Renovation floor / slab

[elite]

I'm in the early stages of renovating a cottage and am considering the options for the ground floor. This was previously quarry tiles mostly onto concrete, but some onto earth.

 

In an ideal world the finished floor would be UFH, but to acheive that I would need to dig down for DPM, concrete and insulation - I can't afford to lose any height in the rooms due to low ceilings.

 

Can anyone advise on depth I need to dig down and the prefered make up of the floor?

 

Also, I am not sure of foundations, the more recent extension (1960s?), presumably has them, but the earlies part 1800(?) I guess might not. How do I know how deep it is safe to dig down?

 

Thank you for any guidance!

 

 

[Gone West]

You should consider whether UFH is the best way to deliver heat to the rooms in an old house. Unless you're putting 300mm+ under the slab you will be spending a lot to heat the ground under the house. Even with 300mm EPS you will be losing around 8% of the heat to the ground.

[elite]

Thanks @Gone West

 

The house is not on mains gas, so the plan is for heating to be provided by ASHP, my understanding is that this runs more efficiently at lower flow temps, so would be better suited to UFH than increased sized rads?

 

How would you tackle this? Just dig out the bare earth area and fill with concrete? 

[Olf]

 

 

 

 

24 minutes ago, elite said:

The house is not on mains gas, so the plan is for heating to be provided by ASHP, my understanding is that this runs more efficiently at lower flow temps, so would be better suited to UFH than increased sized rads?

Correct: you will recover some of the cost of going UFH route by saving space and flexibility on the layout. Warm feet being a 'freebie'

 

On 01/12/2021 at 10:43, elite said:

Can anyone advise on depth I need to dig down and the prefered make up of the floor?

 

Only structural engineer.

Possibly test pits can be done from outside so you don't have holes in the floor, but it is down to SE to decide.

I'd go passive slab:

- sand blinding

- DPM

- depending on how deep you have to/must not go (existing foundations depth may be against you) and how will it be dug up, then (together with your budget) you have options of using EPS or PIR as floor insulation and deciding on thickness - this bearing in mind that investment in extra thickness upfront means payback (lower losses) long term.

- PE separation (250um DPM)

- 100mm (unless SE requires thicker) concrete on top

with perimeter insulation 

 

[Gone West]

2 hours ago, elite said:

Thanks @Gone West

 

The house is not on mains gas, so the plan is for heating to be provided by ASHP, my understanding is that this runs more efficiently at lower flow temps, so would be better suited to UFH than increased sized rads?

 

How would you tackle this? Just dig out the bare earth area and fill with concrete? 

I built a PH with an Isoquick  insulated raft foundation which had 300mm of EPS under the concrete slab. Obviously not something you can do but I would want at least 150mm PIR insulation under a concrete slab. It would mean digging out probably below the foundations which would mean getting advice from a structural engineer. An alternative would be to use an air to air ASHP

which would mean you would only have to carry out repairs to the floor. There are people who are very happy with their A2A systems and The Green Building Forum has contributors who have given details.

[elite]

Many thanks @Olf & @Gone West

 

I'll sound out a friendly SE and then see where it is best to go from there

[fletcher5555]

@elite are you able to provide an update on this thread? What did you decide to do in the end?

[elite]

@fletcher5555 The project has be delayed by a structural wall that needed to be rebuilt or more accurately replaced with a steel. Once the steel is in place, we'll be putting various test pits in as the property has been extended over the years.

[fletcher5555]

Cheers @elite, think we will need to do something similar but please keep us updated as this thread is mega useful