Raising concrete floor level

My house has a built-on porch, the original front door was left in place and a new front door installed, so the porch was a separate room, I have removed the original front door, so the porch is now effectively part of the hall.
Having removed the original door, frame and step, there is a difference in levels between the concrete floors of about 40mm, the porch being lower, the DPC is only about 25mm above the level of the porch floor, so adding 40mm of concrete seems like a bad idea, is there any thin/strong enough insulating foam blocks to make up the level?
I suppose I could either frame around the porch floor with about 1" timber, with a few cross-members to subdivide the span, then fill the spaces with foam, and then cover with plywood.
Alternatively I could forget the frame and just lay about an inch of foam block with plywood over that, how much will the foam "give" over time?
Or, I could just say sod the DPC and fill with cement/concrete, any other thoughts?
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Andy Burns wrote:

Lay 25mm blue or pink foam over a DPM which you carry up the walls and use a bit of rebar or mesh in 15mm of concrete.
Or forget the foam and just use 40mm screed over a plastic DPM.
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EPS (Expanded Polystyrene) and Kingspan/Celotex boards can take floor loads, provided it is spread nicely. This can be done with a screed, or thick plywood.

Lay a DPM (tying into the DPC) and then screed.
Christian.
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Andy Burns wrote:

Plenty of 'thin screed' cementitious options on the market. Most are also waterproof. Try Watco? I dont know their web address but google will.
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Bookworm wrote:

Thanks all,
www.watco.co.uk has lots of floor coatings, coverings and markings, but I didn't see much about screed.
I found some good info on insulating foam blocks at http://www.knaufinsulation.co.uk/output/products/page_272.html
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