Insulation over joists?

[tone]

The joists in my loft are just 70mm. Is it OK to put down a 100mm roll between the joists and then lie a second 100mm so that this layer goes across the joists to meet the recommended minimum depth? Are there any issues (condensation?) that means that the joists should not be covered and left to breath? Is the second layer really worthwhile? The area that this applies to is a small second roof space which I'm not interested in boarding.
Thanks.

 

[felix]

You can cover your joists if you like. The stuff is permeable so the joists can still breathe. The only problem with this will appear if you (or somebody else) ever needs to crawl around in there.

If this is likely to be a problem you can add extra timbers across the joists (only needed in those areas where you will need to crawl) before you put the insulation in. If you are likely to need regular access, cap these with any old slabs of chipboard - like smashed up kitchen units - that you can find.

 

[tone]

Thanks felix. To avoid the possibility of falling through the ceiling at a later date, i'll follow your advice and raise the joist level along one side.
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