Underfloor heating and insulation advice

[bwbwbwb]

I recently came across this forum after finding this post whilst researching underfloor heating. I am hoping to get practical advice on how to approach my problem.

Background context:

• Flat in a converted Georgian house, estimate early 1800s build and 1980s conversion. We are on the lower and upper ground floor and are planning renovations to the whole lower ground floor at once. We had been hoping to install a wet UFH in all rooms. Lower ground floor has airwells to both sides (no ground against the wall).
• We have been here about a year and not noticed any damp issues or humidity spikes, running a good quality dehumidifier and ventilating where we can, although we have no extractor fans at all. We do not run the heating much compared to many, probably around 18-19C in the winter and frequently lower whilst trying to avoid dipping below 15C. Would not expect to run it much hotter with UFH.
• We have thin asbestos-containing floor tiles throughout the ground floor that will be removed by a specialist. We are removing them because we will install a new foul drainage connection which requires a ~250mm deep trench through the floor slab. The ACM means we cannot inspect the floor below, but it seems very solid. We should be able to find out the build up of the floor when we do the trench.
• We had been planning a low profile UFH system, aiming for absolute minimum depth.
• We understand the importance of breathability in our situation and had been factoring this in to plans for wall finishes, but not considered the floor.

The problem:

• We had not factored in the risks of a UFH installation in terms of moisture: both breathability due to impervious insulation, and higher temperature gradient to the ground below increasing evaporation and possibly leading to interstitial condensation.
• Looking in to options, we'd begun to consider a 60-90mm build-up of boards/breathable screed but hadn't got very far before realising there was this condensation risk too.
• Most guidance involves fairly significant build-ups to combat the above mentioned risks (525mm mentioned in the linked post). Realistically, we cannot justify anything near this.

Questions I was hoping to ask:

• Would lime plaster external walls, humidistat controlled extractor fans, a dehumidifier, general moisture management, be sufficient to manage the risk of interstitial condensation or is it always going to be a problem without the proper 'strip it back to the ground' 400mm+ profile with capillary separation etc.?
• Any advice on a UFH installation on top of existing slab, which ensures breathability and avoids the temperature gradient-driven moisture build up? Any way of getting close to 50-100mm or am I in dreamland?
• We used a surveyor who specialised in period properties. One recommendation was to install a french drain along the face of the wall in the airwells to manage moisture. Would this help at all?
• Any other neat tricks to achieve the end goal without building a castle on sand?
• Any predictions on what the floor build up would be in our situation?
• Have I overlooked anything else?

Thanks if you've read all of this. I hope the context is relevant to the questions. Please let me know if I need to clarify or provide more information.

 

[claraclattermole]

So you have a solid floor but then you mention airwells? Is the lower ground floor underground?

 

[bwbwbwb]

So you have a solid floor but then you mention airwells? Is the lower ground floor underground?

I meant that we have an airwell on exterior side of walls. Maybe I've used the wrong word, image searches didn't throw up results I expected!

For clarity: it is a 1.2m wide 'trench' about 1.4m below ground level, running the length of the wall. This means that despite our lower ground floor sitting well below ground level, our exterior walls are not retaining any earth, which I understand can reduce breathability and introduce more moisture to the wall/floor. Hopefully this sketch helps?

 

[montyjohn]

This is a completely theoretical answer as I just don't know.
But if your room at say 20 degrees has a relative humidity of 50%, then the due point is 9.3 degrees C (which is a typical ground temperature apparently).

If your slab is heated to 40 degrees, the ground below is going to be warmer than 9.3 degrees so it's not going to condensate at that point. It will condensate further down where presumably the ground is already moist so who cares?

This is my gut feeling, could be way off????

If the humidity in the house is up at 70%, then the due point is 14 degrees. This would be a little risky I think. Keeping it below 50% would be a minimum in my view.

This might be enough to mitigate the risk of interstitial condensation but I would seek an educated opinion first.

Something I do know about is drainage as it's my day job. Filter drains I design for motorways for example are great at draining down the pavement formation, but, when it rains it's an easy path for overland flow to saturate the highway foundations. But because it drains back down again (thanks to gravity), it means the foundations are just damp long term which is fine.

This is not what you want on your house. Don't give water an easy route into your foundations. Keep filter drains away from the house, and make sure the airwall bit falls away from the house also.
You might get away with a filter drain next to the retaining wall. Just be careful you don't undermine it. I would only consider this if you have a high water table. Otherwise just manage the surface flow properly.
Historic England also advise keeping filter drains away from properties.