Loft Insulation Sanity Check: Rockwool or foil? Ceiling or rafters?

11 Apr 2009
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United Kingdom
I think I've got myself confused between two discrete options for insulating our house, and could do with another opinion …

I'm in a late 18th century terrace, 550-600mm stone walls with rubble infill and a pitched slate roof. It's a cold house, and I'm wanting to insulate it as best I can. There was some rockwool insulation between the ceiling joists in the loft, which I've topped up where accessible, but other areas are covered in boards where the loft space is used for storage and these have been left undisturbed.

Then, thinking at the time that 'it'd all help' I've also fixed some foil insulation to the rafters with the notion of fully lining the space, perhaps without really thinking things through. I've currently done a full length of the house from the purlin to the ridge beam which is where I've stalled.

If the ceiling was insulated properly (ie. lift those boards onto stilts and lie rock wool underneath) then all I'd really be doing with the foil on the rafters is keeping the loft space itself insulated, yeah? Or is there benefit to having insulation to both ceiling and rafters?

The thought that foil may not be best option came to mind while considering fitting a whole house dehumidifier: If I fully foil line (taped and sealed) the loft space then I not sure there'd be adequate intake of fresh air to the loft space to function as intended. If relevant our (electric wet system) boiler and insulated hot water tank are situated in the loft space.

TLDR: am I better doing a proper job with rockwool ceiling insulation rather than lining the loft with foil insulation?

Thanks in advance.
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