Maximum Celotex thickness to use in cavity wall

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We're building an extension and want to use Celotex CW3000 board insullation.

The wall will have a 100mm cavity, so what is the maximum thickness of board we can use. My feeling is 75mm is the maximum to leave a 25mm cavity, but I seen some comments about allowing a 50mm cavity to prevent bridging of the gap through low quality brickwork. Is this the case?

I should add that we are not using variable sized bricks (e.g. reclaimed) for the exterior leaf, but will be using Ibstock Marlborough bricks with the inner leaf made from 100mm hemelite blocks.
 
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Why go to the fuss and expense of rigid board?

Full fill 100mm drytherm is easier to use and install.
 
Why go to the fuss and expense of rigid board?

Full fill 100mm drytherm is easier to use and install.

Celotex CW3075 has an R value of 3.25 (CW3050, R=2.15) whereas DriTherm 32 Ultmate 100mm (for full fill) has an R value of 3.1.

Also the thermal conductivity is better 0.023 W/mK vs 0.032 W/mK

It might be an option if we can't use the 75mm Celotex.
 
I am aware of the thermal performances of both products and the superiority of p.i.r. board, but is the pain and expense of fitting rigid foam actually worth it to the pocket of the installer.

The building may retrieve the difference cost in its lifetime but not yours. ;)
 
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Agree with Nose , from a cost point of view also 75 mm cellotex aint cheap and is a pain to install. MHO is cheaper and easier pay back is achieved via upgrading roof insulation, also all the good that can be done in the walls can easily be lost in the windows at around 1.8w/mk.
 
Why go to the fuss and expense of rigid board?

Full fill 100mm drytherm is easier to use and install.

Celotex CW3075 has an R value of 3.25 (CW3050, R=2.15) whereas DriTherm 32 Ultmate 100mm (for full fill) has an R value of 3.1.

Also the thermal conductivity is better 0.023 W/mK vs 0.032 W/mK

It might be an option if we can't use the 75mm Celotex.

The u value of board is 0.30 @75mm
the u value of full fill is 0.32 @ 100mm
Add to this the thermal resistance of the 25mm air cavity with board, added to the other elements making up the wall you will have a superior U value with the rigid boards, cant give you exact U values as its past midnight but will do so if you want them later
 
Why go to the fuss and expense of rigid board?

Full fill 100mm drytherm is easier to use and install.

Celotex CW3075 has an R value of 3.25 (CW3050, R=2.15) whereas DriTherm 32 Ultmate 100mm (for full fill) has an R value of 3.1.

Also the thermal conductivity is better 0.023 W/mK vs 0.032 W/mK

It might be an option if we can't use the 75mm Celotex.

The u value of board is 0.30 @75mm
the u value of full fill is 0.32 @ 100mm
Add to this the thermal resistance of the 25mm air cavity with board, added to the other elements making up the wall you will have a superior U value with the rigid boards, cant give you exact U values as its past midnight but will do so if you want them later

I would appreciate the U values, but it would have to be for 50mm Celotex and not 75mm as the manufacturers recommend a 50mm wide air cavity to prevent bridging and the total cavity is 100mm.
 
You do realize that the insulation sits tight against the inner leaf therefore ensuring that you have to build the block inner leaf first.

Bloomin pain in the but! :evil:
 

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