1930s House: Need to upgrade the Insulation!



Being a total beginner, and realising the house was a little cold (double bricked with no cavity), I thought the best option was first to check the loft insulation.

I'm afraid I don't know all the terminology, although I've been trying to read up!

I've got a tile roof directly hung over rafters with mortar holding it. There is no felt or membrane underneath and the mortar dust is going everywhere.

The floor of the loft is currently boarded. I've taken some up and measured it out, to find that I've got loose rock wool (or similar) to a depth of only about 60mm, which is the depth of the cross joists (the smaller ones that hold the ceiling up!).

Some of the large joists (that are load bearing) are 200mm from ceiling to top, others are 160mm.

What I'd like to do is:
1. Get some sort of fabric and staple it across the rafters as a kind of barrier to dust falling from the mortar. Doesn't need to be insulating but I suppose it needs to be breathable. What would I need?

2. Take out the old insulation and replace with some new (probably the recycled bottle stuff). I've worked out that I'm supposed to do about 170mm and then cross lay 100mm over the top. Does that just go over the small joists or the large joists as well? Or should I fill to 60mm between the small joists and then cross lay over the top of that?

3. I want to board some of it for storage. Should I board at 200mm (the level of the large joists) using insulated board, and do I need to put in further structural support? Or should I build up those joists to 270mm and fill with full insulation?

The alternative of course is to get a professional in to do the whole job exactly to spec but I suspect that would be expensive.

Any help gratefully appreciated for a newbie.
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