Attaching Stud Wall To Rafters?

[Geps]

Hi,

I'm renovating a 1930s semi and abit unsure about this bit....I need to put in 50mm PIR board against the (solid) external walls and then build a stud wall in front. Upstairs the rafters "cutoff" the corner of the room. I've got a picture to help understand:

//www.diynot.com/network/Geps/albums/9255/35427

Can I just use the rafters to attach my studs to? I was then going to use insulated plasterboard fixed to the rafters and use rockwool to bridge the gao, taping where necessary to form a VP.

The loft has vents in the roof itself so I don't need to worry about the rockwool blocking off soffit vents etc.

Cheers,

 

[noseall]

The 'raked' ceiling will require insulating to a high standard to prevent cold bridging and condensation related damp issues.

It is usually a case of fitting 100mm celotex etc, between the rafters as well as say a 70mm underdraw.

Once you have done this you can then fix to the underside provided you can find screws long enough.

You are ultimately trying to achieve a continuous insulation barrier from the floor up to the horizontal part of the ceiling whereby the loft insulation will take over.

 

[Geps]

Thanks Noseall, My rafters are only 50mm thick so looks like I'll need to put lots on the underside!

 

[noseall]

Also to add; you will also need an air-gap on the cold side of the insulation for venting purposes.

As with a majority of rafters they are only 75mm - 100mm deep. So 100mm plus air-gap equates to 150mm. This usually means adding a timber plant to the underside in order to gain the depth required to accommodate 100mm celotex and a 50mm gap..

 

[Geps]

I was thinking just insulate under them, leaving the 50mm depth of the rafter as the room for ventilation.

It's going to eat into my room space my rather alot!

 

[noseall]

It's going to eat into my room space my rather alot!

Life is tough.

The alternative is black stained mouldy ceilings. Not good.

 

[Geps]

No I'm happy to do it - just like as large a room as possible.

Anyway having spoken to Celotex and LABC the recommended course of action is to screw through the insulation with hammer fixings doing away with the need for a head plate. My rafters are actually 75mm so 25mm board inbetween with the 50mm air gap and then 50mm thermal laminate plasterboard underneath.