Breathable insulation between rafters

[Benguela]

Hi all,

 

I live in a barn conversion with open rafters. What I have at the moment is some kind of foil backed insulation board between the rafters... with part of the rafter exposed towards the interior.

 

Satisfactory in most of the house, but I have one tricky corner where two sections of roof meet that was never insulated and I now want to insulate it.

 

I want to shove some semi rigid breathable insulation between those rafters. Maybe something like Hemspan, but I know there are alternatives made of wood wool. My thinking is it is easy to cut, easy to shape to size, and I can fit it with friction, no fixing needed.

 

Question: once I've got the breathable insulation in between the rafters... how do I finish it? I guess I would ceiling board over the whole lot, but my question is do I have to leave an extra air gap once again between the ceiling board and the insulation? I was kind of thinking that the breathability of the insulation means that I don't need an air gap.

[Temp]

Normally the cold side of the insulation has to be ventilated to the outside.

 

Is there any membrane under the tiles and if so is it an old bitumen like material or a modern vapour permeable membrane? Any writing on it?

 

If its not a vapour permeable type there should be a 50mm ventilated cavity between the membrane and the insulation. Each rafter bay ventilated at eaves and ridge  If it is vapour permeable you can get away with a 25mm non-ventilated void. Some void is normally needed to stop the membrane being trapped between insulation and tile batten. If the insulation presses the membrane to the underside of the battens it can prevent water running down the membrane. Instead it can pool above the batten causing the battens to rot.

 

On the inside of the insulation it would be wise to fit a plastic vapour barrier then plasterboard. You don't need a void unless you want one for pipes or wires.

 

[Benguela]

Hi Temp,

 

Thanks for helping me think this through.

 

Part of the problem is that I don't know exactly what our roof construction is at the moment, except that it was re-done about 5 years ago by the previous owner. 

 

From the inside I can see the exposed rafters with plasterboard either above or in between them. The outside is pan tiles.

 

In some rooms I know it is insulated plasterboard (because we've had to drill it for electrical work) but in other rooms I can see there is plain plasterboard, with some kind of foil-faced insulation above that.

 

How can I figure out what we have exactly? Should I cut or drill into it from the inside to have a look? Or should I go onto the roof and lift a few tiles to have a look?

 

I feel like, without knowing what the roof is at the moment, I also don't really know how to improve it.