When we moved into our home 5 years ago there was a small watermark at low level by the skirting boards of the internal wall which separates our front room from our back room , pointed out to us by the damp and timber surveyor we’d employed.
He showed us the bitumous layer of the damp course lying under the watermark and explained how this was failing and moisture was rising up from the cold sub floor - he said to keep a watch on it, we’d need to replace the physical damp course eventually, as it was likely to break down more.
Today we have suddenly noticed the watermark has appeared again , but is along most of the skirting board now. Concerned, we started researching and thinking!
Reading about damp proofing, I came across an article by Heritage House , this peaked my interest having read other good articles on there about older properties.
What I read has made me question a lot I thought knew about remedial damp work and prevention.
I’m sharing, because it made sense to me and discusses how modern practices are a direct cause of damp problems, especially in heritage homes designed to be breathable.
Reading this has us questioning the cause of this rising damp symptom - the plasterwork on the wall and if we have adequate sub floor and above floor ventilation.
Indeed how do we regulate all of this to eradicate the cause?
It feels like there might be a middle road here.
I wish I understood these things better.
I think I need to go on a course!!
https://www.heritage-house.org/damp-and-condensation/all-about-the-pca/the-damp-con-collection/injection-damp-proofing.html
Most “building dampness monitors” are really wood dampness monitors. Now of course there is a proven connection between damp wood and timber damage, as seen in your bathroom. On the AECB website there is a discussion I read relating dampness readings in masonry to a “wood equivalent” dampness, but it is very subjective. I would start by removing the skirting on both sides of the wall and any non breathing wall covering in the damp area, be that paint or wallpaper. That will start the drying process.
It is true that dampness emanating from masonry draws with it mineral salts from the bricks and mortar. These are often hygroscopic, so you may indeed need to replace some plaster. As to the source of the dampness it could be rising damp, condensation or hygroscopic attraction. Put a wet finger in the air and guess. (Not too wet mind.)
Do you have access under the floor on either or both sides of the wall? Is there damp down there?
How well is the underfloor ventilated? Is one side of the wall unheated? What state is the timber of the removed skirting boards?
All these give clues but none give definite answers.
There is a flaw in the article you linked to. Although many companies inject bricks to make a damp course you should inject the mortar if you decide to take the injected route. The bricks have loads of gaps between them. The mortar is contiguous.
Thanks Tim. I feel you’ve given me some really good starting points there for our own investigation.
Still not sure about damp injection though … it’s not the first time I’ve heard it contested about whether damp injection works or not . Hi
It’s an interesting topic - scary for the householder though 
.
Damp is mostly not covered by home insurance and can be costly to fix, as well as major upheaval to access affected areas.
Owing to all of what we’re experiencing with our period home, I just ordered a book about caring for your Victorian home … I think it’s time to understand the bigger picture. Especially as our problems all seem to be about moisture and ventilation.
I’ll keep the topic updated as we progress.
First thing is first though, we have a bathroom to complete 
…
Resilience and pacing is needed here for our sanity. I keep wondering what this house will throw at us next!!
In regards to the wall covering we have - it’s plaster and Matt paint in both sides.
I’m hoping the plaster might be lime and not gypsum.
It’s easier to access the sub floor on one side than the other as the other side has an engineered wood floor on top of the origal floor boards, so two floors to come up there.
We have a modern lean to kitchen with a concrete floor which replaced the original kitchen along the back of the house about 20 years ago - I guess this has affected the original sub floor ventilation from that back wall.
The other two external walls seem to have good clear ventilation via air bricks.
I wonder how one could drive more ventilation into a sub floor ? Are there other options than air bricks ?
Beware of generalisations about Victorian homes. My first house was Victorian. Built for the railway workers at Carnforth Junction. I’m sure it was very different from yours. The Victorian era also spanned nearly 64 years, and techniques changed. If your house was built without a damp course it would probably be wrong to add one but you mentioned a bitumen membrane, so it would be appropriate to re-damp course if it has failed.
If you have an unused chimney it might be possible to link the underfoor space to the flue. With the added chimney effect you should see significantly increased ventilation. Remember to isolate the airway from your house air, which will probably mean blocking off that chimney without an airbrick to the flue. Remember the conventional airbrick used when blocking a fireplace is to air the flue but your flue will get adequate air from under the floor. The more ventilation there is under your floor the more important a vapour membrane and thick underfloor insulation are. To avoid airwashing your insulation you might need membrane both above and below the insulation.
Our friend @lloydham mentioned we could get our multi fuel stove vented to the sub floor. We underestimated the value in this - thinking it was for the smoke - duh!!
We are a classic example of householders , who don’t think along these lines.
Literally I am kicking myself now!!
We went with the stove we’d already chosen which can’t be vented to the floor.
Now I understand why Lloyd said this to us 
an opportunity to increase the sub floor ventilation!!
About the bitumous damp course , I did wonder if it’s originaI and if we should replace like for like ?
Obviously we have more work to do in the subfloor
- replace the physical damp course
- enable more ventilation to the sub floor
- insulate under the floor with double vapour control layer to avoid *airwashing??
Let me guess about airwashing
… Some form of dry cleaning for insulation ?? 
…where the wind blows the insulation and affects it , not sure how Does it affect the integrity of the insulation or allow draughts through?
I think householders like me, need the term airwashing explaining please Tim.
I’m unpicking what I thought I knew now .
My mistake, it’s called windwashing. This is where moving air permeates your insulation and replaces the air trapped in it. Instead of a nice consistent thermal gradient though your insulation you get cold patches. It is most often seen with glasswool in lofts but affects any fibrous or open cell insulation.
Having a vapour barrier over, and touching the insulation protects it from windwashing. Windwashing also introduces foreign matter into the insulation, which clogs the pores and ultimately makes the insulation useless. Many homes where nice yellow glasswool was fitted in the loft a few years ago now have a black heavy mat with no insulation value at all. The householders are blissfully unaware. The vapour membrane will also stop this from happening.
Insulation needs to be sandwiched between an airtight or vapour barrier on the warm side and a vapour barrier on the cold side to maintain peak performance.
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There is a good chance it isn’t original. Early damp courses were usually slate or engineering blue brick. Maybe the damp course was added to try to stop the same damp patch that you are querying now.
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Thankyou so much for sharing your knowledge and taking time to help us Tim . You are very much appreciated 
The explanation about windwashing really makes sense and I will act on this in my loft.
We have the vapour barrier nearly complete under the sheepwool insulation , and will put a barrier over the top once we’ve completed.
I 'm learning how essential it is to insulate, but at times I have massive wobbles over it when I’m trying to mitigate against moisture and mould, dew points and maintaining
that breathability in our home.
A lot about our home and the modernisations versus the original fittings is becoming apparent - it is the modernised parts of our home which are failing and suffering from condensation build up.
Mostly we see mould and mildew around the gypsum plaster and where the original lat ceilings have been replaced.
The only window in our home which doesn’t suffer from condensation is on an original lime plaster wall.
Even modern homes would benefit from lime plaster with vapour open finishes but it is harder to justify the expense.
Homes have to be modernised, even Victorian ones. It is the only way to become carbon neutral. Associated with that is the headache of finding a new equilibrium between air tightness, comfort and humidity. I don’t envy you. Even my comparatively modern 1936 home has its problems.
What really annoys me about all this is that houses being built and passing regulations today already need retrofitting!
First, you can manage the sources of water. The dominant form will be rain from the outside, then ground water. A picture of the trouble area from the outside would be helpful.
Remedial work to gutters, an addition of a protective rain water splash back render up to 300mm from the ground, digging down to the foundation to install a perimeter drain and crushed rock infill/filter fabric to reduce groundwater sources, a sloped membrane to a French drain/soak away a few meters from the walls to move surface water further away etc.
However, the damp proof course should be installed and if you want to completely eliminate risk you can replace several brick courses with glass foam, load bearing, blocks below grade.
With respect to ventilated voids, they are generally poorly understood. The problem area will be the end grain of floor joist embedded into exterior walls.
