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.