Understanding the House as a System

Canada Mortgage and Housing Corporation (CMHC) have an excellent guide to helping homeowners understand a very important concept: a house is made up of a series of interconnected sub-systems.

To view the full guide CLICK HERE, but for the purposes of windows, doors and siding read below for our condensed version.

Before we continue, we need to define the major components of the ‘house as a system’;

  • The building envelope (roof, walls, windows, doors and foundation) – separates the interior space from the exterior environment
  • The mechanical systems (heating, cooling, ventilation, exhaust fans, humidifiers, dehumidifiers etc.) – provide, remove or regulate heat, air and moisture conditions inside the house
  • The occupants (number of people, their lifestyle, how the house is operated and lived in)

Changes to one or more these systems may have a positive or negative impact on home durability or occupant comfort. 

Heat, Air and Moisture

Heat, air and moisture move in and out of a house through the building envelope because of:

  • Envelope air tightness and insulation
  • Outdoor weather conditions
  • Mechanical equipment in the home
  • Lifestyle of the occupants

Household Air Requirements

Fresh air from outside is required to provide:

  • Ventilation for health (people require a certain amount of fresh air every minute to feel healthy)
  • Combustion air for fuel burning appliances
  • Make up air exhausted by air consuming appliances

There is recognition in the industry that trades are not only responsible for the quality of their work, but also for the impact that their work may have on the house as a system.

Understanding the Science

Three ongoing processes that are continuously underway in a house and have significant impact on energy efficiency, occupant comfort, indoor air quality and the durability of a home: heat flows, airflows, and moisture flows.

1 – Heat Flows
Heat in a house moves from hot to cold by three different processes:

  • Conduction: through solids (ex: glass), frames and walls
  • Convection: baseboard heaters, sealed units (argon gas)
  • Radiation: Low E window coatings, concrete, furniture

All three happen simultaneously: YOU CANNOT STOP HEAT FLOW… ONLY SLOW IT DOWN
Heat flow directly or indirectly affects comfort levels – comfort is #1, not cost!

2 – Airflow
Air always moves from a high pressure to a low pressure; provided there is a path. Pressure differences are causes by three forces:

  • Stack effect: air rises up – can drive warm, moisture laden air into attics where it can cause moisture damage and contribute to the formation of ice dams. It can be reversed when air conditioning results in cooler indoor temperatures than outside, causing exfiltration to occur at lower levels and infiltration of warmer outdoor air at the upper levels.
  • Wind action: wind causes high pressure on the windward side of a house and low pressure on the leeward side causing air to leak into the windward side and leak out of the leeward side.
  • Influence of mechanical equipment: promotes indoor-outdoor air exchange that can be beneficial to indoor air quality (to a point) but increases space heating costs.

3 – Moisture Flow
Moisture is always present in every house and in certain quantities can be beneficial. However, excessive moisture can cause undesirable problems as well. For moisture problems to occur, four criteria must be present:

  • Liquid moisture (rain, condensate, plumbing leak, etc.);
  • A driving force or pressure (wind, gravity, capillary or surface tension);
  • A path (crack, hole, opening, etc.); and
  • Moisture-sensitive material.

Moisture problems can be substantially reduced or eliminated by removing or addressing any one of these four criteria in a situation where moisture problems currently exist.

Movement of Moisture by Vapour Diffusion

Water vapour is made up of microscopic airborne water molecules.  Two terms that must be understood are: Relative Humidity (the amount of water vapour present in the air relative to the total amount of moisture that the air can hold at a given temperature) and Absolute Humidity (the actual amount of water in the form of vapour contained in a given amount of air regardless of the temperature of that air).

Where this can be confusing is when the temperature changes, so too does the relative humidity, but the total amount of water vapour in the air remains the same.  This is because air at different temperatures can hold different amounts of water.  Cold air can hold less water than warm air, so as the temperature decreases, the relative humidity increases for a given quantity of moisture in the air.

The point at which a body of air is saturated with water is called the dew point.  The process by which dew, or condensate, is formed is called condensation (CLICK HERE for more info on condensation and windows).

In general, moisture problems can be handled by (1) removing or dealing with the moisture source, and (2) ventilating to control the moisture that will be produced through cooking, showering, laundering, dishwashing and cleaning.

Condensation and Windows

To help keep windows condensation free we can help chase the problem with technology by building a warmer window.  Centre-of-glass temperatures of insulating glass units can be improved by increasing the cavity width with double glazing, Low E coated glass, and argon gas fill.

House as a System (Seeing it Altogether)


(Diagram and Info from CMHC)

1 – The design and construction of the envelope determine whether and how air and moisture move into and out of the house, and whether air or moisture causes problems for the structure and occupants.

2 – An air barrier helps control drafts and prevents moist air from leaking through the building envelope, condensing and damaging the structure or creating mold problems.

3 – Strong exhaust fans in range hoods or stove-top cookers can back draft furnaces, fireplaces, conventional aspirating water heaters or wood stoves.

4 – Moisture can enter foundation walls from the soils and from the footings. It is important to prevent moisture from contacting materials that have capillary potential.

5 – New windows can affect the building’s air tightness. This in turn can affect humidity levels and air quality.

6 – A vapour barrier on the warm side of the insulation and continuous air barrier protects the building envelope from vapour diffusion, leakage of warm moist air into the building envelope and resulting condensation.

7 – The number of occupants, their lifestyle and use of spaces within the house will affect ventilation needs.

Benefits of the House as a System Approach

For homeowners, the benefits of the house as a system approach include a well-planned renovation that provides greater comfort, improved indoor air quality and long-term cost savings through energy efficiency, durable materials and improved performance of all parts of the system.



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