Insulation 101

Understanding Heat Transfers and Insulation

Energy efficiency of any structure is defined by its ability to control the three basic types of heat flow. These modes of heat transfer vary in importance and magnitude for any structure depending upon the materials used and workmanship employed to construct the building 'envelope'.

Basically heat is transferred in three ways:

Conduction is the transfer of heat through solid portions of a structure. The transfer of heat conduction can be significantly reduced by using insulation. Insulation is a material with low thermal conductivity which thereby resists the flow of heat. This thermal resistance or R-value is a uni-directional measurement of this type of heat flow. The higher the R value, the greater the insulating value.

Convection as it relates to a building structure is the transfer of heat through the movement of air induced by pressure differentials. This can occur due to temperature or humidity differences, wind or mechanical air movement. The term air infiltration, or air leakage, refers to air that enters the structure through gaps and cracks in the building envelope.

Radiation is the transfer of heat by light waves that generate heat upon contact with any material capable of converting the radiation to heat and then releasing this heat as radiation to the air or conducting the heat to adjacent material. Radiation impeding materials, or 'radiant barriers', such as aluminum foil block the radiation light waves and greatly reduce the heat generation.

In Canada, 'radiant barriers' are often used as external facings of rigid board insulations and sometimes in residential attics. Why? As a facer for rigid insulations used as sheathing for the building envelope it acts somewhat as a radiant barrier, primarily to eliminate the reduction of R-value due to aging that all cellular plastic insulations experience.

In attics, when sunlight strikes a roof, the attic space heats up considerably. A radiant barrier installed in the attic absorbs the heat and releases the radiant heat waves back into the attic space as apposed to transmitting the heat into the living space.

Unfortunately, there is still heat buildup in the attic from the hot roofing materials and other structural components that transfer heat through conduction. This means that high levels of thermally resistive (by conduction) insulation such as fiberglass batts or loosefill insulation are required to impede the conduction of heat into the living area. In Canada, radiant barriers are not effective as solar heat gain to warm the attic in the winter months is desired.

Conduction - Install loose fill insulation or fiberglass batts in the attic, fiberglass batts or spray applied polyurethane foams in the walls and fiberglass batts or polyurethane foams in the floors.

Air leakage - Use foam insulations or sealants to seal the holes, crevices and penetrations in the exterior framing envelope of the house. Also install sprayed-in-place polyurethane foam air barriers for commercial and industrial applications.

Radiation - Most insulations which that is installed should have a cellular structure which blocks the flow of heat of radiation. If the cavity is completely filled with insulation, the radiant heat loss from the inside finish to the outside sheathing is virtually eliminated.

Recommended Insulation Levels

Since insulation levels vary for commercial and institutional buildings as specified by the design professional we will present insulation levels as they relate to residential construction. When deciding on insulation levels, the house should be viewed as a whole, for example it makes very little sense to add a high level of insulation in the attic when the exterior walls have low insulation values, or the basement is uninsulated. Since heat loss occurs through all areas of a house, each part of the building envelope which separates the heated interior from the outside needs to be insulated.

The list below gives recommended levels of insulation. Values are shown in both imperial 'R' and metric ('RSI') units.

Minimum Recommended Insulation Levels:

	Traditional Space Heating	Electric Space Heating
Basement Floors	R-8 (RSI-1.41)	R-8 (RSI-1.41)
Basement Walls	R-10 (RSI-1.8)	R-19 (RSI-1.8)
Above Grade Walls	R-17 (RSI-3.0)	R-27 (RSI-4.75)
Ceiling	R-31 (RSI-5.4)	R-40 (RSI-7.04)
Floors Over Unheated Spaces	R-25 (RSI-4.4)	R-25 (RSI-4.4)
Exposed Cantilevers	R-25 (RSI-4.4)	R-25 (RSI-4.4)
Cathedral Ceiling	R-20 (RSI-3.52)	R-22 (RSI-3.87)

Note: R-1 = 0.1761 RSI