Cavity wall insulation

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I live in a Wimpey built semi-detached chalet type house.If, for example, I was to get walls insulated how would this affect the other semi.I mean would the insulation creep through to the other house and would it be advisable to have both houses done,also I had soffits and facias replaced a few years ago and I suspect that builders took the insulation out from the dormers and did not replace it.Can the cavities there be insulated considering the fact that the inside wall is plasterboard,I think! I would appreciate any help with this.

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[Phil Anthropist]

If you phone a local cavity wall insulation company their surveyor should be able to answer these questions. They will also do a free survey, give you a quotation and explain what grants are available.