Cavity & thermal conductivity

[WWilts]

Air at 20degr C has low thermal conductivity. (lambda 0.026 W/mK) Better than blown-in EPS beads.

For small extensions exempt from Building Regs, 75 mm cavity, what value would be added by including cavity insulation? 
Zone 1 for wind-driven rain.

[PeterW]

Only works for still air with no leaks - 50mm EPS is so cheap why would you not bother putting that in …??

[Conor]

Air circulates and flows... And so the heat with it. That's the principle of wool type insualtion. It holds air, and therefore, heat in place. 

[JohnMo]

But the air in the cavity will not be at 20 deg unless you have a huge depth of insulation on the outside wall.

 

The cavity assuming no other insulation will be midway between inside and outside temp.

 

So 20 deg inside and -2 outside, it would be at 11 degs, a lot less if a ventilated cavity.

 

On a cold winter day do you choose an insulated coat or a thin single skin rain coat.  The single skin will trap air, but you will be very cold. The trapped air the installation keeps you warm. 

Edited April 7, 2022 by JohnMo
More details

[ADLIan]

In a masonry cavity wall the thermal resistance of the residual cavity is set at 0.18 m²K/W.

[SteamyTea]

2 hours ago, ADLIan said:

In a masonry cavity wall the thermal resistance of the residual cavity is set at 0.18 m²K/W.

Which is about 5.5 W/m².K. 

 

 

 

[WWilts]

6 hours ago, ADLIan said:

thermal resistance of the residual cavity is set at 0.18 m²K/W.

What thickness of residual cavity is assumed for that?

[WWilts]

found this at designing buildings wiki

 

A range of U-values are indicated below for the purposes of comparison only:

Solid brick wall: 2 W/(m²K)

Cavity wall with no insulation: 1.5 W/(m²K).

Insulated wall: 0.18 W/(m²K).

Single glazing: 4.8 to 5.8 W/(m²K).

Double glazing: 1.2 to 3.7 W/(m²K) depending on type.

Triple glazing below: 1 W/(m²K).

Solid timber door: 3 W/(m²K).