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Cavity Walls

Postby stumeadowcroft » Mon Mar 24, 2008 11:07 am

Hi i am building a front porch made from brick, in regards to cavity how many types of cavity excist as i cannot decide on whether or not to use one skin of brick and one of block ? or use two skins of brick with a gap inbetween for insulation, as the same as my house as it is pre 1960s built could you please also advise do i nee to use a cavity gutter useing flashing at the base of the cavity

regards stuart
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Simply Build It

Postby Degsy » Mon Mar 24, 2008 2:11 pm

Hi Stuart

If your porch is less than 30m2 and you still retain the existing external door between the porch and existing dwelling then the porch will be exempt from Building Regulations.

If it isn't exempt then you will have to make an application to your Building Control dept at the Council.

But basically all elements of the structure should have a minimum U' Value. How you achieve the U'Value will depend on what product you use. If Celotex is used the following should be OK

Floor Slab: 150mm H/C - 50mm sand blinding - DPM - 90mm Celotex - 100mm Concrete = U-value 0.21 (but depends on perimeter/area ratio!)

Walls: (EXT) Brick - 25mm cavity - 50mm Celotex - 100mm Celcon Std Block - Wet Plaster = U-Value of 0.26

Roof: All depends on whether its a pitched or flat roof (Cold or warm roof construction) etc

Doors/ windows have to achieve 1.8 u-value.

Your cavities should be continued to the existing wall and a cavity tray installed above flashing level.

As I say the above info will be purely your choice if the porch is exempt. You should contact your Building Inspector to clarify.
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