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Roof insulation


deuce22
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Hi.

 

I'm trying to understand exactly how building control calculate insulation requirements. A friend is currently refurbing a property in South Wales and has been told to put 75mm PIR in between the rafters and a foil quilt underneath. The rafters are only 4x2s and so he's had to add a batten underneath to get a 50mm air gap. I've put this into the calculator and the U value is 0.39. I refurbed my own property from 2002 regs some time ago and had to put 100mm in between and 50mm underneath.

I'm currently doing a self build and to achieve the 0.11 that I need, I am putting 200mm of mineral wool in between and 150mm on top. Are these foil quilts that good and why doesn't the calculator recognise the U value of them?

Thanks.

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59 minutes ago, deuce22 said:

Are these foil quilts that good?

 

No. There is some very marginal improvements in emissivity (shining out of heat from a surface) and as a radiant barrier (heat protect)  in the right circumstances but largely they're nonsense when it comes to insulation. 

 

If I could I would use organic material in a pitched roof, (cellulose, hemp, woodfiber etc). Much better heat protection and less issues with thermal bypass when using pumped insulation. Also more vapour safe and better in a fire. 

 

In the case of your mate what U-value does he need?

 

What stage are you at in your build? There's some nice examples of good pitched roofs here. @ProDave is one. 

 

 

 

 

 

 

 

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From the diagram it states 0.18 for pitched roofs at rafter level. My own logic says that what he's been told is incorrect, as I had to do more than this with 2002 regs. I can't understand why they've told him this and are they treating the quilt foil as if it is some type of super insulated material.

 

I'll be putting the last floor of ICF on soon and then ready for the roof. I'm trying to work out the most efficient way of insulating it. Thanks for the link, but where do I go once i'm in ProDave's profile?

Screen Shot 2022-04-03 at 11.51.35.png

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Posted (edited)

So, Ive just been speaking with a developer who is building multiple properties and he suggested that I speak with Superfoil as he has used this product on all of his properties. They have given me the following solution. SF60 below rafters, 25mm PIR in between rafters and SF40 above rafters. They have stated that this achieves 0.11.

I have just spoken to them again and told them that I don't want to have the hassle of fitting PIR if I'm going down the foil route and asked if I could fill in between with wool (whether glass, mineral or natural). He stated that I only need to double the thickness of the wool to achieve the same U value of a PIR insulation, but will still need to have the 2 layers of foil.

They have also sent through an alternative, which is 200mm wool in between and SF40 on top. This method seems a lot less hassle than having to cut and gapotape PIR, but I'm still unsure, as so many people say foils are not good enough. I've input these materials into the Ubakus calculator and it doesn't come near 0.11.

Does this seem correct.

Edited by deuce22
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What's the lambda for the foil your talking about? A datasheet would help.. there is no magic insulation solution , but there is a lot of misleading information...

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Posted (edited)
1 hour ago, markc said:

25mm between rafters?

I know, I can't get my head around it all.

 

46 minutes ago, Miek said:

What's the lambda for the foil your talking about? A datasheet would help.. there is no magic insulation solution , but there is a lot of misleading information...

I'm finding that there's a lot of misleading info on a lot of things and it seems like everybody is just trying to promote there own product, whether it's right or wrong. I asked if BC would be happy with this and they said, "if they're not tell them to call us and we'll give them all the information they require. Surely, if this is correct, BC wouldn't even question it.

They only give you an R value, which is 3.67.

Edited by deuce22
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The R value is meaningless without a thickness. Id call BS  without knowing the lambda.

 

I think a lot of salesfolk either don't understand this themselves or they assume the person they are talking to doesn't understand.

 

R-values can be calculated by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/mK). R-values are therefore expressed in m2K/W (or ft2·°F·hr/Btu in the USA

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The above does not apply to multi foil insulation. Here the manufacturers normally quote a total thermal resistance of their product based on the resistance of the core material plus the resistance of a low emissivity air space on both sides. It is the additional battens that can add to the thickness of the roof (or wall) and add to material and labour costs.

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On 03/04/2022 at 11:57, deuce22 said:

I'm trying to work out the most efficient way of insulating it.

When you say efficient do you have a value in mind ? and i assume the most cost effective way to hit it without cutting a lot of PIR

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13 hours ago, Miek said:

The R value is meaningless without a thickness. Id call BS  without knowing the lambda.

 

I think a lot of salesfolk either don't understand this themselves or they assume the person they are talking to doesn't understand.

 

R-values can be calculated by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/mK). R-values are therefore expressed in m2K/W (or ft2·°F·hr/Btu in the USA

The thickness is 75mm. I'll ask them today what the lambda value is.

 

13 hours ago, Buzz said:

When you say efficient do you have a value in mind ? and i assume the most cost effective way to hit it without cutting a lot of PIR

I'll be fitting all the insulation myself. I've been on many jobs and seen what a lot of people do when insulating, it's like a jigsaw puzzle. I won't trust anybody to do it properly. If it's not going to cost a lot more and it makes it easier for me, id prefer to use materials that are straight forward to install. 

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7 minutes ago, deuce22 said:

I'll be fitting all the insulation myself. I've been on many jobs and seen what a lot of people do when insulating, it's like a jigsaw puzzle. I won't trust anybody to do it properly. If it's not going to cost a lot more and it makes it easier for me, id prefer to use materials that are straight forward to install. 

I looked into the foil systems and came to the conclusion they were not for me i forget which one on their website had a good guide to u value build up but seem to recall to get down to .12 still involved putting a large amount of PIR between the rafters with foil on top and below with air gaps between each layer which would be a PITA to install , in the end we opted for batten and counter batten, Procter roof shield membrane , 175mm full fill multi roll between the rafters and 100mm PIR underneath which gets to .12. if you want lower than this just up spec the multi roll, we used 40 but there is 37 , 35, and 32 available at additional cost .

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Think I went through every possible solution for our roof including multi foil.

 

I ended up spray foam, reflective airtight vapour control and 50mm service void.

 

If I had a clean sheet of paper I would consider just having external insulation, no cold bringing to think about, easy enough to install.  All you wood is within the heated environment, so no condensation risk.

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5 minutes ago, JohnMo said:

If I had a clean sheet of paper I would consider just having external insulation, no cold bringing to think about, easy enough to install.  All you wood is within the heated environment, so no condensation risk.

Yes , Yes and Yes 👍👍👍

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image.thumb.png.cd16d0557930a71a2d592cbc1ebabcb6.png

 

 

 

I joists + Dense Pack Cellulose 

 

Cellulose is about €100/m3 inc VAT installed near me. 

 

If there's something better for heat protection/ cost/ sound protection / airtightness/ VOC's/ carbon/ mess/ ease of install

 

I'm yet to hear about it. 

 

 

 

 

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Thanks Iceverge.

 

I have given up on searching for alternative ideas now, I'm fed up with all the misleading information. I'll probably go back to my original idea of 200mm wool in between and 150mm PIR on top, this gives a value below 0.11. 

 

I'll have a look for some cellulose installers near me and compare the costs. 

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2 minutes ago, deuce22 said:

Thanks Iceverge.

 

I have given up on searching for alternative ideas now, I'm fed up with all the misleading information. I'll probably go back to my original idea of 200mm wool in between and 150mm PIR on top, this gives a value below 0.11. 

 

I'll have a look for some cellulose installers near me and compare the costs. 

are you going to be installing the mineral/glass wool and PIR yourself? if so, then seriously consider not doing it! it's not a nice job and took me ages to do just the glass wool and I still need to do the 100mm PIR. if I could go back in time, I'd add battens to below the rafters and pay for cellulose!

 

just my 2 cents.

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I would do all the insulation myself. I agree that it's a nightmare and have done it many times in the past, I'm not looking forward to it at all. I've never fitted PIR on top before, only in between.

 

I've had a search for cellulose installers near me, but can't find anybody.

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17 minutes ago, deuce22 said:

I would do all the insulation myself. I agree that it's a nightmare and have done it many times in the past, I'm not looking forward to it at all. I've never fitted PIR on top before, only in between.

 

I've had a search for cellulose installers near me, but can't find anybody.

maybe search nationally? if it's a specialised field then companies would probably travel.

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19 hours ago, Iceverge said:

image.thumb.png.cd16d0557930a71a2d592cbc1ebabcb6.png

 

 

 

I joists + Dense Pack Cellulose 

 

What is holding all this cellulose on the inside? This image seems to only show vapour barrier which is probably not strong enough? 

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10 minutes ago, oldkettle said:

This image seems to only show vapour barrier which is probably not strong enough? 

 

A good vapour membrane stapled and pinched between SC battons and Rafters would be quite capable. I reckon if you were careful you could lie on it yourself!

 

We have 400mm Above ours and it's fine. 

 

They regularly use netting in the US which looks infinitely more flimsy.

 

Can Cellulose be Used in an Unvented Roof? - GreenBuildingAdvisor 

 

 

I think @Nickfromwales has some cellulose contacts from memory. 

 

 

Edited by Iceverge
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7 minutes ago, Iceverge said:

 

A good vapour membrane stapled and pinched between SC battons and Rafters would be quite capable. I reckon if you were careful you could lie on it yourself!

 

We have 400mm Above ours and it's fine. 

 

They regularly use netting in the US which looks infinitely more flimsy.

 

Can Cellulose be Used in an Unvented Roof? - GreenBuildingAdvisor 

 

 

I think @Nickfromwales has some cellulose contacts from memory. 

 

 

 

This is impressive - I was sure OSB or similar would be required. 

 

So to be absolutely sure: 100/m3 at 400mm means it's £40/m2 installed? Or is it in Ireland since you quoted euros? 

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On 06/04/2022 at 18:15, deuce22 said:

They only give you an R value, which is 3.67.

 

On 07/04/2022 at 08:34, deuce22 said:

The thickness is 75mm

SF60 has 120mm thickness, so that would be equivalent to lambda (or K) of 0.29 - pretty realistic. What is not realistic is that you pay over £21/m2 for that performance (https://www.wickes.co.uk/SuperFOIL-SF60FR-Multifoil-Insulation-Roll-1500mm-x-8m/p/237123) so roughly 6 times more than wool https://www.wickes.co.uk/Knauf-Insulation-Space-Standard-Top-Up-170mm-Loft-Roll---6-47m²/p/109450  Even after adding reflective VLC (like https://passivehousesystems.co.uk/product/phs-hi-thermia-reflective-membrane/) there is a lot of change left.

 

There are caveats to that performance though:

- the quoted R takes into account surfaces and continuity of the material, the core itself is probably on par with wool at between 0.3 and 0.4

- it is reduced to 0 where attached to rafters, so with 600mm centres and 75mm wide rafter effective R drops close to 3, and probably more as it takes some distance to expand from 0 to 120, and compression reduces performance

 

Big worry is also that their BBA certificate has been suspended https://www.bbacerts.co.uk/2022/01/07/december-2021-new-withdrawn-certificates/

 

 

On 06/04/2022 at 18:32, Miek said:

The R value is meaningless without a thickness. Id call BS  without knowing the lambda.

 

I think a lot of salesfolk either don't understand this themselves or they assume the person they are talking to doesn't understand.

 

R-values can be calculated by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/mK). R-values are therefore expressed in m2K/W

 

 Be careful, as your statement is as misleading as salesman talk: R value is not meaningless without thickness. Thermal resistance of a surface is non-zero, yet its thickness is. And even for those cases that indeed can be calculated by the equation above, R can still be better, if k changes with thickness - which frankly is the case for most of the commonly used materials: PIR and phenolic lambda improve with sheet thickness, wool performance drops.

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2 hours ago, oldkettle said:

So to be absolutely sure: 100/m3 at 400mm means it's £40/m2 installed? Or is it in Ireland since you quoted euros? 

 

 

We paid €28 +VAT for 400mm for a cold roof in 2020 per m2. 

 

That's €70/m3 +VAT /m3. It's probably increased a bit and I can imagine the labour for dense packing between rafters might be a bit more. 

Edited by Iceverge
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9 hours ago, deuce22 said:

I've had a search for cellulose installers near me, but can't find anybody.

Do you still need a recommendation? PM me if so and I will give you their details. Not sure he wants to "go public" but I will ask ;) 

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