Building Homes or Extensions/gap between extension walls


Hi Martin,
We have 2 single storey extensions that site side by side at the back of our semi, not sure when they were built, but assume at different times. There is a 3cm wide gap in the middle between each extensions brickwork from top to bottom, that has some sort of fibreboard inserted (I assume insulation boards of some sort) 2 at the bottom 2 at the top, that fill the gap. When it rains the boards get wet (obviously) and I have now noticed damp on one internal wall near the skirting.
Should I seal the gap with external sealant or expanding foam?
Any advice would be welcome


If it was insulation it shouldn't be running all the way through to the outside. It may have been intended as a movement joint if the wall is quite long. Either way it sounds like a pretty shoddy job! You may find the fibreboard is very difficult to remove as it will swell when wet. If you seal the boards in, then water will soak through the adjacent bricks, into the board and you will continue to get damp problems, so it really does need to come out. Once removed you can seal up the joint to prevent more water getting in.

Hope this helps


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Martin Owen


Anything relating to the design and construction of extensions and loft conversions.


I currently work as an Architectural Technologist for, where I design extensions and advise on legal implications.

Degree in Architectural Design and Technology

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