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Removing concrete floor in 70s build

houseyhouse

We have 1970s semi-d that we plan to renovate and extend in the near future. The ground floor is over a foot above ground level. My neighbour is renovating now and says there is no insulation underneath the ground-floor concrete slab. He’s putting insulation on top of the existing floor and then pouring a new layer of concrete screed over. Given that our ceiling is barely 8 foot to begin with, I’m not keen on this solution. What would the cost implications be of removing the existing concrete slab, insulating and repouring vs putting insulation and concrete screed on top of the existing floor? Even better if we could lower the floor further to get more head room downstairs. Is that possible?

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Dudda

I'd much prefer digging out the existing floor and is what I did on my 1979 house. We had circa 75mm screed on 25mm of insulation on top of the concrete subfloor. We had a split level house and had almost three meter ceilings in one area so removed the screed, kept the existing sub floor, put down radon, 150mm of insulation and a 100mm polished concrete floor on top of this. This meant all doors were now 150mm to low. I had a massive concrete ring beam above the front door and had to hack off a large chunk of this to keep the front door height. That was harder than removing the floor. The heights of the one other internal door was also raised. Also put in under floor heating. Outside I'm still trying to fix the outside steps as an extra step is now required (ran out of money / next years problem).

In the other area where we only had 2.4 meter high ceiling we removed everything including the sub floor and dug down another 200mm approx below that. We then put back a new sand binding, radon, concrete sub floor, insulation and the polished concrete. The advantages of this were we included a radon sump (although not in a high radon area). This was also the new kitchen area so ran waste pipes to an island and new sink and dishwasher locations. The original floor level was retained which was important due to the ceiling height but also we have a large slider that goes out to the garden and is level with the patio without any steps.

While retaining the sub floor was less work in digging out and one less concrete pour there are other issues like door and window heights that need to be fixed, outside steps or no longer having a level access for child buggies / wheelchairs. It doesn't work out as big a saving as you'd expect. Advantages in removing the sub floor include the ability to add loads more insulation, a proper radon membrane and sump, flexibility in locating ducts and waste pipes, etc.

Another thing to check is if any internal load bearing walls are built off the sub floor. The load bearing wall at the change in level had its own foundation so could be retained but another load bearing wall was built off the sub floor. Obviously you can't remove the sub floor and dig down while retaining a load bearing wall floating in mid air. This needs to be propped and gets very messy. Can you check the neighbors house (which I presume is the same as yours) and see if he has any load bearing internal walls and if they sit on the sub floor or have their own foundation? That would be very useful and important to know before beginning.

houseyhouse

That’s great information, thank you!

My neighbour also mentioned having to redo the internal doorways because of the change in floor height but I hadn’t thought about the front door being affected.

I’ll have to look into the load bearing walls. Certainly the wall between us and our attached neighbours is load bearing and I think it’s built on top of the slab. I’m guessing the slab for the two houses was poured in one go before the internal walls went in. We can’t go interfering with the shared wall so that might rule out digging up the existing slab for us.

You said you retained the existing floor level when you dug out the slab. Do you think it would have been possible to lower it? I’d love to have slightly higher ceilings and it seems like it wouldn’t be that much extra work if digging out the slab and replacing it anyway.

Dudda

You could have lowered it but in my case it wasn't practical due to the outside ground level. In your case it shouldn't be an issue. You will have to lower the entrance door which isn't a big problem. It's a lot easier than raising the height. Just make sure you wrap the new Radon barrier up to wherever the DPC layer in the wall is.

Yellow_Fern

There are fancy insulation types like vacuum panels that can allow you to get more insulation for the space you have but they cost a lot more.

Still stihl waters 3

Is the sole reason for removing floor to put in insulation op ? And how many square meters is floor area

houseyhouse

https://www.boards.ie/discussion/comment/118318253#Comment_118318253

Insulating to up the BER is the main reason. But I would be delighted if we could get higher ceilings at the same time. It’s about 46 square meters.

houseyhouse

https://www.boards.ie/discussion/comment/118318240#Comment_118318240

Do you headed to know how the cost of something like that would compare to replacing the slab? I’m happy to pay a little more to replace the slab if it would allow us to increase the head height.

houseyhouse

I saw they dug up the floor on Bungalow Bliss last week and it ended up costing the 70k 😬😱 That is exactly the situation I want to avoid!!

Still stihl waters 3

https://www.boards.ie/discussion/comment/118318504#Comment_118318504

The reason I ask is firstly if it makes economic sense to take up the floors to put down insulation if the money can be used elsewhere to improve the ber then that's the way I'd go but as you've 2 separate reasons it's not quite as straightforward, it's going to be costly to do what you're asking if you're starting off with a budget

houseyhouse

https://www.boards.ie/discussion/comment/118319536#Comment_118319536

We’re doing an extension so we need to increase the BER substantially. I got the impression from talking to our neighbour that insulating the floor was very important in that regard. My thinking was if we have to spend money to insulate floors anyway then I’d be happy to pay a bit extra to also lower the floor/ increase the head height. If it’s a big cost difference then it becomes a more difficult decision. I am very reluctant to raise the floors 150mm or whatever the standard insulation thickness is because the ceilings are only about 2450 to begin with.

MicktheMan

Looking at this from another viewpoint...

We renovated a 70's bungalow with a solid concrete uninsulated floor. For various reasons, we decided to leave the floor alone and externally insulate the walls ensuring that the ewi went down approx 1m below the ffl. Worked a treat in the sense that you wouldn't know from feel or touch that the floor had no insulation.

houseyhouse

https://www.boards.ie/discussion/comment/118321612#Comment_118321612

Oh very good! Do you mind me asking what your BER rating is after all the work? We’re doing an extension so we’re going to have to get ours up to a B2 under the current regs.

MicktheMan

https://www.boards.ie/discussion/comment/118322015#Comment_118322015

Poor, the official BER is C1 (but I pay no attention to it as I do not believe in the system of rating for existing housing which results in the BER ... but that's another story).

However:

We maintain a constant (24/7) 20 degC living area and 18 degC bedroom area during the heating season. I measure all energy inputs and monitor the internal temperatures etc. When all inputs of energy actually consumed are calculated and converted back to primary energy usage over the last 7 years, it performs at between 26 & 29 kWhr/m2/yr or A2 BER depending on the actual year in question.

houseyhouse

https://www.boards.ie/discussion/comment/118322287#Comment_118322287

Ok thanks. I agree the BER system has its flaws. Unfortunately, with the new buildings regs we’ll have to get it up to a B2 anyway.

MicktheMan

https://www.boards.ie/discussion/comment/118322416#Comment_118322416

Yes attaining a B2 is one way of complying but there are other ways where attaining a B2 is not feasible... perhaps worth exploring!

Yellow_Fern

https://www.boards.ie/discussion/comment/118318513#Comment_118318513

The difference in price is a lot. More than double for certain.

C. Eastwood

I would not recommend raising the Ground Floor level under any Circumstances whatsoever.

houseyhouse

https://www.boards.ie/discussion/comment/118331880#Comment_118331880

Do you mean, don’t insulate over the existing slab? I agree. I’m surprised the neighbour’s going that route given ceiling heights we have.

C. Eastwood

What I mean is do not under any circumstances raise OR lower the level of a Ground floor. Never.

it will cause many problems.

I Recommend that if you intend to insulate your existing concrete Ground floor, you should engage a Registered Building Surveyor or Chartered Building Engineer, etc. to carry out a survey of the existing house and advise the best way forward. It will cost a lot of money to insulate an existing concrete ground floor. Ensure it is done correctly and certified in accordance with the Building Regulations.

You can however leave the floor as is.

lf it is decided to insulate the ground floor (GF) it must be done in accordance with the Building Regulations (BR). The Technical Guidance Documents (TGD) give advice on how this must be done.

The 5 purposes of the BR are for the Health Welfare and Safety of Persons, Access for people with disabilities, and Conservation of fuel & energy.

These are very complicated and you need a professionally qualified person to survey, advise, supervise and certify when the works are completed. This Certificate of Compliance with the BR is very important when you go to sell the property.

C. Eastwood

Some of the BR are as follows:-

GF must be a minimum of 150 mm above external ground level.

Concrete GF must be minimum 150 mm thick of concrete

Minimum finished floor to finished ceiling must be minimum of 2400 mm.

the new insulated floor must have a maximum stated U value.

Radon sumps must be installed

A Radon Barrier MAY be required. ( put your Eircode in to the Ireland Radon Map on www.epa.ie) if a Radon Barrier is required fit it. If it’s not required- then fit an approved DPM.

The TGD’s are slightly different for Existing Buildings - and New Builds.

C. Eastwood

What I would normally Specify for an existing house - if it is possible-

Powerfloat finished minimum 150 mm concrete on 100 mm hi-density insulation, on 1200 gauge DPM (or) Radon Barrier (RB) on 25 mm sand blinding on minimum 150 well compacted hardcore with Radon sump/s and pipes under the DPM/RB.

In an existing house there will be many problems and many scenarios. Therefore you must retain a professional, specialising in house renovations.

Regards

CE.

houseyhouse

https://www.boards.ie/discussion/comment/118335329#Comment_118335329

Thank you for your detailed response. I really appreciate it. We won’t be doing any work on the house without a professional builder and an engineer’s sign off. I was really just wondering what alternatives there are to insulating on top of the floor slab and how they compare cost wise. I’m just worried about our budget disappearing into things like floor insulation when we also need to add space.

C. Eastwood

Houseyhouse. Thanks for your kind words.

Take a look at what Micktheman said above.

If a budget is a problem then don’t insulated the floor at this time. Use the money to insulate the walls, double glazing and insulate the ceiling in the roof.

There is very little heat lost through a solid concrete floor.

I would not invest money on insulating my floor which is 100 concrete on 1” (25 mm) aeroboard.

You could put laminate on 6 mm approved underflooring insulation on your ground floors.