Damp issue in 1900's building with strange ventilation setup

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Hi all

First post here. I've searched around everywhere for someone with a similar set up to my place and found nothing so had to ask for myself.

I have a ground floor flat of a converted pub built circa 1905 and converted in the mid 90's. It's solid brick wall construction, single glazed sash windows, cement based (I assume) rendered outside, gypsum plastered inside. It has a very large basement, none of the basement comes above ground level which is fairly unusual it seems. We have damp problems from ground floor up to about 1m around most of the external wall areas ranging from a bit patchy to very wet but there is no visible mould, just tide marks. The pavement is a little higher than I expect it was when originally built but it all slopes away from the building quite dramatically and never has any pooling water outside. There are airbricks / grills that are mostly covered by the render and are full of grime making them pretty much ineffective at the moment.

Now I know what a lot of people will be thinking already; cement render, gypsum plaster, old building, blocked air vents. Lots of causes for damp already that seem pretty obvious. However.....

The blocked airbricks I mentioned are ABOVE the internal ground floor level (roughly 25cm above the finished floor). They appear to vent down into the basement via small cavity in the solid wall. It's similar to a periscope vent setup but this is not a cavity wall. It's basically a periscope vent to the basement from above the ground floor level but it's directly built into the brickwork. I'm fairly sure that the closed portion of the vented cavity above the ground floor is only covered by a bit of plasterboard / ply or something and skimmed over. I haven't yet confirmed this though as all the areas with vents are covered by radiators. If this is right then the only thing between the internal wall and the outside air in my living room is a bit of board and plaster! See my illustration of what I think is going on below:

Basement Vent.jpg


The other thing I have noticed is that the only place we have damp is where the vents are located. The only 2 bits of external wall that have no vents are bone dry. The basement itself is a little musty but not visibly wet and usually stays fairly warm. It has a plastic drainage membrane around the perimeter but the damp above was there long before I installed that. If anything it's got slightly better since the membrane went in. Even behind the membrane it's fairly dry and drains freely. This tells me that the vents must be the main issue for some reason but I expect the cement render outside and gypsum plaster don't help the situation. It definitely gets most visibly wet when it's cold outside and the windows are kept shut so I'm fairly sure condensation is the main culprit.

This is leading me to a couple of possible conclusions:
1 - The lack of any solid wall to the outside where the air brick is located is created big cold spots where moisture coming up from the basement and within the house is condensing around leading to concentrated damp patches.
2 - There may be penetrating damp coming in around the vent where the wall is thin and the pavement raised condensing on the back side of the wall inside the vent flu cavity and saturating the walls from within.

I think I'm first going to try and un block the vents and get some air flowing to see if that makes any difference first. If that fails then I might have to fully seal the vents and fill the cavities and put some kind of mechanical ventilation in the basement / house to mitigate the condensation more effectively. Last thing I want to have to do is re-do the rendering and plaster with lime if I can help it. The fact that there are dry areas with no vents tells me this shouldn't be necessary and it's the vents combined with the condensation that are actually causing the problems.

Has anyone ever come across a set up like this? Anyone got any better solutions than my approach? Googling around and scouring forums has lead to finding nothing similar to this setup and specific issue.

Thanks
Will
 
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Insulate the walls, either inside or outside.
A dehumidifier might help, but it'll take a long time to notice much difference.

Your query should have been posted in the Building Forum. I'll ask the mods to move it.
 
It could be that your vents are piping warm, moist air in to your upper wall and the moisture is condensing in the gaps/cavity. Even 9" walls have small gaps between the brick. This is then causing damp.

Replacing the vents and lining the "tunnel" may help. Internal/external insulation wont improve things if you have an air bridge to an internal cold surface. In fact, it will make it worse.

When the house was built, it was unlikely that they intended the basement to be a warm habitat and more like a cold loft. The vents would be there to allow air flow to avoid rot. Now its been tanked and heated the vents are working in reverse and the upper wall acting as a condenser.
 
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Internal/external insulation wont improve things if you have an air bridge to an internal cold surface. In fact, it will make it worse.
The idea of insulation is to avoid cold bridging spots.
As I saw it, there is no airflow between the room and the cellar, simply a conduit between the cellar and the outside, passing via a sealed conduit through that room, which would be insulated by internal insulation.
 
It could be that your vents are piping warm, moist air in to your upper wall and the moisture is condensing in the gaps/cavity. Even 9" walls have small gaps between the brick. This is then causing damp.

Replacing the vents and lining the "tunnel" may help. Internal/external insulation wont improve things if you have an air bridge to an internal cold surface. In fact, it will make it worse.

When the house was built, it was unlikely that they intended the basement to be a warm habitat and more like a cold loft. The vents would be there to allow air flow to avoid rot. Now its been tanked and heated the vents are working in reverse and the upper wall acting as a condenser.
Yeah this is what I'm thinking too. The air from the basement is naturally travelling up through the vents because it's the path of least resistance to the cold external wall above but it's not ventilated well enough to disperse through the vent so is condensing on / within the wall that it meets upstairs and saturating the brickwork. I expect before it was rendered the wall could breathe enough that this wasn't as noticeable an issue but over time the moisture has built up. There isn't any heat source in the basement except the pipework for the radiators upstairs but that said it never gets cold in there, always warmer than above in winter and nice and cool in a heatwave.

I'm going to try first unblocking the vents as much as I can, see what difference that makes in 6 months and go from there. I guess if that improves things then a lining in the vent will help stop moisture condensing directly on the brickwork. Maybe some insulation on the rear of the plastered covering behind the radiator upstairs to mitigate the cold spot slightly will help.

If opening up the vent makes things worse though then blocking it off and fully filling in / insulating the vents and finding a better way to vent the basement might be the better course of action.
 
I'm no builder, but to me it looks like they may have lowered the floor. Only conclusion I can come to with that set up. Probably no help to you though.
If they did it must have been done 50+ years ago, the brickwork in there looks ancient. I don't think they would have bothered lowering it anyway though, the ceiling is already 3m high, hardly seems worth pinching an extra 30cm but i suppose it's possible.
 
Is the ceiling in the cellar Plaster boarded? Are your central heating pipes in the floor lagged with modern pipe lagging?
 

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