How to most economically lay extension floor

[pilsbury]

On the cusp of starting my side kitchen extension. Until very recently I was going to go the full hog and do wet underfloor heating, but with my tight budget, I think this is an area where I'm happy to compromise and just have rads which will probably save damn near a grand all said and done.

So with this compromise, how can I best maximise my savings? Ground level is 360mm below FFL. I'm aware of the make up of the floor, hardcore, sand blinding, DPM, then either concrete or insulation one way or another possibly followed by screed.

What thicknesses would you use for the components for economy? seeing as not using UFH, would you lay insulation above or below the slab? To me the variables are hardcore, concrete and insulation and screed.

 

[tomfe]

I would use your concrete as the finished floor but it takes a bit more care and time to get it the correct level and finish. This will also allow you to use EPS instead of PIR so you save on insulation and screed costs.
However I am sure there are some people on here which will say if you are paying anymore than £4k for your extension you are being robbed.

 

[Doggit]

Does it have to be concrete, in which case it would be hardcore, sand, DPM, concrete, 100mm polystrene (to help bulk out the depth) then screed. But if you can get away with a suspended wooden floor, then you just need celotex between the joists - much cheaper.

 

[pilsbury]

I would use your concrete as the finished floor but it takes a bit more care and time to get it the correct level and finish. This will also allow you to use EPS instead of PIR so you save on insulation and screed costs.
However I am sure there are some people on here which will say if you are paying anymore than £4k for your extension you are being robbed.

Concrete as finished floor would cut out the screed and be ideal, but being a novice and doing it myself might make this hard. The concrete has a limited working time whereas the screed I can fanny about with and faff with the level. What is the reasoning behind a concrete finished floor being able to use EPS instead of PIR? That sure is cheaper insulation if it conforms to BR.

Doing the 40 sqM extension totally myself is going to come in at around £14k - if anyone recons they can get materials cheaper than that, speak up.... and I don't mean buying a handful of dumped blocks of ebay listings for the next year touring the country if you're referring to a £22k full house build!

 

[pilsbury]

Does it have to be concrete, in which case it would be hardcore, sand, DPM, concrete, 100mm polystrene (to help bulk out the depth) then screed. But if you can get away with a suspended wooden floor, then you just need celotex between the joists - much cheaper.

Think a suspended wooden floor might be an issue. It will be linking in to the existing kitchen (solid floor) and also the garage (floor too high to leave a decent gap under joists). Like the thinking though - I may give this some further thought. What screed depth would you recommend over polystyrene? Is it more compressible tan PIR/celotex?

 

[pilsbury]

Just to clarify - roughly speaking 150mm hardcore, 100 EPS, 100 concrete?

 

[Doggit]

Yes, or 75mm concrete, and 25mm self leveling compound.

 

[endecotp]

What is the reasoning behind a concrete finished floor being able to use EPS instead of PIR?

To meet the insulation requirement you can install a lot of EPS or less PIR. (A bit less than 2X IIRC.)
If there is no screed, there is more space available between the ground and the finished floor level. So there is space for using EPS.

 

[tomfe]

You might not get to building regs with 100mm of eps but with 150mm you will.
So:
100mm hardcore
150mm eps
leaves you 110mm for floor finishes and structual slab.

 

[pilsbury]

Just been tapping into Jablite's U value calculator and it recons 100mm of EPS gets to 0.229 U value..... which is bang on BR if I'm not mistaken

 

[tomfe]

Well, there you go. I'd put as much insulation in as possible to keep the correct levels instead of more hardcore or concrete, easier to lay and the benefit of less heat loss.

 

[Doggit]

Now this is one of the things I find odd about insulating floors. As 100mm of concrete is going to act like a giant heatsink, whilst insulation is going to stop it feel like it's freezing, is anything over 50mm of EPS actually going to be doing much good.

 

[endecotp]

Yes. (Huh?)

 

[Doggit]

Well, there ended that discusiion.

 

[pilsbury]

Diminishing returns I guess.... 50mm is much better than none, looks like 100mm is building regs compliant - just an arbitrary figure set by them. Why not double it again? All seems a load of balls to me seeing as the other 90% of my house is solid brick, single glazed and draughty as hell.