Conductive Heat Loss Basics

U-Value. Conduction is heat flow through a material or a building component – like a wall or window. Conductivity, or u-value, measures the ease of conduction. A high u-value indicates a high capacity for heat flow. Fiberglass batt insulation has a u-value of about 0.30. Aluminum has a u-value of 1,111. This means that heat flows through aluminum 3,700 times faster than through batt insulation. This also means that a one-foot long piece of 16-gauge aluminum through-wall flashing would be responsible for as much heat loss as 15 square feet of insulated wall.

R-Value. R-value measures resistance to heat flow. It is the opposite, or inverse, of conductivity, or u-value. R equals one divided by u. This means that things with low u-values have high r-values. Materials with high r-values are insulators.

UA (Delta T). The rate of heat loss through a building component is calculated using the basic equation Q = UA (delta T) . In plainer language, conductive heat loss is equal to the conductivity, or u-value, of a component, times its surface area, times the difference in temperature between indoors and out.

The UA (delta T) equation reveals that conductive heat loss can be influenced, or limited, in three different ways: by lowering u-value – through better insulation; by lowering surface area by using compact forms – forms with low surface area to volume ratios; and, by lowering the temperature differential. As a what not to do illustration of each of these, I like to use the example of warm-air ducts running through an unheated attic. Insulated ducts have less insulation, and therefore higher u-values, than a building's walls, roof, or floor. High u-value means high heat loss. Since ducts are long and skinny, they have high surface area to volume ratios. High surface area means high heat loss. The air inside of the ducts is warmer than room air – so the temperature difference between the inside of the duct and the unheated crawlspace is higher than the temperature difference between the heated room and the crawlspace. High temperature difference means high heat loss. Because of their high u-values, high surface areas, and because they carry especially warm air, ducts in unheated spaces do an ideal job of wasting heat.

UA (delta T) is used by the passive house energy modelling software (PHPP) to calculate conductive heat loss. Heat loss due to air leakage is calculated by the software separately.