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 yesterday at 02:19 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)

[saveasteading]

17 hours ago, SteamyTea said:

desired U-Value.

Excellent graph thanks. May I ask it's origin?

That's for an infinite area or based on some HP/A assumption?

Theres a bit of a kink at 100mm ish thickness which seems wierd and that really underlines how worthwhile 100mm is.

Severely diminishing returns after,  I'd say, 200mm PIR.

And no significant benefit after 0.3? ie a complete waste of resources.

[SteamyTea]

4 minutes ago, saveasteading said:

May I ask it's origin

I just knocked it up from assumed k-values.

7 minutes ago, saveasteading said:

an infinite area or based on some HP/A assumption

It is a 1D model, so edges, which can have greater losses, would need to be added on.

8 minutes ago, saveasteading said:

bit of a kink at 100mm ish thickness which seems wierd

I think that is caused by the charting software, see below for a different curve algorithm (changed to a B-Spline from a Cubic Hermite Spline).

9 minutes ago, saveasteading said:

Severely diminishing returns after,  I'd say, 200mm PIR.

And no significant benefit after 0.3? ie a complete waste of resources.

This is where it gets difficult.  It depends, primarily, on energy prices and installation prices.  UFH would cause greater losses than just heating the air in the building, the building form factor would make a difference as well.

This is why individual buildings need to have heat loss calculations and not have the heating system sized by a rules of thumb i.e. just floor area or number of windows (I have heard a plumber assess the heat loss that way).

 

Losses though the slab are uneven as they can be affected by the room temperature above, the ground type below and if there is any ground water movement nearby.

So probably safer to pop a bit extra in, rather than too little.

 

[G and J]

2 hours ago, SteamyTea said:

This is where it gets difficult.  It depends, primarily, on energy prices and installation prices.  UFH would cause greater losses than just heating the air in the building, the building form factor would make a difference as well.

This is why individual buildings need to have heat loss calculations and not have the heating system sized by a rules of thumb i.e. just floor area or number of windows (I have heard a plumber assess the heat loss that way).

 

Losses though the slab are uneven as they can be affected by the room temperature above, the ground type below and if there is any ground water movement nearby.

So probably safer to pop a bit extra in, rather than too little.

Allow me, (in my post New Year’s Eve socialising, sleep deprived, just driven 3.5 hours and me adrenaline left the building about 5 miles short of home state), to translate into buildspeak.

 

”Dunno.  Hard innit.  Better put enough in to stop anyone moaning, which means too much.  But sod it, it’s cheap enough.”

:-/

[SteamyTea]

  

11 minutes ago, G and J said:

just driven 3.5 hours

That gets me to @Pocsterland, or about a third of my bi-monthly trips up country.  I find filling my mind with thermodynamics problems passes the time.

[SteamyTea]

I must be bored.

Here is a chart showing price comparison between insulation types.  I have only got the prices from 1 site, and have used median k-Values.  To get better results, do your own research.

 

There is little difference between XPS and Phenolic at the 0.1 W.m^-2.K^-1 level, about £10.m^-2.

There is a 0.1m thickness difference though, which may or may not be important depending on how you excavate and dispose of the dirt.

 

 

 

(as usual, there may be errors as I am tired)

[saveasteading]

On 31/12/2025 at 14:30, JohnMo said:

You can also get specialised insulation saws.

Something else I didn't know.

Seems to be a saw tooth but with a wavy profile to avoid sticking, and produces less dust. Between a saw and a knife.  £22

 

The Bahco insulation saw has been specially developed for cutting various types of insulation material

[saveasteading]

On 31/12/2025 at 13:32, JohnMo said:

appropriate compression strength. Not all are equal.

 

I'm aware of that but reminders are no bad thing.

I wonder how well a soft packaging eps would fare beneath 100mm or more of PIR and a grand piano on top. perhaps with a big enough dish under each wheel it would be ok.  This is whimsical: I am not saving amazon packaging to try it.

[Roger440]

39 minutes ago, saveasteading said:

Something else I didn't know.

Seems to be a saw tooth but with a wavy profile to avoid sticking, and produces less dust. Between a saw and a knife.  £22

 

The Bahco insulation saw has been specially developed for cutting various types of insulation material

 

Ive found this very difficult to use on the thicker boards. PIR at least. Works great at 50mm. Useless at 100mm. Better off with a cheap normal saw.

 

[Roger440]

4 hours ago, G and J said:

 

 

”Dunno.  Hard innit.  Better put enough in to stop anyone moaning, which means too much.  But sod it, it’s cheap enough.”

😕

 

Where is this cheap PIR you speak of?

 

[saveasteading]

1 hour ago, Roger440 said:

this cheap PIR you speak of?

So far, simply using online published rates with no allowance for quantity..

celotex or other names all seem to be the same price as listed below, since when I've found cheaper but must check the spec.

eps varies by vendor and many don't do it.

please excuse that the Tab key doesn't work

 

PIR

thickness  £/m2          £/m3

25             4                 161

50mm       6                 121

75             8.4              112

100           10.4             104                 Now found approx   £88/m3

120           12.7              106

 

EPS      

25              2.16           86

50/75/100     all          86

 

unfaced pir

90-100       £8.51       £85/m3

 

as you will see, this bears out what some of you clever people have said about 100mm being the most competitive.

 

[SteamyTea]

7 hours ago, saveasteading said:

100           10.4             104 

 

7 hours ago, saveasteading said:

50/75/100     all          86

 

At those prices (shows it is worth hunting around), and if my spreadsheet is correct, EPS comes out at £30 (0.35m thick) and PIR at £26 (0.25m thick per meter squared.

Those are for a U-Value of 0.1, which in reality, is pretty good.

If there was a 20°C temp difference (quite possible with UFH) and you have 100m² floor area, then:

20°C x 100m² x 24h = 4.8 kWh/day.

That would be for a very cold day.

Edited 6 hours ago by SteamyTea

[Gone West]

12 hours ago, saveasteading said:

The Bahco insulation saw has been specially developed for cutting various types of insulation material

I'm using a Predator saw/knife to cut mineral wool insulation and it's suprisingly effective. It's a tough material, a lot different to EPS/PIR. Worth a try for a tenner from Amazon.

 

[G and J]

3 hours ago, SteamyTea said:

 

 

At those prices (shows it is worth hunting around), and if my spreadsheet is correct, EPS comes out at £30 (0.35m thick) and PIR at £26 (0.25m thick per meter squared.

Those are for a U-Value of 0.1, which in reality, is pretty good.

If there was a 20°C temp difference (quite possible with UFH) and you have 100m² floor area, then:

20°C x 100m² x 24h = 4.8 kWh/day.

That would be for a very cold day.

Is that assuming a BnB floor with the underfloor void being the same temp as outside?

 

One of the reasons I was pleased to go first a solid floor was that the ground under would be a more constant temperature possibly 7C.

[Roger440]

10 hours ago, saveasteading said:

So far, simply using online published rates with no allowance for quantity..

celotex or other names all seem to be the same price as listed below, since when I've found cheaper but must check the spec.

eps varies by vendor and many don't do it.

please excuse that the Tab key doesn't work

 

PIR

thickness  £/m2          £/m3

25             4                 161

50mm       6                 121

75             8.4              112

100           10.4             104                 Now found approx   £88/m3

120           12.7              106

 

EPS      

25              2.16           86

50/75/100     all          86

 

unfaced pir

90-100       £8.51       £85/m3

 

as you will see, this bears out what some of you clever people have said about 100mm being the most competitive.

 

 

Thats interesting.

 

I need to buy more for my workshop. The previous part i did in 100mm (seconds) with 100 x 50 battens.

 

Was going to do 50 +50 this time to reduce the bridging effect of the battens. Though the saving on timber by using 50x50 studs might offset the PIR increase?

 

Plus of course depends what seconds and Co have available at the time.

[G and J]

10 hours ago, saveasteading said:

So far, simply using online published rates with no allowance for quantity..

celotex or other names all seem to be the same price as listed below, since when I've found cheaper but must check the spec.

eps varies by vendor and many don't do it.

please excuse that the Tab key doesn't work

 

PIR

thickness  £/m2          £/m3

25             4                 161

50mm       6                 121

75             8.4              112

100           10.4             104                 Now found approx   £88/m3

120           12.7              106

 

EPS      

25              2.16           86

50/75/100     all          86

 

unfaced pir

90-100       £8.51       £85/m3

 

as you will see, this bears out what some of you clever people have said about 100mm being the most competitive.

 

Just looked back at our quotes from our local Jewsons.

 

EPS70 was less than £47/m3

PIR (foil faced) was less than £87/m3

 

Both for 120mm thick 2.4m x 1.2m sheets held in stock locally at their suppliers.