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Hi...i am a little confused.
In a cavity wall , be the cavity 100mm or 80mm do you have to have a 50mm clear cavity, then 30 or 50mm insulation?
I know you can full fill the cavity with insulation but guess this doesnt help with transfer of damp?
thanks
rob

[quote="canon3379"]Hi...i am a little confused.
In a cavity wall , be the cavity 100mm or 80mm do you have to have a 50mm clear cavity, then 30 or 50mm insulation?
I know you can full fill the cavity with insulation but guess this doesnt help with transfer of damp?
thanks
rob[/quote]Cavity 75mm,insulation 50mm,never a 100mm cavity.

Cavity walls are designed to stop the water that comes through the outer wall, from finding its way into the home.
The only way to guaranteed that insulation placed in a cavity will work is to use expanding foam as you say all the other stuff can get wet and wet destroys the insulation of the whole wall.
The way to insert foam is to drill 12mm holes at 50cm centres and squirt the stuff in.
Unfortunately, expanding foam is not cheap.

Hi
Firstly I guess you are talking new build from your question. The only products you can use have to be approved so you should check with building control.

Secondly it is actually a myth that fulfill cavity wall insulation causes damp in properties.

The cavity wall was designed to stop moisture transfer from outside to inside. If the cavity wall is properly constructed then filling with any BBA approved product will not compromise the function of the wall. The problems arise when there is debris or sloping wall ties in the wall. These act as bridges which allow moisture to travel accross. Therefore the fact is cavitywall insulation shows up building defects but does not cause the defect.

Injecting expanding foam is the most ridiculous suggestion I have heard. It is far cheaper to employ a recognised contractor who can give you a meaningful guarantee. Contact the energy saving trust or your local council for assistance.

By the way expanding foam is NOT approved for this purpose.

Aidan

Hi

What u-values does expandable foam give you , if any ?

Bye.