Loft Insulation

[MG2000]

I have 70mm joists in my roof space, and my insulation is about 120mm. I have been given several options by a builders merchant, as i want to board it out, so any guidence would be most appreciated.

1. Take out existing insulation and put in 70mm of xtratherm or similar and then place chipboard on joists.

2. Leave existing insulation in and put 50mm of xtratherm on top of it betweeb the joists.

3. Lay sheets of xtratherm over joists (i.e joists go north to south, so place sheets east to west) and then place chipboard on top.

Should I be using 18mm or 22mm chipboard??

Any ideas?

 

[Chukka63]

i think out of those options id go for option 3.
thickness depends on joist spacings (regarding minimum you can get away with)400 mm spacings you could get away with 18mm.600mm spacings,you would wanna go for 22 mm