Why not lay new floor joists cross-ways to the old ones, pour in Warmcel 100, and lay chipboard over the top?

This is what I have done in part of my attic

1. New plasterboard ceilings with polythene vapour barrier stapled to joists above
2. Mineral wool between existing 50x75 ceiling joists [short span annex]
3. 100mm foil faced PUR rigid boards [2400mm x 1200mm sheets] cut to size and laid transverse above joists
4. Cheap hardboard [2400mm x 1200mm] laid on top to create a safe walking and storage surface

Not ideal, but much lighter than chipboard and extra joists, which your current joists may not be designed to cope with

Of course you still have to be careful how much weight you put up there

EDF insulated my loft to about 8" for free last summer, so I bought a load of timber and put raised posts at regular intervals along all of my floor joists. My old chipboard floor now lays on top of those posts, leaving the thick insulation where it was, safe and unsquashed. All it cost me was about £40 and a few weekends in te loft, which I imagine will be cheaper than the SIPs. How much are you planning on spending?

Posted By: TerryHave to agree with joe.e - seems the simplest and most cost effective method and retains the floor strength that Mikes loses.

Why do you think my floor has no strength?

100mm of rigid PUR is pretty solid when supported by joists underneath anyway. Laying hardboad over the top means that I can be sure that the surface of the PUR remains undamaged.

Adding more joist and chipboard will add more weight overall - It does not mean you can store much more weight up there - as you have allready increases the self weight of the old attic joists with your new floor

My option is lighter that's all

Thanks loads for all your comments – gave me loads of options to look at and I’ve been doing a bit of digging on prices:

Option 1: SIPS Panels
£42/m² for 150mm thick panels

Option 2: PUR rigid boards with hardboard facing
£10/m² (100mm Celotex) plus £1.60/m² hardboard (3mm)
£12/m² Total

Option 3: Raised floor over insulation
£7/m² plywood (12mm thick) plus £1.80/m² loft roll (100mm) or £2.70/m² Warmcell (100mm)
£9 - 10/m² Total

I’m looking doing about 30m² and I’m not made of money so the SIPS are out. The other two are comparable in price and the Celotex and hardboard sounds like a lot less work (cutting joist raisers etc). A couple of questions, though:
- Is 3mm hardboard enough on top?
- Will the Celotex be compressed where it lays on the joists (only worked with it a couple of times and can’t remember what the compressive strength’s like)
- Will I get a condensation problem in my existing cellulose under the Celotex as I don’t have a vapour barrier at the ceiling level.

I am about to do what you are contemplating, I think at your pricing of £9/sq.m a multifoil insulation on top of your GF with your flooring raised on 1" battens would work out easier to do and cheaper.
Sorry I mentioned the "M" word but lofts get very hot in summer which is the problem I have, in that the bedrooms get too hot in summer. Foils are very good for this. Trying to find a permeable foil.
Frank

Like joe.e I put in a new row of joists above the existing ones separated with 2by2 inch cross battons ie making a large pallet like platform and then toppng off with a mix of 18mm plywood old floorboards and chipboard. I left the old glass fibre insulation around unboarded sections and covered with kraft paper to stop the stuff making me itch. I've got a load of 50mm sheeps wool insulation but I need to supplement this to finish the insulation between the joists. May not get more thermafleece because of huge transport costs so may just use rockwool. Are there any national chains selling warmcell so you don't have massive transport costs?

For a vapour check I cut sections of sisalkraft 420 and put this between the joists. Not a perfect fit as its hard to retrofit with lath and plaster ceilings.

I still have not made a decision on what to put under the rafter. I only need to keep the big swings in temperature away from the stuff I store in the loft. Foil insulation look like an ideal solution but I'm worried about condensation issues.

I might have found the perfect insulation for the job...

Jabfloor 70 is designed for underfloor insulation and to be topped by screed or concrete but it's very strong, light and workable. It also doesn't have the condensation issues thatmight arise from using Celotex or similar over existing insulation without any vapour barrier.

It has the same k-value as mineral wool so I can put up 120mm thick sheets and get a significant performance increase over what's already there.

I am a little nervous about ordering a big batch of the stuff without even having seen it (just in case there's something I've overlooked that'll make it unsuitable). Has anyone used it before?

Just had Rockwarm round because I was interested in the latest grants and they are in the 'scheme'. They quote a cost to me of £199 for adding 8" (I presume this means 200mm) to the existing 75-100mm. Of course this is glass fibre, they don't do anything friendlier, but is it (1) a reasonable price for an area of about 40sqm, (2) a good idea to simply cross lay two layers of 100mm over the existing insulation and joists, (3) better to remove the boarding covering 75% of the floor (which they suggest), and/or (4) a problem to cover any wiring still further?

And did anyone have any comments about the Jabfloor mentioned by AndyShep?

I was house clearing (final push - all done now) at mother's at the weekend and ended up with about 10 pillows, 5 double duvets and I can't think how many cushion innards up in our loft. Quite snuggly up there now.