Hi all! I have just started renovation work on my 1920's house and am after some advice on insulating some stud walls in our attic/loft space.

The top floor of the house, in the attic, has stud walls separating the rooms and stairwell from the rest of the (unheated) roof space. These walls are original lath and plaster over wooden studs. The roof is clay tiles with a felt underlay.

My plan is to fill the space between the studs with flexible woodfibre insulation and then fix rigid woodfibre over the whole lot to help increase the insulation and hopefully limit any cold bridging.

In this scenario do I need to include a VCL and where would it go in the build-up?

I've attached a top-down plan of the wall in case my description above isn't clear!

Thanks in advance!

You have not mentioned plaster. Is that because that's too obvious?! You want an air-tightness layer at very least, which could be plaster, primed and taped at all joints and perimeters, or perhaps Intello and tape prior to plasterboard finish. To be certain, I would suggest you get a WUFI interstitial condensation risk assessment. I am not wanting to advertise but if you were to buy your WF from a merchant with the initials B t E in the west of England you will get the WUFI for free. You should be OK assuming the void beyond is vented as it should be, but safe is always better than sorry.

If you are 'renovating' more than 50% of the thermal element (in the room) you will need BC approval, which means you also have to meet the U value target, or prove why you cannot. The sloping ceilings above need a U value of 0.16W/m2K, while the wall only 'needs' 0.3. I try to get as close as I can to 0.16 (or even better) on all surfaces. You'll only do it once!

Edit: Sorry, you asked where the VCL would be. Well, as above, (a) you may not need one and (b) if you do, it's immediately behind the plasterboard/final finish.

Aha! Julio posted while I was typing!

Posted By: JulioVCL wants to go on the warm side or near the warm side of the insulation ideally. What is access like, tight I imagine.

Yup, the VCL needs to go on the warm side of the insulation.
Perhaps the easiest way would be to put the VCL on the inside of the lath and plaster and then plasterboard over.

Duplex plasterboard has built in vapour barrier in the form of foil layer on the back side.

There seems to be an assumption that a VCL is required in this build-up.
With the vapour open materials noted on the the OP's drawings, if the loft void is ventilated, which is likely in a 1920's house, then I'd be confident NOT to use a VCL. But as is often mentioned on GBF, suppliers will often do a WUFI calc for you, to confirm interstitial condensation likelihood.
Air tighten of course around edges/interfaces with plasterable tapes etc.
The more challenging insulation zone is the inclined ceiling (in terms of depth available and maintaining the ventilation void to the felt, from eaves to ridge.

Posted By: Peter_in_Hungaryplasterboard ceiling

Plasterboard can have many different properties from vapour-open to vapour-blocking, so I think you need to specify more precisely what you mean.

@PiH - That principle of vapour removal from house into ventilated loft has been around for a long time, so yes I would agree with that. Like wise through a suspended timber floor with glass wool below.