Advice on insulating under ground floor floorboards

Hi there,

Thanks all here in advance.

I have done some research but that’s left me with more questions. So I could do with some tips.

I’ve just taken down a lathe and plaster ceiling in the cellar exposing beams and floor board above. Beams are about a foot apart. What choices of materials are there to insulate?
It doesn’t need to look too neat but it is a working / storage space down there.
There seem to to be plenty of gaps in the floor boards, it’s an old house.

There is space to put in PIR / PUR boards if needed.
Are there certain types of insulation to avoid in this kind of situation?


Hi Mick, I used 200mm thick loft insulation as I took advantage of a 2 for 3 promotion offer. In addition to this I stapled a breather membrane over it and taped the overlapping edges. I have found this very effective and quick to do. Rock wool might be better but might be more fiddly to handle.

How damp is your cellar? I’m a little concerned that the ventilation between the floorboards might be required to keep underfloor humidity under control. Your cellar will become cooler and have less ventilation. You may need to address any humidity/dampness issues before the very important job of insulating, for which @lloydham’s advice is spot on. Because of the potential for damp I wouldn’t use natural fibre, particularly around the perimeter.

Hi there,

Thanks for these thoughts.

It’s quite damp. It doesn’t happen a lot In the storm last night for example there’s some dampness on bits of the floor and walls.
I put a humidifier on when it’s like this - It usually starts off near 85% 90%

There is a cellar window which is really old and drafty but currently good ventilation!

Is this a matter of balancing things out?


Good point Tim, my apologies for omitting a couple of things. I removed cobwebs and years of dust/ mud from the inside of the airbricks and ledge. The external surface of the air bricks appeared rounded and seemed to be funnelling the wall runoff water inwards so I fitted metal vents (silicone sealed on the top) to deflect water. I have checked and this has stopped water ingress with no further issues - dry. Please see picture for clarification.

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How is the water getting in? If it is soaking through the walls and floor you have a major job to fix it, which must be done before reducing ventilation.

My first house was a terraced property in Carnforth and the cellar was extremely damp. Actually the whole house was, but we are discussing cellars in this thread.

I removed many years of whitewash from the walls (a dreadful job) and then painted them white with epoxy resin paint that was sold at that time for dairies. It was allegedly proof against high pressure washing. It soaked very well into the bricks before setting and cured the wall dampness. The floor was cobbles and as I wasn’t really using my cellar I just left it like that but nowadays I would take it up and build a whole new floor with new binding, DPM and screed.

I’m sure that two part epoxy paint isn’t part of any modern approved product list but it was wonderful (and low fume). I worked in agriculture at the time and just used the tools I was familiar with.

Sorry. I’m just an old man taking a chance to reminisce!

Hi @Tim_Gilbert,

Do you think, long term, there is a risk of bricks spalling if painted with something like epoxy resin?

Epoxy and moisture

I’m not trying to rain on your reminiscence, but the basement where we are currently staying suffers from exactly that, compounded by one-sided ventilation.

In terms of insulating floors from the underside, @lloydham you have probably seen it’s similar to doing it from above but in reverse, plus some support battens… like this EBS example

I agree that I wouldn’t use epoxy resin now, at least not on a wall without good ventilation on the other side, which precludes cellars. It is really to keep moisture out of walls, rather than in. As I mentioned, it was sold for dairies, where obviously considerable washing down is required.
As for spalling, I don’t know. It would be a risk in frost prone walls.

As for the article that you linked to, on decent concrete the epoxy sits on top and can be worn off, damaged and become scruffy. I my case is soaked into the porous bricks and was therefore protected from those risks. As it had soaked in it does mean that if it proved to be the wrong choice, removing it would be a real headache if not impossible.

Hi, I improvised by using 12mm packaging strapping and stapled this in place to support insulation- very quick.

With a patch of airtightness tape over each staple?

How and where did you fix the membrane to the wall?

@Mick_Chesterman, the membrane is the top layer, so when working from below it goes in place first. When insulating other surfaces put the membrane nearest the habitable space.

[quote=“Tim_Gilbert, post:11, topic:832”]
With a patch of airtightness tape over each staple? no- the nail tear strength was rated at 106N and with friction fitted insulation held in place by packaging strapping, I am not expecting any tears.

How and where did you fix the membrane to the wall?
[/quote] I tucked excess membrane between the insulation and the wall.

I checked on my improvised handy work a couple of weeks later and nothing had moved. The underfloor area was dry and the rooms above warmed up far quicker and became comfortable. If there had been a problem I could rapidly dismantle anything I had done. In this instance I was working from below - not a lot of room for manoeuvre- see pic

Thanks for the clarification and correction. I have edited my post where incorrect.

You have a nice clean, as these things go, course of engineering bricks that would have been ideal for taping membrane to the walls. Do they constitute your DPC?

Yes, but there are also external indications of a previous liquid DPC treatment (>20 years old)

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