Posted By: jackmccabeI know a parge coat is usually recommended but considering the unevenness in the walls it would be quite costly with materials and time.

FWIW a parge coat on a rough surface doesn't need to use that much more material as long as you don't care about its appearance. You can just follow the roughness.

I was thinking of putting a breathable membrane behind the timber battens directly against the brick, is that a bad idea?

It sounds OK to me, as long as there are no sharp bits on the wall that might poke holes in the membrane.

Or nowadays perhaps one of the airtightness paints.

Thanks for the suggestions, looked at airtightness paint but worked out crazily expensive.

Think we'll go with the membrane as it is just a lot easier and quicker right now.
We've already put quite a few battens on the wall and pretty easy to take them off and just screw back over the top vs a parge coat.
Also walls are very dusty and crumbly which would probably need sorting for a parge coat.

It's just one floor we are currently working on but I will look to parge coat the next floors when we IWI them.

I was not going to wade in on this one, but I will, to back up or follow on from Jonti's comment. @jackmccabe, you have already said the wall ''has areas where the brick work is in a bade state and where windows have been bricked up". While I accept that an airtight membrane is likely to be airtight (!) I would, particularly on a wall as you describe, be very wary of it not being sealed at the edges. If it is not you run the risk of thermal by-pass which could, at worst, be about as bad as if you had not put on an air-tightness layer. @djh and @tony are right - the parge coat does not have to be flat. You'll have to make adjustments to your battens whether or not you have a parge coat.

If we cannot persuade you away from the membrane can I make a suggestion? Make a 'picture frame' of parge coat around the perimeter and, using an approved primer and good quality air-tightness tape, tape the membrane in to the 'picture frame'. You then also have an 'air-tightness edge' to work to when you come to IWI the rooms below or above.

Incidentally have you seen the latest (English - don't know where you are) gov't guidance on IWI? I don't agree with everything they say but their suggestion, where you are *not* using a vapour-open insulant such as wood-fibre, is that there should be a *min 25mm ventilated* cavity behind the insulant. I have always understood that if a ventilated cavity is left it should be min 50mm. I have not yet come up against a BCO invoking the guidance but it may come. Have you had a condensation risk assessment done? (If I am teaching you to suck eggs, please accept my apologies!).

I definitely agree with all your points although at this point we have put all the timber battens on the wall, so parge coating would be even more difficult at this stage.

We are doing the insulated plasterboard on 25mm battens as specified by Kingspan, who did the condensation risk analysis. They did not mention anything about a parge coat or any air tightness membranes so it was sort of a last minute thought.

As for the issues sealing the membrane I will make sure it is well taped and it will be taped to the ceiling/roof membrane and I will use blowerproof to paint down between the joists.

Unfortunately a lot of issues arise not just due to lining builders up and schedules, but the fact they look at me like I am insane when I say a lot of these things. I will probably look to do the rest of the floors using breathable insulation like wood fibre and get someone in who is a bit more aware of the kinds of things I am trying to do.

Thanks for all suggestions and help.