Building Services Engineering 5th Edition Handbook

Heat loss calculations 61
Y of composite building elements. Chartered Institution of Building Services Engineers data are
used throughout and representative values are given. Calculation of building heat loss allows
load estimation for heating equipment.
Thermal resistance of materials
The thermal resistance of a slab of homogeneous material is calculated by dividing its thickness
by its thermal conductivity:
R = l λ
where R is the thermal resistance (m 2 K/ W), l is the thickness of the slab (m) and λ is the thermal
conductivity (W/mK). Resistance to heat flow by a material depends on its thickness, density,
water content and temperature. The latter two parameters result from the material’s location
within the structure. Insulating materials are usually protected from moisture and the possibility
of physical damage as they are of low density and strength. The thermal conductivity of masonry
can be found from the bulk dry density and the moisture content, which depends on whether it
is exposed to the climate or is in a protected position. Table 3.1 shows data for building materials
taken from the CIBSE Guide, to which further reference can be made.
EXAMPLE 3.1
Find the thermal resistance of a 110 mm thickness of brickwork inner leaf.
From Table 3.1, λ = 0.62 W/mK and l = 0.11 m. Therefore:
R = 0.110
0.62 m2 K/ W
= 0.1774 m 2 K/ W
EXAMPLE 3.2
A designer wishes to replace 200 mm thick heavyweight concrete blocks in the design of
a wall with fibreboard having the same thermal resistance. What thickness of fibreboard
could be used?
The values of λ are 1.63 W/mK for the heavyweight concrete block and 0.06 W/mK for the
fibreboard.
R (concrete) = 0.200
1.63
R (fibreboard) =
l
0.06