Loft insulation: replace or lay on top?

[Poiup]

Last year we bought our first house and the survey we had mentions the following:

The roof is insulated with mineral wool quilting of variable age that in general is no more
than approximately 100mm thick and in some areas is untidy, so there is scope for
improvement. To meet current standards insulating materials such as mineral wool
quilting should be laid between and across the horizontal ceiling joists to an even
overall depth of at least 300mm, taking care not to block any ventilation openings or
cover any recessed light fittings.

Before the winter comes I want to improve the situation but I was wondering what would be the best way to go about this.
The loft is quite filthy and the mineral wool seems to be quite old and very uneven. If I disturbe it significantly, it almost disintegrates. I was therefore thinking to bite the bullet and replace the lot but I am wondering, would it make sense? Should I just add a new layer on top? Potentially I can borrow a thermal camera from work to see where more insulation is needed.
Also, I have some downlights that, according to the manual, can be buried in rockwool so I was thinking to use that instead of mineral wool.
Another question is: do I have to put 2 layers perpendicular to each other's or can I just lay one layer on top of the other in the same direction? I guess two perpendicular layers would be preferred but will also make moving around in the loft very hard.
Any suggestion?

 

[blup]

Any additional insulation will help, I took the old stuff out and cleaned up all the muck underneath, brick dust, wood shavings etc that had had accumulated over the years and following rewires and the installation of new central heating (plumbers see voids as informal trade waste receptacles).

Just getting rid of all that (commercial vac) improved the atmosphere no end.

Blup

 

[Poiup]

Any additional insulation will help, I took the old stuff out and cleaned up all the muck underneath, brick dust, wood shavings etc that had had accumulated over the years and following rewires and the installation of new central heating (plumbers see voids as informal trade waste receptacles).

Just getting rid of all that (commercial vac) improved the atmosphere no end.

Blup

So your suggestion would be to clear all that old stuff and replace with new insulation instead of just adding one layer on top of the existing one

 

[Mr Chibs]

If you don't want to be that thorough, just add on top of what you have.

It won't make any noticeable difference if it's laid perpendicular (or not) to the first layer.

Try to have cables on top of insulation.

You can cover lights if they are the correct type for this.

 

[blup]

So your suggestion would be to clear all that old stuff and replace with new insulation instead of just adding one layer on top of the existing one

Either.

Blup

Edit.

I added a membrane to the underside of the rafters which succeeded in stopping the fine layers of dust that had accumulated over the years

 

[JohnD]

You say it is filthy.

How old is the house, and is the roof felted or can you see the underside of the tiles?

If it is not felted it will get dirty again.

Old loft dust, and the fibres from yellow fibreglass, can trigger athsma attacks so I would want it cleaned out for good. The modern mineral wool, if treated with Ecose (will be marked on the wrapper) does not shed irritant dust and fibres.

There are ways to floor a loft despite overlays of insulation. I used counterbattens. Decide this before adding more insulation.