U-values

U-values

Open Ended

What factors might affect the rate at which thermal energy passes through a closed window? List as many as you can think of.

The rate of energy transfer depends on:

• The temperature difference (inside-outside)

• The area of the window

• The thickness of the window

• The thermal conductivity of the glass/air

Multiple Choice

A bigger temperature difference will cause the rate of energy transfer to

• increase

• decrease

• stay the same

Multiple Choice

Energy transfer will increase if the window has a surface area that is

• large

• small

• impossible to tell

Multiple Choice

Making a window thicker will

• increase the rate of energy transfer

• decrease the rate of energy transfer

• make no difference

Multiple Choice

A window with high thermal conductivity will transfer energy

• quickly

• slowly

U-values

To simplify calculations, building materials are given a rating called a U-value.

The units of a U-value are W/(m^2 oC)

Open Ended

Write out what

W/(m^{2 o}C)

means in your own words, using a proper sentence.

W/(m^2 oC)

This is the rate of energy transfer in watts (joules per second) for each square metre of window, and for each degree celcius temperature difference across it.

Can you see what factors have been incorporated into a U-value to make it simpler to calculate?

thickness & thermal conductivity

Open Ended

energy transfer rate = U-value * area * T difference

If a window has

a U-value of 2.8,

an area of 2m²,

and a temperature difference across the window of 10^oC ,

what will be the rate of energy transfer across it?

Give your answer in watts to the nearest whole number.

2.8 * 2 * 10 = 56W

Multiple Choice

A cavity wall has a U-value of 1.7. What would be the energy transfer rate for a single outside wall that is 4m wide and 2m tall, if the temperature inside is 20^oC and outside it is 5^oC?

• 1360 W

• 272 W

• 204 W

• 56 W

What was the temperature of the hot metal plate?

give your answer in the next slide.

Multiple Choice

What was the temperature of the hot metal plate?

• 100 deg C

• 80 deg C

• 60 deg C

• 25 deg C

Which material is the best insulator?

give a reason for your answer in the next question.

Open Ended

Which material is the best insulator. Give a reason for your answer

The table gives information about four types of insulation that could be used for insulating the cavity walls of houses.

Which one of the types of insulation in the table would be the best to use for cavity wall insulation?

Multiple Choice

which one of the types of insulation in the table would be the best to use for cavity wall insulation?

• felt wool

• mineral wool

• polyurethane foam

• rock wool