Insulated suspended concrete floors.

[Oz07]

Browsing these recently and most manufacturers have a panel which fits between the beams and one which goes over the top. Like this. 

 

But then I see a manufacturer who does it with the insulation underneath and in-between and i think you end up with cold bridges where the beam bears onto the supporting wall. Have they allowed for this in their u values? I suppose you could mitigate with a sheet over the top but just seems a massive design error to me. Is it that the bridges are inconsequential on a large floor or do others agree? I'm just trying to guess why they would design like this?

[Nickfromwales]

Beamshield is the same, and also covers the underside.

 

I would go for the system that keeps cold air from the ventilated cavity getting to the beams, for obvious reasons, and then use PIR over the top.

 

I see a lot of developer sites using the cheaper / faster (poorer) version so there’s your answer , but it is good to see some at least trying.
 

Using blocks is just neanderthal level building, with the cost and grunt of slinging lorryloads of them around site.

 

Developers have gone with this to reduce costs, the extra insulation is just an accidental side effect of their bean-counting. 

 

The beams will always bear on to blockwork due to the point loadings / weight upon them, and the cold bridging is negligible. 

[saveasteading]

20 minutes ago, Oz07 said:

manufacturer who does it with the insulation underneath 

This has been around for decades and is simple and effective. I've never used it and so haven't checked the relative cost.

22 minutes ago, Oz07 said:

Have they allowed for this in their u values? 

They will give a a value for a central cross section. The overall effect of the beam bearings depends on the size and shape of the building and is reality, not a flaw... any beam has the same.

The main benefit of this is in insulating under the beams, in vacant space, and saving insulation above.

 

Best is to work out the relative costs every way , if inclined.

 

At the internal wall I assume the eps block is cut to fit.

 

Another advantage will be the finish. With concrete blocks they don't always sit tight and you have to brush a sandy mortar into the gaps.

And lighter to handle of course.

 

 

[Oz07]

I think you guys are missing what im saying. The bottom photo is how that system is finished. As in the slab sits directly on top of those beams which bear directly on top of the masonry at the walls. So an unmitigated cold bridge. The top system has the underside of the beams exposed but has a 75mm (or thicker) sheet of insulation over the whole top so no unmitigated cold bridge. It just seems a massive own goal to me but perhaps I don't understand cold bridges correctly?

[G and J]

I agonised over things like the cold bridges with insulated bnb floors then I did the numbers on a solid floor with lots of underfloor insulation.  I know bnb floors bypass a lot of underground issues and are often the only option (as in our last house) but if you’ve the choice why not check the relative costs of a solid floor for your build.  

[Oz07]

I did ground bearing last place but got some apple trees im lopping so think gonna play safe with suspended

[saveasteading]

4 hours ago, Oz07 said:

you guys are missing what im saying

Not at all.  There is a cold bridge but it is not completely cold if thought through with insulation above.

A building is a cold bridge between indoors and outdoors... but it is relative.

 

4 hours ago, Oz07 said:

play safe with suspended

If near trees then the foundations will be deep and the trees won't affect the slab.

The main downside of  suspended floor is the essential draught under it. You lose the benefit of the natural insulation of the earth. 

[Oz07]

3 minutes ago, saveasteading said:

Not at all.  There us a cold bridge but ig is not completely cold if thought through with insulation above.

A building is a cold bridge between indoors and outdoors... but it is relative.

 

If near trees then the foundations will be deep and the trees won't affect the slab.

The main downside of  suspended floor is the essential draught under it. You lose the benefit of the natural insulation of the earth. 

The system is designed by manufacturer. That bottom one is the finished article excluding dpm. Literally finished floor slab. On top of concrete beams which bear onto walls which touch the ground and are exposed to outside air. Of course you can potentially mitigate with insulation on top but who knows if then complies with bba cert. At least the top system has a nice chunky 75mm sheet on top of beams before concrete

[Oz07]

Also certain trees mean deeper foundations which mean suspended floor is the better option incase of heave. 

[ADLIan]

55 minutes ago, Oz07 said:

I think you guys are missing what im saying. The bottom photo is how that system is finished. As in the slab sits directly on top of those beams which bear directly on top of the masonry at the walls. So an unmitigated cold bridge. The top system has the underside of the beams exposed but has a 75mm (or thicker) sheet of insulation over the whole top so no unmitigated cold bridge. It just seems a massive own goal to me but perhaps I don't understand cold bridges correctly?

The floor edges are dealt with as a linear thermal bridge separate from the insulation (& Uvalue) in the general floor area. The ground floor/wall junction is a difficult one when preventing/minimising the thermal bridge. This is more difficult with a suspended concrete slab that bears on inner masonry - 'overlapping' the cavity wall and floor insulation is one option. This is further complicated with beam and block. Relatively simple where the beams run parallel to the wall (2D thermal analysis) but probably requiring a 3D thermal analysis where the beams run into and bear on the wall. Again extending any cavity insulation to well below the floor insulation will help.

[saveasteading]

1 hour ago, Oz07 said:

suspended floor is the better option incase of heave. 

A bold assertion without knowing the detail.

Everything on its merits.

[Oz07]

Its not that bold. A normal ground bearing floor is susceptible to heave, suspended one less so. I have trees very close to proposed house. Better to plan for the worst. 

 

I started the thread as I was interested in the difference between the cold bridges on the 2 different systems. I wasn't really looking for a discussion on whether to use ground bearing or suspended floor. This always happens on this forum. I can see why it has quietened down with ai. 

[Nickfromwales]

3 hours ago, Oz07 said:

The system is designed by manufacturer. That bottom one is the finished article excluding dpm. Literally finished floor slab. On top of concrete beams which bear onto walls which touch the ground and are exposed to outside air. Of course you can potentially mitigate with insulation on top but who knows if then complies with bba cert. At least the top system has a nice chunky 75mm sheet on top of beams before concrete

I did one in Gravenhill and the B&B had regular blocks, and then we put 200mm of PIR and 100mm of concrete; reinforced by 2 layers of mesh.

 

The design and storey rods help you to calculate how low the B&B needs to be set down to allow the extra insulation. 
 

75mm of EPS is pretty crap. 

[Oz07]

35 minutes ago, Nickfromwales said:

I did one in Gravenhill and the B&B had regular blocks, and then we put 200mm of PIR and 100mm of concrete; reinforced by 2 layers of mesh.

 

The design and storey rods help you to calculate how low the B&B needs to be set down to allow the extra insulation. 
 

75mm of EPS is pretty crap. 

75 on top would be 225 in total with the 150 between beams. I think you can up the spec of the top sheet to achieve .08 u value according to the websites. 

 

But again we are proving my point here. The thread wasn't set up to debate whether beam and block is good or not. Just what appears to me the glaring cold bridge in one of the options above compared to the standard way these 'warm' b&b floors usually work.