External Wall Insulation Below DPC?

[EinTopaz]

Hello all,

 

Now the temperature has got a little colder, i've noticed the floor in our rear extension appears to be colder than the floor in the rest of the property. Long story short, original property is is suspended floor, extension done 2023, piled, concrete floor floor with 100mm kingspan in there, then 6" concrete over the top, which the inner skin wall was built on.

As the floor concrete meets the inner skin block wall with no thermal separation in this case, i suppose the prevention of a bridge here is entirely down to how well the PIR was done in the wall cavity and indeed how low it goes... I can have that inspected etc. But from chatting the problem through with AI, it suggested dealing with this externally rather than trying to mess around with the wall cavity. It suggested running 500mm tall XPS external insulation boards from just below DPC, down into the ground. So in my case... essentially 200mm visible about ground level, 300mm below ground level. Then to hide the visible bit, an angled plinth atop, and brickslips over the front /side faces. 

Has anyone done this before? Looking at most EWI / Render jobs i can see online they usually run from DPC upwards to avoid a bridge I guess. But in this case is it fine to do from DPC downward? maybe leave a 100mm gap from DPC to where the plinth starts.

 

thanks

 

Ged

[SimonD]

It's standard practise with proper Ewi to run plinth insulation from DPC for a calculated distance into the ground - it's just not done a lot in the UK. So really not a problem. XPS tends to be the default but EPS is fine too  (there are a good few studies where EPS is used subgrade without any issues and of course it's used without problem as part of slab buildup. Good practise also asks for some pea gravel or similar for drainage just below the plinth in the ground.

 

HTH

[MikeGrahamT21]

I did the plinth insulation as part of the whole EWI system, using XPS.

 

 

[JpManning]

Before spending on the plinth, it's worth confirming where the heat is going. On a cold morning compare the floor surface temperature mid room with the strip along the external walls using an IR thermometer or thermal camera. If the edges are a couple of degrees colder, you have a perimeter bridge and the plinth is worth doing. If the whole floor reads much the same, it's more likely the finish: tiles on a dense slab feel far colder underfoot than a suspended timber floor at the same temperature, and 150mm of concrete is slow to warm.

As your inner leaf sits on the slab, heat can leak from the slab edge through the blockwork and down into the ground beam, so also check that the cavity fill runs down to at least the level of the underslab PIR rather than stopping at DPC.

SimonD is right that it's normal practice. If you go ahead, keep the plinth and its capping below the DPC so nothing bridges it, leave any weep holes clear, keep finished ground level 150mm below DPC, and bond the XPS rather than fixing through it below ground.

[EinTopaz]

thanks for all the replies guys. I didn't actually know there was a technical term for this so thats reassuring. "Plinth Insulation" ?

if I mention that terminology to a EWI company, will they likely know what im referring to? Im surprised it didnt come up when i asked AI. Though the process it walked me through does sound somewhat consistent with how its being described on here. 

 

 

[MikeGrahamT21]

15 hours ago, EinTopaz said:

thanks for all the replies guys. I didn't actually know there was a technical term for this so thats reassuring. "Plinth Insulation" ?

if I mention that terminology to a EWI company, will they likely know what im referring to? Im surprised it didnt come up when i asked AI. Though the process it walked me through does sound somewhat consistent with how its being described on here. 

 

 

Yes they should definitely know what you are talking about. This is an updated Best Practice Guide which I followed when i did mine: https://www.inca-ltd.org.uk/wp-content/uploads/2024/04/INCA-EWI-Best-Practice-Guide-RENDER-V1B-01-10-24.pdf

 

Gives all the detailed drawings showing relevant detailing for just about every situation you're likely to come up against during EWI installation.

[Redbeard]

Good call on the INCA guide @MikeGrahamT21 but, @EinTopaz don't necessarily expect the 'blokes on the tools' to be keen on what some see (and some avoid) as 'sub-DPC work', to be avoided in case of future dampness claims...

 

Agree with @SimonD re EPS sometimes being suitable for the plinth layer. Some system providers insist on XPS, while at least one (Possibly Back to Earth? - 'twas a long time ago) used a denser grade of EPS for the plinth. The rationale is that XPS being completely impermeable could, theoretically at least, cause capillary movement of moisture from the plinth (below DPC) to above. The next bit mixes concepts of 'Breathability' and porosity, but bear with me... The thinking, as I understand it, is that using EPS increases the chances of any sub-DPC moisture being wicked away *through* the EPS and ('somewhat breathable') render rather than rising and breaching the DPC. (Normal stuff re 'softening' the layer below the base layer, and French Drains etc. in order to deal with potential moisture before it can even *think* of rising...)

 

As for the insistence of some on using the metal (thermal bridge?...) or plastic base-rail as well as a plinth layer... Last time I did EWI I tried, so that no-one in future could quibble on any claim ('You didn't do it according to 'the system'...') but the plastic base-rail distorted and gaps resulted. In the end I ripped it out, pointed up the adhesive at the bottom of the 'above DPC layer' and simply butt-jointed. (This, BTW, was on a wall where the 'above DPC' was done first on the rail, and the base layer 'retro-fitted'. By far the better way was that used by a colleague in the NW, to lay the 'below DPC' base-layer first, and then build off that. No wibbly 'tray' required).

 

In the OP's case, where there is to be no EWI above the 'Plinth layer', there are 'capping details' (usually seen at eaves and verge level). I would strongly suggest using an 'Upstand Capping Detail' with a flashing over than a 'Downstand Capping Detail'. The 'Downstand' one is mechanically fixed before the insulation goes in (?gaps?) and sealed with Silicone mastic, which in site conditions almost certainly has less of a life than we'd wish.