DPM cut short

[Bobo]

Concrete slab installed in extension. 

 

DPM was cut flush with top of slab with no allowance to lap over onto blockwork/dpc. Also not fully flush as can see tip of dpm in some areas and not in others, so potentially a bit low in some sections. 

 

Blockwork not started yet. 

 

Is this ok? Or does it need to be fixed and how? 

 

Builder suggesting lapping some dpm onto existing and into block/dpc. Am unsure how this can be done in reality as concrete poured so insulation and dpm are tight up against rising wall so no space to get in at it. 

 

Thanks

[markc]

Do you have a pic or quick sketch to show us the detail? as its an extension there are circumstances where you want/need to break the DPM to prevent channelling of water and collecting in cavity etc.

[MortarThePoint]

Is the current DPM between the insulation and poured concrete?

 

A sketch or photo would help understand

[Bobo]

Thanks for coming back to me.

 

See diagram of current. Photo to follow

 

dpm cut off sketch.pdf

[MortarThePoint]

I suspect the orange DPM would only really need to extend up another ~100mm join with the DPC (blue) to perform its function * . The DPM doesn't need to cross the cavity as far as I understand. You have a break in the damp protection between the top of the orange and the blue DPC.

 

Is the first course of blockwork against the concrete floor in place yet?

 

* Please confirm it is only against damp not gas (eg Radon etc)

[Bobo]

Photo as of today.

 

Cant see how we would get the "green" line on sketch into place and properly lapped given how tight this is.

[Bobo]

The membrane happens to duplicate as both radon barrier and dpm.

 

However not high radon area so we are more concerned about preventing water ingress and rising damp down the line.

[MortarThePoint]

Just now, Bobo said:

Photo as of today.

 

Cant see how we would get the "green" line on sketch into place and properly lapped given how tight this is.

 

That answers my question, the first course of blocks is in place (pink in the sketch below). Drastic, but you could remove that course of blocks to join to the orange DPM, but I'm sure someone has a better idea

 

[Bobo]

Thanks MortarThePoint.

 

That's what I had concluded but was wondering if there is any other solution.

 

How about if green line did not turn down to meet the orange but was carried straight out onto slab for say 150mm?

 

[MortarThePoint]

1 minute ago, Bobo said:

The membrane happens to duplicate as both radon barrier and dpm.

 

However not high radon area so we are more concerned about preventing water ingress and rising damp down the line.

 

Ah, that's very different then. To protect against gas you need to have it continuous across the cavity. You either have to have a radon barrier or not and if you have to have it it has to be installed correctly. Could be very expensive to correct later if building regulations get worked up about it, or your Warranty provider.

 

Is the material you have referred to as DPM actually a certified radon barrier.

 

To key options come to mind:

1. Take down the first course of bocks to expose the orange DPM/Radon barrier and make a good solid joint to new sections.
2. Apply a Radon barrier on top of the concrete floor slab, across the cavity. This would require a cavity tray on top of the radon barrier which is a nuisance.

[MortarThePoint]

9 minutes ago, Bobo said:

How about if green line did not turn down to meet the orange but was carried straight out onto slab for say 150mm?

 

I think it's more about the Radon gas side of things. Something in your favour is it looks like there is around 50mm of edge insulation.

Edited May 24, 2021 by MortarThePoint

[MortarThePoint]

11 minutes ago, Bobo said:

How about if green line did not turn down to meet the orange but was carried straight out onto slab for say 150mm?

 

I think your idea would work against the damp as the perimeter insulation may not wick water, but it wouldn't satisfy the Radon requirement as far as I know, but my knowledge there is very limited.

[MortarThePoint]

The suggested fix in green is likely to be the most pragmatic solution. They will have to scrap out the perimeter insulation a good depth (e.g. 75mm), tuck in some double sided butyl tape and the attach the green portion. Pig of a job unfortunately

 

I think this is the sort of tape you'd want.

https://www.permagard.co.uk/radbar-double-sided-tape-dpm-tape

Edited May 24, 2021 by MortarThePoint

[Bobo]

Thanks for your help MortarThePoint

[MortarThePoint]

1 hour ago, Bobo said:

Thanks for your help MortarThePoint

 

No worries, happy to try to help. I've found these bumps in the road can be stressful, but are part of the journey. Solve them promptly and completely so they don't come back to haunt you and cost much more to resolve

[Carrerahill]

3 hours ago, Bobo said:

Concrete slab installed in extension. 

 

DPM was cut flush with top of slab with no allowance to lap over onto blockwork/dpc. Also not fully flush as can see tip of dpm in some areas and not in others, so potentially a bit low in some sections. 

 

Blockwork not started yet. 

 

Is this ok? Or does it need to be fixed and how? 

 

Builder suggesting lapping some dpm onto existing and into block/dpc. Am unsure how this can be done in reality as concrete poured so insulation and dpm are tight up against rising wall so no space to get in at it. 

 

Thanks

Who cut it - if not you, have them solve it, that's a right bog up. 

[MortarThePoint]

@Bobo just double check they have used the correct sheet material for the membrane as well since they may not have understood what was required and I can't see any red membrane showing in your photo (actually some Radon membranes look to be green as well). I expect joints would need to be overlapped and taped as well which may not be the case for DPM. Potentially a bigger problem if they have used the wrong membrane, but it may make for an easier solution as it potentially narrows down your options as you'd probably have to have a fresh membrane over the top of the concrete slab. It would be a pain to live with during the build though, so you may prefer to go a 'perimeter and infill' approach where you put gas proof DPC in all the walls sticking out enough (e.g. 100mm) and then later, when less likely to get damaged, you stick in areas of gas proof membrane attached to the DPC using that gas proof tape. Hopefully obvious, but that gas proof DPC would have to be under ALL walls, so include internal walls. You'd be making a continuous barrier out of the DPC and later applied membrane.

 

You don't seem fully convinced you need a Radon barrier. Double check that as I don't think there are any grey areas in it from a regulations perspective. Maybe you wanted it as you are outside (but near) a required area, but is it in the existing house and if so how are you joining to it?

Edited May 24, 2021 by MortarThePoint

[Bobo]

6 hours ago, Carrerahill said:

Who cut it - if not you, have them solve it, that's a right bog up. 

Agreed re bog up. 

[Bobo]

5 hours ago, MortarThePoint said:

@Bobo just double check they have used the correct sheet material for the membrane as well since they may not have understood what was required and I can't see any red membrane showing in your photo (actually some Radon membranes look to be green as well). I expect joints would need to be overlapped and taped as well which may not be the case for DPM. Potentially a bigger problem if they have used the wrong membrane, but it may make for an easier solution as it potentially narrows down your options as you'd probably have to have a fresh membrane over the top of the concrete slab. It would be a pain to live with during the build though, so you may prefer to go a 'perimeter and infill' approach where you put gas proof DPC in all the walls sticking out enough (e.g. 100mm) and then later, when less likely to get damaged, you stick in areas of gas proof membrane attached to the DPC using that gas proof tape. Hopefully obvious, but that gas proof DPC would have to be under ALL walls, so include internal walls. You'd be making a continuous barrier out of the DPC and later applied membrane.

 

You don't seem fully convinced you need a Radon barrier. Double check that as I don't think there are any grey areas in it from a regulations perspective. Maybe you wanted it as you are outside (but near) a required area, but is it in the existing house and if so how are you joining to it?

Thanks MortarThePoint. 

 

"Green line" solution in progress. Assured radon membrane has been used¿. Removing insulation to expose dpm and tape new section. Will be awkward getting insulation back into place neatly. Hope solving one issue doesnt cause a thermal bridging problem! 

[MortarThePoint]

2 hours ago, Bobo said:

Thanks MortarThePoint. 

 

"Green line" solution in progress. Assured radon membrane has been used¿. Removing insulation to expose dpm and tape new section. Will be awkward getting insulation back into place neatly. Hope solving one issue doesnt cause a thermal bridging problem! 

 

You've bags of room to get insulation in there. If you've used EPS there up until now it could be replaced with PIR and that's 2/3 the thickness for same performance. You could research spray foam as may work well to fill the gap. Likely 20mm of PIR plus spray foam a nicely workable solution.

[Bobo]

22 hours ago, MortarThePoint said:

 

You've bags of room to get insulation in there. If you've used EPS there up until now it could be replaced with PIR and that's 2/3 the thickness for same performance. You could research spray foam as may work well to fill the gap. Likely 20mm of PIR plus spray foam a nicely workable solution.

Thanks