Cold bridge created by groundworker mistake...how to resolve?

[Thorfun]

the groundworker made a mistake with the size of the insulated slab for our entrance hall porch. the design is for a 200mm EPS300 raft that joins with the basement walls as follows (hopefully it makes sense):

 

 

the green bit in the screenshot above. We measured it out but the mistake is that he thought the measurements we did were to the outside of the slab but they were to the outside of the 100mm insulation and so I am left with 100mm on either side that is now a cold bridge. hopefully the photos below show what I mean with the blue areas in spray paint being the timber walls that should sit just over the insulation.

 

 

the slab has 2 layers of A393 mesh in them which is supposed to have the usual 50mm cover at the sides and is 200mm thick.

 

potential solutions:

 

1. use a disc cutter to remove some of the slab and put some insulation in, maybe 50mm? but this will go through the top mesh sheet resulting in there not being any cover for the mesh on the sides for the top sheet.

 

2. remove the 200mm thick concrete slab on the outside of the blue line entirely and put the full 200mm deep insulation in there but, again, I'm worried about the lack of cover for the rebar.

 

3. another solution could be to simply put some insulation on top of the slab but I'm not sure how that will fit in with the timber frame sat on top as it's obviously not been designed to have 100mm insulation there but as there's 75mm mortar and coursing block and then the soleplate it would probably be ok as 100mm insulation would then cover up to the top of the sole plate. e.g.

 

 

so that could potentially work and will reduce the impact of getting rid of the cover on the mesh. 

 

We're planning a 75mm sand and cement screed internally so there's no height for insulation internally.

 

Can anyone recommend a course of action listed above or something else I've not thought of please? the groundworker is coming back tomorrow and I'd like to have a plan of action for him.

 

 

 

[Hobbiniho]

i would say given the relatively small size of the concrete pad, complete removal of the concrete shoudnt be too much hassle, you can then move the insulation to the correct size and fit new rebar and concrete, cutting the slab down and putting insulation in will still leave you with the insulation sticking out too far which will result in it being hacked off with a rusty old saw to get the flashing fitted.... and you will still have the problem of rebar not having the required coverage

[Thorfun]

49 minutes ago, Hobbiniho said:

i would say given the relatively small size of the concrete pad, complete removal of the concrete shoudnt be too much hassle, you can then move the insulation to the correct size and fit new rebar and concrete, cutting the slab down and putting insulation in will still leave you with the insulation sticking out too far which will result in it being hacked off with a rusty old saw to get the flashing fitted.... and you will still have the problem of rebar not having the required coverage

thanks but there's just not time to take it all out and redo it. scaffolding is being put up, coursing blocks are being laid and timber frame is coming Monday for erection.

[PeterW]

Can the TF company modify it as it’s a porch and make it bigger ..???

[Thorfun]

16 minutes ago, PeterW said:

Can the TF company modify it as it’s a porch and make it bigger ..???

hmm.....that might be a fantastic idea. I'll have to have a look at the plans and consider how it will look from the outside and then speak to the timber frame company (assuming it's not too late to make that sort of change!) and see. but it's a damn good idea, thanks.

 

just mentioned it to the lovely other half and she pointed out that it will change the pitch of the roof on the porch. but still worth the conversation with the TF company.

 

[Nick Laslett]

Can you not just have an eps block on the side covering up the exposed concrete? Like if you were doing an external EWI on an existing build. 

[saveasteading]

I don't think the loss of end cover woild matter. The slab should  all be within dpm so won't ever get wet.

The 50mm mostly makes it convenient to place.

[Thorfun]

17 minutes ago, Nick Laslett said:

Can you not just have an eps block on the side covering up the exposed concrete? Like if you were doing an external EWI on an existing build. 

yeah, that was my option 3. and I was thinking of wrapping the whole building up to sole plate level with some EPS anyway so it would be the same here too.

[Thorfun]

17 minutes ago, saveasteading said:

I don't think the loss of end cover woild matter. The slab should  all be within dpm so won't ever get wet.

The 50mm mostly makes it convenient to place.

pretty much what the groundworker said too.