EPS70 vs PIR price

[Oz07]

I've only had a quick look online will have to be a bit more thorough.

Is it right that roughly you need double thickness of EPS 70 to achieve same Performance as PIR?

I cannot find EPS70 for half the price of PIR. Am i missing a trick here?

[Jeremy Harris]

2 minutes ago, Oz07 said:

I've only had a quick look online will have to be a bit more thorough.

Is it right that roughly you need double thickness of EPS 70 to achieve same Performance as PIR?

I cannot find EPS70 for half the price of PIR. Am i missing a trick here?

 

 

You need about 1.55 times the thickness of EPS for the same U value as if using PIR.  It's the ratio of the relative λ values that matters, so EPS with a λ of about 0.034 W/m.K and PIR with a λ of about 0.022 W/m.K gives a ratio of 0.034 / 0.022 = 1.55

[Russell griffiths]

I have found eps to be about a third of the cost of pir 

[Alexphd1]

There was a really good thread over on greenbuild forum that give you a set u-value and the thickness of different kinds of insulation eps, xps pur etc) and a rough cost. Think the uvalues where 0.2, 0.15 and 0.1.

[Oz07]

Thanks all 

@Russell griffithswhat suppliers are you using?

I've just been doing some quick googling and perhaps better prices to be had via merchant?

Sure 150 pir looked to be around 50 quid with 150 eps 30 quid. 

Tbf if only need 1.55 thickness then 225mm ish eps must be getting to bregs u value. .13?

[Jeremy Harris]

58 minutes ago, Alexphd1 said:

There was a really good thread over on greenbuild forum that give you a set u-value and the thickness of different kinds of insulation eps, xps pur etc) and a rough cost. Think the uvalues where 0.2, 0.15 and 0.1.

 

 

It's not hard to calculate U values using basic principles.  You need to calculate the R value (from the material λ values for each layer material and thickness), add the surface R values for either side (typically 0.13 m².K/W internally and 0.04 m².K/W externally) and then just add all the R values together.  Take the reciprocal and you have the U value.

 

For example, for a simple wall with 12mm OSB both sides, no thermal bridging and a core of 200mm of EPS, then the R values are calculated like this:

 

External resistance = 0.13

12mm OSB, λ = 0.13 W/m.K, R = 0.092

200mm EPS, λ = 0.034 W/m.K, R = 5.405

12mm OSB, λ = 0.13 W/m.K, R = 0.092

Internal resistance = 0.04

 

Total R = 5.759 m².K/W

 

U value = 1/5.759 = 0.174 W/m².K

[Russell griffiths]

1 hour ago, Oz07 said:

Thanks all 

@Russell griffithswhat suppliers are you using?

I've just been doing some quick googling and perhaps better prices to be had via merchant?

Sure 150 pir looked to be around 50 quid with 150 eps 30 quid. 

Tbf if only need 1.55 thickness then 225mm ish eps must be getting to bregs u value. .13?

Insulation cart. 

[Jeremy Harris]

I've knocked up a simple spreadsheet for calculating U value, and included some typical material λ  values, and a simple calculator for working out the λ  for an air gap.  The calculator adds the thermal resistance of the internal and external surfaces and includes provision for walls. floors etc with up to 6 layers of different materials.

 

It's a bit rough and ready, but seems to work OK.  To help prevent accidental deletion of calculation and fixed data cells I've locked the worksheet, but if anyone wants to unlock it the password is BuildHub

 

If anyone finds any bugs let me know and I'll try and fix them.

 

Simple U value calculator.xls

 

 

[Oz07]

Thanks @JSHarris. I've generally been finding u value calculators on companies websites i.e. jablite to help me work out U value. I guess your spreadsheet is a form of this except just input the raw data? I will download good to have.

Cheers @Russell griffiths will look

[Oz07]

Jeremy are you building regs U values right or is it outdated? Thought floors were now .13

[Jeremy Harris]

4 minutes ago, Oz07 said:

Jeremy are you building regs U values right or is it outdated? Thought floors were now .13

 

That's what should be in the spreadsheet, at least that's what's showing on the master copy, 0.13 W/m².K for floor and roof, 0.18 W/m².K for the walls and 1.4 W/m².K for the doors and windows.

[Oz07]

Sorry my bad you're right. Didn't have to enter a password my end

[Jeremy Harris]

6 minutes ago, Oz07 said:

Sorry my bad you're right. Didn't have to enter a password my end

 

Phew, that's a relief!  You don't need any password unless you want to modify any of the locked cells.  I locked them just to prevent accidental finger trouble deleting the formulae in them, so only cells that need to have values changed are unlocked and can be edited as needed.

[Adam2]

Thanks @JSHarris, in L1A I thought the min values were different and the ones in the spreadsheet are the model house values. Or did I misunderstand? which won't be a surprise. Am curious as my walls plasterboard/air gap/65mm EPS/150mm concrete/65mm EPS come in at about 0.23 which is > the  BR wall value of 0.18. Yet our SAP assessor says, based on the overall design, that our design is a pass.

 

[Jeremy Harris]

12 minutes ago, Adam2 said:

Thanks @JSHarris, in L1A I thought the min values were different and the ones in the spreadsheet are the model house values. Or did I misunderstand? which won't be a surprise. Am curious as my walls plasterboard/air gap/65mm EPS/150mm concrete/65mm EPS come in at about 0.23 which is > the  BR wall value of 0.18. Yet our SAP assessor says, based on the overall design, that our design is a pass.

 

 

The limiting fabric values aren't really that significant in Part L1A now, as a lot of the time the house may need to exceed those values in order to get a "pass" in SAP, I believe.  Those values in the spreadsheet are only there for reference, they aren't used in any calculations and could be just deleted easily enough.

 

I'll check the calcs.  What's outside the outer layer of EPS?

[Adam2]

OK thanks, wouldn't want people thinking they were a min target to hit/improve upon - not saying that's a bad thing of course but with the title it does imply the are. Not wanting to detract from the spreadsheet, that + the other one you provided are great resources for many.

[Adam2]

10 minutes ago, JSHarris said:

 

The limiting fabric values aren't really that significant in Part L1A now, as a lot of the time the house may need to exceed those values in order to get a "pass" in SAP, I believe.  Those values in the spreadsheet are only there for reference, they aren't used in any calculations and could be just deleted easily enough.

 

I'll check the calcs.  What's outside the outer layer of EPS?

Mostly will be render

[Jeremy Harris]

Just now, Adam2 said:

OK thanks, wouldn't want people thinking they were a min target to hit/improve upon - not saying that's a bad thing of course but with the title it does imply the are. Not wanting to detract from the spreadsheet, that + the other one you provided are great resources for many.

 

 

I've just checked, and with only 130mm of EPS in total you aren't going to have very good wall insulation really.  Ignoring the concrete, 130mm of EPS on it's own is only going to be about 0.25 W/m².K, which is a bit grim really. 

 

Running the calculations by hand for a build up that's (from inside to out): 12.5mm plasterboard - 50mm service void - 65mm EPS - 150mm concrete - 65mm EPS - 15mm cement render I get 0.23 W/m².K

 

Double checking with this online calculator : http://www.changeplan.co.uk/u_value_calculator.php gives a significantly worse U value of 0.27 W/m².K, mainly because the default values of λ  for the EPS and the concrete seem to be a bit worse than the values I selected.

 

[Adam2]

Hmm, thinking we could apply an extra 50mm EPS on the inside - mainly due to exterior window reveal depths becoming pretty chunky for a contemporary design if applied outside. Should get us down to 0.18 or thereabouts. Presumably this wouldn't lead to condensation risks. Alternatively if externally applied extra EPS could mount windows on some form of extended support built off the concrete (think I saw this on another thread where this was reducing reveals but also improving thermal performance).

[Jeremy Harris]

15 minutes ago, Adam2 said:

Hmm, thinking we could apply an extra 50mm EPS on the inside - mainly due to exterior window reveal depths becoming pretty chunky for a contemporary design if applied outside. Should get us down to 0.18 or thereabouts. Presumably this wouldn't lead to condensation risks. Alternatively if externally applied extra EPS could mount windows on some form of extended support built off the concrete (think I saw this on another thread where this was reducing reveals but also improving thermal performance).

 

 

Why EPS and not PIR?  Switching to PIR would make a useful difference, as it has a λ  of around 0.022 W/m.K, versus a λ  of around 0.034 W/m.K for high density EPS. 

 

Two 65mm layers of PIR on their own would give a U value of about 0.165 W/m².K, or your wall build up with PIR in place of EPS would give about 0.16 W/m².K, which is heading towards a sensible design target (although still not great). 

 

The notional fabric values in Part L1A aren't a bad aiming point, and they quote a wall U value for design of 0.13 W/m².K, which seems pretty sensible.

[Adam2]

7 minutes ago, JSHarris said:

Why EPS and not PIR? 

Planning on Nudura so EPS. If we went with own additional insulation that of course could be PIR for the width/U benefit. Looks like that would get down to 0.15.

We'll probably live in the house ~5 years so need to consider cost/benefit over this period. Though of course we may change our minds after we finish building it ?

 

[Jeremy Harris]

7 minutes ago, Adam2 said:

Planning on Nudura so EPS. If we went with own additional insulation that of course could be PIR for the width/U benefit. Looks like that would get down to 0.15.

We'll probably live in the house ~5 years so need to consider cost/benefit over this period. Though of course we may change our minds after we finish building it ?

 

 

Adding 50mm of PIR to the inside EPS (or better still the outside, if possible) and reducing the internal service void down to 25mm (just to mitigate the wall build up thickness) would bring the U value down to around 0.15 W/m², which is getting there, and certainly a lot better than the original build up. 

 

There's a chance that the specific type of high density EPS used in Nudura may have a slightly better λ  than the figure of 0.034 W/m.K that I've used in these calculations.  Might be worth digging around in their specifications to see if they list it.

[ADLIan]

Appr Doc L1A does not give specfifc U-values apart from the backstop, worst case, values - this gives a lot of design flexibility. The U-values in Jeremy's spread sheet are those used in the model dwelling (Table 4 in  AD L1A) used to assess compliance using SAP.

 

Remember in Jeremy's spread sheet to always enter cavity width (cell B25) in order to give correct thermal conductivity for use later in each construction. @JSHarrisCould this be defaulted to R=0.18 which would be good enough for most instances and not be so thickness dependant.

 

The spread sheet only gives an approximation and should not be used for Buildng Reg purposes as it does not correct the U-value for thermal bridges, mechanical fixings thru the insulation, air gaps in the insulation etc

 

[Oz07]

Ha I thought these u values had to be hit now regardless of other things to offset. Learn something new everyday.

Think I'm going to go with 200mm eps under floor. gives around .14 based on p/a ratio

[PeterW]

I nearly went 250mm EPS until I found some 150mm PUR on Seconds & Co at a cheap price. Was easier than digging down another 100mm.