Heat is always on the move. Whether it is the teapot going cold or the warmth from our home’s natural and artificial heating systems, heat tries to migrate to colder areas.
Several types of heat transfer affect us: radiant, convection, conduction and phase change.
Radiant heat transfer is one that we know well.
The sun, although 150 million kilometres away, emits electromagnetic waves that reach us even in the cold north and make us feel warmer.
Living things also radiate heat. A barn with livestock is much warmer than the temperature outside, while putting all the children in one bed in cold houses of the past, was much warmer than individual slumber.
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Convection refers to heat moving within air or water caused by the movement of molecules.
Air and water molecules expand and become lighter, which is why warm air and warm water rise and cool air and cool water fall.
This explains why the temperature at the ceiling is often warmer than at the floor in a building heated with forced air, why it’s colder below a window than above and why low areas in the field may be more prone to early frosts.
Conduction is the heat transfer that takes place molecule to molecule.
It requires objects to touch each other, such as a frying pan heating up on an electric stove.
Phase change is when material changes from one phase to another.
Evaporation is the most common example of phase change in our environment, when water changes from a liquid to a gas. Significant absorption of energy occurs at the point of change.
We will use this type of heat transfer to our advantage in the future with new drywall materials that absorb daytime heat with invisible internal phase change and release it in the evening when the temperature drops.
Understanding phase change also explains why swimming pools use so much energy.
Water cools as the surface water evaporates, requiring the water to be heated again.
This heating can often take as much energy as heating the nearby residence for the entire winter. The loss of energy to phase change in a swimming pool is the greatest argument for the use of a thermal pool cover in summer.
Convection is probably the type of heat transfer that is of greatest concern when it comes to our homes.
No insulation was installed in the days when stud walls were first built. It was just boards on the inside and outside and air in between.
Because warm air rises and cool air falls, the wall would be warmed by sources inside the house such as the fireplace, stove and radiators.
The warm wall would then warm the air in the stud cavity, which would rise and contact the outer wall of the cavity. The air would then cool and drop, creating a continuous circular current.
The convection currents would continually drain energy from the room, prompting the development of between-the-stud insulation.
It was initially wood chips and later fibreglass, cellulose fillings, mineral fibre and most recently, sprayed foam.
Convection loss also refers to heat loss that takes place when warm air leaves buildings through gaps, holes and electrical outlets, even with the presence of insulation.
Air and vapour barriers such a tarred paper, polyethylene, foil, door and window gaskets and sealed electrical pans were developed to prevent these convection losses.
Convection takes place in window cavities, which is why newer windows have a smaller space between panes than what was previously used. The narrow space mitigates against easy convection currents, thereby reducing heat loss.
Conduction heat transfer takes place when materials touch, which is why heat is lost when warm drywall touches a stud and the stud touches the siding.
A thermographic photo shows just how dramatic the loss can be through a stud wall compared to the insulated cavity between the studs.
Prevention of such thermal bridging is why many builders install sheet insulation, most often foam sheets, on the inside or outside of stud walls. It’s also why structurally insulated panels or insulated concrete forms rely on continuous insulation to prevent heat loss.
Radiant heat loss has not been studied well in terms of building technology.
Radiant barriers such as aluminum foil can prevent radiant heat loss or gain.
Foil has its proponents for wall use, but its most typical application is on the underside of roof rafters to reflect heat back to the roof and away from the attic space.
Heat is also lost through radiant energy, particularly through windows. New window technologies, such as low-emissivity or low-e coatings are designed to prevent radiant heat loss.
Heat transfer or loss is not a simple matter influenced by just one factor. Building technologies must reflect that complexity to provide effective energy saving solutions, holding back nature one degree at a time.
Will Oddie is a renewable energy, sustainable building consultant with a lifetime interest in energy conservation. To contact Oddie, send e-mail to energyfield@producer.com.
