Measure the surface area of the roof you want to use in your comparison. To do this, write down the width and length of each flat surface on the roof. Multiply each pair of numbers and add the results together for the total area, or A.
For example, given a house with a simple sloped roof, both sides of which measure 40 by 20 feet, this equation would look like (40 --- 15) + (40 --- 15) = 1,200.
Subtract the lowest annual exterior temperature from the interior temperature of your house, measured in degrees Fahrenheit. This is the temperature difference, also known as "T.
For example, given an inside temperature of 60 degrees and an outside low of 16, this equation would look like 60 - 16 = 44.
Find the capacitance of your current roofing material or other traditional roof system, if you're working with a hypothetical situation. This is expressed as a U-value, a number less than 1 that indicates the percentage of heat the material allows to pass through it. Charts with common U-values can be found in both books on heating and on several websites.
For example, given a house with a built-up roof, no attic and R2 ceiling insulation, this number would be 0.197.
Multiply the various values, with A --- U --- "T = Q, the heat energy loss in British Thermal Units, or BTUs, per hour. Most structural insulated panels have a U-value of approximately 0.17, so multiply A --- 0.17 --- "T to find the energy loss of a hypothetical roof of your dimensions with structural insulated paneling installed. Compare the two numbers.
For example, given the data calculated in the previous steps, the equation would be 1,200 --- 0.197 --- 44 = 10,402 BTU/h. With structural insulated paneling, the equation would be 1,200 --- 0.17 --- 44 = 8,976 BTU/h, saving 1,426 BTUs hourly.