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How Your Home Works

Control of Heat Flow

 

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What Does Insulation Do?

Insulation is like a giant sleeping bag. It wraps the house in a layer of material that slows the rate at which heat is lost to the great outdoors.

Remember that heat always flows from warm to cold, and it moves in three ways: by conduction, convection and radiation.

Still air does not conduct heat well and is a relatively good insulator. However, in large spaces such as wall cavities, heat can still be lost across the air space by convection and radiation. Insulation divides the air space into many small pockets of still air; this inhibits heat transfer by convection. At the same time, the insulation material reduces radiation across the space.

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What Is the Insulation Value?

Insulation works by trapping small pockets of air.

Years ago when the choice of insulation was limited, the measure of insulating effectiveness was thickness. Products have changed, and that rule of thumb can no longer be applied.

Insulation is now manufactured and sold by its thermal resistance value (called the "RSI" value) – a precise measurement of resistance to heat flow. The higher the resistance value, the slower the rate of heat transfer through the insulating material. One brand of insulation may be thicker or thinner than another, but if they both have the same RSI value, they will control heat flow equally well.

Insulation materials and RSI values

Measuring Up

This Web site uses metric measurements and values and gives imperial equivalents in parentheses, e.g., RSI 3.5 (R-20). We use certain measurements as expressions – for example, what we refer to as a 38 × 89 mm stud is commonly known as a 2 × 4. In these cases, we have chosen not to include the unit of measurement. We include imperial equivalents because most retrofit projects take place in houses that were built using this system of measurement.

Some insulation materials are marked with both RSI and R-values. RSI values indicate thermal resistance in metric terms (Resistance Système International), and R-values represent imperial measurements. Be careful not to confuse the two.

For insulation to work properly, it must be installed properly. There are specific instructions for installing insulation in attics, basements and walls. Some common guidelines, however, apply wherever insulation is installed:

  • The insulation must fill the space completely and evenly. Any blank spots or corners will allow convection to occur, sometimes allowing the heat to bypass the insulation completely.
  • Wherever possible, avoid thermal bridges. As the name suggests, a thermal bridge is any solid material (such as a wall stud) that directly connects the warm side of the envelope to the cold side. Steel studs, for example, are very conductive and can reduce the effective wall insulation by 50 percent. When insulation is installed on one side of the thermal bridge, it acts like a roadblock, reducing heat flow.
  • It is also important to install the appropriate thickness of insulation for the size of the space and, when using loose fill, at the proper density.

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How Much Insulation?

Wood studs provide a thermal bridge, and gaps in the insulation allow convection currents.

Your choice of how much insulation to add will depend on many factors:

  • Local housing codes may specify minimum levels of insulation that should be added when upgrading.
  • How much insulation is already in the house will partly determine how much you need to add.
  • How the house is built will determine how much insulation may be practically added.
  • Other work you are doing along with re-insulating may make it practical to add higher levels of insulation.

Guideline to follow to determine insulation levels for different areas of a house.

Note: Each zone on the map represents an area that experiences a similar number of degree-days. Degree-days are a measure of heating demand based on the difference between the average daily outdoor temperature and 18°C (65°F). Cumulative totals for the month or heating season are used to estimate heating energy needs.

The following table is a good guideline to follow to determine insulation levels for different areas of a house.

Zone A B C D
Walls RSI 3.0 3.6 4.1 5.3
R 17.0 20.0 23.0 30.0
Basement walls RSI 3.0 3.0 3.0 3.0
R 17.0 17.0 17.0 17.0
Roof or ceiling RSI 4.5 5.6 6.7 9.0
R 26.0 32.0 38.0 51.0
Floor (over unheated spaces) RSI 4.7 4.7 6.7 9.0
R 27.0 27.0 38.0 51.0

The following table is a good guideline to follow to determine insulation levels for different areas of a house.

You may wish to compare your plans with the Model National Energy Code for Houses for your province or territory. The thermal resistance values listed in the code are more accurate than the ones listed in the table above because they take into account the type of framing and heat loss through the framing materials.

You can also obtain information about the recommended minimum levels of insulation that correspond to your home's method of construction, region and heating fuel type.

Write to:

Housing Program
Office of Energy Efficiency
Natural Resources Canada
580 Booth Street, 18th Floor
Ottawa ON  K1A 0E4
Fax: (613) 996-3764

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