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I’m in a dilemma about what to do re: insulating my BISF house.

Initially I considered EWI but we are having a two-storey side extension built eventually in traditional blockwork with internal insulation, so no need to do the side wall in EWI as it will become an internal wall.

It would leave only the front of the original house and the upstairs rear, as the downstairs has already been extended on the ground floor and is traditional construction.

So I’m now considering taking off the fibreboard in the living room and front and back bedrooms and insulating those from the inside as a cheaper option.

Does anyone know if there’s enough space in the wall cavity to do this?

Furthermore, would it create problems with heat/condensation if I did it?

The other query this raises is that we want to eventually render the exterior to be one flat profile rather than split in the middle, but I gather the movement between top and bottom sections would cause render to crack, taking me full circle to EWI as that would be a 90ml coat for the wall finished in a silicone render with far less chance of cracking.

So two questions:
1. Has anyone had experience of successful flat rendering outside without EWI?
2. Can you fit proper block insulation inside the walls?

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Thanks Marc, I am thinking still go EWI purely as it will enable a flat render finish with minimal chance of cracking. And as it’s only front of original house and first floor rear that needs it it should keep cost down. I’ve seen lots done on our road and it looks good.

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Here’s an installation by Alsecco

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Hi Danny,
reading into your project, I personally would still consider the EWI option which should come in somewhere between 6 and 8.5k for the house. (EWI to the existing structure blending seamlessly into silicone render over block over the extension walls.

Modern Silicone render shouldn’t crack once the EWI has been applied to the outer walls of the original house due to the tolerance of the insulating boards and the silicone render itself.

The steel cladding to the upper elevation does protrude slightly further forward than the rendered walls below, however, in order to create a fully flat finish, a slightly thicker insulation board is used on the lower elevation to maintain a completely flat surface for rendering.
Your extension could then be built in block rather than brick which should produce a cost saving which could go toward carrying over the same silicone render coating to the newly built walls.

You can of course insulate internally providing that you leave an air gap between the insulation and the outer wall.

We do have a post by ED that you may find interesting, during which he internally insulates several rooms in his own BISF house.

Here’s the link http://bisfhouse.com/internal-insulation-project-of-bisf-bedroom-by-ed/

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