After over 125 years of weathering how porous or permeable are the bricks themselves?
From personal experience I can recommend Stormdry masonry cream. It was easier to apply than I imagined.
As it is invisible when dried it is suitable for spot treatments without giving the wall an odd appearance.
Your comments are really helping me build a clear picture here.
If it weren’t for knowing the plaster is so affected and the seasonal temperatures so extreme in this room, I’d opt for new window, fix the pointing, storm dry on the masonry, add a vent and not bother with the vapour open lime plaster -
But despite not seeing the mould on the walls , you can smell it .
Re ventilation and Aereco - what type of vents are these .
I have a single room Vent Axia mechanical heat recovery vent which we bought for the bathroom, but didn’t fit yet because of structural disruption.
I’m wondering about using and fitting this in the bedroom or would it be too much ?
Re ventilation throughout the house, I think single room vents and open windows are what we’re looking at . Re heating - eventually it will likely be a heat pump or single room heaters - we are insulating and airtighting where we can. On the ground floor , under floor insulation is something we intend to complete eventually after a damp course fix. The mortar pointing at sub floor level is very crumbly too and I wonder if / how we fix this before we fit any insulation under the floor.
The VentAxia in the bathroom permanently running at a low level for background ventilation (and boost mode on humidistat or on the light switch) will be a big improvement for the house as a whole. Basically you need background ventilation to effectively or best deal with humidity build up.
The Aereco we have are the EHT in wall vents and we have a VentAxia extract Tempra fan in the bathroom (similar to your unit but without the heat recovery). So basically a background extract fan such as these Vent-Axia units and in wall room Aereco vents work well together. The Aereco vents provide the in coming fresh air (passively humidity controlled) to balance out the air extracted by the Vent-Axia. I would keep the Vent-Axia units to just the wet rooms, ie no need to fit in the bedrooms. Whereas the in-coming air is needed to control that balance between in/out air-flows.
I agree with @zapaman for a possible long term solution but you already have the MVHR unit. You could fit that in your target room, possibly moving it between rooms in the future. I seem to remember that you bought the higher powered and diameter model, which makes swapping out with something else a bit of a problem. That model is really for a large or damp room. Hopefully after your retrofit you will end up with a small dry room, so the prospect of reducing the diameter of your vent and making good might put you off the temporary solution.
My preference is for suitable MVHR whenever practical, with no other permanent vents (in or out). If that is your aim then you will need a smaller unit for the smaller room.
Edit: oh dear. I see you bought the MVHR in November 22. Definitely time to get it fitted. But bathroom or daughter’s bedroom? That is the question. Even having it in the bathroom will help a little in the bedroom, as some of that condensation may well originate as vapour in the bathroom. Given the system specification I think it should go in the bathroom asap and then go for a smaller unit in the small room.
Could you make the external wall a trombe wall?
I dread to think what’s behind the plaster board. My first guess is very mouldy Victorian wallpaper on the original plaster.
You definitely need forensic suits and breathing equipment to clear the room.
I love this idea! I had to Google it and found the following link.
Our gable end wall is south facing, but it’s externally covered in a network or piping and wiring . How would the heat in the cavity affect all of this ?
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/trombe-walls#:~:text=The%20Trombe%20wall%20is%20a,enter%20the%20indoor%20[4].
I’ve been wondering if someone might like the larger Vent Axia I have and swap me for a smaller one .
And buy an Aereco for the bedroom.
Definitely will have mask and coveralls. We don’t want to breathe in any nasty particulates.
I realised another potential problem - the ceiling in this room is also plasterboard with a moulding attaching attached. Above that is 30 cm of sheep wool.
If we’re going to all the trouble replacing the gypsum wall coverings with lime - what do I do about the ceiling. The old lath and lime plaster ceiling is completely gone on this side of the house. What is our best approach with the ceiling?
I’m starting to wonder about the lime with aerogel board or clay board options mentioned.
@Tim_Gilbert
@zapaman
@Clive_Durdle .
When I think about costing and how much more labour intensive one option might be to the other - versus cost of materials ??
On another note , I went to visit my neighbour last night. Her house still has all the original lime plaster upstairs. I went into the same little bedroom with the same external wall situation as ours. Her bedroom, also with a storage cupboard ram jammed full of assorts in plastic bags and boxes is as dry as a bone. She stated she never has had a damp problem . In fact - her upstairs was noticeably, far more temperate and comfortable than ours.
I couldn’t have been more impressed and I’m certain that vapour open lime is the way to go.
Other things I may have overlooked .
This bedroom was originally part of one large room with two windows, which was split into two rooms by a stud wall also covered with gypsum plasterboard.
In my mind , the plans for the room were to replace only the vapour closed gypsum plaster wall coverings on the two external facing walls - then it occurred to me - will we actually create a thermal envelope by only changing the wall coverings of the external two walls and the ceiling? 
Are there problems or benefits still to be had with our focus area?
When you installed the insulated ceiling did you cut the aligned wall plaster back to the brick (to reduce thermal bridging) and run your vapour control layer down the exposed brickwork. No? I would probably not have done either.
I fear that you are going to need to disturb the edges of your ceiling. Any IWI can then be taken up to the top of the wall and the two insulations overlapped and abutted to remove thermal bridges. The old plaster needs to be removed anyway, unless you believe that there is no dampness that high on the wall.
Changing your proposed insulation and/or wall finish won’t alter the above.
It is fortuitous that you were able to visit the same room in the “same” house in a different time zone. I take it from your description that it is also split be a partition wall.
You may have the answer by copying the past or maybe you could be inspired by the past for the answers of the future. Perhaps a lime based plaster with integral insulation? You could look into Diathonite.
Hi Carla, we had a similar small bedroom, with South & East solid brick 1910 external walls, one party semi neighbours wall & one internal lath & plaster. Gaps round window where sashes had been removed & some pointing issues.
Was definitely the coldest room in house in winter.
But we didn’t have persistent damp or mould & mostly the original lime plaster was in good condition so were able to retain it.
Didn’t want to loose much internal space as room only 2.7 x 2.8m.
Our retrofit assessment recommended a target u value of 0.4 W/m2K .
We compared prices & overall build up thickness for PIR, aerogel, wood fibre & cork.
Aerogel spacetherm wallboard was the thinnest but is very expensive & not easy to fix/work with (we don’t have a nail gun!)
PIR/plasterboard is next thinnest but with cost of battening & membrane the price mounts up.
Cork & woodfibre/lime plaster are similar build ups but cork is more expensive.
We’ve been trying to use breathable natural fibres where possible so went for 80mm wood fibre (stieco therm) with lime plaster (Adaptavate) & mechanical fixings (ejot) (basically a hybrid of the ecomechants/back to earth/Ty mawr recommended methods).
We had a new triple glazed window and sealed & insulated the lintel, sill & reveal with a combination of tape, diathonite, Pir & plasterboard to fill up the ‘sash boxes’ (ok, not so natural or breathable!)
We couldn’t find any window manufacturer who would supply without trickle vents either.
We used parge coats of diathonite (~2mm thick) around floor joists to ceiling below and above up into wall in loft. The woodfibre continued up through ceiling into loft about 100mm.
When we did the loft we similarly parged around edges & left excess intello which we were able to bring down and join with tape/plaster to the wood fibre.
Around perimeter of external walls we infilled between joists with natrahemp batts approx 600mm into room under floor.
If/when we insulate walls in room below we should hopefully be able to join up contiguously through the diathonite layer.
We did the work ourselves except for the lime plastering, cost of materials was around £700 inc. woodfibre, fixings, tapes, plaster, clay paint, delivery & government diy insulation penalty charge.
It was completed just last month so haven’t had a winter yet, it’s been so hot in last week that hard to say if we’ve cracked the decremental heat - but the room does have a good ‘feel’ about it!
Hope this helps. D&H
That looks like a well researched and planned job. Well done. Please post an update in the spring, if not sooner, with your winter experience.
I think we might have to look into replacing the lime plaster base layers ourselves.
I’m not confident we’d be good with the final finishing coat though.
I briefly mentioned brise soleil above. If summer overheating remains a problem this link could be useful.
Thanks Tim. I’m rather delayed responding to all this great feedback, but I I did take a little look at this and just read some more .
About the Brise soleil - looking at it I read about orientation and how the the effectiveness of brise soleil can be reduced - our front is approx 130 Southeast facing.
*Horizontal brise soleil ‘shelves’ will not work as well on east or west elevations as the angle of the sun is lower ― sunlight will pass underneath. In these situations, a system that uses multiple vertical blades in front of glazing may be more effective.
Another factor is when it’s hot in the room in question. Even throughout the night when it’s considerably cooler and the sun has long before gone around the side to the back, the room remains sweltering.
I think the bricks are heated through the day and then release the heat into the room throughout the night. I wonder if improving the u-value of the wall with good lime parge layer / lime cement and lime finishing coat lime will be sufficient to enable a more temperate room - or if wall insulation may also be needed.
A brise soleil across the window would help somewhat too it seems.
The ceiling in the bedroom is also just flimsy layer of plasterboard, with 30cm of sheepwool on top. I’m considering lowering the ceiling to add in more insulation too.
Interestingly and a boost to my confidence around replacing the gypsum with lime , was a recent trip to my neighbour who still has all her lime walls in tact - there is no sign of damp and the rooms are lovely and temperate. It has convinced me to restore the lime walls as completely as I can.
Indeed her house and my house together are the perfect example of what goes wrong when a Victorian house built to be breathable has the layers replaced with non breathable and moisture becomes a problem.
Her home is my envy !!!
Although a brise soleil is usually associated with windows, I know of one family that fitted one along their entire south wall. The brickwork remains cool. The slats of a brise soleil can be vertical if you prefer but should pass left to right rather that to and from a window.
You can build shades for east or west facing walls/windows and they will have some effect but if you have serious overheating then vertical blinds will be best. Something looking like a Venetian blind would be good for windows and vegetation can shade the walls. In winter the blind can be removed but vegetation is generally fixed.
Lowering ceiling height will help make a room warmer, whatever the time of year, however purge ventilation to expel hot air at night will be improved.
I have nothing to add to your lime plans, apart from “
”.
I am about to fit a shade for my WSW back wall. [Edit: ground floor only.] To avoid thermal bridging it will not be fixed to the house but freestanding 20cm from it. The depth will be 1.75m. That is the length of the solar panels that I’m going to fit above my ground floor windows.
As you can see, I’m killing two birds with one stone. I should get an extra kW output, to contribute to my heat pump requirements.
That is a great plan Tim ! I love it 
