I saw a program a while ago (forget what it was) where they recommended that you overlay new insulation across the rafters over the insulation which is already there (mineral roll type), in order to bring the depth up to the recommended level (ours is 6 inches deep).
However, I haven't seen this mentioned anywhere else and it would make access to the roof from inside using the rafters pretty difficult should this be required.
Is this something I should be doing, or not?
It certanly reduces heatloss to next to bugger all, but as you point out, it makes it hard to use the loft space.
It doesnt kill draughts in the loft insualation either. Rockwool is not so good without a draught stopper.
Consider transverse joists across original,. rockwool filled and plated with flooring chip.
If you pay attention to the edges of thus where the loft soffit vents are bound to be, and seal teh slab against draughts, its about as good as it gets,
On this subject: I recently did the energy assessors course and was suprised to find how little effect extra loft insulation has. Playing with the software it seems that the first 100mm of insulation will reduce your heating bill by 12%, an extra 100mm will reduce your heating bill by only an additional 2%. Cheap as rockwool is, you won't get any return on your investment over 200mm.
The above figures vary depending on construction type but the proportions are about the same.
Surely that depends on the other heat losses, via walls, draughts, fenestration and so on?
Yes, your right it does but the principle that the first 100mm of insulation does 90% of the work is the point I wanted to make.
Indeed.
In my house the domainant heat loss is two inglenook chimneys, and the ventilation the BCO insisted on..
They didn't mention it with ours. Not so grand perhaps but still substantial fire openings and flues.
I am still ruminating on the log burner. With a bit of effort I can put a duct under the floor which allows me to go for a stove over 4.9kW. Convincing myself that something with a 10 year payback is worth doing is less easy. Hetas engineer and plumber availability is another issue. I'm inclined to wait for the current rush to die down as people realise that a sustainable fuel is not necessarily convenient or cheap.
regards
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