Cavity or not?

[wfoot]

I am considering converting an old single storey extension into a kitchen.

Only problem is the walls are single brick thickness. I was looking at using 30mm cavity wall boards to insulate. They are supposed to give a U value of 0.35, so is there any need for a cavity between the boards and the brickwork?

If so how large a cavity is sufficient?

Gratefully

 

[noseall]

apart from an insulator a cavity helps prevent damp from entering any secondary internal walls.

 

[^woody^]

Do you mean single brick (215mm) or half brick (102.5mm) thick?

A 30mm board (celotex?) on the inside is insufficient to acheive a .35 u-value. What will the inside skin be?

A 30mm board will only acheive .35 u-value for the whole wall with a cavity and perhaps 110mm aerated block internally.

You will need to increase the insulation and include the internal leaf in the calculation.