Perimeter to floor ratio / floor construction

[garyo]

I'm not sure if this should come under the building or flooring section?

My perimeter to floor ratio is 2.1. That's off the scale on the local authority guides that you see, meaning a huge amount of insulation would be required to achieve an adequate thermal standard.

As this is down to the thermal bridge that's around the perimeter of the wall - where the concrete slab meets the inner leaf of the cavity wall - is there a way I can construct the floor upside down:

25mm chipboard
100mm (say) kingspan
concrete
DPC
hardcore
earth

An immediate problem I can see is that the DPC is vulnerable to getting battered by the insulation and chipboard floating around, but I'm just thinking out loud really.

The room is a 4M x 1.8M porch, probably with a stud wall separating it into 33% / 66% sections.

Gary

 

[RedHerring]

Is the porch within the insulated envelope of the building, in which case it applies. If not, it doesn't apply.

What's wrong with an insulation upstand around the perimeter to isolate slab from internal skin?

 

[garyo]

Yes, I call it a 'porch', but it's really a front extension. 33% of it will be an open extension to the downstairs hall/lobby area, and the remaining 66% will be separated by an extra thick insulated stud wall (just so that I can choose whether to heat the outer part, rather than any attempt to meet any kind of regulations).

I've heard of having a kingspan upstand in the past, but it's not something I commonly see when looking at other peoples floor designs. So in that situation the slab is 100% floating/insulated and would I lay 25mm chip board up to DPC level, being careful not to damage the 25mm of exposed DPM?

I've been using these kinds of lookup tables to arrive at a design, and they don't tend to cover the upstand solutionhttp://www.greenspec.co.uk/documents/materials/insulation/UvalueOfElements.pdf

Gary

 

[RedHerring]

I'd personally use 25mm insluation for the upstand, taped temporarily in place. Concrete to dpc level, which is also top of insulation upstand. Even if no finished floor on top of concrete skirting will cover top of inslutaion upstand.

 

[Chukka63]

i thought an insulated upstand was standard these days.and you may not see it as it is probably hidden behind the plaster/skirting etc.

 

[garyo]

I like the sound of that. Perhaps I've not noticed it on cross sections because the 25mm might not be all that visible at that scale.

So the DPC is much wider than the inner leaf, to cover/protect the upstand - makes sense. I guess the DPM is pretty well protected then too.

Excellent - I just need to figure out how to calculate the main under-slab insulation thickness now.

Cheers.

 

[Chukka63]

dpm on top of insulation too and run up the wall.last time i did this,it was 100mm of kingspan or similar.

 

[RedHerring]

So the DPC is much wider than the inner leaf, to cover/protect the upstand - makes sense. I guess the DPM is pretty well protected then too.

Not sure I follow that.

After hardcore, sandblinding, then dpm which goes up wall past inner skin dpm or laps onto it. Insulation, including upstand. Another layer of separation which can be dpm, (It does not need to act as dpm). Then concrete up to level of wall dpm. Trim of any exposed dpm or if it's tight against wall you can trim it off 50 mm above wall dpm. It, and upstand, is covered by plaster/skirting.

 

[RedHerring]

Incidentally, a couple of things:
Trick 1; place the upstand insulation first (or have it ready) deep enough to reach from sand blinding to wall dpm, then the under slab insulation will hold it in place.
Tip 2: The separation layer between insulation and concrete need only be half the thickness of the dpm proper.

I'm not sure if this separation layer needs to come vertically to the top of the concrete.

 

[jm2000]

On my extention i used 50mm kingspan...i had it all ready...building controll came and said "i want 100mm"...i had the concrete on its way as well.

I am a bricky and work on building £ 10m upwards projects for the likes of Sheperds and balfours....i have never seen them put 100mm of insulation in yet.....even building control bloke said its all gone mad.

 

[garyo]

When I say extra wide DPC, I mean the brickwork DPC being extra wide to overlap the 25mm vertical insulation. So I'd be looking at this kind of cross section:



Gary

 

[mikric]

Are you sure you have done the P/A calc correctly ?
You say a porch, surely only 3 sides are exposed ?
Area 7.2m2 Exposed perimeter 4+1.8+1.8=7.6m
: P/A =1.05
100mm kingspan be more than enough, even if it were a detached building.

JM2000, its all about the P/A ratio, usually on large buildings it works out a lot lower.

 

[freddiemercurystwin]

Your dpm is beneath the insulation. The membrane above the insulation is just a thin polythene sheet or similar to stop the concrete bleeding into the insulation gaps.

 

[hotrod]

You're over complicating the detail gary. There is no need for 2 DPMs or an extra wide DPC. You just need a DPM under the insulation then run it up past the upstand and overlap it with a std 100mm DPC on your inner skin.

Take a look at the .dxf / .pdf details on the Jablite site and all should be crystal clear.

hth

 

[garyo]

Thanks - I'll take a look at the jablite site.

Re: my P/A calcs - you could be right. The porch is being built into a crank/dog-leg in the front wall, so actually only two and a half walls are exposed to the elements.

I'd asssumed the entire permeter would be used because it's the perimeter dictating how much thermal bridge there is from the ground up through the inner skin, regardless of whether it's exposed or not?

On the basis it's the exposed perimeter, and it's the outside brickwork measurement (?) then it's (4.5 + 1.76 + .89) / 7.92 = 0.90 Excellent!