Insulation of houses
Much detailed work has been done on this topic
and a few results are quoted below.
| Part of house |
Rate of loss of heat (Wm-2) |
| Glass window |
112 |
| Double-glazed window |
60 |
| Single brick wall |
66 |
| Cavity wall |
19 |
| Foam filled cavity wall |
10 |
| Un-insulated roof |
30 |
| Loft with 50mm glass wool |
10.4 |
| Loft with 75mm glass wool |
7.8 |
| Solid floor |
30 |
All these figures are for a temperature difference
across the surfaces of 20
oC. Of course, even perfect insulation is no use at all unless
you cut out all the draughts!
U values
It is useful for architects to know the
rate of loss of heat per square metre through a material when the difference of temperature
across its faces is 1K. Clearly this will also depend on the thickness of the specimen so the
values given below can only be regarded as approximate.
Example problem
Calculate the thickness of a single sheet glass need in a 0.5 m2 window to maintain a rate of loss of heat through it at 2 kW. Assume a temperature difference across the glass of 15oC.
Using dH/dt = - kAdq/dx
2000 = 0.8x0.5x15/dx therefore thickness (dx) = 0.8x0.5x15]/2000 = 0.003 m = 3.0 mm
Student investigation
Keeping warm in bed can be very important, especially for old people. In this investigation you will study the insulation properties of blankets and continental quilts of various types.
Fill a metal can with water at 100 oC and fix the probe of a digital thermometer to the outside of the can. (If such a thermometer is not available a thermocouple will do instead.)
Place the quilt or blanket over it and record the tem¬perature at regular time intervals.
Repeat the procedure for several types of bed covering. Which type would you recommend?
