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.
NORA Advanced Hydronics
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