Cavity/Floor Detail

[jayc89]

Looking at the cavity/floor detail for our extension. A few points for context;

 

• Existing house does not have any DPC
• Existing floor level is just below ground floor level, by approx 50mm, (and surprisingly, given the lack of DPC, has no damp...)
• Some of the final room layouts will span both the existing and extension floor space.

 

We're aiming for a sensible design, but not necessarily Passivhaus criteria, so are trying to follow the Golcar detail as near as possible. This is what we've come up with;

 

 

Ideally we'd have gone for insulation under the slab and UFH ran through the slab, not needing screed at all, but because some of the UFH loops need to span both the existing and extension floor space and we won't knock the existing walls down until the extension's water-tight, I'm not sure that would be possible.

 

The extension's final floor level is to remain at the same height as the existing final floor level, so we'd end up with some funky DPC set up for the extension detail we choose.

 

Does the proposed look sensible? Would you recommend any changes?

[ETC]

Put the DPM above the slab and look out for cavity tie locations in relation to the height of the tanking.

[jayc89]

Tie locations is a good shout. Why put the DPM on top of the slab? I generally thought it was a good idea to keep the slab on the dry side.

[ETC]

Differential settlement, reducing the height of any tanking membrane and connecting the inner wall DPCs to the DPM. Speak to your local Visqueen representative - he/she will do you a site specific detail. Don’t forget about radon if you are in a radon area.

[jayc89]

I guess a consequence of putting the DPM above the slab would be having the DPM exposed during construction and it almost certainly being compromised.

[ETC]

1 hour ago, jayc89 said:

I guess a consequence of putting the DPM above the slab would be having the DPM exposed during construction and it almost certainly being compromised.

The DPM will only be laid once the building is water tight and everything but the floor insulation and screed are to be completed. So it will not be exposed or compromised during construction.

[jayc89]

Just leaving a 100mm lap on the DPC to tape onto?

[Iceverge]

Ditch the XPS and just put EPS beads right down to the foundation. French drain outside. 

 

We didn't have any cavity tray either. 

 

If you can't have any French drain I would consider a perforated pipe at the base of the cavity draining out and down to a soakaway. 

 

I would add perimeter insulation to the slab too. 

 

 

 

 

Edited October 27, 2021 by Iceverge

[jayc89]

21 minutes ago, Iceverge said:

Ditch the XPS and just put EPS beads right down to the foundation. French drain outside. 

 

We didn't have any cavity tray either. 

 

If you can't have any French drain I would consider a perforated pipe at the base of the cavity draining out and down to a soakaway. 

 

I would add perimeter insulation to the slab too. 

 

 

 

 

 

What's the reasoning behind dropping the XPS?

I thought the introduction of Thermalite blocks, plus the full fill cavity would negate the need for a perimeter strip, is that not the case? (I did plan to include a perimeter strip where the screed joins the (currently external) original property's wall.

[cwr]

I suspect that perimeter strip would do more good than the Thermalites. Look up the u value's. I would do both. 

 

Your XPS detail is same as I had originally specified. I let my builder convince me to drop the cavity tray and blown EPS to the bottom of the cavity. Seemed reasonable at the time, but what I hadn't anticipated was how much the cavities would get filled with dropped mortar. No matter how many times I asked them to be careful... With hindsight a cavity tray would have caught the droppings which the brickies would have had to somehow clean out. Maybe that was their real reason to suggest no XPS and tray...

 

 

[jayc89]

37 minutes ago, cwr said:

I suspect that perimeter strip would do more good than the Thermalites. Look up the u value's. I would do both. 

 

Your XPS detail is same as I had originally specified. I let my builder convince me to drop the cavity tray and blown EPS to the bottom of the cavity. Seemed reasonable at the time, but what I hadn't anticipated was how much the cavities would get filled with dropped mortar. No matter how many times I asked them to be careful... With hindsight a cavity tray would have caught the droppings which the brickies would have had to somehow clean out. Maybe that was their real reason to suggest no XPS and tray...

 

 

 

Assuming I'm using this calculator correctly - https://www.changeplan.co.uk/u_value_calculator.php

 

Without a perimeter strip the wall construction would be around 0.19 W/mK and with a 25mm PIR perimeter strip it reaches 0.16 W/mK

Edited October 27, 2021 by jayc89

[cwr]

Sounds about right.

And with UFH def do the perimeter strip.

[Iceverge]

Our Foundation With perimeter insulation internal U value 0.1689

 

 

 

And without the PIR 25mm upstand. 

 

It's a small improvement but worth it I think. 

 

 

[Iceverge]

 

3 hours ago, jayc89 said:

What's the reasoning behind dropping the XPS?

 

 The EPS beads are easier to install tight to the wall and cheaper.

 

Like @cwr says mortar droppings are your enemy. I would prefer to have them at the bottom of the cavity rather than sitting on top of my XPS insulation. Golcar used mineral wool batts to avoid this but this also requires conscientious blocklayers to make sure batts are installed in the proper way. I did spend 2 days with a homemade brush and long shovel cleaning all the cavity ties and collecting all the mortar droppings. I had very skinned knuckles by the end. It may not be an option in a narrower cavity. 

 

Below would be my preferred arrangement should I do it again and I would insist the block layers used boards to catch any droppings. 

 

[jayc89]

10 minutes ago, Iceverge said:

 

 

 The EPS beads are easier to install tight to the wall and cheaper.

 

Like @cwr says mortar droppings are your enemy. I would prefer to have them at the bottom of the cavity rather than sitting on top of my XPS insulation. Golcar used mineral wool batts to avoid this but this also requires conscientious blocklayers to make sure batts are installed in the proper way. I did spend 2 days with a homemade brush and long shovel cleaning all the cavity ties and collecting all the mortar droppings. I had very skinned knuckles by the end. It may not be an option in a narrower cavity. 

 

Below would be my preferred arrangement should I do it again and I would insist the block layers used boards to catch any droppings. 

 

 

Sorry, I have lots of questions

• I assume the red line is the DPM - why run that over the EPS, rather than keeping the EPS dry?
• Are there any wheepholes etc on the first external course to allow water to escape into the french drain?
• Why only two lightweight blocks and not either/or;
  • 1 below to fully join the cavity and slab insulation?
  • 1 above to improve the slab perimeter insulation?
• Why DPC below the first lightweight block AND above the last?
• Is there another internal DPC at the finished floor level, or is that mislabeled? 
• Whats the benefit of the blocks running along the bottom of the cavity?
• Are you aware of any similar details being approved by BCO?
• 100mm vs 150mm conc slab? I guess both would be reinforced?

[Iceverge]

58 minutes ago, jayc89 said:

I assume the red line is the DPM - why run that over the EPS, rather than keeping the EPS dry

 

Personal preference. So the concrete doesn't seep into the insulation when it's poured and so the EPS isn't sitting in a swimming pool of water before the concrete is poured. I don't like the idea of 2 layers of impermeable material anywhere in a building as water can't escape from between. The EPS won't be wet underneath if you take care to ensure the water table never gets above its base. 

 

 

1 hour ago, jayc89 said:

Are there any wheepholes etc on the first external course to allow water to escape into the french drain

 

No, the main purpose here is to ensure water from outside doesn't get in. The blocks are somewhat porus anyway and any moisture will drain away through them over time. I did drill some larger holes to allow rain water escape during construction. 

 

1 hour ago, jayc89 said:

Why only two lightweight blocks and not either/or;

• 1 below to fully join the cavity and slab insulation?
• 1 above to improve the slab perimeter insulation?

 

 

I put many iterations through Therm to see what performed best. I did try a third block below but the gain was tiny. Ideally they would be directly in line with the insulation but in the real world blocks only come in certain sizes. It's vital the aerated concrete blocks are kept dry. Hence keeping them away from the bottom of the cavity. 

 

1 hour ago, jayc89 said:

Why DPC below the first lightweight block AND above the last?

 

The one below is a strip of membrane and the one above is an extension of the whole DMP barrier extending over and down the cavity.  The 100mm block above it is to give a level surface to screed the floor from. You don't want to screed on top of a DPC as it'll tear. 

 

1 hour ago, jayc89 said:

there another internal DPC at the finished floor level, or is that mislabeled? 

Mislabeled

 

1 hour ago, jayc89 said:

Whats the benefit of the blocks running along the bottom of the cavity

This is not the detail we built but rather an improved one I would have preferred. Our actual foundation is stepped twice so various levels of fully filled wall were raised to give a level platform to work from. It's called deadwork in Ireland. Not sure about the UK. Also begining a cavity wall directly from the foundations will be difficult as it'll be too uneven and rough. 

 

1 hour ago, jayc89 said:

Are you aware of any similar details being approved by BCO

 I will direct you towards our as built design if I can find it. 

 

1 hour ago, jayc89 said:

100mm vs 150mm conc slab? I guess both would be reinforced

 

100mm to save concrete. A production builder near us uses this with reinforcing fibers and no mesh. We used 150mm with mesh. Total overkill in my opinion but our builder wanted it and didn't charge extra so hey ho. 

 

One thing that is vital with no subfloor is to compact the substrate thoroughly. Otherwise it may sink and you will get cracked tiles etc. 

[jayc89]

What about something like this? EPS Beads all the way down, french drain at the foot of the external leaf, but the DPC/DPM is slightly different;

 

 

The thinking being, by keeping the Celcon/Thermalite and the EPS on the dry side, it should remain more thermally efficient. 

[Iceverge]

I'd have at least one course of full fill cavity at the bottom. It'll give the masons a good chance to get the first cavity row spot on. Foundations can be too rough to start from IMO. 

 

How are you going to deal with the DPC ( red vertical line ) at thresholds?  How are you going to pour your floor level without any walls to screed from?

 

I've included a link from the thread in which I discussed this 2 years ago. Below is what actually got built.

 

 

 

 

 

There was a couple of issues. 

 

The block layer didn't look at my drawings and put the first row of lightweight aerated concrete blocks onto the "deadwork". I wanted them kept up a course to keep them out of any damp as they wick water like crazy and become ineffective. 

 

The DPM below the EPS created a complete swimming pool before laying the insulation and pouring the concrete. I'm dubious as to how well it was dried out before they poured it as it was all finished while I was away at work. The finish on the concrete wasn't great, I suspect because it was laid in the pouring rain.  We had a single layer of 200mm EPS. I imagine the concrete juice went between the boards also. 2 x 100mm layers staggered with the DPC over would have been better. 

 

The blocklayer (who was excellent at laying blocks BTW) added a second DPC (yellow). I assume because of convention of having a DPC above ground level on the internal leaf. I didn't want this. I couldn't figure out how water would make its way across the 250mm cavity and up the wall. The only way I could imagine this happening was if flooding came within 100mm of the floor level. We'd all be done for then as we're on an elevated plinth 100m above sea level. 

 

1.This extra Yellow DPC created a slip plane for the wall at a critical airtightness junction.

2.It trapped water for months and months between the yellow and red DPC's. I'm dead set against 2 layers of impermeable material in a structure build up. Water will get in but it can't then dry out. If it can't dry it's gonna die!

3.It got torn to shreds at the thresholds (6m out of 44m perimeter) from foot traffic. So bad I actually cut it out and threw it away.  

4.There was so much damp between the 2 DPC's I began to wonder if it was creating a capability route for water to come up from the bottom of the cavity. Exactly what we wanted to avoid. 

 

One robust solution I've seen is to have a paint on DPC on the inner leaf. A consideration I didn't think about at the time is cavity ties below the DPC. You should clear this with your SE.

 

 

 

Here's a thread about this from 2 years ago before we started. 

 

 

 

 

[jayc89]

7 hours ago, Iceverge said:

I'd have at least one course of full fill cavity at the bottom. It'll give the masons a good chance to get the first cavity row spot on. Foundations can be too rough to start from IMO. 

 

How are you going to deal with the DPC ( red vertical line ) at thresholds?  How are you going to pour your floor level without any walls to screed from?

 

I've included a link from the thread in which I discussed this 2 years ago. Below is what actually got built.

 

 

 

 

 

There was a couple of issues. 

 

The block layer didn't look at my drawings and put the first row of lightweight aerated concrete blocks onto the "deadwork". I wanted them kept up a course to keep them out of any damp as they wick water like crazy and become ineffective. 

 

The DPM below the EPS created a complete swimming pool before laying the insulation and pouring the concrete. I'm dubious as to how well it was dried out before they poured it as it was all finished while I was away at work. The finish on the concrete wasn't great, I suspect because it was laid in the pouring rain.  We had a single layer of 200mm EPS. I imagine the concrete juice went between the boards also. 2 x 100mm layers staggered with the DPC over would have been better. 

 

The blocklayer (who was excellent at laying blocks BTW) added a second DPC (yellow). I assume because of convention of having a DPC above ground level on the internal leaf. I didn't want this. I couldn't figure out how water would make its way across the 250mm cavity and up the wall. The only way I could imagine this happening was if flooding came within 100mm of the floor level. We'd all be done for then as we're on an elevated plinth 100m above sea level. 

 

1.This extra Yellow DPC created a slip plane for the wall at a critical airtightness junction.

2.It trapped water for months and months between the yellow and red DPC's. I'm dead set against 2 layers of impermeable material in a structure build up. Water will get in but it can't then dry out. If it can't dry it's gonna die!

3.It got torn to shreds at the thresholds (6m out of 44m perimeter) from foot traffic. So bad I actually cut it out and threw it away.  

4.There was so much damp between the 2 DPC's I began to wonder if it was creating a capability route for water to come up from the bottom of the cavity. Exactly what we wanted to avoid. 

 

One robust solution I've seen is to have a paint on DPC on the inner leaf. A consideration I didn't think about at the time is cavity ties below the DPC. You should clear this with your SE.

 

 

 

Here's a thread about this from 2 years ago before we started. 

 

 

 

 

 

Thanks, that's super useful!

 

The DPC may need dropping. I was trying to keep it above 150mm from ground level just for an easy lift, but given the existing house doesn't have a DPC at all, my BCO might be more lenient...

 

I was going to forget about the floor insulation and slab until I'm water tight (and two of the existing walls are knocked down so it can all be poured at the same time). Leaving an additional 100mm of the lower internal DPC for the DPM to lap under when I'm ready. Given the slab will be the finished floor, I was planning on using something self-leveling/self-compacting like Cemfloor HPC.

[Gordo]

On 26/10/2021 at 23:07, jayc89 said:

Tie locations is a good shout. Why put the DPM on top of the slab? I generally thought it was a good idea to keep the slab on the dry side.

With a suspended floor the DPM/radon barrier should be fully supported. With the high GLs I’d suggest wrapping Dpm up inner leaf to 9” above GL with a stepped DPC over

Edited October 28, 2021 by Gordo

[Gordo]

On 27/10/2021 at 17:27, ETC said:

The DPM will only be laid once the building is water tight and everything but the floor insulation and screed are to be completed. So it will not be exposed or compromised during construction.

DPM/Radon barrier carpet laid before insulation. Tape radon barrier to exposed tails of wide stepped DPC

[jayc89]

22 minutes ago, Gordo said:

With a suspended floor the DPM/radon barrier should be fully supported. With the high GLs I’d suggest wrapping Dpm up inner leaf to 9” above GL with a stepped DPC over

 

16 minutes ago, Gordo said:

DPM/Radon barrier carpet laid before insulation. Tape radon barrier to exposed tails of wide stepped DPC

Are you suggesting the same thing here? DPM below insulation and then slab? Lapped up the inner side of the inner cavity wall leaf to above ground level?

[Gordo]

51 minutes ago, jayc89 said:

 

Are you suggesting the same thing here? DPM below insulation and then slab? Lapped up the inner side of the inner cavity wall leaf to above ground level?

I was referring to the first detail with suspended floor slab. Most recent details show ground bearing slab. IMO I’d place DPM /radon barrier on top of insulation under screed/slab but it’s a matter of preference TBH. But if GL high I would wrap DPM up inner block. This detail falls down at doors and maybe overkill. As long as cavity ending is 9” below DPM level DPC it’s not necessary to wrap. I’d also leave outside block work perpends at cavity ending open for drainage,

 

all in all the most recent details are excellent 

Edited October 28, 2021 by Gordo

[jayc89]

Here's what I'm currently thinking;

 

• 750mm wide trench (as I can only get 600mm or 750mm wide digger buckets)
• 175mm wide cavity
• Foundations are 550mm thick meaning the brickies have less work below ground level.
• French drain around the external perimeter to protect the EPS beads below DPC.
• Brickies to fit both the internal/external DPCs.
• I'll tape the DPM onto the internal leaf DPC once the roof's on, before fitting the floor insulation/UFH/slab.

 

This provides a U-Value of 0.144 at the join between the wall and the slab and a general u value of 0.183 for the rest of the wall.

 

Thoughts?

Edited October 29, 2021 by jayc89

[ETC]

VCL?