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I need to insulate a totally clear loft in a standard 3 bed semi built around 1930. I am not certain what to use to achieve the required depth of 270mm. also want to board the loft area after insulating. what is the best product to use for thermal efficiency and how could i raise the height of the standard 4 x 2 joists? Thanks tooly

The figure of 270mm of insulation, relates to the use of glass fibre insulation only.
Glass fibre is not very good for lofts as it looses its insulation qualities when it gets wet, if you compress it by laying things on top it doesn't work as it relies on the air trapped inside to work. It is transparent to water vapour and the wind blowing over it does suck the trapped warm air out.
For a loft a closed cell insulation like polystyrene is far more practical, it is to all intents water proof, wind proof, you can lay things on top of it, without it compressing and loosing its insulation qualities and it is far more effective in as much as, you get a better result from a thinner layer.
If you pour foam between the joists you can be sure of a tight fit. With glass fibre it is difficult to fit tightly and the result is lots of gaps for the heat to escape through

Perry525,
Isn't what you are suggesting a horrendous fire risk?
end

Stoneyboy, I'm glad you raised this point as, there has been a lot of misinformation put out by the fibreglass industry.
If you check out the press you will see that the polystyrene industry are about to run an advertising campaign to clarify the fire safety of their products.
There is one UK manufacturer that makes both products and claims therefore to be neutral-look at their site.

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Hi tooly,

The best way to do this would be to buy some rigid celotex or kingspan insualtion. it comes in 8x4 foot sheets usually and a range of thicknesses, so you can cut and lay it between the exisiting joists and get it to line up just fractionally less than the top of the floor joists. You can get data sheets for these products off the internet. just search for them on google. it will tell you the insulating performance figures. Then you can compare them to fibre glass, loose fill or whatever else. If your happy then board straight over the joists. If you want to improve this further you can fix additional joists running adjacent to the existing ones. And add another layer of insualtion, then chipboard over. personally if you only need a small storage area then only fix additional joists were you need them, lay celotex between them, then chipboard. and cover the rest of the loft space with encapsulated insualtion (space blanket or similar) cut a square of rigid insulation and stick it to the inside of the loft hatch with gripfill too. Remember never cover any electrical cables!