Garden Office Roof - insulation

[rs6mra]

Hi Guys,

I now have all my rafters in place with 11mm OSB fixed on to them. The finish is to be felt tiles with a felt underlay (IKO).

The rafters are 95mm deep and I plan on using 70mm foil backed insulation - Does this leave enough of an air gap between the insulation and the OSB or will i have to reduce the thickness of the rigid insulation boards? This also means there will be no space between the Celotex and the pasterboard or T & G boards that will be installed later.

Should I have laid my breather membrane before fixing the OSB boards? (I think so (hammer) ) I take it that its not too late to now fix this underneath the OSB?

Any advice or suggestions as always would be greatly appreciated. I have also included some photographs.

 

[rs6mra]

Any suggestions?

 

[RobinBHM]

I think if I was doing this, I would put the insulation on top of your osb, then over board with 18mm osb, fill all gaps with foam, tape all joints with silver tape and felt over. This would create a warm roof with no need for ventilation.

If you insulation inside, between the rafters you will have a thermal bridge at each rafter (timber is not as good an insulator as celetex). Also you will need to vigilant in filling all gaps between rafters and insulation.

If you want to fix the insulation to the inside, I'm not sure there is any need for the breather membrane. Felt roofing can be fitted as a warm roof so there is no advantage to including any breather membrane -and you arent intending to leave an air gap at this point anyway. If you fix your insulation tight to the osb, and tape around all the rafters to form a vapour barrier then this will work as a warm roof. If you want a vapour barrier can be stapled to the inside of the rafters before plasterboarding -which is the correct place for a vapour barrier (as near to the inside as possible, since you want to prevent any warm moist air reaching the cold side where condensation will form).

If you want a cold roof construction, then you probably need a 50mm air gap to be effective and you will need soffit / fascia vents (with insect mesh).