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Insulating old mass concrete house

  • 08-06-2022 11:16am
    Registered Users Posts: 16 rogash

    Hi There,

    I'm currently renovating an old cottage/house based in the west. The house is single story and comprises a mass concrete wall about 4 inches thick with dpc installed with a dash finish. And has a modern extension out the back with a cavity wall (this is ok).

    I'm trying to come up find the most suitable way of doing the insulation for the old building and am trying to figure out the best practice of using external insulation with a stone finish on the outside (I love stone & working with it) the idea would be a jacket around the house with solid cavity and stone with lime finish to protect.

    Old concrete wall(5 inch thick)>insulation (4 inch thick)>natural stone(6 inch thick with lime mortar) with the stone being wall tied to the existing structure.

    For the insulation, the most suitable and cost-effective seems to be EPS (expanded polystyrene which is currently common with external insulation builds) and the material properties seems to allow vapor to pass through albeit small.

    See attached pdf of the idea for a better understanding of the plan.

    My main concern is around dampness and the breathability of the structure - I don't want water being trapped behind the eps and causing problems. from a logical point of view, it seems similar to the external insulating method but instead of using a silicone render (breathable) I'm using lime and stone(breathable) but has a standalone foundation.

    So the question is has anyone done a similar project before and gained any experience on this method or similar or has any reference material/articles/standards on this topic? most articles I read contradict or don't actually give useful info.

    Thanks any thoughts are welcome.

    Post edited by rogash on


  • Registered Users Posts: 1,535 ✭✭✭ Dudda

    Are you sure the existing external wall is only 5 inches thick? That implies 4 inch block and half an inch of plaster each side giving 5 inch. That's quiet thin for an external wall. I'd be worried about that existing wall supporting the weight of the stone wall which won't be light and need lateral support.

    Also if going to all this trouble go for thicker insulation, at least 150mm. EPS isn't as good thermally (which is why it's cheaper) as more expensive PIR insulation so I'd put it in thicker EPS. It's not like you can come back in 5 years and make it even thicker.

  • Registered Users Posts: 16 rogash

    Yes, it is surprising how strong the existing mass concrete wall is (it's not block) the workmanship for the 1950s is astounding no signs of shuttering etc from the various levels. I am going to leave the existing dash for the reason of not negatively affecting the structure.

    Yes, that is why I am installing a reinforced concrete foundation to dab for the natural stonework and lime and then tie it with wall ties (not sure which ones yet testing out tiger wall ties atm) to the existing concrete structure.

    I know 50mm isn't much (apart from on your nose) but I'm not out to create a super-efficient insulation house just cozy and comfortable. That 50mm would in turn increase the facit/sofit rafter extension which would encroach on the existing windows (roof goes down to meet wall (existing roof is grand)) making the building lose its character and decrease sunlight.

    The reasoning for going with the EPS solid cavity was from case studies from Kore and approved systems from NSAI for pumping cavity and doing external insulation.

    Has anyone had any experience with these kind of systems?