Question

Many decisions are made on the basis of the payback period: the time it will take through savings to equal the capital cost of an investment. Acceptable payback times depend upon the business or philosophy one has. (For some industries, a payback period is as small as two years.) Suppose you wish to install an extra 8.0 cm of insulation in the attic of a house. The house already had 15 cm of fiberglass insulation in the attic and in all the exterior surfaces. Take the single story house to be of dimensions 10 m by 15 m by 3.0 m. Ignore air infiltration and heat loss through windows and doors. If energy cost $1.25 per million joules and the insulation was $4.10 per square meter, then calculate the simple payback time (in y). Take the average ΔT for the 120 day heating season to be 14.2°C. (Assume that the floor is also insulated, and that thermal conductivity of fiberglass is the same as glass wool.)

Answer 1