By James Dulley, Published June 12 2009
Heat transfer pivotal to more efficient homeDear Jim: I see so many ads for products and services to make my home more efficient that I get confused. Can you explain a bit about how a house loses and gains energy so I can better evaluate marketing claims? – Ron M.
Dear Ron: Before making any energy improvement decisions, it is wise to get a sound understanding of how a house loses (winter) and gains (summer) heat. The first step in this understanding is learning the basics of heat transfer.
Heat energy flows in all directions equally and is not affected by gravity. To the contrary, hot air flows upward because it is less dense than cooler air. If you put a heating element in the center of a metal block, the bottom will get as hot as the top and sides.
There are three modes of heat transfer that affect your house. Conduction heat transfer is heat flow through a material. An example of this type of heat transfer is how the handle of a cast iron skillet gets hot when it is on a stove top. This is how heat flows through an insulated wall.
The temperature difference on each side of the wall determines how fast heat conducts through it. For a given wall insulation R-value, if it is 68 degrees indoors and 28 degrees outdoors (40-degree temperature difference, about twice as much heat will be lost through the wall than when
it is 48 degree outdoors (20-degree temperature difference).
Convection heat transfer occurs in fluids and gases, which are mobile. When it is windy outdoors, blowing cold air increases the heat loss from the walls. This creates the winter chill factor you hear about in weather forecasts. Similar to conduction, when the temperature difference is doubled, the heat transfer is also basically doubled.
Radiation heat transfer does not need a transfer material or contact between the hot and cold surfaces. This is how the sun heats the Earth through space. Radiant heat transfer is generally more of an issue during the summer, but it cannot be ignored during winter. It increases exponentially with the temperature difference. This is why a very hot roof can make the entire house hot during summer.
Now that you understand the various ways your house gains and loses heat, you can better evaluate the needs of your own home. For example, if there is particularly cold wall in your house and it faces the northwest, there likely are both conductive and convective heat loss modes.
Making sure the wall is well insulated is the first step to reduce both modes. To further reduce the convective heat loss, plant evergreen trees or build a privacy fence to block the force of the wind. Since heat also flows downward, add insulation to the band joist immediately above the foundation to reduce conductive heat loss there. Also, use foam caulk to seal along the top of the foundation.
Don’t forget the typical heat loss areas of windows, doors, fireplaces, etc. Windows and doors lose heat by conduction, convection and air infiltration through leaks. Make your fireplace (glass doors) and chimney (damper sealers) as airtight as possible when not in use.
Send inquiries to James Dulley, The Forum of Fargo-Moorhead, 6906 Royalgreen Drive, Cincinnati, OH 45244, or visit www.dulley.com