Hello all,
New to the forum and just wondering if I could get some (urgent) advice on a dilemma please. Before I get into the detail – Apologies if some of this is a little excessive. Just wish to provide as much information as possible to help with the decision making process.

First of all, about my home. It’s a mid-terraced property, not exactly sure on the age but I’ve been advised it may have been built around 1920, maybe even earlier. Solid red brick walls with no cavity. Had external wall insulation fitted a few years ago. Now onto the dilemma:

I am in the process of renovating my front room on the ground floor. We have knocked down the wall to bare brick and now planning on the plastering phase. However, there are a few areas of concern. Firstly, there is one area which is slightly damp due to a leaking chimney which has now been resolved. I guess this will take a while to fully dry out. Another is traces of moisture in the mortar, three rows/brick levels from the ground up (hope that makes sense). It seems to be the case in many properties on our street. Unsure of the cause but a few builders have stated it’s just the way the houses were built or a failed damp proof measure. Which could be timely and costly to resolve. I wonder if it could also be due to the fact that these houses previously had wooden floors which were removed due to rot and a concrete layer was applied. Its possible correct measures weren’t followed during this stage. One thing I have noticed is an air brick visible outside the property at pavement level, partially paved over too! Internally this air brick is not visible as I guess its covered by the cement floor. Several neighbours have tried various damp proof treatments followed by renovation plaster, bonding and finally skimming the walls. This has helped some of the previous damp issues but unfortunately, signs of damp in certain areas reappear on the lower parts of the walls.

For the reasons above, I would like to go with fixing graded roof battens first followed by plasterboard. However, I have read about the risk of condensation forming in the space between the board and the bare brick. Should I be concerned about this? The articles I have read suggest installing some form of insulation and vapour barrier but the detail is brief and there is no further information of precise products to use. Is it an absolute requirement to install some form of insulation? Would expanded polystyrene such as the following suffice:
https://www.insulationshop.co/20mm_white_polystyrene_board_for_external_wall_insulation.html

Other questions I have are:
1) Do I need to apply any sort of membrane/sheet material on the bare brick (at lower level) to prevent the moisture from coming into contact with the timber/plasterboard, etc? Or should I just use some form of plastic spacer/trim to prevent the timber from coming into contact with the wall? One suggestion was to use something like the following - https://www.toolstation.com/rhinovent-pro-breathable-membrane/p92016

2) Should I treat the graded roof battens with wood preservative? Or as they're designed for roofing and areas prone to moisture, will the battens be ok as they are.

3) Before fixing the plasterboard, must I fit another form of membrane/sheet type material. I have read that it is a good idea to fit something to form a vapour barrier. Is that the correct method? If so, which material/product can I use?

I think that should do it for now, hope it all makes sense. Apologies again for the long post and all the questions. Just want to ensure I spend both time and funds wisely and complete the work in a correct manner.
I look forward to any suggestions and would be very very grateful on receiving any advice.
Many thanks – Sufyaan :)

Before doing anything I would suggest you find the cause of the damp. From what you say it sounds like the ground outside has been built up over the years to cover the air brick which would have been about 6" below the DPC. If you have a failed damp proof course it is relatively easy to put right and if you are handy is a DIY job hiring a machine from a hire shop and injecting the brick with a DP liquid.
I would be very cautious about trapping in damp regardless of the technique employed to protect the dry lining. If you cannot solve the damp then my approach would be to plaster certainly the lower metre of wall with a lime plaster. That is what would have been used on the house originally I reckon.

How does the outside level compare to the internal floor? That might give you a clue of what is happening. Are you absolutely certain you do not have a cavity wall? I had an 1890's house with cavity and builder told me it was impossible, that was until I had taken a brick out and showed him.

Hello and welcome to the forum.

Posted By: SufyaanP84First of all, about my home. It’s a mid-terraced property, not exactly sure on the age but I’ve been advised it may have been built around 1920, maybe even earlier.

Do you have and/or can you get hold of the title deeds? They may show the age. Old maps/photos/council records are other places to look for information.

Another is traces of moisture in the mortar, three rows/brick levels from the ground up

Is it just the third row (the mortar above or below the row BTW?) or is it all the mortar below that row? How much is traces? Is it everywhere or just in patches? Measured, felt or seen to be damp?

As revor says, finding the source of the damp is the first step before taking any action. Where is external ground level in relation to the damp? Is it possible to excavate a bit outside to get a clearer view? Is the EWI normal rendered EPS? Is it in good condition and does it cover the affected area? Where (how high) was the original wooden floor? A diagram may help to explain.

For myself I would prefer to plaster the wall with lime plaster rather than use any membranes or plasterboard. It will help to make the wall airtight, which is good, and will allow moisture to evaporate through it if necessary.

BTW, I'm not sure why you would want/need to use graded roof battens rather than ordinary treated battens if you do go down that route. In any case they have been treated during manufacture so there's no need to treat them again, except for cut ends of course.

From your description it sounds like you will be 'renovating' (adding or replacing a layer to) more than 50% of the thermal element (in this case the wall) in the room you are stood in. The target U values in Approved Document L1B of the Bldg Regs will apply. get -out clauses may apply, but if my understanding of your situation is correct you will have to pay the fee (£150 where I live) and argue the toss if you cannot achieve a U value of 0.3W/m2K.

The issue you are looking for (or rather to avoid) is interstitial condensation, caused because as you warm the room up you cool the wall down, so the dew-point (the point at which water vapour becomes liquid) moves in, to a point where it can grow mould, rot out joist ends and timer lintels, and associated nasties.

I would also not use battens on the cold side.

''You caught that there's EWI already installed? So there may well be less of a concern about the U value.''

Ooops! No. Ignore that, then!

Thanks djh.