# Homework Help: Wall Cavity Insulation Question?

1. Jul 23, 2014

### jake96

1. The problem statement, all variables and given/known data

Hi guys, I have read this question over and over and have no idea where to start, would anyone be able to help.

The Question

Up to 20% of all heat lost in domestic dwellings occurs through walls. Many modern Australian homes are built by constructing walls using ‘cavity concrete blocks’ which have at least a 75 mm air gap or ‘cavity’ as part of their design.

Consider a home built using cavity concrete blocks and compare this to using two other construction materials; brick and aluminium siding. By investigating the heat flow through an external wall,

a) outline the range of assumptions you make.

b) determine, using appropriate calculations to support your answer, the required equivalent
thickness of a home's wall, in order to give the same amount of insulation as a cavity concrete
block wall if the wall is made of
i) brick.
ii) aluminium siding.

c) Discuss comparisons between the individual construction materials.

Thanks

2. Relevant equations

no idea

3. The attempt at a solution

don't know where to start

2. Jul 23, 2014

### CWatters

They are asking you to compare the thermal performance of three different wall constructions.

Cavity block
Solid Brick
Aluminum (I'm not familiar with this type).

How about you start by calculating the thermal resistance of a cavity concrete block wall ? They are made up of a sandwich of concrete block - air - concrete block. Should be possible to find the data you need (eg typical thickness of each layer and thermal resistance of each material).

3. Jul 23, 2014

### jake96

ok, ive found the thermal conductivity of the four materials
air: 0.024
Aluminium: 205
Brick: 0.15
Concrete: 0.55

and found thickness for concrete block (in accordance to pictures from homework sheet)
concrete (30mm)-air (75mm)- concrete (30mm)

would this be the resistance of the cavity concrete block?
=.03/.55+.075/.024+.03/.55 (thickness/conductivity)
=3.234Km^2/W

4. Jul 23, 2014

### jake96

also using a wall of 1mx1m so its not a hassle in calculations

5. Jul 23, 2014

### Staff: Mentor

Good. Now, how thick would these other individual materials have to be to give this same resistance?

Chet

6. Jul 23, 2014

### jake96

Brick
3.234=x/.15
=485mm

aluminium
3.234=x/205
=663m

this seems a little extreme for the aluminium

7. Jul 23, 2014

### Staff: Mentor

Well, aluminum has a very high thermal conductivity.

Chet

8. Jul 24, 2014

### jake96

so these sort of calculations would be correct considering no other outside influence,

in reality this value would be much lower due to reflection and other attributes

9. Jul 24, 2014

### Staff: Mentor

What kind of reflection is involved with heat conduction?

Chet

10. Jul 25, 2014

### Lyr4x

What formula did you use to find the thermal conductivity?