Installing PIR boards in Rafters - advice sought

[Tim_B]

Hi, I am looking for some advice on insulating my loft.  I want to put PIR boards to cover the rafters.  The rafters are only 73mm deep so in order to provide the regulation 50mm air gap it seems best to just fix the boards straight to them rather than try and infill. 

I assume screws are the best way to fix them to the rafters, If I am using 100mm thick board how long do the screws need to be for best purchase? I had assumed 110mm screws would be OK.

There are quite a few longitudinal binders in the way, I assume it is OK to just cover them in the board as they aren’t too thick.  I don’t imagine its safe to remove them, unless the boards would replace that stability?

The house is a terrace with exposed breeze block at the gable ends.  Would I also need to clad this?  If so can I fix straight to the wall or does this need an air gap too?

Finally the floor has fibre insulation between the joists which extends to the roof membrane.  If I then place board over the rafters would this mean that in fact there is no air circulating?  Not sure if this closes the air gap I am making for the boards.

[Radian]

Why do you want to put PIR against the rafters? You've effectively trying to convert a cold (ventilated) roof into a warm roof and that's not trivial. To achieve beneficial insulation for the house it's going to be far more cost effective and safe (for the fabric of the roof) to simply add insulation to the floor of the loft.

[joe90]

You are far better adding insulation to the existing , creating a warm roof far more tricky to get right.

[Temp]

20 minutes ago, Tim_B said:

Hi, I am looking for some advice on insulating my loft.  I want to put PIR boards to cover the rafters.  The rafters are only 73mm deep so in order to provide the regulation 50mm air gap it seems best to just fix the boards straight to them rather than try and infill. 

 

The issue is the 50mm void must be ventilated at the eaves and ridge. Ventilating at the ridge means every rafter bay as there is no lateral air flow due to the rafters.  So the best solution involves replacing the ridge tiles with a ventilated ridge rather than individual vents.

 

Some care is needed at the eaves to ensure the air goes up the 50/75mm void rather than into the floor or the loft itself which you are trying to keep warm... but it's doable.

 

I would use 125mm fixings for 100mm insulation.

 

Under the insulation fit a vapour barrier (plastic sheet) or possibly foil tape the joints in the foil covered insulation. Then plasterboard.

 

 

[Radian]

1 hour ago, Temp said:

So the best solution involves replacing the ridge tiles with a ventilated ridge rather than individual vents.

Unless the roof has a breathable membrane between rafters and battens, but somehow I suspect we are talking about an older, bitumen based roofing felt here. Out of interest what alternatives to ridge venting are there?

[nod]

Your going to cause your loft to condensate There will be so much that you will think your roof is leaking