Hi, I'm a newbie to DIY doctor that I hope someone in your forums can help with my questions.
Firstly, I've just recently had my loft space insulated by I believe 300-350mm of rockwool or similar fibre rolls (done by professional installers). It is a huge space in an old tenement building and the insulation has been placed between the deep joists without actually going over the tops of the joists at all. I want to put some boards in for walking and a bit of storage. The boards laid across the joists will be only 2 wide in most of the loft, 3 wide in one section (ie laid boarding will be mainly 1200 wide and there is at least that distance again to the eaves). I'm not boxing or sealing the ends of the boards to the rafters, ie they will be "open ended" (with the insulation beneath). The possible problem is that the gap between the top of the insulation and board will not be much at all, if any. Is there much risk of condensation between insulation and boards, given the boards are only partially covering the insulation and there are open ends towards the eaves? If there is any real risk, will drilling occasional "air holes" in the boards be of any use? The roof has already got proper ventilation units installed in each section of the attic, on both sides.
Secondly, I have some spare rolls of fibre (I had started buying some myself before the installer did the whole job). Is it worthwhile to lay out the extra rolls in the attic space on top of the existing installation (but across the joists, at right angles) - but not blocking any of the ventilation facilities in the attic and not blocking the ends of the boards mentioned in the question above?
It would be great to get any guidance on these questions. Thanks...