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I am building a cavity wall extension - 1x100m inner leaf, 1x 100m outer leaf cald with 100mm facing stone. Cavity is 100mm to include 50 mm celotex How do I construct openings for windows/doors whilst maintaining strenght insulation and dpc to current building regs please?

chiefy,
If I am understanding you question correctly then the answer is to use cavity closers for 100mm cavities. See your local builders merchant.
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