Granite flag stones / tanked walls

[bustergreen]

Hi everyone,

Me and my partner have recently brought a 1850s cottage in Cornwall. It's split on 3 levels, the lower has one wall completely under the ground, two walls are adjoined by two other cottages in the terrace, and the third opens up to a kitchen which is at ground level.

We knew the room suffered from damp when we brought it so are not surprised by that. How ever when we pulled the carpet up you could see some of the original granite flag stones underneath a sandwich of cement screeds and some sort with some bitumen lining. We decided to remove some more of the screed to reveal the stones, whilst initially most of the stones were dry, they now seem to be absorbing whatever moisture is in the ground. Since this has happened there is now a lot of condensation forming on the wall which is underground. It's all been tanked with some kind of special paint, apart from a strip behind a rayburn and another where the stairs meet the wall, everything down there is cement render and then painted again with some sort of plastic paint.

In an idea world we would like to get rid of all the cement and plastic stuff and go back to the stone and lime. But we are a bit concerned with the amount of moisture appearing. So we are a little dubious to continue incase we open a even bigger can of worms.

Just looking for some advice or some one who has done something similar and could let us know how damp things got before it all dried out (if it did).

 

[Stevers]

At this time of year it could easily be condensation, or as I like to say 'sweating'. The limestone floor in an alcove in my basement that has been bone dry all winter and spring has now developed a fairly significant damp patch, and I reckon that's condensation given how dry it was earlier in the year. That alcove now has a bookcase and cupboard that I've built into it with a well ventilated void behind it. I'll make sure that any joinery that touches masonry is treated and had at least two coats of paint and then I'll live with it.

The Blue Lias limestone slabs had also been cement screeded and it's remarkable how much better the room is without the screed. The apparently dry floor will always need to 'breath' (paper goes soft on it) and the room will therefore require permanent ventilation - in my lit basement this is via a trickle vent in my one modern window, plus the extra vents I put in at the top of the two alcoves, and my of course a pair of 'draughty' single glazed sash windows. To add complication there's a utility room with a Sheila Maid at the other end that's in constant use to dry clothes, so for me, ventilation, with a bit of targetted warmth in winter, is the way to go.

[Edit] Happy to report that now temperatures (and humidity) have fallen, my damp patch has gone, but I have every confidence that in future years it will return for a few weeks in late summer.

 

[bustergreen]

Hi,
Cheers for the reply it's good to hear about what other people are finding out / know from experience.

We ended up leaving a big fan on for a night just to see what happens and all sighn of condensation has gone, this was with open windows etc as well. Most of the damp on the floor has cleared apart from the worst bit.

Were going to try and clear the majority of the cement this weekend and possibly re point the slabs together with lime to see if that aids on drying things out as well.

 

[a twig]

Where the wall has been tanked there will also most likely be a lot of trapped moisture which will be making that wall colder and thus promoting the condensation as well.

Great to hear the fan made a difference as it’s a good sign you are on the right track

 

[bustergreen]

Hi everyone.

Just thought I'd do a follow up on the floor / room. The room seems to "sweat" when it's a humid day or after it's rained and the sun comes out. Also we have recently started to light the Rayburn up that's in the room, this seems to make the room "sweat" (condensation on some walls and granite floor seems to become damp) as well. Other wise on a rainy / cold / windy day it dries out with in 4-5 hours.

I'm guessing as the air is warmer it can hold more moisture, so a cold surface will produce condensation.

Next week we are having an extraction fan installed with a small heat exchanger in it, hopefully that will help a little. We are also getting ready to point in-between the granite slabs with lime, and are also getting some of the plastic paint off the only non below ground level wall which we hope to lime render later on.

As always, any comments or questions are appreciated !