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Building a first floor cavity wall above a ground floor solid wall

Hello There
We are adding a first-floor bedroom on the side of the property. Ground floor walls are 1930’ 250mm walls with an empty cavity between the two brick skins. The new first-floor wall will be a 302mm brick/block standard cavity wall. The outer face of the new wall to be aligned flush with the existing ground floor brick face. This means inner blockwork will be about 50mm projecting out from the existing inner brickwork. How to support the bottom of the first-floor blockwork? Thank you so much in advance?

4 Answers from MyBuilder Bricklayers

Best Answer

In cases like this building control will allow you to match the size of the existing cavity
Hope this helps


Answered 25th Mar 2021

There's a couple of ways to do this
1: reduce the new walls to a 50mm cavity to match the existing but instead of using rockwool insulation which is standard for brickwork I'd use 50mm celotex as its thermal efficiency is about 6x better than rockwool so you won't lose the thermal efficiency even with the reduced cavity size of anything it should be better.
2: timber frame with a Brickwork outer skin. Use a 6inch 145mm studwork c24 timbers for the internal walls and the external brickwork goes up on the outside. Then fill inbetween the studs with celotex.


Answered 25th Mar 2021



Answered 15th Jan 2023

you maintain the same cavity and purchase a better quality insulation


Answered 25th Mar 2021

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