Insulating vaulted ceiling - between rafters

teaspach

Hi,

I am renovating an old cottage. One room in the cottage is converted from a dairy, and was rebuilt in 1986. I have removed the ceiling from this room, and the joists. I have replaced the joists with a vaulted type ceiling. The roof is tied at the wall plate using oak beams, and then more beams follow the line of the roof to make up an A-frame. The plan now is to nail plasterboard to the ceiling rafters, between each A-fram. I am wondering what kind of insulation I can put behind the plasterboard. The rafters are 100mm. The temptation is to stuff the gap between the rafters with insulation, and slap on the plasterboard, but I know that will cause dry rot. I read somewhere that I need a 50mm air gap between the roof felt and the insulation. Is that correct? If so, then what is the best option: 50mm rockwool/fibre glass or 50mm Kingspan or insulated board or blow in pellets? Which provides the best insulation?

Any advice really appreciated...

Vandy West

Do you have an engineer/architect on this project? By taking out the ceiling joist at wall plate level you are putting more stress on the rafter. Do the oak frames support a purlin (beam) that supports the rafter? Either way you should get the rafter size checked.

The best you could do insulation wise is leave 50mm gap (if you have sarking felt you need air space), put Xtratherm or Kingspan insulation tightly fit between rafters, and insulation backed plasterboard below rafters.

teaspach

Vandy West wrote: »

Do you have an engineer/architect on this project? By taking out the ceiling joist at wall plate level you are putting more stress on the rafter. Do the oak frames support a purlin (beam) that supports the rafter? Either way you should get the rafter size checked.

The best you could do insulation wise is leave 50mm gap (if you have sarking felt you need air space), put Xtratherm or Kingspan insulation tightly fit between rafters, and insulation backed plasterboard below rafters.

Hey, thanks for your reply. The ceiling joist has been replaced by 5x3 inch oak beams which tie the roof down at the wall plate. So existing ties have been replaced with the oak.

I'm not sure what sarking felt is. It's just regular black felt. What thickness Kingspan can I use if I have 100mm rafters, and have to leave an air gap.

riclad

I have an old,attic to insulate,clarify re air gap, is that an air gap between roof tiles,roof felt and the new insulation layer.
When you say put plasterboard below rafters ,
where exactly is that .
IS there a vapor barrior needed on the roof felt, beside
the airgap.