The house we have just moved into currently has 4"(100mm) roof joist and rock wool type insulation to about the same depth. I have a free supply of rock wool from a building project that over ordered so I was thinking of adding more up to at least the recomended 270mm. The outside areas are easy as it can just be laid ontop of existing insulation. The central section needs to remain a usable borded storage area so was just going to run additional 6 or 8" beams at 90 degrees to the current beams and fill with insulation and board on top does this seam reasnoble.

the vapour barrier should be under the insulation to avoid condensation. If the boarded area is open to the house the insulation and v.b. should be above it, not under it.

I did the same, except I used 2×4 and put them on 'stilts' – It made for much easier insulation installation

Like the idea "scienceofficer" that would keep the cost of the wood down. Its amazing how the simple things pass us by at times.

Scienceofficer, how did you secure the 'stilts' though, you must have had to bracket every single still in place which I imagine took time and money?

Screws, pre-drilled at an angle prior to moving the whole lot up there. I didn't use a single bracket.

It certainly took some time, but I had it. 25 person hours I think.

I assumed that buying sufficient insulation, and then packing it in nice and tight under the boards would work, is that not true?

and then packing it in nice and tight under the boards would work, is that not true?

I'm not sure 'tight' helps. The insulation material serves the purpose of preventing convection in the air by restricting it to small pockets. I mentioned the vapour barrier because the air inside a house is relatively damp due to sweating, respiration, cooking and washing, and it's best to prevent that air diffusing into the insulation (by an impervious layer of plastic) so that it doesn't cool to its dew point and condense liquid water which may then drip down and cause problems

barnes is right about the insulation. I don't know about the moisture barrier though.

I think 100mm is enough, I have not seen any evidence that 270mm is significantly better than 100mm, in my experience of designing enclosures for liquid nitrogen the effectiveness of insulation drops off over a certain thickness and given that the difference in temp is only ever likely to be 30c max I personally think this is bunkem made up by the industry or some civil servants…

One of my mates is an epc assessor and he cant justify it..

That aside insulation is so cheap you might as well if you want..

the effectiveness of insulation drops off over a certain thickness

the rate of heat transmission is inversely proportional to the thickness and proportional to the temperature difference, so each doubling of the insulation thickness halves the heat loss, and you can run into diminishing returns.

the rate of heat transmission is inversely proportional to the thickness and proportional to the temperature difference, so each doubling of the insulation thickness halves the heat loss, and you can run into diminishing returns.

exactly.

Go for it. 270mm or more is well worthwhile. Toys19 is wrong.

I did the same and it made a really big difference. Even in the hot summer now the upstairs stays cool and never boils like before. Don't worry about vapour barrier provided you have some through ventilation in the loft and airspace above. Do seal the loft door carefully with gasket though.

If you put this lot in without good ventilation above, your stuff in the loft will get damp with winter condensation.

mountain carrot how much insulation did you have before?

Make sure you add more insulation to your cold water tank and overflow pipes.
The people who insulated my dads loft failed to do this with catastrophic results ie flooded loft and bathroom ceiling on the floor 🙄

Who is doing the best deals on insulation atm?

http://www.save-energy-uk.co.uk/cavity-wall-loft-insulation-information/The-correct-prodedures-for-installing-loft-insulation.htm

http://www.diydoctor.org.uk/projects/loftinsulation.htm

Who is doing the best deals on insulation atm?

B&Q are doing the 200mm triple rolls at the moment for £3 – as cheap as it gets