Cavity wall - OK or NOT

[STEVEROG]

I'm planning to remove the existing hardwood frame window and door in the rear extension of my property and replace with UPVC french doors. I'm not sure if the existing cavity walls are OK the way they are or if I need to sort them out.

The top sketch of the two i've attached shows how I think the walls are currently built. SpecificallY, I think the internal single skin block wall has been taken all the way to the outer brick wall. I can't tell for certain but 'tapping' the wall with my knuckles at the point where I would extect a more hollow sound of a cavity behind plasterboard it sounds like solid wall!
We don't have any sign of damp at the moment but this might be because of the lashed up timber conservatory (not shown) build on the back which would of course keep the outer brick wall dry.

I believe that the second sketch (middle one) is probably how it should be built?

I did also consider building a new small section of cavity wall to the right of the new french door as shown in the 3rd (bottom) sketch but this would introduce the pain of trying to match the finish brickwork on the outside.

Any comments/suggestions?

View media item 49563

 

[r896neo]

Cavity walls were usually built like that with reveals fromed by simply returning the inner leaf to the inside of the outer face. There should then be a vertical dpc between the outer face and the returned blockwork possibly a strip of insulation as well depending on age of the house. This vertical dpc is what stops damp coming from the outer leaf to the inner

Your hollow theory only applies to very modern houses which have cavity closers rather than the above method which is still used widely but with a strip of insulation to stop cold bridging.

Like this
http://www.polypipe.com/cms//produc...99101572d4e0d5437cfafc2jpg-20120210161415.jpg

 

[r896neo]

Oh ande if you did want to build the extra section of wall you need to open the cavity and continue it to the reveal as you have shown in your sketch.