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Cavity & thermal conductivity


WWilts

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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.

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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 by JohnMo
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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).

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