Insulating old window and door void

14 Oct 2021
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United Kingdom
I am looking for some guidance on insulation. I am undertaking some preliminary work before I start the main building project.

I am having a window removed and blocked up and a door changed into a window. The wall is cavity wall construction - 100mm brick, 60mm cavity, 100mm brick. It was proposed by the builder to put Celotex insulation in the cavity. The cavity is not insulated anywhere else. I was planning to insulate the house nearer the end of the project as I have not decided if I will insulated internally, externally or within the cavity.

is it worthwhile or required of me to put anything in the newly blocked cavities?
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