I have 900 square feet of loft to insulate and various rolls of 100mm rockwool that someone has left behind. There is lots of old rubbish and broken glass left between the rafters and the gaps are quite small rectangles. The space between the rafters does not appear to be standard and so to do it properly I'm going to need to cut every roll down to size. It's going to take days and is a daunting prospect.
So I'm tempted not to cut all the rolls to size in order to get the stuff between the rafters. How 'bad' is it if I just lay the rolls across the rafters? I was wondering if ending up with an air gap might actually end up being better? I wasn't planning to put the extra 170mm across the top of the rafters to conform to current specifications as I was led to believe it doesn't actually result in much extra saving beyond the first 100mm.
its unadvisable to leave gaps as you can get condensation forming in the area, Its best to remove the old stuff and renew the lot, I know its a horrible job, but the new insulation will save you a lot of money in the long term as its a better quality
Water vapour can easily pass through rockwool, so there will be no problem from this point of view.
However, warm air rises by convection, and warm air is pulled out of a home by the passing wind. So leaving spaces will allow your heat to escape, both by the open gaps at the end of the joists and through the joists themselves by conduction, the joists are connected to the walls and roof, offering plenty of routes for your heat to escape.
Most people take the easiest and cheapest route, when it comes to loft insulation, placing the insulation between the joists. This is actually a poor way to insulate a ceiling as those joists add up to a vast amount of wood. Which is connected to the warm air in the room on one side and the cold air of the loft on the other providing a perfect escape route for their heat by conduction, placing insulation between the joists and covering the joists in the loft as well, will at least help retain some of the heat that will usually escape to the cold air.
I also spoke to an ex-clerk of the works who was in the trade all his life and he concurred that it would be OK to do this. Judging from what Perry says, it will actually be better than laying between the rafters because I can roll out much of the insulation over the rafters without leaving any gaps.
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