Final answer:
To determine the percentage reduction in heating cost from added insulation, the concept of R-value is key. Without specific R-values for the insulation, an exact answer cannot be calculated, but more insulation typically leads to significant energy savings relative to the proportions given in the thickness.
Step-by-step explanation:
Calculating the percentage drop in heating cost due to the addition of insulation requires an understanding of the concept of thermal resistance, or R-value, which is a measure of how well a particular material can resist heat flow. The R-value is directly proportional to the thickness of the material. Therefore, by adding more insulation, the R-value increases, making the house more resistant to heat loss.
To begin with, we have 15 cm of fiberglass insulation and we add an additional 8.0 cm, resulting in a total of 23 cm of insulation. As the R-value increases with the added insulation, the thermal resistance of the attic goes up, resulting in reduced heat flow out of the house, thus reducing heating costs. The exact percentage by which the heating cost is reduced would depend on the initial and final R-values, among other factors, which are not provided in the question. Therefore, we cannot calculate the precise percentage reduction without this information.
However, in a theoretical scenario, assuming the increase in R-value due to the added insulation proportionally reduces the amount of heat lost through the attic, and knowing that heat loss through the attic constitutes a significant portion of a house's total heat loss, it's possible to achieve substantial energy savings, potentially reflected in one of the answer choices provided.