How to ‘finish’ a concrete floor in cellar?

[Cellarman]

Hi everyone,

I’ve recently moved in to a Victorian property (1852) which has a cellar. It’s currently used for storage and is fairly damp - i’ve just setup a dehumidifier to begin the drying out process. I’m looking to DIY it in to a more habitable space.

I have found tons of advice about breathable walls, but very little about the best method for finishing floors.

It has a solid concrete floor (so, not breathable) and i’m wondering what’s the most suitable way to cover it with something a bit more pleasant (tiles, wood etc.).

- I have limited headspace, so something like Tŷ-Mawr’s Sublime floor system wouldn’t work
- Do I just put down a DPM then floor as normal?
- Will this trap moisture and cause problems? Is the moisture already trapped under the concrete anyway?

It’s not clear to me at the moment if there is damp coming up through the floor. It seems to be coming through walls (near air bricks), but can’t be sure until it is properly dried out.

Appreciate your help!

 

[Cubist]

Is the wall in the photograph fully exposed out-doors or is the ground level a little below the air-brick we can see?

I'm guessing the latter and that the feature on the left is the remains of an old coal-chute. Has this been properly sealed and is that seal effective still? Also, I'm assuming too that the air-brick faces onto an access road or path; can driven rain or splashes from vehicles be a possible cause? May also be worthwhile checking your gutters and rainwater drains from the roof during heavy weather.

 

[Cellarman]

Is the wall in the photograph fully exposed out-doors or is the ground level a little below the air-brick we can see?

I'm guessing the latter and that the feature on the left is the remains of an old coal-chute. Has this been properly sealed and is that seal effective still? Also, I'm assuming too that the air-brick faces onto an access road or path; can driven rain or splashes from vehicles be a possible cause? May also be worthwhile checking your gutters and rainwater drains from the roof during heavy weather.

Exactly that. Most of the wall is below ground level. The airbricks face out on to a driveway so very good call on vehicle splashes. Would you suggest blocking up the airbricks and ventilating mechanically instead?

I appreciate your reply!

I’d also appreciate anyones thoughts on how to finish a concrete floor.

 

[Kearn]

Concrete isn’t permeable, so is already effectively a membrane, pushing moisture toward the walls. You don’t want to be introducing more barriers down there, and certainly not blocking up natural ventilation, you need more of that and cross flow.

Cellars are naturally damp, always will be, so their use today is limited and moisture management is ongoing and essential. Moisture is coming from the floor and from the walls as they’re underground, this will not change. Also looks like it’s leaking through the chute and air brick form outside, this can be addressed.

I wouldn’t plan to use it for anymore than storage of non perishables - wine is always a good choice. Get as much natural airflow, plus forced if necessary. Make sure the walls are lime plastered / lime washed so moisture can be drawn out of them and ejected from the space. They appear to be limewashed currently which saves you a removal job.

You could reinstate a breathable floor - permeable bricks / slabs into soil, which would help moisture evaporate, but frankly I wouldn’t bother as it’s a major effort and won’t gain much as the moisture will just keep coming.

What is the goal for finishing the floor? Any kind of wood will rot. This is a functional utility storage space, consider it external, so concrete floor would suffice.

The mouldy areas do appear to be possibly exaggerated by water ingress, block the water getting in, not the air. I’d reinstate airflow via the chute.

 

[Cellarman]

@Kearn thanks so much for the thoughtful reply.

I was hoping to have it as a semi-habitable space (ideal would be for an occasional drinks/cigar room and/or gym) but embracing the fact there will be some moisture by having all breathable materials. Mechanical ventilation would sustain air quality and force additional moisture out. I’d be happy with a tiled (limestone?) floor.

Do you feel this is just not possible? Would the only realistic way to achieve this be through proper waterproofing? (sump etc).

Additionally, I am confused by conflicting advice regarding concrete’s permeability.

from Peter Ward is a good example - he states that the bitumen isn’t breathable, but the concrete is (hence why moisture is being trapped between them). His solution is to remove the bitumen, but he doesn’t suggest what can be put on top of the concrete instead (as everybody in the comments questions!). There’s a similar query on this forum from @leifinlisbon https://www.periodproperty.co.uk/forum/threads/breathability-of-floors.18086/ that raises similar confusion on the same topic.

 

[Lacan07]

Concrete is a bit confusing tbf, I've seen examples of floors with damp patches where I assume its failed to resist the moisture beneath. I would say as a material its the worst of both worlds. If it were me I wouldn't bother treating it as a breathable surface and in your instance whether its pumping moisture into the wall is the least of your worries as much of that space is below ground level.

Old cellars are considered limited spaces these days, my neighbour has one and moans about the mould. When you think of these huge underground conversions taking place everyday in smart locations around London I suspect you're talking proper modern waterproofing, i.e keeping the moisture out utilising tanking, sumps, pumps and membranes, etc. All designed and built from scratch, whether it holds out of course is another matter. I suspect the issues will be coming down the line, not that the people rich enough to pay for these conversions would care that much.

 

[Kearn]

Concrete is a bit confusing tbf, I've seen examples of floors with damp patches where I assume its failed to resist the moisture beneath. I would say as a material its the worst of both worlds. If it were me I wouldn't bother treating it as a breathable surface and in your instance whether its pumping moisture into the wall is the least of your worries as much of that space is below ground level.

Old cellars are considered limited spaces these days, my neighbour has one and moans about the mould. When you think of these huge underground conversions taking place everyday in smart locations around London I suspect you're talking proper modern waterproofing, i.e keeping the moisture out utilising tanking, sumps, pumps and membranes, etc. All designed and built from scratch, whether it holds out of course is another matter. I suspect the issues will be coming down the line, not that the people rich enough to pay for these conversions would care that much.

Sadly they’re just hiding the issues. Can’t see the chaos behind the nice lined and painted internal box. Sunos fail, things break, drains clog, nobody knows until it gets very bad!

 

[Lacan07]

Sadly they’re just hiding the issues. Can’t see the chaos behind the nice lined and painted internal box. Sunos fail, things break, drains clog, nobody knows until it gets very bad!

One of my clients owns a swanky property space in the heart of London, just off Shaftsberry Avenue, next to one of the big theatres, ground and basement. Its fitted out as a commercial photographic studio. Keeping the basement dry is a never ending struggle, even the mildest of rain floods the basement. Its come to the point where he's installing thick perspex sheets on the walls. Its probably worth millions, I wouldn't give you 50K for it.

 

[Flyfisher]

Cellars were an important feature of houses in the days before refrigerators and when coal reigned supreme for home heating. Today they are just incurably** damp holes in the ground. We have a cellar and despite having air bricks front and back and the previous use of a 24/7 dehumidifier, its humidity was and is always up in the high 90%s . . . where 'high' was usually 98%+. I say previous use of a dehumidifier because I gave up after a few years because it made little or no difference - unsurprising really given the effectively infinite amount of moisture in the surrounding ground.

Nowadays we use it for storage of wine, beers and homemade jams, chutneys and cider - basically anything in glass containers that will be unaffected by the high humidity, while taking advantage of the cool conditions - for which a damp cellar is admirably suited. There are some changes that are really not suited for change. What's that old saying about making a silk purse from a sow's ear?



** I say 'incurably damp' but of course as Lacan07 has pointed out, it can be cured if money is no object but in the grand scheme of things is there really any point? Life is short and I'm not wasting mine on trying to dry out a damp cellar that will still be damp long after I'm also in a hole in the ground.

 

[Cellarman]

Thanks everyone!

Having run the dehumidifier for nearly 48 hrs things are drying out nicely. Humidity is at around 60% and still dropping. The outdoor air is fairly humid at the moment so this is positive, but there hasn’t been any rain yet. I’m convinced there’s water ingress through the coal chute or airbricks so this will need solving.

I have a multi split AC system for another part of the house and I might connect up a unit in the cellar to control things more permanently (i.e. without having to empty the dehumidifier tank twice a day!). Ultimately I guess i’ll have to see how it performs over several warm/damp periods to assess its suitability beyond storage.

I’ve told the wife it’s a 3 year project so time is on my side. I’m yet to tell her about the lifetime commitment of maintaining it!

 

[Flyfisher]

I have a multi split AC system for another part of the house and I might connect up a unit in the cellar to control things more permanently (i.e. without having to empty the dehumidifier tank twice a day!).

Might be worth considering a condensate pump.

When I was running a dehumidifier, I also got fed-up with emptying the water tank so I rigged it up to a condensate pump, which collects all the water from the dehumidifier into its own tank and then when nearly full automatically pumps the water out through a flexible pipe upwards and out of the cellar. I routed the pipe through an air-brick and into a nearby surface water drain. It easily raised the water the necessary six feet or so and meant no more daily checking and emptying of the dehumidifier tank.

I think this is the one I used (I'm not sure exactly where it is now I'm not using it any more, so can't check for certain), but a quick search shows there are plenty of different models out there. Just a thought.

 

[chriswarwickrd]

Your cellar looks very familiar.... be interested in what you landed on @Cellarman

 

[Cellarman]

Your cellar looks very familiar.... be interested in what you landed on @Cellarman

View attachment 15591

They do look similar! I’d suggest yours looks in slightly better condition.

I’ve not made huge progress yet - i’m expecting it to be a multi-year DIY project. My goal over the last few months has been to see if I can control the humidity.

At the beginning of summer humidity was over 90% down there. Having the dehumidifier on constantly for about a week made a big difference and removed a lot of surface damp (which was mainly on the floor).

I have since partly installed https://www.ventilationland.co.uk/en_GB/p/vent-axia-multihome-aep-advance-368m3-h-humidity-sensor-bluetooth-and-euro-plug/19608/ and humidity is around 65-75% now. I say partly installed as the extract is just blowing towards an external air brick and isn’t fully ducted yet. I plan to do this next, then add a silent extractor fan (bathroom style) at the other end of the cellar to encourage positive airflow (extracting dry air from the house into the cellar).

I think this will make a difference, but it’ll be hard to tell until next summer. In the meanwhile, i’m using the space for my home gym and storage.

Longer term, I had originally wanted to make a cosy tv area down there, but I don’t think this is viable without doing a whole damp proofing membrane/pump setup. It’d also need digging down another foot for insulation (regs) and head room. All of this obviously comes with its own issues. So i’m likely going to go for an exposed/industrial look down there - limewash walls, exposed pipes, ducted cabling etc. I hope I can then turn it in to a small games/drinks space with decent wine storage.

I’d been keen to hear your ideas and any advice you have had. Unfortunately all the tradies I have spoken to just suggest a full cellar conversion, and have no idea about dealing with lime, airflow etc.

 

[chriswarwickrd]

Thanks @Cellarman that's really helpful. I've also managed expectations with my boss that it's not going to happen overnight...

Your approach sounds sensible and that's a good early result with the smart extraction fan. It looks as though you also watched the Peter Ward videos. This blog/article I also found helpful in case you haven't seen https://www.kezzabeth.co.uk/2020/11/why-your-cellar-walls-need-to-breathe-and-why-were-not-tanking.html

Our relative humidity is currently ~55% at 13C so not terrible albeit mid winter. However, the coal shute sits under a side access path and there is some water ingress.
The other half of the job is improving drainage/DPC on the exterior of the property (photo below with arrow to the other side of the covered coal shute) which runs along side wall in cellar. There was previously a soakaway/gravel either side of patio tiles but I have dug out about ~0.5m to dry out the wall and considering options (unlikely DIY).

I've only recently bought a dehumidifier (again similar model to you!) and not expecting it to be a game changer. We currently have our laundry washer/dryer in cellar too (different room) which we will use it for (also need to improve ventilation for these units).

The main thing I am still questioning is whether to improve ventilation AND add 'breathable' Sika slurry to external walls to provide additional protection from water ingress, OR ventilation and lime wash only, OR ventilation and leave exposed.

As to your original question on flooring, I'm also looking to improve this (likely after walls) and haven't landed on a definite answer so not sure I can be much more help. The consistent advice seems to be to also maintain breathability so as to not adversely affect walls e.g. lime-based screed potentially with microcement in areas of holes/need levelling.

 

[NecessaryWing]

Hi,

I have a cellar in our Georgian cottage. I would say very good ventilation is crucial if you want it as a living space. Most cellars I’ve come across (including my own) suffer from high radon levels. This is the second biggest cause of lung cancer in the Uk. I would say it is essential to measure the air quality down there before you start considering it as a habitable space. Airthings monitor is good but not cheap. You can also hire them through UKHSA - the government body that deals with such things. A neighbour converted theirs into a cinema. When I saw they had no ventilation I leant them my monitor and the levels were so high that UKHSA said not to use it at all until it was mitigated. We live in a very low radon area. Radon comes from the decay of uranium which is found in the ground. When it decays into radon (a gas) it seeps through into cellars. Since they are usually poorly ventilated the levels are often very high.

To get the radon and humidity low in our cellar we installed large air bricks on one side and two dMeV extractor fans on the other (these are designed to run 24/7). This brought air quality to a level where it could be used as living space.

Is your floor particularly damp? Does it have obvious signs of damp? If not, you could leave it as it is. Most of our village have cellars and I’ve seen lots with concrete floors that don’t get many issues. A limecrete floor with glapor is ideal but with limited head room I see why you might not want to do it. You can usually get some extra head room when digging down. Older house have shallow footings. So long as you don’t dig deep enough to undermine you can often gain 10-20cm or so even on older houses.

Regarding the walls the best way I have seen it work for cellars in our village are stud walls with a a breathable plasterboard. Traditional fat lime will be too permeable to penetrating damp. Tanking or anything non breathable is a total no go. NHL will work better for damper areas but you will still likely get patches coming through on the paint work.

In summary:

> definitely check air quality - primarily radon before using it as living space. High radon levels are common in cellars and the risk to health from radon is no joke.
> To ventilate a dMEV extractor fan is the way forward in my opinion and worked really well for me. It’s best to have air bricks on one side for intake and the dMEV on the other (if you have two externals walls that is.
> I would install stud walls with a breathable plasterboard. The gap between the cellar walls and plasterboard will eliminate penetrating damp spoiling the painted finish.
> If the floor doesn’t look overly damp with damp patches just leave it and apply a suitable low profile flooring on top.

Hope that helps.

 

[NecessaryWing]

To add- this is our reading currently;



It is winter so absolute levels are low. No dehumidifier. I found big differences when the air turnovers per hour increased to over 3. We have a large 50 m² cellar with 2m of height giving 100 m³ of volume. The two fans extract 350 m³ per hour. It’s important to have lots of intake vents with them of course.

 

[NecessaryWing]

Thanks @NecessaryWing. Do you use a specific radon monitor? Or is a standard air quality monitor (e.g with PM2.5) sufficient?

I have this one:



The radon monitor in it is accurate - it was developed by CERN and is a patented design.

Radon levels should only be interpreted once you have a month average but ideally a 3 month average. It’s usually a simple fix if they are high. The extractors lowered ours a lot - they went significantly below the habitable threshold of 100 Bq/m³

 

[ThornhillMike]

To add- this is our reading currently;

View attachment 15608

It is winter so absolute levels are low. No dehumidifier. I found big differences when the air turnovers per hour increased to over 3. We have a large 50 m² cellar with 2m of height giving 100 m³ of volume. The two fans extract 350 m³ per hour. It’s important to have lots of intake vents with them of course.

@NecessaryWing I am looking at something similar for our basement (c200 cu m). Ours is four-roomed and we are likely to make use of most of them. I'm thinking an extractor each in the back two rooms and an additional air brick in the front two. How noticeable is the air movement when the fans are running? dMEVs vary significantly in price; do you have any recommendations on particular features, or brand? Thanks. Mike

 

[NecessaryWing]

@NecessaryWing I am looking at something similar for our basement (c200 cu m). Ours is four-roomed and we are likely to make use of most of them. I'm thinking an extractor each in the back two rooms and an additional air brick in the front two. How noticeable is the air movement when the fans are running? dMEVs vary significantly in price; do you have any recommendations on particular features, or brand? Thanks. Mike

That's a large space to ventilate. The key is to experiment in terms of what system is required. To get decent air quality for use as habitable space you want 4-6 air turn overs per hour. Assuming your basement is 2m high thats 400 m^3 of air so you likely need extraction rates at least around 1200 m^3 per hour. I use Tornado dmevs but none of them would give close to that. I would imagine some in-line fan designs might deliver that. It also depends on your set up down there. If there are things like 90 degree turns then you may need fans on internal walls between rooms to help ventilate the air through the space better.

With a large space like that I would personally install some more air bricks - the larger the better (https://www.castironairbricks.co.uk/). I think standard single air bricks would be too small to ventilate that space. Put some high extraction rate fans in the back and see what sort of air quality that gives.

 

[a twig]

1200m3 per hour is a lot, we are talking units like this typically used in commercial premises and requiring a 12” diameter hole in the wall, are you sure about your sums?

I think Mike’s post already had the volume at 200 cubic meters