Loft Boarding - Clear Insulation or Crush it?

[twixx]

Ive just moved into a 12 year old home. It has roughly 200mm of insulation (2 sheets of 100mm), but suffers from the "modern house syndrome" of only having 38x75mm joists. This means my insulation sits very proud.

I'm looking to board it out. Now, conscious that the existing joists aren't the most supportive (though I'm sure they'll be fine for the occasional human traipsing around up there, and storing of Christmas decorations) in order to board out, I'm going to lay a subframe.

I have googled until my hearts content, and liaised with many people. The consensus is do not crush the insulation.

I've narrowed it down to 3 options

- lay additional CLS timbers 38x89x2400, crush the insulation and suck it up. This would give me a raised floor of roughly 164mm (75+89), meaning there'd be a potential squashing of 36mm.

- the next option is 47x100x2400 sawn treated, this would give me a total height of 175mm, and a squash of 25mm.

- finally, do I lay 47x150x2400 sawn treated, and give myself a void of roughly 25mm between insulation and chipboard flooring.


2 things;

- CLS is much cheaper and lighter, though it doesnt seem to be recommended as being as hard wearing.

- Sawn Timber is heavier, this could affect my existing joists?


I'm looking to do it "right", rather than cut corners.

What would you do/advise?

 

[Mottie]

20mm? That’s just over 3/4”. I suspect you mean 200mm? I think the recommended thickness is now 270mm if you’re looking to do it 'right'. What about those 'loft legs’. 175mm tall. And how about using interlocking loft flooring - that should spread the load?

 

[twixx]

My bad, yeah it’s roughly 200mm. May even be 270.

I looked at the LoftZone stuff and loftlegs but can save a few pennies using timber.

Question is, what timber?