1965-1972 concrete slab

[Sophiae]

Good evening my family of experts 

 

I pray all of you are well.

 

Did a bit of digging and found out the property (single floor detached bungalow) was built between 1965-1972 as part of a larger development at the time.

There was no insulation put underneath the concrete slab for the full property, and no Radon membrane either.

 

When I bought the property it was assumed it was low risk. A couple of weeks ago the BCO said we need one as it is high risk area. 
I went on the Radon UK website and said it was higher risk and needed protection. 
 

I am in the middle of getting building regs drawings for a proposed single story rear extension (planning granted) and want to know what do I do that would protect me and my little man?
All suggestions are welcomed.

 

[Mike]

19 minutes ago, Sophiae said:

I went on the Radon UK website and said it was higher risk and needed protection.

That's a good source of information. They also have some information about the most common solution - a retrofitted sump - here: https://www.ukradon.org/information/reducelevels_sump

[TonyT]

Get the architect to specify a product and produce a detail for it to ensure it’s correctly installed 

[Sophiae]

1 hour ago, Mike said:

That's a good source of information. They also have some information about the most common solution - a retrofitted sump - here: https://www.ukradon.org/information/reducelevels_sump

Thank you so much. 

[Sophiae]

40 minutes ago, TonyT said:

Get the architect to specify a product and produce a detail for it to ensure it’s correctly installed 

Will do. 
what about the insulation? Shall I still excavate, insulate then lay new concrete for each room? 

[Mike]

8 hours ago, Sophiae said:

Shall I still excavate, insulate then lay new concrete for each room?

Not for the Radon. Only if you have other good reasons to do so - for example as part of a package of comprehensive insulation measures or to install underfloor central heating. But even then replacing the floor is one of the last things to consider.

[saveasteading]

Any radon gas coming up through the ground will take the easist route. A standard house on a development is less likely to be an outlet than the drains around the estates, as they are lower, surrounded by gravel, and vent to air.

So don't worry about radon, but put a barrier in the new area according to the rules.

[TonyT]

12 hours ago, Sophiae said:

Will do. 
what about the insulation? Shall I still excavate, insulate then lay new concrete for each room? 

Ask the Architext who is familiar with your local authority requirements it’s one way to ensure a compliant design

[Sophiae]

18 hours ago, Mike said:

Not for the Radon. Only if you have other good reasons to do so - for example as part of a package of comprehensive insulation measures or to install underfloor central heating. But even then replacing the floor is one of the last things to consider.

I need insulation 😞 I discovered there are none. 

[Sophiae]

@ETC would you kindly help me figure this out. 
Finally after 4 weeks or so I got the B-regs done. BCO happy but sent me this note yesterday. 
“the section through the foundations, oversite and dpc is missing a second radon barrier Tray but I can explain on site providing I inspect before you build”. 
 

I want to understand what he means prior to his visit as I won’t be able to be there on every occasion (childcare issues). Any drawing would be greatly appreciated. 

Edited July 6, 2023 by Sophiae

[Iceverge]

Strange detail that. Not to fear, plenty of expertise here.

 

Are you planning on UFH? 

 

A simpler detail for the floor might be. 

 

1. Finished floor. 

2. 100mm powerfloated concrete slab with UFH stapled to insulation. 

3. Slip membrane. 

4. Insulation ( as much as you can fit) EPS is cheaper and better than PIR in my opinion but thicker. 

5. Radon Membrane. It's just a slightly thicker sheet of plastic with all tapes welded/taped so does the job of the DPM. 

6. Sand binding. 

7. Hardcore. 

 

You'll have a significant thermal bridge at the wall floor junction there. You need an aerated concrete block in line with the insulation and to take the cavity insulation down to the plane of the floor insulation. 

 

150mm full fill EPS beds would be my choice as it can be used to insulate below ground level and is harder to muck up and will give a U-Value of 0.21W/m2K. 

 

I might do a drawing....

[Sophiae]

2 minutes ago, Iceverge said:

Strange detail that. Not to fear, plenty of expertise here.

 

Are you planning on UFH? 

 

A simpler detail for the floor might be. 

 

1. Finished floor. 

2. 100mm powerfloated concrete slab with UFH stapled to insulation. 

3. Slip membrane. 

4. Insulation ( as much as you can fit) EPS is cheaper and better than PIR in my opinion but thicker. 

5. Radon Membrane. It's just a slightly thicker sheet of plastic with all tapes welded/taped so does the job of the DPM. 

6. Sand binding. 

7. Hardcore. 

 

You'll have a significant thermal bridge at the wall floor junction there. You need an aerated concrete block in line with the insulation and to take the cavity insulation down to the plane of the floor insulation. 

 

150mm full fill EPS beds would be my choice as it can be used to insulate below ground level and is harder to muck up and will give a U-Value of 0.21W/m2K. 

 

I might do a drawing....

A drawing would be much appreciated. I couldn’t understand your layers. Apologies (visual aid needed). 

[Nickfromwales]

22 minutes ago, Sophiae said:

A drawing would be much appreciated. I couldn’t understand your layers. Apologies (visual aid needed). 

Have a look here and watch the video  

 

https://www.mbctimberframe.co.uk/passive-house/passive-foundations/

[Iceverge]

 

 

Points to note

 

1. I've moved the Radon membrane between the two layers of EPS and forgone the Sand binding as the EPS will protect the membrane. You could then use 150mm X 2 of Floor insulation then which will give you a U value of about 0.11. 

 

2. I've put a French Drain at the perimeter which is always a good idea to keep the bottom of the walls dry and prevent splashing. 

 

3. The cavity insulation extends well below the floor insulation. This will ensure any mortor droppings don't fill the cavity and create a thermal bridge here. 

 

4. The PIR upstand is "pinned" in place by the floor insulation. Otherwise it comes loose during the floor pour. 

 

5. Let the slip membrane extends well beyond the internal leaf during the pour and trim it back later. It'll protect the Radon membrane and the PIR upstand during screeding. 

 

6. I've included the block sizes to allow for normal cavity tie spacings. 

 

 

 

 

 

 

 

[JackofAll]

58 minutes ago, Iceverge said:

 

 

Points to note

 

1. I've moved the Radon membrane between the two layers of EPS and forgone the Sand binding as the EPS will protect the membrane. You could then use 150mm X 2 of Floor insulation then which will give you a U value of about 0.11. 

 

2. I've put a French Drain at the perimeter which is always a good idea to keep the bottom of the walls dry and prevent splashing. 

 

3. The cavity insulation extends well below the floor insulation. This will ensure any mortor droppings don't fill the cavity and create a thermal bridge here. 

 

4. The PIR upstand is "pinned" in place by the floor insulation. Otherwise it comes loose during the floor pour. 

 

5. Let the slip membrane extends well beyond the internal leaf during the pour and trim it back later. It'll protect the Radon membrane and the PIR upstand during screeding. 

 

6. I've included the block sizes to allow for normal cavity tie spacings. 

 

 

 

 

 

 

 

What is the slip membrane for?

[Nickfromwales]

1 minute ago, JackofAll said:

What is the slip membrane for?

It's unnecessary with a concrete pour afaic, but necessary with a Flocrete or liquid screed. Radon is down low with Kore and MBC, so doesn't need much more protection other than at the perimeter, where it is exposed for a while. The upper level of EPS becomes the 'sacrificial layer' and protects it.

[Sophiae]

1 hour ago, Iceverge said:

 

 

Points to note

 

1. I've moved the Radon membrane between the two layers of EPS and forgone the Sand binding as the EPS will protect the membrane. You could then use 150mm X 2 of Floor insulation then which will give you a U value of about 0.11. 

 

2. I've put a French Drain at the perimeter which is always a good idea to keep the bottom of the walls dry and prevent splashing. 

 

3. The cavity insulation extends well below the floor insulation. This will ensure any mortor droppings don't fill the cavity and create a thermal bridge here. 

 

4. The PIR upstand is "pinned" in place by the floor insulation. Otherwise it comes loose during the floor pour. 

 

5. Let the slip membrane extends well beyond the internal leaf during the pour and trim it back later. It'll protect the Radon membrane and the PIR upstand during screeding. 

 

6. I've included the block sizes to allow for normal cavity tie spacings. 

 

 

 

 

 

 

 

My arrangement was made to match the pre-existing floor type (solid concrete). The only difference is that existing slab doesn’t have any insulation as it was built pre-insulation era. The extension is rather small 4X9m at the most as the walls are 35cm housing the 2023 150mm cavity insulation. The existing cavity walls will have the upgraded beads installed into them as we only managed to add 25mm PIR to them prior to plastering. More than that would eat more of my living space which is already small. 
 

Yes I intend to use UFH as radiators are a health hazard for my son, so I needed to make up the floor layers like that to have the most energy efficient floorings I can get. 
The existing building floors will be subsequently dug and insulation will be laid and concrete poured again. A Radon Sump will be installed to mitigate all Radon from the original build. 
 

Going back to the BCO comment about a second tray. Where would that be on the drawing as I failed to understand?. 

[Sophiae]

This is what I followed. But now he wants a second one? 

[saveasteading]

I expect there is a misunderstanding. One is enough, and can also act as the dpm..

but the original design wasn't quite right so maybe he meant something else. It's great that he's being helpful

 

[Sophiae]

9 minutes ago, saveasteading said:

I expect there is a misunderstanding. One is enough, and can also act as the dpm..

but the original design wasn't quite right so maybe he meant something else. It's great that he's being helpful

 

Thank you. What do you see is wrong about the original design. It was suggested here and the only thing I did was increase the PIR to 150 instead of the 100. What seems to be the problem? 

[Sophiae]

Could it be that?  

TD-JUTA.GP4_.016-Ground-Bearing-Slab-Perimeter-Detail-Membrane-Below-Slab.pdf

Edited July 6, 2023 by Sophiae

[saveasteading]

Yes. The radon barrier needs to continue to the outside, so that any radon gas disappears into the atmosphere. So the 'second' layer may be what is shown dark blue: a strip up the wall and out.

 

I don't know Juta products. They seem to cost more though as they are red and have "radon barrier" printed on them.

[Sophiae]

4 minutes ago, saveasteading said:

Yes. The radon barrier needs to continue to the outside, so that any radon gas disappears into the atmosphere. So the 'second' layer may be what is shown dark blue: a strip up the wall and out.

 

I don't know Juta products. They seem to cost more though as they are red and have "radon barrier" printed on them.

Thank you very much for explaining. I won’t be buying their products, I found their diagrams very useful so I wanted to share with copyrights intact. 

[Iceverge]

6 hours ago, JackofAll said:

What is the slip membrane for?

 Stop any chance of the EPS floating if the concrete was too watery and to protect the perimeter strip and DPM during screening. Also if you use PIR isn't there a risk of the aluminium foil reacting with the cement and making bubbles in the concreting?

 

 

Thought it was best practice but more than happy to be corrected. Less components is always better than more! We didn't use one by the way. 

[Sophiae]

4 minutes ago, Iceverge said:

 Stop any chance of the EPS floating if the concrete was too watery and to protect the perimeter strip and DPM during screening. Also if you use PIR isn't there a risk of the aluminium foil reacting with the cement and making bubbles in the concreting?

 

 

Thought it was best practice but more than happy to be corrected. Less components is always better than more! We didn't use one by the way. 

I plan to use VCL on top of the PIR for protection. I believe a small investment here goes a long way.