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How Your Home Works

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House Performance

We expect our homes to provide shelter from the sun, rain, wind and snow, and we expect them to keep us warm and comfortable. We also expect them to be sturdy and durable.

A number of factors work together to meet these needs. These include the building shell, the outside environment, the mechanical system and the occupants themselves.

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The Building Envelope

The building envelope is the shell of the house that protects us from the elements; it comprises the basement walls and floor, the above-grade walls, the roof, the windows and the doors.

We expect a lot from the envelope: it must provide structural support for the walls and roof, protect the structure from deterioration, allow for natural lighting of the interior and serve as a means of getting in and out. Finally, the envelope must separate our warm and comfortable controlled indoor environment from the weather outside.

To maintain our indoor environment, the envelope must control the flow of heat, air and moisture between the inside of the home and the outdoors.

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The Envelope and Heat Flow

Heat moves out of the house in all directions.

As part of the controlled indoor environment, we add a heating system to overcome the cold Canadian winters. We try to build our homes so that we don't heat the Great Canadian Outdoors. We try to keep the heat in!

But heat will move wherever there is a difference in temperature. Basically, heat flows from areas of warmth to areas of cold.

Many people believe that because hot air rises, most heat loss will be through the ceiling. This is not necessarily so. Heat moves in any direction – up, down or sideways – but always from a warm spot to a colder one. A heated room over an unheated garage will lose heat through the floor. Similarly, heat loss can occur through walls in the basement or crawl space, as well as above the ground. Heat moves to the cold. It's the job of the envelope to control the flow of heat between the indoors and outdoors.

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How Does Heat Flow?

Heat flows in three distinct ways. In a part of the house envelope, such as a wall, heat can move in any or all of three ways at the same time.

  • Conduction. Heat can be transferred directly from one part of an object to another when particles bump into each other. For example, the heat from a cast iron frying pan is transferred to the handle and eventually to your hand. Some materials conduct heat better than others. Insulation works by reducing heat flow with tiny pockets of air, which are relatively poor conductors of heat.
  • Convection. Heat can also be transferred by the movement of a fluid, such as water, or air. In an uninsulated wall space, for instance, air picks up heat from the warm wall and then circulates to the cold wall, where it loses the heat. Some heat is also transferred by the mixing of warm and cold air.
  • Radiation. Any object will radiate heat in the same way that the sun radiates heat. When you stand in front of a cold window, you radiate heat to the window and so you feel cold, even though the room temperature may be high.

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The Envelope and Airflow

Uncontrolled airflow through the envelope can be a major source of heat loss and can lead to other problems. Since warm air can carry large amounts of water vapour, airflow is also the main means by which moisture is carried into the envelope.

Under winter conditions, air is forced through the building envelope. Air moving out carries heat and moisture, while air moving in brings uncomfortable drafts and dry winter air.

For air to move from one side to the other, there must be a hole in the envelope and a difference in air pressure between the inside and outside. The difference in air pressure can be caused by any combination of the following:

  • Wind effect. When wind blows against the house, it creates a high-pressure area on the windward side, and air is forced into the house. There is a low-pressure area on the leeward side (and sometimes other sides) where air is forced out.
  • Stack effect. In a heated home, the less-dense warm air rises and expands, creating a higher-pressure area near the top of the house. Air escapes through holes in the ceiling and cracks around upper-storey windows. The force of the rising air creates lower pressure near the bottom of the house, and outside air rushes in through cracks and openings around the lower floors.
  • Combustion and ventilation effect. Appliances that burn fuels such as wood, oil or natural gas need air to support combustion and provide the draft in the chimney. Open chimneys and fireplaces tend to exhaust lots of air. This air is replaced by outside air drawn in through the envelope. This is why a room often becomes drafty when there's a fire in the fireplace.

Ventilation fans in the kitchen and bathroom, central vacuum systems, stove-top grills, clothes dryers and other exhaust fans also cause this effect.

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The Envelope and Moisture

Moisture can cause concrete to crumble, wood to rot and paint to peel; it can also damage plaster and ruin carpets. In its many forms, moisture is a major cause of damage to building components.

Moisture can be solid, liquid or gas (water vapour). It can originate from outside as ground water in the soil or as ice, snow, rain, fog and surface run-off, or from inside in the form of water vapour produced by the people in the house and their activities, such as washing, cleaning, cooking and using humidifiers.

In its different forms, moisture can move through the envelope in a number of ways:

  • Gravity. Water running down a roof or condensation running down a window pane shows how gravity causes water to move downward.
  • Capillary action. Water can also move sideways or upward. Capillary action depends on the presence of very narrow spaces, as with lapped siding or porous materials, such as concrete or soil. (Think of how a paper towel absorbs water.)
  • Diffusion. Water vapour sometimes moves directly through materials by diffusion, depending on the difference in water-vapour pressure and the material's resistance to this pressure.
  • Air movement. As water vapour, moisture is carried by moving air. This can happen, for example, where there is air leakage through a crack in the house envelope.
  • Far more moisture can be carried by airflow through a small hole in the envelope than by diffusion through building materials.

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Condensation

One small air leak can let through 100 times as much moisture as will travel by diffusion over a much larger area.

Water vapour becomes a problem when it condenses and becomes liquid. This happens at 100 percent relative humidity, when the air cannot hold more water vapour.

A typical example is condensation on windows. Air contacting a cold window loses heat. As it cools, it loses its capacity to hold water vapour, and condensation occurs on the surface of the window.

Condensation on an extremely cold window appears as frost. Because the interior surface of a single-glazed window is colder than that of a double-glazed window, a single-glazed window is more likely to have problems with condensation or frost buildup, even under conditions of lower humidity. Condensation is more likely to occur in humid areas of the house, such as the kitchen and bathroom.

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The House as a System

Condensation occurs when moist air meets a cool surface.

Before beginning any retrofit work, it's a good idea to review what's involved and to understand how your work might affect other aspects of the house. If you think things through and plan carefully in the early stages of work, the work will meet your expectations and you will have fewer unpleasant surprises.

A house is made up of components that work together to form an integrated system. The performance of one component depends on its relationship with other components in the same system. Your home's ventilation and heating components, construction materials, their assembly and the behaviour of occupants all interact – a change to one can affect all others.

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