrier is a below-grade membrane or coating used to prevent moisture from migrating through building walls and floors. Asphalt emulsions and polyethylene are two types of moisture barriers.

The Weakest Component will Fail First

A chain is only as strong as its weakest link. This statement is applicable to many instances of failure in building science.

Thermal bridges may be considered the weakest path of thermal resistance. Thermal bridges occur at the studs or where the insulation is deficient. The wall or ceiling surface will be colder at these points and moisture will condense more readily. The moisture will attract dust (leaving behind dust marks) and mould growth.

Materials Will Not Deteriorate if One of the Conditions for Deterioration is Absent

Wood will rot if subjected to all of the following five conditions:

• presence of water
• mould spores
• oxygen
• optimum temperature
• a food source for mould

Rusting of steel requires:

• water
• oxygen

Water and Buildings Do Not Mix

Water in all of its forms (liquid, solid, gas) can affect the performance and service life of buildings. Water is one of the ingredients for wood rot and rusting of steel. Building codes contain many provisions for keeping water away from building materials.

Water Vapour Condenses at the Dew Point

Relative humidity (RH) is a percentage that indicates the amount of moisture in the air relative to the maximum amount the air can hold at that temperature. For instance, when air at a given temperature contains all the water vapour it can hold at that temperature, it has a RH of 100%. If the humidity exceeds 100%, moisture will begin to condense from the air. Warm air can hold more moisture than cool air, which is why moisture will condense on cold surfaces such a windows and at thermal bridges.

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