Another loft insulation question

[Rat]

I have recently installed insulation in my loft and am about to install a further 15cm (wickes rockwool) across the existing roof joists (spaced too far apart to floor). I have purchased 5, 3.2m 7" by 2" joists to run across the existing joists to install chipboard flooring. The flooring will only be done for the center 1.6m width under the apex of the roof.

My question is with regards ventilation. Will there be a condensation problem under the area of flooring? The supports of 7" (or 178mm) will raise the floor only 3cm above the insulation (150mm). The ends are open to air flow.

Could condenstation be an issue?

Should I:
Add vent holes to the sides of the joists after 1.6m?
Use only 100mm insulation?
Lift the insulation and put a vapour barrier under the boarded area?

Or can I get away without doing anything else?

Thank you

 

[masona]

Use only 100mm insulation?

Yes,no need for 150mm insulation,an extra 50mm would not make any difference to heat loss.

The ends are open to air flow.

No need to do anything else.

Add vent holes to the sides of the joists after 1.6m?

If you want to,then the way round it,is to put approx 3" x1" flat on top of the joists in opposite direction then T&G flooring on top which will allow air flow across the joists.

Also make sure the 7" x 2" joists are resting on the wallplate.