Cardboard in the attic (insulation)

Cardboard doesn't have a terrible U-value, roughly the same as rockwool. But that's in cellulose fibre form, rather than sheets. I doubt you could get a tight seal with it in box form as there would be too many air gaps. I suppose it would be possible to cut pieces to lay it flat.

I'd however worry that it would retain moisture, not what you want next to roofing timbers. Wet insulation isn't good insulation. Commercial insulation is also treated against fire, insects, etc, which random cardboard won't be:

But if cardboard is all the insulation you have... it works for the homeless folks.

Theo

It depends how well you want your roof space to burn in the even of a fire. Rock wool or even sheep's wool is a much better alternative.

That said I have a piece of expanded polystyrene sat on top of my cold water header tank.

Well, I was told it was a fire hazard when the laggers came along. Brian

Back in in 1989, I worked in a drawing office, doing CAD work. The rest of the office worked on drawing boards. The drawing office was directly above the loading bay, with the doors open all day. I was fine, sat at a computer, but all the draughtsmen stood at their boards, on top of a 2" stack of cardboard exhibition signs, to keep their feet warm.

1. Risk of cardboard absorbing moisture and holding it
2. Risk of rodent infestation

The boxes are up there anyway. I'd guess they'll burn less readily laid flat than in their original form.

What's happened to all the Rockwool products. The link about had several options for Rockwool. Now seems anywhere that has it is selling at double the price and only fibreglass is left at the prices I remember.

Cardboard insulation does work, but it's flammable, intolerant of water, goes mouldy, hosts insects & rats, and bends out of shape when it gets wet. If you truly can't afford anything better, it's an option, covered with some nonflammable insulation like fibreglass/rockwool or plasterboard. Otherwise I wouldn't.