External Concrete Plinth - Should I Remove?

[HandSlander]

Moved into this c. 1900 Victorian Terrace a few years ago, and although I haven’t had visible damp internally, I’ve had major humidity issues with certain rooms constantly well over 70% humidity even shortly after thorough ventilation.

The property is an end of Terrace, and all external walls have a c. 150mm height render as pictured with spaced air bricks, it appears to be painted cement. Reviewing my legal pack from purchase, it appears that “damp proofing” was done in 2010 by Kenwood plc, consisting installation of 4x air bricks, full length “vertical damp proof course” and in the case of what is now the most humid room, “tanked” walls on the full length (of that room only).

My question is, should I remove this cement render from the brickwork to help reduce humidity and moisture? I’m not sure if a Victorian (slate?) DPC exists in the brickwork underneath. I don’t want to worsen things but this humidity issue needs to be resolved, I’m sick of running a dehumidifier 24/7! Also advice on clean/easy removal appreciated, I have a respectable arsenal of DIY tools including an SDS Hammer Drill...

 

[noseall]

No.

 

[HandSlander]

No.

Do you care to elaborate on this?

Perhaps I understated the problem I'm facing. My 2yr old son's room (the pictured one with bay window) has a near constant ambient humidity level of ~70-80%, with daily condensation forming on all of the single-glazed window panes, significant mould formation and with evident paint degradation and damage to the wooden sash windows due to all the moisture. Clearly this is not healthy, for my family nor the building. I've eliminated all internal sources of moisture. I believe the internal walls were re-plastered by a damp proofing company in 2010 (Kenwood) which I'm guessing is keeping the moisture out of the internal wall surface and within the brick work where it is evaporating into the house...

 

[noseall]

Do you care to elaborate on this?

Humidity in homes is created by humans. Kids are resilient.

 

[HandSlander]

Hmm... you seem to have your mind made up.

The below Temperature (blue) and Humidity (purple) chart is for the room. Windows open the humidity drops to the outside ambient 55% RH level... however you can see how steeply the humidity spikes back up when the window is closed again - this is with the door to the room closed. I am certain having lived with this problem for 3 years that it is not being caused by our use.

 

[Notch7]

You need to identify if the humidity is due to damp in the masonry or because the internal walls are very cold and allowing condensation.

If your walls inside arent showing damp, then the issue is possibly the walls are too cold -in which case, internally insulate with say 50mm celetex.

reducing humidity is a bit involved -ventilation helps with exchange of air, but lots of ventilation will allow external cold air laden with humidity to enter. Generally reducing humidity requires lots of heat combined with ventilation.

are the walls cavity or solid?
Im guessing the floor is suspended -is the floor insulated?

I wouldnt remove the plinth, that on its own wont be a major cause of humidity. If you remove it, you will most likely destroy the brick face, causing you another problem.

There are waterproofing agents that are available for treating brickwork and render -the professional ones are based on silane.

 

[noseall]

Hmm... you seem to have your mind made up

You seem a touch obsessed with humidity. Throw that sensor in the bin.

 

[dontbelieveawordofit]

tanking is a last resort, and usually the worst and incorrect solution. it creates cold surfaces and stops any natural escape of moisture.
I can’t stand outdated sash windows that aren’t fit for modern living or the obsession with them.
rip it out and replace it with a modern pvc sash.
can we get a picture of the bay window from the inside?

 

[HandSlander]

are the walls cavity or solid?

I'm not certain but as the walls are about 300mm thick or more my assumption is cavity

Im guessing the floor is suspended -is the floor insulated?

Yes suspended timber floorboards, not insulated to my knowledge

I wouldnt remove the plinth, that on its own wont be a major cause of humidity. If you remove it, you will most likely destroy the brick face, causing you another problem.

Hmm fair point, I read that cement plinths can cause humidity, this link has a replacement with a Natural Hydraulic Lime (NHL) as a base with Roman Cement as a finish coat... do you think this would do anything? https://www.standardheritage.uk/uncategorised/un-damp-proofing-listed-building-kent/

There are waterproofing agents that are available for treating brickwork and render -the professional ones are based on silane.

Am I right in assuming this will help to reduce penetrating damp but not capillary action through the brickwork? I'm no expert, but from what I've read the main purpose of having a DPC above ground level is so that the damp can evaporate out of the brick work into the atmosphere. My concern is that the cement plinth may be preventing this evaporation, and the moisture is evaporating into the cavity and sub-floor instead.

 

[HandSlander]

tanking is a last resort, and usually the worst and incorrect solution. it creates cold surfaces and stops any natural escape of moisture.

This was done in 2010 by the prior owner. Is there anyway to reverse this or remediate? The wall that was tanked belongs to the pictured room and has the biggest humidity issues in the house...

I can’t stand outdated sash windows that aren’t fit for modern living or the obsession with them.
rip it out and replace it with a modern pvc sash.

Thanks for the suggestion, other than 2 large sash bay windows all other windows in our property are double-glazed PVC or Aluminium. However given the size and the fact we live in inner London replacements are really expensive and requires (Westminster) council approval - so not a quick fix.

can we get a picture of the bay window from the inside?

Sorry I don't have a photo

 

[JohnD]

My 2yr old son's room (the pictured one with bay window) has a near constant ambient humidity level of ~70-80%,

How, and when, is the room ventilated?

If you can't see the slate DPC, have a look under or beside your door sills, where it is often visible. Unless you are on severely sloping ground, it will be at the same height all round the house. if you can dig a trial pit beside the house at some convenient point, you can look at the walls below paving height, and try to work out where ground level used to be when the house was built. The DPC will have been two bricks above that. It is very common for idiots to heap flowerbeds and raise paving against houses to bridge the DPC.

Run a hosepipe onto that paving and see if the water runs towards the house, or away. Please show us the guttering, rainwater downpipe, and gully nearby, and the condition of the roof and flashing over that bay.

Have you got a water meter?

 

[HandSlander]

How, and when, is the room ventilated?

Via the large sash window which is frequently opened or left on the screwed in window lock posts.

If you can't see the slate DPC, have a look under or beside your door sills, where it is often visible. Unless you are on severely sloping ground, it will be at the same height all round the house. if you can dig a trial pit, you can look at the walls below paving height, and try to work out where ground level used to be when the house was built.

Good idea, let me do that. There is no area without the plinth (other than our new extension) so I will have to dig or remove the plinth to see.

Have you got a water meter?

No, only mains feed so no ability to check for sub-floor leaks.

 

[JohnD]

I edited my earlier post with some other ideas....

 

[HandSlander]

The dig-down attempt below plinth

 

[HandSlander]

Run a hosepipe onto that paving and see if the water runs towards the house, or away. Please show us the guttering, rainwater downpipe, and gully nearby, and the condition of the roof and flashing over that bay.

The paving is just rubbish crazy paving laid straight on clay soil and as such has no drainage other than natural. As you can see on the left of the image in the OP, the cement plinth is extended to cover the ground area left of the bay and the main down pipe runs into a ground drain next to the building via a hole in the cement (over which in the picture I have a drain protector). You can also see part of the cover to the main manhole for the property.

The bay window guttering was replaced a couple years ago by a professional roofer who also inspected the bay roof. It is all sound and we've had no leaking issues from that bay roof or guttering since.

By the way @JohnD thanks for your good ideas and help so far