Example 1 - Calculation procedure for showing the fabric insulation meets the requirements
A detached, four storey office building 45 m x 13 m in plan and height 15 m is to be constructed with 55% glazing, using windows and glazed personnel doors which have metal frames with thermal breaks containing sealed double-glazed units with 12 mm air gaps and a low-E coating. No rooflight glazing is proposed. The remaining exposed walls and the roof are to have U-values of 0.6 W/m2K and 0.45 W/m2K respectively, with the ground floor being uninsulated. Personnel doors are to have a total area of 14 m2 and there are to be vehicle unloading bay doors with an area of 27 m2 and a U-value of 0.7 W/m2K.
diagram
Proposed building
Calculate the area of each element and then calculate the rate of heat loss from the proposed building by using the table to J3.3 as follows:
table
Notes:
1. (45 + 45 + 13 + 13) x 15 = 1740 m2
2. 55% of 1740 + 14 + 27 = 998 m2
3. 45 x 13 = 585 m2
Notional building
The area of openings in the proposed building is more than the basic allowance in Table 2 to J3.2. So the basic area allowances of 40% of exposed wall area for windows and 20% of roof area for rooflights should be assumed for the notional building.
The exposed perimeter of the ground floor of the proposed building is 116 m and its area is 585 m2. So the ratio P/A is 0.2, which indicates the U-value of the ground floor is 0.36 W/m2K (from Appendix F, Table F1). This is less than the standard U-value in Table 1 to J3.2 so 0.36 W/m2K must be the value used in the notional building calculation.
The rate of heat loss from the notional building is calculated in the same manner as for the proposed building as follows:
table
The rate of heat loss from the proposed building is less than that from the notional building and therefore the requirements of the Technical Standards are satisfied.