Cross-Batten advice

[verloren]

I've just moved into a 1970's house that currently has around 100mm of fiber-glass batt insulation in the roof. My current plan to improve this is:

1. Add some loose fill to close off existing gaps (after 35 years the existing insulation doesn't adequately fill the joist spaces.
2. Raise the level of the joists by cross-battening.
3. Top off with more batts, then put down chipboard flooring for some light storage.

Two questions:
1. How deep can I make the cross-battens and keep them structurally sound? The common advice seems to be to use 3x1 laid flat, but obviously that doesn't leave room for more insulation. On the other hand putting in (say) 2x6 would presumably require some sort of cross-blocking to keep the floor stable. and some way of fixing it to the existing joists?

2. Would it be better to skip steps 2 and 3 and use the Knauf Supadeck product ( http://www.knaufinsulation.co.uk/output/products/page_392.html )? And if so, does anyone know where to find it?

The roof has a W-truss arrangement (I think that's what it's called), so the area on each side outside the W will just be piled with batts to a suitable depth without any flooring. The relatively small area to floor means I can afford a notch above basic if necessary.

-v-

[Static]

Common sense is about your only restriction on size of timber cross members. You should choose a size that is taller than your proposed additional insulation and wide enough to work with. ie. your adding 175mm of insulation then go for 200/225mm deep timber and 35 or 50mm wide. Next thought should think about length of each piece to man handle into place etc. As long as the cross battens are sensibly nailed into the existing then it should remain stable

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(Any reliance you place upon any advice given will be at your sole risk)

[verloren]

Thanks Static. I'm a little concerned about having timbers tall enough that they will tend to fall over, but I guess if I have them fastened at their base and boards connecting the tops then they should be stable.

Oh, and to answer part of my own question, I've found out that the supadeck boards are sold at Travis Perkins. Haven't checked on price yet.

[Deluks]

They should be fine once screwed into place. If you're worried though then get a few of these

Unless you have a trade account, forget travis perkins, too pricey. 8x2 chipboard flooring panels are about a fiver each from B&Q or Wickes, much more economical. Oh, and screw don't nail!

[muffinn]

How far apart were you going to make them - I am considering the same solution in a 30s house

[verloren]

Deluks - the advantage of the insulated board is that I don't have to raise the joists at all - I can just lay them over the existing joists. That greatly cuts down the amount of work required, and reduces the height of the end solution (the roof is already low enough that I can't stand up).

muffinn - I hadn't really decided, but probably 16" - whatever would make efficient use of standard batt sizes.

[Static]

If you go for the polyfoam deck you dont need any cross battening.

Oh and spacing of cross battens should be just wider than the rolls of insulation

__________________
(Any reliance you place upon any advice given will be at your sole risk)

[muffinn]

So with ceiling joists insulated with 70s standard fibre insulation under good floor boards is it viable to just put polyfoam deck on top of the existing floor boards?

[Static]

If you plan to leave the existing flooring down, then an air gap may be sensible. In then end you should probably do what is recommended by the insulation manufacturers to get their guarantee

__________________
(Any reliance you place upon any advice given will be at your sole risk)