40m2 concrete extension options

madaboutblue

Anyone ever consider mass concrete as an option for a single story 40m2 extension? Can do all form work and pouring myself and would be insulated internally? Good option or totally avoid?

baxterooneydoody

Perfect way to build but put the insulation on the outside, no need to step the raft just let insulation down past floor min 150mm and base coat and acrylic render finish which is waterproof

BryanF

https://www.boards.ie/discussion/comment/122691468#Comment_122691468

And Wall insulation connects to insulation under raft

mickdw

All good in theory until the external finish allows water in at a window detail or crack or whatever.

Rows Grower

https://www.boards.ie/discussion/comment/122711072#Comment_122711072

In fairness mick you could say that about every building method.

mickdw

https://www.boards.ie/discussion/comment/122711147#Comment_122711147

Not so much as this system is entirely depending on the external plaster to keep the water out.

baxterooneydoody

https://www.boards.ie/discussion/comment/122712001#Comment_122712001

Most bases for the acrylic/silicone renders are waterproof

Mellor

https://www.boards.ie/discussion/comment/122712057#Comment_122712057

and if they crack, the wall is no longer waterproof. General most walls don’t rely on the finish to protect the whole structure

Mellor

https://www.boards.ie/discussion/comment/122712057#Comment_122712057

and if they crack, the wall is no longer waterproof. General most walls don’t rely on the finish to protect the whole structure

baxterooneydoody

https://www.boards.ie/discussion/comment/122720037#Comment_122720037

The waterproof acrylic and basecoat would have to crack (highly unlikely with reinforced fibreglass mesh bedded in the basecoat ) then the insulation would have to let in moisture through and on through the waterproof basecoat behind the insulation and finally the moisture would have to make its way through 225 mm of vibrated concrete, that's a lot to have to go wrong on one wall for it to let in moisture. The Scandinavians and Europeans have been building this way with decades, it's a great way to build if done correctly. As added protection you could get the plant to add sika or similar into the concrete mix but unnecessary in my experience

Mellor

https://www.boards.ie/discussion/comment/122720325#Comment_122720325

The insulation itself isn’t waterproof. Neither is concrete, vibrated of otherwise. Why do you think we have membranes under slabs?

So the render is doing all the work. cracking is not uncommon around differential settlement at junctions. But the big risk points are windows and doors where the render has to waterproof a reveal.

mickdw

https://www.boards.ie/discussion/comment/122720325#Comment_122720325

If that is your understanding of weatherproofing, you really are on the wrong track.

If water breaches the render in these systems, its in. No debate. It can travel on either surface of insulation. It will find a construction joint in the concrete or a poorly compacted section and penetrate.

The biggest issue is that the appearance of problem internally can be a long way from the actual leak.

Maybe OK in a sheltered back garden where people get away with hollowblock structures too but for severe exposure, not a hope.