Cheapest way to achieve the effect of 270mm insulation with a boarded loft?

andrewmoorcroft

I have a loft with 4" joists insulated with 2" rockwool and capped with 8' x 2' T&G chipboard flooring for storage. They are not screwed down.

Ive seen previous threads which suggest 'building the joists up', which appears allot of heavy expensive wood leaving a 'thermal conductive bridge' or doing similar but with cross battern joists to achieve cross laying.

I was in a builders merchants last week and saw a roll of 30mm thick multilayer stuff that claimed to be as effective as 210mm traditional insulation. The roll was an inconvenient width to fit between joists, so i was thinking of pulling the boards up, cross laying it, and putting the boards back on top (locally squashing it) between the boards and joists.

Any other ideas on cheap effective ways to achieve what i'm trying do do would be great. Before anyone says it, yes i do need the storage area, and i dont want to insulate the roof to keep my loft warm.
Thanks. :beer:

Doozergirl

If it's Tri-iso super 10 (or the likes) then you wouldn't want to fit it between joists anyway. It acts like a blanket and you tape it all together like an even bigger blanket.

H reckons you can do what you're saying you want to do. we're using it with attic trusses for the loft conversion and you do need ventilation gaps either side when you're laying it in the roof, but I guess it's not necessary on the loft floor.

It's not exactly cheap though.

andrewmoorcroft

Doozergirl wrote: »

If it's Tri-iso super 10 (or the likes) then you wouldn't want to fit it between joists anyway. It acts like a blanket and you tape it all together like an even bigger blanket.

H reckons you can do what you're saying you want to do. we're using it with attic trusses for the loft conversion and you do need ventilation gaps either side when you're laying it in the roof, but I guess it's not necessary on the loft floor.

It's not exactly cheap though.

Thats the stuff. I'd need around 50 square metres at £550!!!
Its expensive but what's a practical cheaper alternative to achieve the insulation required AND boarded?

Do you think i should pull it tight across the joists or let it sag a little before putting the boards back down?

Do you think it matters that it will be squashed where its gets trapped between the joists and boards? I think it should 'fluff out' to the full 30mm between joists without a problem.

I think i'll give the manufacturer a call to check before i buy.

Update: My local builders merchant has a booklet showing £95 + VAT for 10 metres but the pack priced instore at £95 is actually 20 square metres so idea looks more attractive.

kje_2

You could try space boards. You can't walk on them, except where there is a joist underneath to support the board, but can place boxes on them. Maybe, you could put these under your current loft boards and the loft boards would spread your weight as you walked over them which would make them suitable for walking areas in your loft.

http://www.space-insulation.com/board.html

Available from B&Q. Might be worth a try depending on their cost.

economiser

I insulated between the rafters rather than disturbing the flooring which has the advantage of keeping the loft space frost free. Raising the flooring obviously limits head room which would have been a problem for me but maybe not in your case.

andrewmoorcroft

kje wrote: »

You could try space boards. You can't walk on them, except where there is a joist underneath to support the board, but can place boxes on them. Maybe, you could put these under your current loft boards and the loft boards would spread your weight as you walked over them which would make them suitable for walking areas in your loft.

http://www.space-insulation.com/board.html

Available from B&Q. Might be worth a try depending on their cost.

Thanks. Looked at these but they are only 50mm thick so i would need to quadruple layer them to get the thickness. Their web site suggests laying them on top of existing boards and then adding another layer of floor boarding to enable you to walk on them and i want to be able to walk everywhere in the loft because it's like another room.

andrewmoorcroft

economiser wrote: »

I insulated between the rafters rather than disturbing the flooring which has the advantage of keeping the loft space frost free. Raising the flooring obviously limits head room which would have been a problem for me but maybe not in your case.

Thanks. My roof space is huge so no problem with losing height. I do not want to insulate my roof space, i want to stop the heat being lost to the loft in the 1st place.

andrewmoorcroft

Update: I contact the suppliers of Triso and Superquilt. Both say that the product is NOT suitable in horizontal applications (must be sloped), must be ventilated, and a whole load of other issues. From what i could gather, the main issue appears to be that it creates a waterproof / vapour shield blanket that would stop moisture escaping through the ceilings and cause the house to become damp.
They said If it was suitable for my application it would have been marketed as such and they would be able to double their sales.

I also called building consultancy. They said a big no to the foil stuff and for similar reasons had reservations about the friendly encapsulted wool stuff that DIYers use. They suggested using polystyrene blocks on top of my existing boards covered with 3mm hardboard.