Crazy Rafter Roll insulation cost

[MortarThePoint]

25 minutes ago, JOE187 said:

    Rockwool Flexi in 600mm or 400mm  x 1200mm at 100 or 140mm thickness  would be my recommendation.

 

 

Thanks, I do like the look of Rockwool but for our design I need to eliminate any added formaldehyde and I thing that has some (likely a very small amount). The Lambda isn't the best, but as @ADLIan points out probably doesn't make much odds:

 

λ=0.032   175mm   R=5.47

λ=0.035   175mm   R=5

λ=0.038   175mm   R=4.60

[I need to leave a ~50mm air gap within the 222mm rafter depth]

 

I'll need additional material under rafter to hit what I need and I am considering 60mm of Pavatherm which has λ=0.038 adding R=1.58 or 50 or 75mm of DriTherm λ=0.032 adding R=1.56 or 2.34. Next time I'd go full fill rafters (225mm mineral wool) with sarking board outside (possibly Pavatex).

[JOE187]

 The rockwool is 0.35 and at 175mm in slab form  would work out  about £9 m2 for flexi or £8 m2 for rw45 just depends if it work for your studs and rafters!

[MortarThePoint]

13 hours ago, JOE187 said:

 The rockwool is 0.35 and at 175mm in slab form  would work out  about £9 m2 for flexi or £8 m2 for rw45 just depends if it work for your studs and rafters!

 

That's a far more sensible price and I think Rockwool is less likely to puff up and restrict the ventilation gap.

 

If I'm worried abut the ventilation gap, I guess I can use a mesh though

    

https://www.insulationsuperstore.co.uk/product/insulation-support-netting-2m-x-100m-white.html  ~£0.26/m2

 

@NSS you appear to have used some cool looking green straps to hold your rafter insulation in place. Can you recommend a product there?

[NSS]

6 hours ago, MortarThePoint said:

 

@NSS you appear to have used some cool looking green straps to hold your rafter insulation in place. Can you recommend a product there?

 

Not sure which picture you're referring to?

[MortarThePoint]

43 minutes ago, NSS said:

Not sure which picture you're referring to?

 

[NSS]

22 minutes ago, MortarThePoint said:

 

 

Ah, the "cool looking green straps" is in fact just green garden twine stapled to the rafters ?

[MortarThePoint]

1 hour ago, NSS said:

 

Ah, the "cool looking green straps" is in fact just green garden twine stapled to the rafters ?

 

Brilliant, shows that you did a very neat job of it.

[MortarThePoint]

1 hour ago, NSS said:

 

Ah, the "cool looking green straps" is in fact just green garden twine stapled to the rafters ?

 

Are you happy with how in insulation worked out. Looks easy to install.

[NSS]

2 hours ago, MortarThePoint said:

 

Are you happy with how in insulation worked out. Looks easy to install.

 

Yes, we're happy. There are those who doubt the performance claims for multi-foil products (ours is Actis), and we've not lived in properties with other insulation for comparison, but it does what we expected.

 

Easy and (importantly for us) very clean to install.

[MortarThePoint]

Here are the prices I have been quoted. It's interesting to see how much the price/performance figure varies (from 50p to almost £5).

 

[t]    Name         lambda  cost     Price/Performance

90mm   FrameTherm     32  £8.80/m2   £3.13 [lambda*cost/t]
90mm   FrameTherm     35  £5.70/m2   £2.22
140mm  FrameTherm     32  £?.??/m2   £?.?? {£14.11/m2}
90mm   OmniFit Slab   35  £4.05/m2   £1.58
100mm  RafterRoll     32  £14.15/m2  £4.58
75mm   RafterRoll     32  £10.80/m2  £4.61
100mm  LoftRoll       44  £1.15/m2   £0.51
150mm  LoftRoll       44  £1.69/m2   £0.50  {£2.16/m2}
200mm  LoftRoll       40  £?2.60/m2  ?£0.52 {£3.32/m2}
180mm  OmniFit Stud   34  £18.05/m2  £3.41

Vari   Rockwool Flexi 38  various    £1.76  (e.g. £4.65/m2 for 100mm)

 

All prices are from the same source except {prices}. You pay a large premium for self supporting rolls. OmniFit Slabs probably represent the sweet spot for Knauf between the rafters. It highlights how cheap the LoftRoll insulation is. I have heard of builders stapling this at rafter level and I can understand why.

 

FactoryClad has a very large minimum order quantity.

 

I have heard of price fixing of insulation in some territories and, given how expensive it is, I am not surprised. There must be a lot of fraud around it as well since someone could easily claim to have used something twice as expensive and it's hidden away, plus hard to tell even if you can see it.

[MortarThePoint]

I have 222mm rafters and need to leave an approx. 50mm air gap so was planning to use 175mm of insulation between the rafters and 75mm under the rafters (all Mineral wool). Here are some example build ups:

• 75+100+75mm Rafter Roll 32, R=7.81 cost £35.75/m2
• 90+90mm FrameTherm 32 and 75mm Rockwool 38, R=5.63+1.97=7.6 cost £21.09/m2 [NB: RockWool not Formaldehyde free]
• 180+70mm Rockwool Flexi 35/38, R=5.14+1.84=6.98 cost £12.23/m2 [NB: RockWool not Formaldehyde free]
• 90+90+90mm OmniFit Slab 35, R=7.71 cost £12.15/m2
• 90+90mm OmniFit Slab 35 and 75mm DriTherm 32, R=5.14+2.34=7.48 cost ~£14/m2
• 90+90mm OmniFit Slab 35 and 60mm PavaTherm 38, R=5.14+1.58=6.72 cost ~£16.50/m2
• 90+90mm OmniFit Slab 35 and 50mm EcoTherm PIR 22, R=5.14+2.27=7.41 cost ~£14.60/m2
• 120+50mm EcoTherm PIR 22, R=7.73 cost ~£22/m2

If I were to do it again I would 100% use counter battens outside the roofing membrane and fill the rafters.

 

Improving a U=1.3 area by 0.1 saves approximately 3p/m2/yr with an air source heat pump.

[dT=13C, U=0.13, 6mo-->7.4kWh/m2, E=£0.05/kWh --> £0.37p.a./m2]

Edited June 30, 2021 by MortarThePoint

[Thorfun]

On 07/06/2021 at 14:22, MortarThePoint said:

Next time I'd go full fill rafters (225mm mineral wool) with sarking board outside (possibly Pavatex).

 

21 hours ago, MortarThePoint said:

If I were to do it again I would 100% use counter battens outside the roofing membrane and fill the rafters.

this is what I've decided to do. no need for sarking board outside as far as I know (in the south of England at least) as the BCO has said it's fine to have the relevant membrane touching the insulation and then counter battens/battens for roof tiles. 

 

we're having 197mm rafters so I will fill that with 200mm of some form of mineral wool (I quite like the look of that omnifit for a price performance material) and put Roofshield membrane above taught and touching the insulation.

 

our architect specified a 50mm gap and very expensive Kingspan phenolic insulation. so not only a lot of money but also a pain to install! even when I take into consideration the extra timber cost for the counter battens, Roofshield membrane and labour for the roofer I'm still a lot better off fully filling with cheaper mineral wool and slightly thicker PIR internally.

[Russell griffiths]

Anybody that uses rockwool needs the head looking at, it is the most hideous product known to man and needs banning. 

Knauf on the other hand is a pleasure to use. 

[Russell griffiths]

I used the knauf omni fit, you could make a coat out of it, not scratchy at all. 

I have a load of rockwool acoustic here and fitting it today I’m seriously considering throwing it all in the skip. 

 

Roofwise i did 200mm omnifit then 70mm pir under the rafters. 

[Thorfun]

5 hours ago, Russell griffiths said:

Roofwise i did 200mm omnifit then 70mm pir under the rafters. 

that's what I'm thinking but 100mm PIR instead of the 70mm. 

 

5 hours ago, Russell griffiths said:

I used the knauf omni fit, you could make a coat out of it, not scratchy at all. 

this is good to know and another tick in the box for the omnifit. thanks.

[MortarThePoint]

8 hours ago, Russell griffiths said:

Roofwise i did 200mm omnifit then 70mm pir under the rafters.

 

2no. 100mm Omnifit Slab 35?

 

That make up should be good and warm. R_ins=9.0. Is that targeting U=0.14?

[MortarThePoint]

On 30/06/2021 at 15:33, MortarThePoint said:

Improving a U=1.3 area by 0.1 saves approximately 3p/m2/yr with an air source heat pump.

[dT=13C, U=0.13, 6mo-->7.4kWh/m2, E=£0.05/kWh --> £0.37p.a./m2]

 

Don' want to draw too much attention to this consideration as I know people's priorities are many and varied. In part it was a note for my own consideration of the pros and cons. Anyway, I made a mistake in the first sentence as it was supposed to be "Improving a U=0.13 area by 0.01 saves approximately 3p/m2/yr with an air source heat pump." The mistake is obvious looking at the next sentence, but I wanted to correct that. so:

 

Improving a U=0.13 area by 0.01 saves approximately 3p/m2/yr with an air source heat pump.

[dT=13C, U=0.13, 6mo-->7.4kWh/m2, E=£0.05/kWh --> £0.37p.a./m2]

[saveasteading]

To complete the sum, do we then divide (.01/.013)  1/13 of £0.37, to obtain 3p p.a./m2 saving?

 

Thanks  for this, it is good to have this info when playing around with options.

[MortarThePoint]

17 minutes ago, saveasteading said:

To complete the sum, do we then divide (.01/.013)  1/13 of £0.37, to obtain 3p p.a./m2 saving?

 

Thanks  for this, it is good to have this info when playing around with options.

 

That's right.

 

The raw maths is: Heat Flow Q = U*A*dT

Heat Loss E_heat = time*average_heat_flow = (days*24hours/day) * U * A * (Average_dT)

For me average dT across 6 month 'heating season' is 13K (i.e. 13 Celsius) --> E_heat = (183days/yr * 24hours/day) * 0.13W/m2K * 1m2 * 13K = 7422Wh/yr = 7.4kWh/yr per m2.

That is heat energy, to understand what I'll pay, I need to know how much heat costs me. Using an ASHP with a COP of 300% (reasonable) and an electricity rate of £0.15/kWh electricity, I can calculated that heat costs £0.15 / 300% = £0.05/kWh heat.

7.4kWh * £0.05/kWh = £0.37/yr per m2.

 

It's all linear with U so to work out differences you can just scale that figure, so if the difference is 0.01 then the cost difference is (0.01 / 0.13) = 1/13 of £0.37/yr /m2.

 

When I work this sort of thing out I keep thinking I am making a mistake, but it's how it works (at least for marginally changes on a good base figure).

Edited July 2, 2021 by MortarThePoint

[MortarThePoint]

I didn't realise how cheap Gas is and that makes the number lower still. It seems Gas costs around £0.03/kWh [1] so that marginal rate becomes closer to  2p p.a./m2 saving. Prices can change of course, but that would need a whole lot of change.

 

https://www.ukpower.co.uk/home_energy/tariffs-per-unit-kwh

 

I secretly hope someone points out I've missed a factor of 10 or something because spending an extra £5/m2 to improve the U-value by 0.01W/m2K is so far from being financially sensible (payback >150 years). Even knowing this, I find myself irresistibly drawn to improving U-values. Even the embodied carbon side of increased insulation is very debateable with decades of payback time, but that's another story.

[MortarThePoint]

For my scenario* I can multiply a surface's U value by £2.85 and it will give me the annual cost of heat lost. Example, U=0.13 --> 0.13*£2.85 = £0.37 p.a./m2. Or dU = 0.01 --> 0.01*£2.85 = 3p p.a./m2

 

Quite handy for me to remember "£3 per U per m2 per year" [I know there's no such thing as 'a U' but it's a way of remembering it]

 

183days * 24h/day * 13K * £0.05/kWh = £2.85 [careful swapping between kW and W of course]

 

* My scenario is dT = 13 Celsius, 6 months and an ASHP with adjusted COP of 300%.

Edited July 2, 2021 by MortarThePoint

[MortarThePoint]

5 minutes ago, MortarThePoint said:

Quite handy for me to remember "£3 per U per m2 per year" [I know there's no such thing as 'a U' but it's a way of remembering it]

 

If you've ever been in any doubt, I do make this stuff up as I go along ?

 

I do think it's useful though. There's a window with a U-value of 1.2 and an area of 1m2. "£3 per U per m2 per year". That window is costing me £3.60 of lost heat per year.

 

Windows are a bad example as they can create drafts as they are so heat leaky.

 

How much heat lost through that bit of loft. U-value of 0.11 area of 15m2. "£3 per U per m2 per year". That loft is costing me 0.11*15*3=£4.95 of lost heat per year.

[sean1933]

I'm just going through this delightful decision at the moment. I have 220mm rafters and was aiming for a u value of 0.12 (rafters at avg.600 centres).

Having previously looked at pricing 2yrs ago I had planned to use 230mm Frametherm40 + 70mm PIR below.

 

Now I have come to ordering and I'm looking at pricing again. One thing for me is I hate noise so have started looking at density in comparison to stay Rockwool RTA45 (kg/m3). From what I have found the Frametherm 40 is only 13 kg/m3, 35 is 25kg/m3 and 32 is 32kg/m3. Does anyone have any experience of these densities in a pitched roof (I have 11mm OSB above rafters, planned for 38mm service battens and 15mm or double 12.5mm plasterboard below)? Should I just bite the bullet and go for the 35?

 

Nobody appears to mention the Frametherm40... which is substantially cheaper than 35 or 32. Is there a reason for this and has anyone used this?

Here is my quotes just to highlight the differences (over £3k between 40 and 35)!

			Unit price	M2 per unit	m2 price	TOTAL for 400m2
100mm PIR			£32.95	2.88	£11.44	£4,576.39
70mm "			£26.75	2.88	£9.29	£3,715.28
50mm "			£18.50	2.88	£6.42	£2,569.44
25mm "			£12.50	2.88	£4.34	£1,736.11
140mm Knauf FrameTherm Roll 40			£28.59	9.14	£3.13	£1,251.20
90mm Knauf FrameTherm Roll 40			£30.50	14.25	£2.14	£856.14
140mm Knauf FrameTherm Roll 35			£35.00	4.45	£7.87	£3,146.07
90mm Knauf FrameTherm Roll 35			£38.50	6.84	£5.63	£2,251.46
140mm Knauf FrameTherm Roll 32			£40.50	3.19	£12.70	£5,078.37
90mm Knauf FrameTherm Roll 32			£44.50	5.13	£8.67	£3,469.79

 

If I had my own way I'd go pumped cellulose but sadly on the south coast the only installer I have found wants horrendous money so a no go.

 

 

[ADLIan]

The difference in density between all those mineral wool products will have no affect on the overall acoustic performance. Price of all insulation products has increased massively in the last 12 months - perhaps by 50-60%!!

[MortarThePoint]

2 hours ago, sean1933 said:

One thing for me is I hate noise so have started looking at density in comparison to stay Rockwool RTA45 (kg/m3). From what I have found the Frametherm 40 is only 13 kg/m3, 35 is 25kg/m3 and 32 is 32kg/m3. Does anyone have any experience of these densities in a pitched roof (I have 11mm OSB above rafters, planned for 38mm service battens and 15mm or double 12.5mm plasterboard below)? Should I just bite the bullet and go for the 35?

 

Be careful here as when I looked at partitions it seemed like the whle Rockwool thing being good for sound was a myth: