R-Value

This is a number indicating the ability of a substance to resist the flow of heat. The higher the R value, the better it acts as an insulator.

Typical R-values of building components can be found in Manual J or go to: http://www.coloradoenergy.org/procorner/stuff/r-values.htm for a free listing of R-values.

U value

(Learn and understand U value. It is the foundation of a load calculation.)

The U value is the number of BTUs that pass though one square foot of substance in one hour’s time when there is one degree temperature difference. The U value is the reciprocal* of the R value. A U-value can to said to be the rate of conduction

U=1/R

*A reciprocal of a number is 1 divided by the number. The reciprocal of 20 is 1/20 or .05.

Suppose you had a six-inch thick, R- 19 fiberglass insulation batt, and you wanted to know how much heat will pass through it. First, determine the U value.

U= 1/R

U= 1/19

U=.0526

Therefore, .0526 BTUs pass through 1 square foot of the batt each hour when there is one degree temperature difference.

If the batt measured 2’ x 8’ we would have a total area of 16 sq. ft. We could then say that .8414 BTUs (16 sq. ft. x .0526 BTUs). Pass through the entire batt in one hour when there is one degree temperature difference.

Carrying our example just one step further, if the temperature on one side of the batt is 20 degrees and on the other side it is 70 degrees, then we would have a 50-degree temperature difference (70–20). Therefore, 42.08 BTU’s (.8414 BTU’s x 50 degrees) would pass through the batt in one hour.

Simply stated, use the following formula to determine the amount of heat gained or lost through any substance:

BTUH = U x TD X AREA

=.0526 x 50 X 16′

= 42.08

Once you know the U value, the square footage and the temperature difference, you can calculate the BTU heat loss or gain per hour through a substance. However, walls, floors and ceilings are not made up of just insulation. A wall may be made up of brick, sheathing, insulation and sheetrock, each of which has its own R value. The R value of each component must be added together to obtain the total R value. Then we simply take the reciprocal (1/R) and get the total U value of the wall. For a list of R-values, locate a table titled “R Values of Common Building Materials” in Manual J8, or go to:

http://www.coloradoenergy.org/procorner/stuff/r-values.htm

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Single appliance –Table 504.2(1) Sizing vents

What size B vent is needed for a 160,000 BTUH, naturally ventilated appliance if the total vent height is 18’ and the lateral 2’?

Under the height column you have to choose either 15’ or 20’. Remember this: The taller the vent the more capacity it has, therefore, if the 20’ row is used the vent may be under sized. Always use the shorter height.

In this case use 15’. Now use the 2’ lateral and select a vent size under NAT. A 5” vent will handle only 150,000 BTUH, while a 6” vent will handle 225,000 BTUH, therefore select a 6” vent.

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