National Oilheat Research Alliance 23
of air in cubic feet in the room by multiplying
the length times the width times the height.
The cubic footage of the room is then multiplied
by the DTD and then by the infiltration
factor. Here is how it looks in this example:
L x W x H x DTD x Infiltration factor = infiltration
loss in Btus per hour.
10 x 15 x 9 x 70 x .018 = 1,701 Btuh
Now calculate the conductance heat loss
through the walls, ceiling (if there’s a cold
space above), floor (if there’s a cold space
below), windows, doors and skylights.
The formula for all these losses is the same:
L x W X DTD x U.
Length times width gives the area and DTD
is the design temperature difference. U represents
the U-Value, which is the assembly’s
ability to conduct heat from one side to the
other. A lower U-value represents less heat
loss. In fact, U-value is the inverse of R-value,
which is an assembly’s ability to resist the
flow of heat. If you know the R-value, simply
divide 1 by the R-value and you’ll get the Uvalue.
(1/R)=U or (1/U)=R
Starting with the windows: There are two 3′
x 5′ windows on one wall and one 6′ x 5′ window
on the other. Windows have U-values on
their stickers when they are installed. However,
there is much data available in heat loss computer
programs to help determine the U-value
of existing windows.
Assume you have standard wood framed,
double-pane, low-E windows, which have an
average U-value of .36. To find the window
heat loss, total the square footage of all the
windows and count them as one big window—
it’s easier that way. In this case, the room has
60 square feet of window area. We multiply
60 times the DTD of 70° F and then multiply
again by the U-value of .36, resulting in a window
loss of 1,512 Btuh.
Walls are next: Use the net area of the walls
(wall square footage minus window square
footage) and multiply by the DTD and U-val-
Insulating Materials Thickness (inches)
1 3 ½ 5 ½ 8 10 12 14
Blanket and bat type
Glass Fiber 3.10 11 19 26 32 38 44
Rockwool 3.70 13 20 29 37 44 51
Loose fill type
Figure 5
Cellulos (macerated paper or pulp) 3.70 13 20 29 37 44 51
Wood fiber, redwood, hemlock or fir 3.33 12 19 27 33 40 46
Mineral wool (glass, slag or rock) 2.20 8 12 18 22 26 32
Vermiclulite, expanded 2.30 8 12 18 23 27 33
Perlite, expanded 2.70 9 15 22 27 32 37
Board type
Glass fiber 4.00
Polustyrene (Styrofoam) 4.50
Polyurethane 6.25
Urea-Formaldehyde 5.00
The resistance (R) factor shown may vary slightly with various manufacturers.