Final answer:
The percentage reduction in heating costs after adding insulation can't be determined with the provided information. However, increasing the insulation's thickness improves its R factor and thus its insulating properties, which reduces heating costs by decreasing energy needed to maintain a stable temperature inside the house.
Step-by-step explanation:
To calculate the percentage reduction in heating costs after adding extra insulation in the attic, we need to understand the concept of the R factor, or thermal resistance, which indicates the insulating properties of a material. The R factor is determined by the ratio of the thickness of the insulating material (d) to its thermal conductivity (k). This means the greater the R factor, the better the insulation, and vice versa. The relationship between the rate of conductive heat transfer and the R factor is inversely proportional.
In the given scenario, the house already has 15 cm of fiberglass insulation, and by adding an additional 8 cm, we increase the material's overall thickness, thereby intending to improve its R value. While the exact percentage reduction in heating costs cannot be calculated with the provided information alone since it requires the initial R value and the corresponding heat transfer rate, it is clear that increasing the insulation's R factor leads to a reduction in heat loss through the attic. Therefore, heating costs would be reduced because less energy is required to maintain a constant interior temperature.
In general, improving insulation can be a cost-effective way to enhance a home's energy efficiency, especially in areas with significant temperature differences between indoor and outdoor environments. Thicker batts may be more appropriate for colder climates to ensure higher R values and thus greater insulation efficiency. Remember, though, that improving insulation is just part of a comprehensive approach to energy saving that can also include addressing air infiltration and installing energy-efficient windows and doors.