DIY Doctor UK DIY Forums

Hi
I am after some advice if that’s OK, as I am going round in circles here as I don’t have the knowledge - however I am really trying and it would be good for me to understand. I am the landlord of a property – small terrace built around 1900. The now ex-tenant never opened a window, dried washing inside the property and didn’t put the heating on, despite repeated requests from my letting agent. Consequently, my lovely house is now full of condensation. Had a RICS surveyor go out who confirmed this. The surveyor stated he saw no evidence of rising damp, but the floors would need to be taken up to confirm which I intend to do. In his report he states:

Internal repairs should include replacement of spoilt plaster coverings; consideration may be given to introducing insulated plasterboards to improve thermal performance of the walls and reduce the impact of low surface temperatures.

When re plastering, appropriate plaster coverings should be applied. If significant salt contamination remains in the masonry, typically trades prefer using a cavity membrane or salt inhibitor to protect new plaster finishes.

Recommended Actions:
• Remove spoilt and damaged dry-lined coverings – consider replacement with insulated plasterboard
• Replace salt contaminated plaster to lounge party wall

My letting agent had plasterer’s go in to quote for removing the wet plaster and/or heavily salted plaster and replace. That’s fine – were I am confused is regarding the internal insulation as I have read in houses of this age, that due to them being solid walls, internal wall insulation can cause more damp problems – and I could really do without introducing more damp! This is the email from my letting agent. Can anyone let me know if what is being suggested is adequate and would solve the problem?

Many thanks for any help!

I asked the plasterer about the effects of using insulated boards, or installing internal insulation and then plastering over, on the ability of the wall to breathe. The plaster did think this would be a problem, particularly as the gable wall is made of engineering brick – which has very small mortar gaps and is impermeable, so doesn’t breathe much anyway. He has suggested knocking off all the affected plaster and installing pressure treated timber battens directly to the brickwork, then screwing the new insulated boards to these before a plaster skim. This would give the benefit of some insulation and leave an air gap behind the boards to allow air to circulate.

On the party wall side, they are suggesting removal of all the affected plaster and then reboarding in the same way (with battens, but normal plasterboards) which should prevent them bridging the existing DPC. They did take out a piece of board from the wall where the kitchen door handle had already damaged it (the tenant had slammed it against the wall). The plaster board was very wet, but the brick wall behind was dry, suggesting its not rising damp but condensation). Upstairs, the only badly affected plasterwork appears to be in the alcoves either side of the chimney in the front bedroom. These areas would be treated in the same way as the gable wall in the kitchen.

The firm wouldn’t be offering any warranty on re-occurring damp issues (the only firms that would are the specialist damp companies) and they are a small two-man outfit (not VAT registered). However, we have used them before and had no complaints. They have quoted £1200 for the work downstairs and a further £600 for the bedroom upstairs, inclusive of all materials and waste disposal.

Hi girlie26,
You have my sympathy trying to sort out this problem of condensation. Unfortunately, if you have tenants who have a lifestyle like your previous tenant, you will always have condensation problems. Ventilation is one solution but generally tenants don't like it and will make moves to seal it off or switch it off.
The suggestion of fitting battens and then insulated plasterboard can lead to issues of ponding at the bottom of the wall because ventilation between the battens will be from inside the house and condensation will form on the inside of the outside wall and run down it.
Gable wall - if the wall is dry-lined, remove this and fit insulated plasterboard direct onto the wall using sticky foam making sure the integral DPMs are sealed together. It's probably worth sealing the outside of the wall with silicone solution to limit rainwater ingress.
Party wall - if the surveyor saw no evidence of rising damp it is difficult to understand why a party wall should have salts coming out of it. It sounds like it is a plastered wall (rather than dry-lined) so clean it and repaint it. If you are on a hill and the floor the other side of the party wall is higher than yours, or you neighbour has a bathroom against the party wall, this may be why there are salts appearing.
There is no economic and guaranteed solution to your problem, yes, you could insulate the outside walls externally but this is big money.
You should also bear in mind that the uprating of the EPC rating of rented properties has not gone away. The current requirement is E but the new legislation which has recently been put on hold was for C - this may affect the choice of insulation you specify.
Regards S

Thanks so much for your comprehensive and very useful reply Stoneyboy. And thanks for your kind words too - I am so upset at the state of the house, it's going to be a costly refurb, and all just for the sake of opening a few windows!

Party wall - you are quite correct the house is on hill. I'm at the bottom. I have to say though, I have owned the property since 2006 - and this is the first time I have had a problem with the party wall. I have checked about the bathroom for next door, and from what I can ascertain, it is not against this party wall. Next door's chimney is on the party wall side - I don't know if this makes any difference as they don't have an open fire - chimney breast all boarded up. The surveyor suggest the salts were down to bad plastering 'Whilst the staining has the appearance of rising damp, issues can likely be attributed to salt contamination of the newer plaster. Replacement plaster was apparently poorly specified and/or applied, hence the issues of contamination.'

The gable wall is dry lined using dot and dab. I note your suggestion of sticky foam as opposed to the pressure treated timber battens suggested by the plaster due to ponding. I have read that sticky foam can allow damp to penetrate. Is this not the case?

I have read another solution could be to render the wall with a hard motar mix - 3:1 - with a waterproofer in it. The render, when dry, could then be skimmed with a finishing plaster coat. This would keep the damp within the wall. But if I have understood this may be no good for epc rating. Property is currently D rated.

As regards the external sealant, would something like this be any good?
https://www.amazon.co.uk/DRYSEAL-PENETR ... B007FDWF6M

Hi girlie26,
Party wall - very unlikely to be down to bad plastering or application, you have eliminated the shower room theory, it’s most likely to be the higher floor levels and no vertical DPM on your neighbours side. Get the wall below your neighbours floor level injected with silicone fluid then use the strong render mix with integral waterproofing solution.
Gable wall - solid walls are difficult because there will always be more permeable areas/courses where rainwater can get into or even through the wall. If you use one of the clear waterproofing liquids this may limit the problem, the product you suggested is not suitable.
Whichever method you use for board fixing it will be a compromise. Sticky foam is the least likely to degrade under damp conditions.
Rendering the wall internally with a 3:1 mortar mix will have its own problems because the wall is likely to move and cracks will appear in the render which will be fissures for water to exit the wall, usually forming salt spurts.
Regards S

Thanks once again for all your help Stoneyboy- really appreciate it!