PIR vs glass mineral wool

[BartW]

Hi everyone,

 

I am nearing completion of my (already approved) drawings, and am now down to a fine nitty gritty. I need to propose the drawings to the TF supplier, and with that in mind I need to make up my mind about the insulation, thus the external wall buildup.

 

I have played around with all sorts of calculators, read the forum about the decrement delay etc, and... still no wiser.

 

So I hope you may be able to give me a final steer.

 

Some basic facts:

- we need to achieve 0.15 uValue

- we are building using a TF construction with 140mm studs

- TF supplier will supply the frame, erect, and VCL all around

- insulation and internals are down to us

- externally we are having a combination of natural slate from Cuppa Pizzaras and Sliva Birch clad as 44mm vertical battens

 

 

My considerations:

- PIR helps achieve the driven uValue with a smaller buildup

- PIR is flammable right?

- PIR has lower decrement delay

- packing PIR tightly is more challenging to unskilled

- mineral wool is better to handle, and transport, store on site, etc

- in order to achieve 0.15 I would need to use layers of rockwool, if on its own

- the build has got a budget cap, and I worry that layers of rock wool will brake the piggy bank, and add a considerable amount of time to the build

 

What if:

- I use 150mm packed rock wool in a 140mm stud (is it a healthy thing? Would that give a benefit? Or should I stick to 140 / 140)

- put a single layer of PIR internally @ 80mm

 

I did some calcs, and I seem to be achieving as per the below:

 

 

Am I on the right path?

 

Of course, I would love to be doing things "the proper way", but money is a detrimental factor here. I hope just by building and not giving in to poor quality mass developer builds, I am already "making a difference"

 

 

On a separate note:

 

 

I am trying to design a balcony / terrace roof insulation, and whilst I am ok to have a step up from the bedroom to the terrace (above the dining room), I wonder if I could be doing this slightly differently.

 

Currently, getting at a typical warm roof buildup with OSB/PIR/OSB, all upon rafters. Could I reduce the thickness of the PIR above to, say, 50mm, pack rock wool in-between rafters, and finish off with a insulated plasterboard? That area of ceiling is less than 10m2.

 

Similarly, we are having a section of a pitched roof that will be clad in natural slate. Is doing the PIR between rafters + PIR under the ok way to do it? I wouldn't necessarily want to increase the roof thickness beyond what's required.

 

Many thanks!

 

Bart

 

 

 

 

 

 

 

 

 

 

 

 

 

[PeterW]

26 minutes ago, BartW said:

Currently, getting at a typical warm roof buildup with OSB/PIR/OSB, all upon rafters. Could I reduce the thickness of the PIR above to, say, 50mm, pack rock wool in-between rafters, and finish off with a insulated plasterboard? That area of ceiling is less than 10m2.

No - that’s a hybrid build up and would need ventilated cavities etc and a nightmare to detail - just make it a warm roof or look at the minimum joist sizes you need for the shortest span and run the terrace in those. 
 

28 minutes ago, BartW said:

What if:

- I use 150mm packed rock wool in a 140mm stud (is it a healthy thing? Would that give a benefit? Or should I stick to 140 / 140)

No benefit as insulation depends on trapped air and you’re excluding that

 

29 minutes ago, BartW said:

put a single layer of PIR internally @ 80mm

You’ve created an air gap for no reason..?? Seems no point to this build up. 
 

Why not 140mm of Rockwool plus 40mm of PIR or 52.5mm insulated plasterboard ..? Or 90mm of PIR and 25mm over ..?? 
 

31 minutes ago, BartW said:

the build has got a budget cap, and I worry that layers of rock wool will brake the piggy bank, and add a considerable amount of time to the build

Insulation per “uValue” is broadly comparable per square meter - yes you need 50% more EPS depth for a certain value vs PIR, but PIR is around 50% more expensive than EPS so it levels out. Similar with Rockwool and PIR, you won’t make huge savings here. 
 

 

[joe90]

Have you thought about blown cellulose insulation?

[TonyT]

You could fill the 140mm timbers, then overlay a sheet over the top at right angles, tape this etc, then fix battens for a service void.

[BartW]

32 minutes ago, PeterW said:

1 hour ago, BartW said:

put a single layer of PIR internally @ 80mm

You’ve created an air gap for no reason..?? Seems no point to this build up. 

 

Sorry, I should have clarified. The what if scenario assumes doing both, not either or.

 

 

16 minutes ago, joe90 said:

Have you thought about blown cellulose insulation?

 

I have. It was big money when I checked.

 

11 minutes ago, TonyT said:

You could fill the 140mm timbers, then overlay a sheet over the top at right angles, tape this etc, then fix battens for a service void.

 

I am assuming that is in reference to the above wall buildup?

[joe90]

1 minute ago, BartW said:

have. It was big money when I checked.

Problem in this country is no one hires out the pumps (but I really like it as an insulating material).

[Jason L]

i have recently been looking at this, how about - 

using Recticel Eurothane PIR which is one of the more affordable ones.

have 140mm of it between the studs, and 30mm on the inside of the studs should achieve around that u-value and reduce the overall thicness of the wall. 

[TonyT]

18 minutes ago, BartW said:

 

Sorry, I should have clarified. The what if scenario assumes doing both, not either or.

 

 

 

I have. It was big money when I checked.

 

 

I am assuming that is in reference to the above wall buildup?

yes, I used 100mm first, siliconed this into the stud wall, 40mm then on top with staggered joins not matching the previous layer. Again siliconed the joins and foil taped. That’s me flush with the timber and then another 40mm fixed over the studs sheets laid horizontally. Siliconed and taped- then the usual battens, VCL and plasterboard.

[Thorfun]

if you use 140mm of Frametherm 32 between the studs you can reduce the internal PIR to probably around 40mm or 50mm to get those uValues. Also, in your uValue calculations you haven't added your external cladding. that will also reduce your uValue so probably best to add it now.

 

oh, and it's a breathable membrane on the outside not a VCL. ? 

[ProDave]

40 minutes ago, Thorfun said:

if you use 140mm of Frametherm 32 between the studs you can reduce the internal PIR to probably around 40mm or 50mm to get those uValues. Also, in your uValue calculations you haven't added your external cladding. that will also reduce your uValue so probably best to add it now.

 

oh, and it's a breathable membrane on the outside not a VCL. ? 

Nice (relative to many other mineral wool products) to work with and will give a much longer decrement delay than PIR it's what I used in this house.

[Thorfun]

7 minutes ago, ProDave said:

Nice (relative to many other mineral wool products) to work with and will give a much longer decrement delay than PIR it's what I used in this house.

and it's what I plan to use in my house. ?

[BartW]

Ok,

 

So:

- Frametherm 32 is a lot more expensive than 35, and only makes a difference of 0.005 u value. So my choice of the 35 version is deliberate (or call me cheap )

- with my buildup, as in the original post reducing the PIR from 80 to 70mm means going up to 0.1501 uValue. So I kept the 80 to stay on the safe side.

- sounds like my choices are feasible?

 

4 hours ago, TonyT said:

yes, I used 100mm first, siliconed this into the stud wall, 40mm then on top with staggered joins not matching the previous layer. Again siliconed the joins and foil taped. That’s me flush with the timber and then another 40mm fixed over the studs sheets laid horizontally. Siliconed and taped- then the usual battens, VCL and plasterboard.

 

Why not use a single sheet of 140mm? Surely It is cheaper and quicker to do a single layer rather than 100 + 40?

1 hour ago, Thorfun said:

Also, in your uValue calculations you haven't added your external cladding. that will also reduce your uValue so probably best to add it now.

 

I omitted it because it was not making any significant difference. Also the program was getting confused with layers of free air and two lots of battens setting the slates, and the cladding off the structural wall.

 

 

4 hours ago, Jason L said:

i have recently been looking at this, how about - 

using Recticel Eurothane PIR which is one of the more affordable ones.

have 140mm of it between the studs, and 30mm on the inside of the studs should achieve around that u-value and reduce the overall thicness of the wall. 

 

That was my original choice, but the decrement delay, and the fuff of precisely slotting bits of the PIR in-between the structural studs has me rethinking my choices. In an ideal world I would do rock wool throughout, but then the wall buildup increases beyond feasible (and so does the labour).

 

 

 

[Thorfun]

53 minutes ago, BartW said:

Frametherm 32 is a lot more expensive than 35, and only makes a difference of 0.005 u value. So my choice of the 35 version is deliberate (or call me cheap )

- with my buildup, as in the original post reducing the PIR from 80 to 70mm means going up to 0.1501 uValue. So I kept the 80 to stay on the safe side.

- sounds like my choices are feasible?

that's interesting as I went through these same calculations and I get 0.12W/m2K with Frametherm 32 and 80mm PIR. but now I think about it that uValue calculator puts the thermal conductivity of PIR as 0.023W/mK whereas you can buy 0.020W/mK PIR (obviously costs more though!) which is how I would've got to the lower uValue. I still thought Frametherm 32 made a bit more of a difference though.

 

your choices are definitely feasible and that is exactly what I'm doing and our architects didn't have a problem with it at all. so I'd say crack on!

 

58 minutes ago, BartW said:

I omitted it because it was not making any significant difference. Also the program was getting confused with layers of free air and two lots of battens setting the slates, and the cladding off the structural wall.

 

fair enough. it doesn't make a huge difference but, as they say, every little helps. (other supermarket advertising slogans are available)

[PeterW]

1 hour ago, BartW said:

 

That was my original choice, but the decrement delay, and the fuff of precisely slotting bits of the PIR in-between the structural studs has me rethinking my choices. In an ideal world I would do rock wool throughout, but then the wall buildup increases beyond feasible (and so does the labour).

So make a jig that allows you to cut at 2.5° and cut the PIR with a slight bevel to it. Push it home and foam the edges and it will be tight in and sealed. 
 

Why would you go with rockwool throughout..?? Mix of materials is a good thing. You can also tape the outer PIR and it will give you a VCL from the foil facing 

[TonyT]

1 hour ago, BartW said:

Ok,

 

So:

- Frametherm 32 is a lot more expensive than 35, and only makes a difference of 0.005 u value. So my choice of the 35 version is deliberate (or call me cheap )

- with my buildup, as in the original post reducing the PIR from 80 to 70mm means going up to 0.1501 uValue. So I kept the 80 to stay on the safe side.

- sounds like my choices are feasible?

 

 

Why not use a single sheet of 140mm? Surely It is cheaper and quicker to do a single layer rather than 100 + 40?

 

I omitted it because it was not making any significant difference. Also the program was getting confused with layers of free air and two lots of battens setting the slates, and the cladding off the structural wall.

 

 

 

That was my original choice, but the decrement delay, and the fuff of precisely slotting bits of the PIR in-between the structural studs has me rethinking my choices. In an ideal world I would do rock wool throughout, but then the wall buildup increases beyond feasible (and so does the labour).

 

 

 

I was building as the shortage of PIR/PUR boards I think due to a factory fire somewhere in Europe and I was also putting 200mm in the suspended timber floor between joists  so buying 100mm meant spare could be used elsewhere in the build.

[BartW]

3 minutes ago, TonyT said:

I was building as the shortage of PIR/PUR boards I think due to a factory fire somewhere in Europe and I was also putting 200mm in the suspended timber floor between joists  so buying 100mm meant spare could be used elsewhere in the build.

 

I remember those times. 2 - 3 years ago I lost a small fortune to buying of 150sq metres of PIR for a domestic refurb / extension project for my client...

 

12 minutes ago, PeterW said:

Why would you go with rockwool throughout..?? Mix of materials is a good thing. You can also tape the outer PIR and it will give you a VCL from the foil facing

 

Decrement delay, and other associated posts and opinions come to mind

 

Really, I am down to choosing between PIR on PIR vs PIR on rock wool. Money seems not far off, although then the difference being:

 

 

decrement delay vs wall thickness. I can afford the latter. Can I afford the former? Would I tell the difference?

 

 

 

[Thorfun]

2 minutes ago, BartW said:

PIR on PIR vs PIR on rock wool

for me rock wool between the TF wins hands down simply for the ease of installation. the last thing I want to be doing is cutting PIR to size to fit! *&^% that.

[Buzz]

Hi @bart , where have you found the most competitive prices for insulation ? need to start pricing my roof up ?

[BartW]

6 minutes ago, Buzz said:

Hi @bart , where have you found the most competitive prices for insulation ? need to start pricing my roof up ?

Generally insulationshop.co for some products, but equally a lot of general google / online shopping for others. 
 

I did not write down the sources I was pricing these from (useful I know...), but was getting the below prices just from shopping. I did not speak to anyone, or any of my usual builders’ merchants, some of which I get better rates than others. 
 

I hope this helps:

 

 

Structural walls insulation:	M2
Supply only cavity insulation	210	Celotex XR4120 per 2.88	£38	£2,771
Supply only internal face insulation	264	Celotex GA4050 per 2.88	£17	£1,558
Supply only VCL	264	Superfoil VCL per 75m2	£85	£299
Supply only internal battens	456	Sawn Green 50x25 per 4.8	£4	£380
MIneral wool for intermediate floor + gym + loft storage	134	Loft roll 150mm Knauf 44 per 9.18m2	£20	£292
Structural roof insulation:
Supply only deck PIR	77	Celotex XR4150 per 2.88m2	£50.00	£1,337
Supply only VCL	77	Superfoil VCL per 75m2	£85	£87
Pitched Roof insulation:
Supply only PIR between studs	70	Celotex XR4150 per 2.88m2	£50.00	£1,215
Supply only PIR internally	80	Celotex GA4030 per 2.88m2	£13.00	£361
Supply only VCL	80	Superfoil VCL per 75m2	£85	£91
			SUBTOTAL	£8,391.43

 

[PeterW]

Why are you using Superfoil VCL..??! There is £300 saved, it’s pointless here. 

[Buzz]

Thanks @bart so your pitched roof is coming in at £1667 for 70m2  or £24 m2 ? what u value will that give ? 

Edited January 14, 2021 by Buzz

[BartW]

40 minutes ago, PeterW said:

Why are you using Superfoil VCL..??! There is £300 saved, it’s pointless here. 

Which part?

 

38 minutes ago, Buzz said:

Thanks @bart so your pitched roof is coming in at £1667 for 70m2  or £24 m2 ? what u value will that give ? 

it’s 80m2. Although I don’t remember why I allowed that much. The pitched prt of my roof is only 10.2m x 4m at the longest. I think I may have put a proviso for front and rear gable in that as well as the wall that’s holding the ridge. 
 

I basically quantified square metres @ 80m2 and deducted for the rafter part, hence the line above it @ 70m2. The predicted uValue is 0.13 when using the kingspan calculator, but I stipulated 0.14 when using cheaper PIR products. 
 

 

 

[Jason L]

Hi. Can you explain what decrement delay is please? 

[BartW]

3 minutes ago, Jason L said:

Hi. Can you explain what decrement delay is please? 

I would love to, but I am hardly getting to grips with it myself ?

 

I will let the veterans take a lead on that. I will happily listen, and read some more myself ?

[Buzz]

So £21m2 to get to .14 in pir , thanks i have somewhere to start ?, going to need somewhere close to 400m2 ?.