I am in the process of insulating & boarding the loft, after raising the floor height to get in 250mm of Rockwool insulation, and put in a new 1.5m x .075m loft hatch.
I was just considering the materials (and the cost) that would be best to use if I was to put some kind of ceiling on the underside of the roof rafters, along with jablite insulation between the rafters. It’s not a proper habitable room so plaster board is not necessary, and it will be 3-5 years before its used as a proper playroom as the little one in under 3 at the moment.
My dad has suggested using plywood, but this would not be cheap for a loft with a floor area of 3.5m x 11m, at £25 for each 4’ x 8’ x 18mm sheet, I suspect 5mm sheets would be in the £6.50 region if I can find them.
I was just looking at 4’ x 8’ x 3mm Hardboard which is only £5.28, the cheapest option so far. I wasn’t shore whether to put the smooth side facing up or down though, down would give a better finish to the loft ceiling and it wouldn’t require painting.
IHO you will eventually be using the room as a habitable room so must meet the building regs. If this is the case the joists must be strong enough to take habitable loads. Then you would not insulate the floor but the space between the roof rafters! You should also build proper stairs as loft access ladders will not be suitable for todlers.
I am in the same position as I want to use the roof as a store room but maybe later a habitable room so I am going to insulate the rafters, put on plasterboard. For insulation see Google "jablite.co.uk" or go to B&Q and start asking. At the end of the day you should draw up some plans and if you get that done and approved you have 5 years to start.
You would need proper floorboards I would use 19mm ply but consider access for wiring underneath and no insulation. I am going to put in some rooflights. A biggish project but do it properly and you will have added value. All the best!
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