Floating floor over insulation - polystyrene?

[Wiredcharlie]

My extension will have a floating chipboard floor over solid insulation over concrete. In order for the bricks to course up, I have a depth of 150mm for the insulation and the chipboard. Could I save money by using a thinnish phenolic layer and a polystyrene layer? Anyone know where I can find u-values for polystyrene?

Any ideas much appreciated.

Tony

 

[ColJack]

you'll only save money in the short term..
use PIR or PUR foam rather than polystyrene.. it's much stronger and much better U values..

 

[mikric]

125mm of jablite/polystyrene will give you the required U-value on your extension floor + 22mm flooring sorted

 

[ColJack]

MINIMUM required, but is there any rules about going above and beyond?
is there any advantage to doubling the minimum?

 

[mointainwalker]

Expanded polystyrene ( white and crumbly ) U values

50 mm = 0.8
100 mm = 0.4

Extruded Polystyrene ( usually blue /light yellow and hard ) U values

50 mm = U 0.6
100 mm = U 0.3

PUR

50 mm = U 0.5
100 mm = U 0.25

@ColJack

With PUR being 400%- 500% more expensive than EXP for a 60% improvement in insulation value, you really do need to put some numbers to where you think heating costs are going, especially when talking about a floor which is going to be cooler ( assuming no UFH ) than the rest of the structure and therefore losing less heat per m2.

 

[Wiredcharlie]

I'm not sure I've got my head around this U-value thing yet. But would I be right in thinking that 125mm polystyrene is U 0.32 ?

My thinking:

if Polystyrene 50mm = U 0.8

then

polystyrene 25mm = U 1.6

so for 125mm (5 x 25mm)

5 x 1/1.6 = R

1/R = 0.32

But I thought the floor had to be U 0.22
Is this something to do with the P/A ratio that I don't understand?

 

[mointainwalker]

You have mainly got it but in an unusual fashion.

Iit is difficult to work with U other than for heat loss, soI prefer to use R as you can just add R values together and then revert to the final U value..

1) The relationship is that R x U always = 1

2 Therefore 50 mm exp polystyrene has a U of 0.8 and so a R of 1.25.and 25 mm has a value of half of this i.e. R 0.625

125 mm exp polystyrene has an R value of 5 x 0.625 = R 3.125

Using 1) the U value of 125 mm exp polystyrene is 1/3.125 = U 0.3 (approx) .

Current build up gives you

1) 125 mm exp poly = R 3.125
2) 22 mm chipboard = R 0.17
3)Surface Resistance = R 0.10

Total = R 3. 4 ( approx) = U 0.3

If you need U 0.22 ( and I don't know the regs ) and you want to stick with exp poly, then you need, 175 mm which changes first line of above calc to R = 4.375 and the total R to 4.65, just a touch more than needed.