raising joists for loft insulation

[coolpran]

I know this question has been asked before, but none of the answers seem definitive.

I want to insulate my loft and still be able to use it for storage.
The current joists are 100mm high X 50mm and I want to increase their height so as to fit in 270mm of insulation and board on top of it.

I am thinking of fitting these 175mm X 50mm wooden planks (http://www.diy.com/diy/jsp/bq/nav/nav.jsp?action=detail&fh_secondid=9276000) at 90 degrees to existing joists.
Fixing these with angle brackets at regular intervals (http://www.screwfix.com/prods/61576...Angle-Bracket-Heavy-Duty-50-x-50mm-Pack-of-10).
Then I am going to put 1 layer of 100mm insulation between existing joists, and another 170mm on the raised joists, leaving about 5mm gap for ventilation/wiring.
On top of all this, I will be fitting standard chipboard (http://www.diy.com/diy/jsp/bq/nav/nav.jsp?action=detail&fh_secondid=9276437) with screws.

The span of the roof is 4.8m (W) X 9.5m (L) and the joist run lengthwise, i.e. 8 joists at ~60cm gaps covering the 4.8m width.
I will try and attach pictures of the roof truss and joists for clarity.

My questions are
- is the recommendation to raise the joist at 90 degrees or along existing joists?
- does raising the height by 175mm seem excessive considering I am going to board on top? can I get away with raising by 100mm only?

 

[JohnD]

cross-battening at right-angles to the existing timbers adds more rigidity and spreads the weight better, it also enables you to lay the new insulation across the old so there will not be joints on top of each other. Lay the battens to suit the width of your boarding, and screw then all together.

you can also lay rigid foam slabs, and lay your flooring on top, instead of cross-battening. I have not tried that method myself, though.

You are looking for a total depth of insulation of 200-250mm

If using rigid foam you need a bit less depth than if just using fibre quilt.

 

[coolpran]

thanks JohnD.

what is this rigid foam? is it same as Knauff space board ? do you have links to this product?

is fibre quilt same as glasswool, i.e space combi roll (http://www.diy.com/diy/jsp/bq/nav/nav.jsp?action=detail&fh_secondid=10186453)?

sorry, this is my first time doing a loft and am not used to the lingo.

 

[JohnD]

yes, space board is one expensive sort, you can also foam in 2440x1220 slabs and cut it yourself (about £25 per slab IIRC if you get Exp polystyrene slabs)

I have not got a link

Fibre is glass fibre, mineral wool, or (my favourite) the white stuff made from recycled plastic bottles which does not make irritating dust or fibres lodge in your skin,, eyes and lungs.

 

[coolpran]

thanks for the advice guys.
i will calculate the costs for using space board or cross-batten supports and take the decision

 

[PaulJR]

My concern with your plans is that all this extra weight is being carried by your ceiling joists.

The upshot of adding the joists and chipboard will be your ceilings will sag and crack.

The chipboard is a bit on the thin side for a 600mm span.