Using Table $2\mathrm{.}7,$ determine the percentage change in the U value of a wall by adding an air gap.

Show images of the calculations from the tables.

b$.$A cavity wall is to be fitted with insulation. Using Table $2\mathrm{.}8$ in Study Guide $2,$ determine the U value of the wall if the insulation is to be $50$ mm of polyurethane foam with the wall being made of $2$ layers of brick.

c$.$Determine the annual heat loss per unit area for such a wall located at a place where the annual heating degree days $($relative to a base temperature of $15\mathrm{.}5$C$)$ are $1800.$

Note:

To determine the heat flow per unit area, you can simply use an area of $1$ m$2$ in calculations.