Calculating Net Area
Having determined the design temperature difference and computed (or selected) an overall heat transmission coefficient for each construction material or combinations of materials, you are now ready to calculate the net area of each surface exposed to the outside or adjacent to an unheated or partially heated space.
The best procedure for calculating surface area is to work from a building plan. If one is not available, you will have to make your own measurements. Measurements for calculating net area are taken from inside surfaces (i. e., inside room measurements). You will not be concerned with structural surfaces (e. g., walls, ceilings, floors) between rooms and spaces heated at the same temperature, because no heat transmission occurs where temperatures are constant.
Calculating the total area for each surface should be done as follows:
1. Multiply room length by room width to determine floor and ceiling area.
2. Multiply room length (or width) by room height to determine the outside wall area for each room.
3. Multiply door width by door height to determine the surface area for each door.
4. Multiply window width by window height to determine the surface area for each window.
Whether you use room length or room width in calculating the outside wall area will depend upon which wall surface is exposed to the outside. In some cases (e. g., corner rooms), both are used and require at least two separate calculations (i. e., room length X room height, and room width X room height).
Add the calculated surface area of each outside wall (see Step 2 in Sizing Systems Using Coefficients of Heat Transmission) to obtain the gross wall area for the structure. Subtract the sum of all door and window surface areas from the gross wall area. The result will be the net wall area for the structure. Multiply the net wall area by the heat loss in Btu per hour per square foot to calculate the heat loss through the walls.