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Comment by oz on November 28, 2008 at 3:19pm

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Comment by oz on November 21, 2008 at 12:25am

Heat Loss is somewhat unusual in the building sciences because the math appears simple. In fact, we choose simple math because it gives accurate estimates with minimal work. A house experiences heat loss in three ways: conduction, radiation, and infiltration. We do three separate calculations and sum them to arrive at the overall heat loss for the house. Today, I will discuss only the formula for conduction. For the other two, I highly recommend taking the class.

Perhaps the easiest form of heat loss to define and control is conduction. Conduction is minimized by insulation, or high R-value walls. R is for Resistance to heat loss. The simple formula for heat loss due to conduction is:

Hmax= (1/R-value) x A x ΔT

where Hmax is the maximum possible flow of heat in BTU/hr for the house
1/R-value is conductance expressed in BTU/Hour*s.f.*°F
A is the surface area of the house in square feet
and ΔT is the change in temperature across the wall in °F

Don't get scared off by the math! This is easy and it actually sheds light on how to build the house. One BTU (British Thermal Unit) is the amount of heat energy that raises the temperature of one pound of water by one degree farenheit. The first problem we solve in class is: how many BTU's are required to make a cup of tea? (hint: a pint is a pound the world around)

The beauty of the conductive heat loss formula is that it tells us immediately that heat loss depends on 3 factors: 1/R-value is the amount of insulation, A is the surface area of the structure, and ΔT is the change in temperature across the building envelope. More insulation (higher R-value) means less heat loss. Bigger houses (with bigger surface Areas) will require more energy to heat. And, of course, it makes sense that the colder it is outside (the greater the ΔT), the more heat we will need inside to keep warm. The formula allows us to put numbers to our intuitive grasp of heat loss and begin to make decisions about our house.

It only takes about 15 minutes to calculate the conductive heat loss for a house by choosing an R-value from the insulation label, adding up the square footages of walls and roof, and looking up the 30-year minimum average daily temperature for the coldest day of the year. What makes this class fun, and what we spend time on in class, is building an intuitive knowledge of heat loss that enables us to design truly energy efficient, comfortable homes.