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FreeBear

Deleted_User said: My gut feeling (based upon how the house feels this year as opposed to previous years) is that the woodburner flu is doing a brilliant job of insulating the house by keeping the loft space warm. So probably limited return on investment adding loft insulation?

The stove is not insulating anything. Having a warm loft means there will be less heat loss from the rooms below - Heat loss is a function of temperature differential. So the colder it is on the other side of a wall or ceiling, the greater the heat loss.

Adding extra insulation to the underside of the roof won't do much to keep the house warm. If anything, it could cause problems in the future. i.e. If the roof develops a leak, any insulation between the rafters could trap moisture rather than letting the water drip down to the ceiling below. A damp patch on the ceiling would alert you to a leak so that you could get it fixed before any real damage occurs. Trapping the moisture in the roof could lead to rot setting in before you notice there is a problem.

Assuming you already have ~200mm of loft insulation, adding more is not going to make much difference even with a flue heating the loft space. So on that score, you are correct with the assumption that there is a limited return on investment. But having 300mm will improve your EPC rating (maybe by one point) which might score brownie points when coming to sell.