Rear extension timber wall build up

[AndyFExtension]

Hi.

 

I am self building a rear extension on 1950s semi. The walls are cavity brick block (no insulation).

 

I will have to raise one wall to within 300mm of the neighbours extension so I am having to build the wall (or at least the external face) and raise it in one go.

 

I have put together the following wall build up. Keen to get your opinions.

 

constraints are:

initial build access and future maintenance

fire regs

limit on value of thermal performance given the U values in the rest of the property.

cost (to a certain extent)

 

out to in:

cedral weatherboard

Counter battens

breather membrane

25 or 50mm PIR

Breather membrane

18mm osb

140mm frame therm

vcl

38mm batten

18mm osb

plasterboard (foil)

 

Many thanks

 

[PeterW]

Need to get the BCO to confirm that is ok from a fire protection perspective. They may also require fireline boards on the inside. 
 

Also think you have your OSB in the wrong place on the inside - should be studs /VCL/OSB/batten/PB

[AndyFExtension]

Thanks Peter.

 

I have spoken to BCO and they are happy with the build up from a fire regs perspective (because of the cedral). Although on reflection I asked the question of fireline boards and they just said my proposed build up was fine - so need to confirm that!

 

on the OSB point I wanted to have the osb and PB together to aid internal fixing in the future and also it provides a greater distance (68mm) between the inside and the VCL so less chance of puncturing the VCL unwittingly.

 

[Russell griffiths]

If you cannot see it, why waste money on cedral how about just a cement board jointed with intumescent sealant. 

[AndyFExtension]

Thanks I did consider that, however I first discounted it because I wouldn’t be able render it and then raise wall without cracking the render.

 

is the cement particle board suitable for permanent exposure to the elements without covering (with render) but sealing the gaps?

[AndyFExtension]

Thanks for the comments.

 

I spoke with the manufacturer of weatherkem boards and they confirmed that painting would be suitable for this application and provide a decent service life.

 

on the PIR front, what are your views on having that external to the osb sheathing and sandwiched between two layers of breather membrane?

 

are there issues for different thicknesses say either 25/50/100?

 

I am aiming for a u value of max 0.18, although can go lower obviously