Here are a couple of pictures to give an idea of existing:

Structure - https://ibb.co/8cTjDG2

Insulation - https://ibb.co/d4Bv6tv

Thanks for the response.

Is there a particular quilt that you would recommend? Should I look for a breathable option?

If it was me doing it, I would pack the studs off the wall with packers possibly short lengths of plastic pipe at 800cc fixings insulation in all voids. Vapour barrier and plasterboard.

I don’t like using sheeps wool, too expensive and there have been problems with moths and other insects as treatment seems patchy and once in they are a big problem. Inorganic best , my choice R32 fibreglass batts

@tony - I was going for sheepswool because of the breathability and wanted to minimise the risk of condensation issues. Is there an inorganic product that's comparable?

@Peter_in_Hungary - the studs are 11" apart. The advice I received on the battens was to leave an air gap between the insulation and the VCL? As I say, another company said this wasn't necessary.

@michaelf - Do you know how much mineral wool exists between the studs and as important what is its condition. Without knowing this you won't know what to plan for the target upgrade u values for the wall.

Messy as removing the lath and plaster will be, theres no point in leaving it as those studs need insulating properly.

When stripping the plaster get yourself a high volume extractor and have it pulling the air out of the room through a cutout in a boarded window opening. Weve got a 50-60 m3/min extractor while we're working in one end of an open plan living area and its keeping the dust out of everywhere except the immeadiate work area.

If the wall is to be breathable, it has to be breathable all the way through, including the felt on the outside and the insulated plasterboard. If some parts are breathable and others not breathable, then humidity can permeate into the breathable bits and become trapped.

One option would be to remove the bitumen felt and replace with a breathable membrane, but that seems excessive as you said. Another option is to use non-degradable materials like glass wool or phenolic boards, with a VCL or foil face on the inside to keep most of the humidity out of them, and at least some ventilated gaps between insulation and felt to allow any remaining humidity or condensation to escape.


Pro joiners might fix battens across the existing studs, at a metric spacing to suit modern plasterboard and collated screws. If you are DIYing you might have more time to trim each plasterboard to fit the 19thC stud spacings. We tried both ways, swings/roundabouts, but keep at it the results will be worth the effort!

Little bit controversial calling it insulation, need 100’s of millimetres of insulation and a target u-value first

No need for ventilation gap in walls. Nicer if you could use thermal board instead of plasterboard in order to mitigate the thermal bridging effect of the studs. With a good vapour barrier there will be very limited vapour to condense, only that in the void, so ensure vapour barrier is well done and joined to adjoining barriers and to windows, doors and ducts.

Yes, re thermal board, I find bitumen felt, especially if it has string in it is never a problem, as string absorbs water and breathes it out to the outside. Regs don’t require a void or ventilation in a wall! Sun will keep it all dry.

Yes , if wood is below 12 % moisture content, the usual case in a lived in house then it won’t get eaten by woodworm as they can’t digest dry wood, won’t get wet rot either.