Airtightness

[Iceverge]

Excellent effort. Only one thing I'd change. You will loose a lot of heat via the route of the yellow line.  We took the EPS beads well below the DPC and dispensed with the cavity tray. 

 

 

Have a look at the below post about this. The second 2 diagrams tell a lot. 

 

 

Steel beams far from the perimeter of your slab might not benefit much from a thermal break as the path to loose heat to the outside will be so long. 

 

 

@OldSpot had some good insights RE insulation DPMS and UFH clips here. It might be worth considering. 

 

 

[matthyde83]

6 hours ago, Iceverge said:

Excellent effort. Only one thing I'd change. You will loose a lot of heat via the route of the yellow line.  We took the EPS beads well below the DPC and dispensed with the cavity tray. 

 

 

Have a look at the below post about this. The second 2 diagrams tell a lot. 

 

 

Steel beams far from the perimeter of your slab might not benefit much from a thermal break as the path to loose heat to the outside will be so long. 

 

 

@OldSpot had some good insights RE insulation DPMS and UFH clips here. It might be worth considering. 

 

 

Great thanks for that… yep been through those. Redone this. 
 

understand the point on maybe 100mm EPS, membrane then 100mm PIR.  Is that going to give a significant difference?  Or much better 2x 100mm PIR. 

 

I also haven’t taken the slab over the block work in this version as won’t have the EPS to shutter it. 

the willows - foundation detail (mh).pdf

[OldSpot]

EPS is fine to use below DPM whereas PIR isn't due to water ingress.

PIR will give a better u-value for equal thickness but if you have the depth/space, EPS usually gives better bang for buck. There is also research to suggest that PIR degrades much more than EPS over time.

 

With regard to the slab build up, a number of people here (not me) have done away with the reinforced slab and screed scenario and simply used a single reinforced slab. The slab can also contain UFH pipes.

Build up would be, from bottom up - with DPM inserted either below or between insulation:

• hardcore
• insulation
• slab.

If you search the forums it will throw up some debate on this build up.

 

HTH!

[Iceverge]

@matthyde83

 

Apparently it's ok to use EPS for staples so long as you get long ones. That would let you use all EPS and no PIR. Hurrah! 

 

@OldSpot Would you put a layer of plastic above the EPS to stop the wet concrete drying into the EPS or is it worth bothering with? Personally I don't like trapping any material between two layers of impermeable material but my fear is that concrete juice would go between the boards and also the boards might float? 

 

 

Edited January 27, 2022 by Iceverge

[matthyde83]

11 hours ago, Iceverge said:

@matthyde83

 

Apparently it's ok to use EPS for staples so long as you get long ones. That would let you use all EPS and no PIR. Hurrah! 

 

@OldSpot Would you put a layer of plastic above the EPS to stop the wet concrete drying into the EPS or is it worth bothering with? Personally I don't like trapping any material between two layers of impermeable material but my fear is that concrete juice would go between the boards and also the boards might float? 

 

 

at 200mm though would EPS performance suffice?  I know that a lot of people on here have gone for 300mm but I can’t easily get that.  Various reasons but insulation on top of slab is my way to go. 

[OldSpot]

@Iceverge

Looks like you can use staples in EPS from that thread and long ones would definitely be better!

I've got a piece of 150mm EPS that I'll stick a couple of 60mm staples in and see how they hold although reading through the other thread with a gun looks like you're okay - I didn't have one.

 

Quote

Would you put a layer of plastic above the EPS to stop the wet concrete drying into the EPS or is it worth bothering with? Personally I don't like trapping any material between two layers of impermeable material but my fear is that concrete juice would go between the boards and also the boards might float? 

For the cost I think I would. You only need a vapour barrier which you lap can lap up the walls rather than a DPM as that's been put in lower down.

So, EPS ---> DPM ---> EPS (or PIR) ---> vapour barrier ---> UFH pipes stapled then pour the concrete.

[OldSpot]

@matthyde83

 

Rather than using a 300mm thick piece do it in layers (100/100/100). You can brick bond it so you overlap the joints and it's a lot easier to cut at that thickness!

[matthyde83]

8 hours ago, OldSpot said:

@matthyde83

 

Rather than using a 300mm thick piece do it in layers (100/100/100). You can brick bond it so you overlap the joints and it's a lot easier to cut at that thickness!

yep but if I have just 200mm would you go 200mm PIR, 200mm EPS or 100mm EPS and 100mm PIR?

 

50mm chemical screed

[Simplysimon]

if affordable 200mm pir next option 100mm + 100mm eps

[matthyde83]

On 23/01/2022 at 11:43, Iceverge said:

Excellent effort. Only one thing I'd change. You will loose a lot of heat via the route of the yellow line.  We took the EPS beads well below the DPC and dispensed with the cavity tray. 

 

 

Have a look at the below post about this. The second 2 diagrams tell a lot. 

 

 

Steel beams far from the perimeter of your slab might not benefit much from a thermal break as the path to loose heat to the outside will be so long. 

 

 

@OldSpot had some good insights RE insulation DPMS and UFH clips here. It might be worth considering. 

 

 

So revisiting this as I’ve now started on site.  We dispensed with the cavity tray and planned for beads all the way down.  Up to cill height on external brickwork and building control/warranty have come round and said we absolutely MUST have a cavity tray and they won’t do an off without.  Help!

[Buzz]

Did they give you any recommendations or alternative solutions  

[matthyde83]

Just now, Buzz said:

Did they give you any recommendations or alternative solutions  

Nope.  Just demanded there be a cavity tray.  We haven’t done interior block work so they suggested taking out the mortar 5mm or so at DPC and beading a tray in with silicone there then tape to interior block at a higher level. 

[matthyde83]

2 minutes ago, Buzz said:

Did they give you any recommendations or alternative solutions  

My favourite bit however is that we need 2 DPM’s.  Have one under the slab that we will lap up the outside of interior wall.  They want another under screed that also laps up to cavity tray.  We have a level threshold at back so our cavity tray will effectively be block and half above FFL inside. 

[Iceverge]

1 hour ago, matthyde83 said:

So revisiting this as I’ve now started on site.  We dispensed with the cavity tray and planned for beads all the way down.  Up to cill height on external brickwork and building control/warranty have come round and said we absolutely MUST have a cavity tray and they won’t do an off without.  Help!

 

Oh dear. 

 

Well 2 things. First we've had some pretty savage driving rain but no issues without cavity trays. 

 

Second, there's always a solution. 

 

Can you provide a cross section diagram and a picture of the buildup as it currently stands?

[Buzz]

10 minutes ago, matthyde83 said:

We have a level threshold at back so our cavity tray will effectively be block and half above FFL inside. 

That somehow doesn't seem right !

[matthyde83]

6 hours ago, Iceverge said:

 

Oh dear. 

 

Well 2 things. First we've had some pretty savage driving rain but no issues without cavity trays. 

 

Second, there's always a solution. 

 

Can you provide a cross section diagram and a picture of the buildup as it currently stands?

I drew this last night.  This is what they’re asking for.  Atm the cavity tray isn’t in, we’re up to plinth brick height on the exterior skin and have the Marmox blocks in on interior.  Obviously no insulation and screed yet. This is a standard detail, at the back we’re level threshold so the garden will be two bricks higher, so there we’ve stepped the DPC up another two bricks so the cavity tray and DPM would effectively be two blocks higher than FFL  

[matthyde83]

The additional problem I have is how do I get the thermabeads injected in under the cavity tray without piercing the DPM.  They’ll need to be injected from the outside so drilled through outside brick and the paving slab which I’m sure the installers are going to love me for!

[Miek]

2 hours ago, matthyde83 said:

The additional problem I have is how do I get the thermabeads injected in under the cavity tray without piercing the DPM.  They’ll need to be injected from the outside so drilled through outside brick and the paving slab which I’m sure the installers are going to love me for!

Looking at your drawing it looks like you could drill from inside to blow the beads below the tray. 

 

Did BC want to see weeps at the bottom of the tray? 

 

Are you in a radon area? The DPM detailing needs to be continuous through the cavity tray as I understand it. 

 

The tray will be a fiddly thing to retrofit but if BC are happy with a 5mm chase ' bodge' then that's at least pragmatic of them.

[matthyde83]

1 hour ago, Miek said:

Looking at your drawing it looks like you could drill from inside to blow the beads below the tray. 

 

Did BC want to see weeps at the bottom of the tray? 

 

Are you in a radon area? The DPM detailing needs to be continuous through the cavity tray as I understand it. 

 

The tray will be a fiddly thing to retrofit but if BC are happy with a 5mm chase ' bodge' then that's at least pragmatic of them.

problem is that would be drilling through the DPM that’s lapped up the outside of the inside leaf. 
 

made the call to remove the paving slabs on top of DPC, that way we have 50mm to join cavity tray to DPC then bed the slabs on top. 

[Iceverge]

What reference is your BC working to? 

 

A quick Google found this image on the NHBC website showing a non continious cavity tray. 

 

[matthyde83]

1 hour ago, Iceverge said:

What reference is your BC working to? 

 

A quick Google found this image on the NHBC website showing a non continious cavity tray. 

 

Yep I found that.  That’s the standard dealing with rising water. 
there’s then one below dealing with water ingress from outside 

 

[matthyde83]

I cannot see why we need 2 DPM’s though. Only thing I think is we were talking about how do we get wall ties in and that’s when he said

[Iceverge]

I cannot see what that cavity tray achieves. 

 

Kore don't have it in their MI's

 

https://www.kore-system.com/wp-content/uploads/2018/12/KORE_Fill_Cavity_Wall_Design_Guide_2018.pdf

 

 

 

 

So much of building is based on " cause we always do it that way," with almost zero interrogation of why something is done. 

 

You need to achieve 2 things here. 

 

 1.Prevent any drips of moisture that may may be blown through the outer leaf (driven rain)  or condense on the inner surface of the outer leaf from running down the wall and making their way in the tops of windows doors etc. 

  The cavity tray needing to extend all the way across the cavity is nonsense in my opinion as by stating that, you've admitted than moisture will be against the outer face of the inner leaf and the battle is well and truly lost. 

 

  In fact if I was to build a cavity wall  house again ( unlikely! )  I would use angled PVC profiles as "drip catchers" rather than DPC plastic as cavity trays. They're always full of holes post build and often hold many KGs of mortar droppings often leading to moisture crossing the cavity. The underside is rarely correctly insulated giving rise to cold surfaces inside the house and more condensation.  More negative than positive. 

 

 

 

Issue 2. 

 

You need to prevent ground moisture getting inside your floor. 

 

Imagine submersing your house to floor level in a huge tub of water. The DPM needs to prevent water getting in. A continuous DPM up the outer face of the inner leaf as shown (should not be breached by Cavity ties) achieves this. I would be very slow to put any DPM vertically  on the outer leaf as it will prevent any moisture that gets into the cavity from draining to the outside. 

 

I would always include a French drain always if I had the opportunity.  Lower than the bottom of the insulation and free draining to a lower plain. In effect it uses gravity as a DPM. Very reliable. 

 

 

 

[matthyde83]

3 hours ago, Iceverge said:

I cannot see what that cavity tray achieves. 

 

Kore don't have it in their MI's

 

https://www.kore-system.com/wp-content/uploads/2018/12/KORE_Fill_Cavity_Wall_Design_Guide_2018.pdf

 

 

 

 

So much of building is based on " cause we always do it that way," with almost zero interrogation of why something is done. 

 

You need to achieve 2 things here. 

 

 1.Prevent any drips of moisture that may may be blown through the outer leaf (driven rain)  or condense on the inner surface of the outer leaf from running down the wall and making their way in the tops of windows doors etc. 

  The cavity tray needing to extend all the way across the cavity is nonsense in my opinion as by stating that, you've admitted than moisture will be against the outer face of the inner leaf and the battle is well and truly lost. 

 

  In fact if I was to build a cavity wall  house again ( unlikely! )  I would use angled PVC profiles as "drip catchers" rather than DPC plastic as cavity trays. They're always full of holes post build and often hold many KGs of mortar droppings often leading to moisture crossing the cavity. The underside is rarely correctly insulated giving rise to cold surfaces inside the house and more condensation.  More negative than positive. 

 

 

 

Issue 2. 

 

You need to prevent ground moisture getting inside your floor. 

 

Imagine submersing your house to floor level in a huge tub of water. The DPM needs to prevent water getting in. A continuous DPM up the outer face of the inner leaf as shown (should not be breached by Cavity ties) achieves this. I would be very slow to put any DPM vertically  on the outer leaf as it will prevent any moisture that gets into the cavity from draining to the outside. 

 

I would always include a French drain always if I had the opportunity.  Lower than the bottom of the insulation and free draining to a lower plain. In effect it uses gravity as a DPM. Very reliable. 

 

 

 

Yep fully agree with all of your points.  I think our DPM 1 achieves point 2, assuming we only put ties above that level (we’re quite high with DPC due to terrace at rear being level threshold - so first ties will be three blocks up). 
 

Point 1 I agree with but I had that debate with BC and he was extremely persistent that it needed to go in.  We then had a debate about the Marmox blocks that he told me were pointless. So that was nice too.  I particularly like it when he asked me what ventilation I was having in addition to MVHR!!!!

[Iceverge]

I think you need to get another BC. 

 

What's your detail for the threshold at the rear like?