Question for the insulation/ heating experts.

I've got a detached house with a modern boiler, 8" of insulation in the roof and double glazing. It has cavity walls which, apparently, are too narrow for insulation. It seems to have loads of ventilation for the underfloor and the cavities, can this be blocked or restricted?

We are bl**dy freezing! Any ideas to help warm it up would be appreciated. Interior and exterior wall insulation has already been discussed and probably not feasible

If you do block up the ventilation, you might find that your house gets damp and mouldy. Mine's had quite a few vents blocked by the previous owner, and there is a mould problem in a couple of rooms because of it.

Insulated plasterboard is not that dear and makes a big difference.
Standard boards are around £5, 30mm £16 and 50mm £23. Just dot and dab as with normal boards. They're a lot stiffer too, so getting a good finish is easier.

aerogel boards are available – won't be cheap though.

you need to work out where the heat is leaking, so you can insulate that. sounds like it could be the walls but it could be something else.

shouldn't lose a lot of heat to underfloor ventilation unless wind is blowing through at a rate. the air under there really should stay fairly still.

Insulate under the ground floor if possible.
Get thick curtains for winter.

as above really thick curtains help but tidy ones are expensive.

also we recently had our attic floored off which has made a massive difference.

Hell of a draft under the floor, which is why I wondered about at least restricting the vents. I was worried about damp though.

I think under floor insulation is the way forward, though selling may be easier.

Thanks for comments though