Advice to retrofit solid floor insulation

[Sophiae]

Any advice would be greatly appreciated.

 

I am about 6 weeks from finishing off a rear extension which would have the open plan living, kitchen/diner area. 
I’m using 150mm Rigid insulation for the floors and warm roof deck to be as energy efficient as possible within budget. 
 

My problem will be the existing building floors as I have upgraded the loft and wall insulation. I absolutely can’t afford any rise in floor level as the ceiling is already below 2.4m.
The existing space is about 80-90m2, 3 rooms, one bigger open space and a side bathroom. 

The question is, assuming the existing solid/concrete floor isn’t insulated, how doable is it to dig the floor for 30cm or more, lay down 100-150mm PIR then follow by UFH and screed? 

[jayc89]

58 minutes ago, Sophiae said:

Any advice would be greatly appreciated.

 

I am about 6 weeks from finishing off a rear extension which would have the open plan living, kitchen/diner area. 
I’m using 150mm Rigid insulation for the floors and warm roof deck to be as energy efficient as possible within budget. 
 

My problem will be the existing building floors as I have upgraded the loft and wall insulation. I absolutely can’t afford any rise in floor level as the ceiling is already below 2.4m.
The existing space is about 80-90m2, 3 rooms, one bigger open space and a side bathroom. 

The question is, assuming the existing solid/concrete floor isn’t insulated, how doable is it to dig the floor for 30cm or more, lay down 100-150mm PIR then follow by UFH and screed? 

 

Doable. I've done it. We were lucky that we could get a micro-digger and dumper into the house to make our lives easier. I certainly wouldn't consider it in a house that was being lived in though, it made a right mess.

 

If I was to do it again though, I wouldn't use screed, I'd use the conc as the finished floor, tying UFH to rebar in it, which should also save some depth. 

[Olf]

Very doable, but watch for:

- what is your current floor construction? Make a test dig in a place you can easily fill in if you decide not to progress. I made 20x20cm hole under fireplace hearth and got good idea of the scale of a challenge

- BC: agree what floor buildup is acceptable so you know exactly how deep you have to dig. I ended up with 150mm hardcore+50mm lean concrete blinding (what BC reqested and pi$$ed me off)+150mm PIR+100mm slab with rebar and UFH: total of 450mm meaning digging 350mm below existing slab. Don't think about reducing insulation thickness to save digging: it is not a passivhouse, your floor temperature will be at least 30°C  and the last thing you want to do is to send too much heat into the ground.

- test hole should also give you idea about soil condition: some are easier to excavate, some (clay...) are proper pain.

- it can be done by hand, great exercise for back muscles

- plan timing: do it after the extension is done and the area leading to entrance should be the last and done like F1 pitstop, otherwise life is challenging for few days

[Kai casswell]

I've done it as well, but we weren't living in the house and we could get a micro digger in for bits. We also hire a little motorised dumper. We kept all the waste and crushed it down to hardcore. 

 

I also forced my family to help:

 

[Sophiae]

OMG, you people have absolutely brightened my morning. I nearly haven’t slept thinking about it and now knowing I am not crazy thinking it too could very much work. 
 

So here’s my plan, once BC is back to check on the roof, I will ask him to check the floor and we should agree on the make up. 
 

I am all in favour of reusing everything we will excavate, so it should all be reused in the base below the insulation. I believe 150mm Celotex will do the job. 
Then it’s a decision on whether to use UFH the traditional way (in screed) or what @jayc89 suggested. Would it be possible to send me some pictures of your UFH? I am interested in seeing how you fit it in. 
 

@Kai casswell thank you so much for the pictures, I can see me doing very much the same, no family though so paying a handyman to help should be ok for a few days. 

@Olf I shall ask the guys to dig a hole like you recommended in one of the corners and see what we find. 
Is there any other way to find out what’s underneath the slab? 

 

[jayc89]

AFAIK your sub-base has to be graded as there is a certain specification it has to adhere to, so I wouldn't have thought it would be as easy as relaying the rubble you've dug up, but it should be useful for non structural use cases (landscaping etc). FWIW, I used recycled MOT 1, which is essentially crushed concrete, but done in a way to ensure it complies with that specification.

 

This is how I would lay a slab next time - 

 

[Olf]

12 minutes ago, Sophiae said:

Is there any other way to find out what’s underneath the slab?

Not really, going under the foundations would be harder...

By the way, as you already dug for the extension, BC should have very good idea about ground conditions, so you test hole in the floor will not need to go too deep: just enough to asses existing floor buildup.

 

13 minutes ago, Sophiae said:

I am interested in seeing how you fit it in

Something like this: UFH pipes tied (with cable ties, easier than rebar wire) to the mesh that is sitting on 'chairs' (but other options of keeping the rebar elevated are possible)

 

[Sophiae]

@Olf Thank you so much. I can see it all clearly now. 
Yes, I am very much counting on BC to visit ASAP and help me figure out the final verdict on the floor for the original build. I feel excited knowing this is doable, and seeing your pictures makes it all coming true for me soon too. 

 

@jayc89 Thank you for the video. The space is massive compared to mine, but I see what you mean. I will definitely try as much as I can to reuse everything I dig out, if not for the base, the for the garden like you suggested. I have a slope which I am not happy about in the front and in the garden so will use to level both if possible. 

Edited May 9, 2023 by Sophiae

[Sophiae]

Another follow up question if I may. 
Do I need a screed on top of the Concrete or 150mm of concrete (skipping the whole screed layer) would be enough? 
PIR insulation boards above or below concrete work better when using UFH?

[Dave Jones]

150mm hardcore (100 absolute minimum), 100mm celotex or 150mm EPS (cheaper), 75mm screed.

[Sparrowhawk]

2 hours ago, Sophiae said:

Another follow up question if I may. 
Do I need a screed on top of the Concrete or 150mm of concrete (skipping the whole screed layer) would be enough?

Hi @Sophiae and welcome to the forum. Funnily enough I was reading old threads yesterday on this and no-screed. Here's a selection from 4-5 years ago. They do focus on the problems that can happen with no screed and I'd like to hear from forum members if the workmanship of the slab has improved in their experience since then?

 

 

[Conor]

2 hours ago, Sophiae said:

Another follow up question if I may. 
Do I need a screed on top of the Concrete or 150mm of concrete (skipping the whole screed layer) would be enough? 
PIR insulation boards above or below concrete work better when using UFH?

 

100mm/150mm concrete would be your final layer. So set the top of this later at the current height of your floor slab and work down.

 

As you'll be flooring directly on tjis, it needs to be properly mechanically floated and finished well to ensure a very smooth and level surface for tiling etc. Make sure the people doing this job know this.

 

PIR is fine under concrete, with proper DPM etc in place.

 

Wishing you the best, the mess will be colossal for a few days, but totally worth it.

[Dave Jones]

the issue you will have concrete as a finished floor WILL NOT be flat and level no matter how much you try. Screed will.

 

So by using concrete you are guaranteeing you will have to then level the floor after. 

[jayc89]

The guy who did our screed also works with a 10mm aggregate flowing concrete and he said it comes out pretty level. If his screed work is anything to go by (tiler said it was the best floor he'd ever worked on), I'm inclined to believe him. (Although, caveat, I haven't seen it for myself). 

[Dave Jones]

22 minutes ago, jayc89 said:

The guy who did our screed also works with a 10mm aggregate flowing concrete and he said it comes out pretty level. If his screed work is anything to go by (tiler said it was the best floor he'd ever worked on), I'm inclined to believe him. (Although, caveat, I haven't seen it for myself). 

 

screed yes, concrete no. Different products.  Screed everytime.

[Sophiae]

I am ever so grateful for all your replies. 
And in conclusion there is a good justification in applying screed as it is almost the only guarantee towards having a level surface for the finished flooring, unless I get a hot shot OCD like myself who aims for nothing short of perfection to have the concrete all level 😂. 
 

Understanding the pros and cons it makes me think should I choose screed, I will need to not go more than 100mm for the concrete, then UFH in screed of 65mm as standard. Otherwise I risk raising the floor again which I can’t afford height wise. 

 

[Nickfromwales]

On 08/05/2023 at 10:19, Sophiae said:

Would it be possible to send me some pictures of your UFH? I am interested in seeing how you fit it in. 

 

The 1st 2 images are pipes in the concrete slab, cable tied to the steel mesh, and the 3rd image is of pipes laid on a slab which then got screeded over.

[haythorn_1]

Interested to see how you get on @Sophiae. I asked a similar question a few months ago and from the responses it seemed it would be possible to match the level of two concrete slabs poured separately:

 

 

 

[Dave Jones]

you dont need concrete and screed.

 

hardcore - insulation - screed. 

[Sophiae]

On 10/05/2023 at 07:12, Dave Jones said:

you dont need concrete and screed.

 

hardcore - insulation - screed. 

How come? I know you need concrete as part of your flooring as I don’t have anything else for a base for the property. 

[Sophiae]

On 09/05/2023 at 19:19, haythorn_1 said:

Interested to see how you get on @Sophiae. I asked a similar question a few months ago and from the responses it seemed it would be possible to match the level of two concrete slabs poured separately:

 

 

 

I will keep you posted. 
i have an appointment on Monday with a Grounds engineering company who will come round and give me a quote.

he already said there is likely 50mm of EPS as insulation from his experience of the year it was all built. 

[Sophiae]

Good evening everyone

I pray you are all having a good weekend.

I’ve been trying to get more information on my existing floors and just found out that the property was built between 1965-1972 and the solid floors (concrete) are assumed to have zero insulation. 
Now, how would I know what is the depth of that concrete slab other than dig lol ;0) 

If I dig, it would be placing 150mm rigid PIR then UFH and 65-75 screed. Would that be reasonable? 
What do I do about the DPM? 

[Olf]

On 27/05/2023 at 22:54, Sophiae said:

the property was built between 1965-1972 and the solid floors (concrete) are assumed to have zero insulation. 
Now, how would I know what is the depth of that concrete slab other than dig

It is probably 100mm, but that does not change you situation a lot: you need to dig up enough for the new structure to finish at existing floor level.

 

On 27/05/2023 at 22:54, Sophiae said:

If I dig, it would be placing 150mm rigid PIR then UFH and 65-75 screed

over 100-150mm slab over 50mm blinding over 100-150mm compacted hardcore. This much digging is necessary.

By the way, using PIR is typically the most cost effective way, as offets cost of digging and disposing against more expensive insulation. However, there are cases (soil easy to work with, can be used locally eg to level the outside) where using EPS (even though 250-300mm needed) is better choice. Unlikely here, but do your maths.

On 27/05/2023 at 22:54, Sophiae said:

What do I do about the DPM?

Over the blinding layer (under insulation), binded to the existing DPC. That is likely some bitumen stuff, so Synthaprufe/Black Jack type of liquid membrane to link old and new together.

Also another layer (not DPM as such, can be thinner, but also can use leftovers of DPM if enough left) over the insulation so screed does not flow through any missed gaps and reduces to 0 risk of any chemical reaction with aluminium film.

[Sophiae]

On 09/06/2023 at 09:06, Olf said:

It is probably 100mm, but that does not change you situation a lot: you need to dig up enough for the new structure to finish at existing floor level.

 

over 100-150mm slab over 50mm blinding over 100-150mm compacted hardcore. This much digging is necessary.

By the way, using PIR is typically the most cost effective way, as offets cost of digging and disposing against more expensive insulation. However, there are cases (soil easy to work with, can be used locally eg to level the outside) where using EPS (even though 250-300mm needed) is better choice. Unlikely here, but do your maths.

Over the blinding layer (under insulation), binded to the existing DPC. That is likely some bitumen stuff, so Synthaprufe/Black Jack type of liquid membrane to link old and new together.

Also another layer (not DPM as such, can be thinner, but also can use leftovers of DPM if enough left) over the insulation so screed does not flow through any missed gaps and reduces to 0 risk of any chemical reaction with aluminium film.

Thank you so much @Olf. 
I am starting with the extension first as I need a place to sort of use whilst I embark on the rest of the property. 
I very likely will need to go as deep as I need to get it all level with the extension and guarantee the 150mm insulation.

The plan is to also hire a crusher so that I can recycle everything that comes out from the concrete and add extras rather than use skips and buy all new. 
 

If you don’t mind I will keep writing here seeking further advice as I start. 

[Sophiae]

Posted on another thread. What do people think of using these instead of hand machines saving on labour costs as my back will not permit me doing the work. 
How close to the load bearing wall should I be digging? 
 

http://www.dceplanthire.co.uk/diggers