Floor slab insulation. Test my logic please?

[SteamyTea]

5 hours ago, G and J said:

PIR instead of 120mm each of polystyrene and PIR

You are still comparing overall thickness though (I know you had a (expletive deleted) up are limited on thickness). 

What you need to do is pick a U-Value (i.e. 1.2 W/m².K) and compare the prices to achieve that, not compare prices to fill the gap up.

4 hours ago, saveasteading said:

10 year return would be my judgement

Not if you had made that decision in 2015, 6 years before our energy prices doubled.

[Roger440]

5 hours ago, JohnMo said:

But you also need to buy the correct grade for appropriate compression strength. Not all are equal.

 

 

 

Now thats a bit of a minefield if ever there was one...............

[Roger440]

2 hours ago, SteamyTea said:

 

Not if you had made that decision in 2015, 6 years before our energy prices doubled.

 Doesnt building control use 15 years as its guide as to insisting on stuff, at least in a retrofit situation?

[SteamyTea]

1 minute ago, Roger440 said:

 Doesnt building control use 15 years as its guide as to insisting on stuff, at least in a retrofit situation?

No idea.

Would be OK for windows and doors, but not very good for slab insulation.

[SteamyTea]

Use this simple cut out guide to work out your ideal insulation thickness for your desired U-Value.

 

[G and J]

9 hours ago, SteamyTea said:

You are still comparing overall thickness though (I know you had a (expletive deleted) up are limited on thickness). 

What you need to do is pick a U-Value (i.e. 1.2 W/m².K) and compare the prices to achieve that, not compare prices to fill the gap up.

Not if you had made that decision in 2015, 6 years before our energy prices doubled.

We ended up with 335mm to 345mm to fill to get to FFL which includes screed with UFH.  I am keen on a good thickness screed so wanted circa 100mm.  So for us the insulation thickness was given. 
 

We want the biggest living space we can fit in to our narrow plot so our walls haven’t the most fantastic U values so to an extent the floor and ceiling can sort of compensate a bit, so I targeted 0.1 for the floor. 
 

Going from the architects suggested 0.13 U value to that felt ok, cost wise.  Beyond that felt silly. 

[G and J]

11 hours ago, saveasteading said:

coarse cut handsaw and a big bench

I’d strongly agreee, using a piece of wood knelt on as a guide for me really sped it up.  
 

Both polystyrene and PIR made mess, but for me only PIR was unpleasant. 

Edited 9 hours ago by G and J
Typo

[Conor]

Just use a layer of 50mm PIR on the bottom, with 100mm on the top. Your local merchant will have 1000s of these in stock, at good prices, as that's what the building trade uses. They won't have any /much 75mm PIR or EPS. As a consequence , 50+100mm will work out better value than 2x 75mm. Thin EPS is a nightmare to work with as it just snaps. Foil faced PIR holds together better, can be cut more precisely, despite it being "harder" to cut than EPS. You also can't have steps in your inusaltion, so just allow the screed to take care of those level variations.

 

(Our floor is 50mm eps, 50mm PIR, 100mm PIR, 50mm screed. 50mm eps at bottom as we changed from 100mm to 50mm screed last minute)