Moonshine

External cavity wall - 140-150mm cavity?

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I have been looking at potential constructions of masonry cavity walls and getting a decent U value (circa 0.17).

 

I have been going on the basic wall with a 125mm cavity as below;

 

  • Plasterboard on dot and dab
  • 100mm thermalite block
  • 75mm Kooltherm K108 Cavity Board
  • 50mm cavity
  • Rendered 100mm dense block 

 

This gives a U value of circa 0.167, however the Kooltherm K108 is expensive (75mm £21/m2 ex VAT https://insulationhub.co.uk/product/kingspan-kooltherm-k108-75mm-cavity-board/) with a premium performance thermal conductivity of 0.018 W/mK, where as standard PIR seems to have a thermal conductivity of 0.022 W/mK.

 

If the Kooltherm K108 is swapped out for standard PIR of 0.022 W/mK (e.g. Celotex CW4000) to get a U value of 0.17, the PIR would need to be 90mm, and the resulting cavity 140mm, or 150mm if a standard thickness of 100mm was used.

 

Celotex CW4000 can be brought in 100mm for £15.70/m2 ex VAT (https://www.insulationexpress.co.uk/celotex-cw4000-cavity-wall-insulation-board)

 

Also by going to a standard PIR there is more competition, and a better chance of getting a deal.

 

That is a saving of £5 a m2, which over 250m2, saves ~£1,250 for the same performance thermally.

 

The downsides i can think of are.....

 

  • loosing internal floor space
  • harder to cut 100mm vs 75mm
  • Increased lintel sizes
  • increased tie sizes

 

What other issues / cost implications are there?

 

I am not sure its worth it for a  ~£1,250 saving

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Or.... i could ditch the PIR all together and use something like full fill Superglass Superwall 32 (£9.20 /m2 ex VAT), and fully fill the cavity at 150mm (https://www.roofingoutlet.co.uk/products/superglass-superwall-32-cavity-wall-insulation-batt-150mm)

  • Plasterboard on dot and dab
  • 100mm thermalite block
  • 150mm Superglass Superwall 32 (0.032 W/mK)
  • Rendered 100mm dense block 

This also gives a a U value of 0.17.

 

So over 250m2

  • 75mm Kooltherm K108 Cavity Board - £5,250 ex VAT (125mm cavity)
  • 100mm Celotex CW4000 - £3,925 ex VAT (150mm cavity)
  • Superglass Superwall 32 - £2,300 ex VAT (150mm cavity)

 

not sure what the pitfalls are of having a full filled cavity with glass wool (handling would be an issue), but also not having a clear cavity to allow any water that gets in to get to the bottom of the cavity 

Edited by Moonshine

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26 minutes ago, Moonshine said:

not sure what the pitfalls are of having a full filled cavity with glass wool (handling would be an issue)


my 200mm cavity is full filled with 2x 100, Dritherm 32 Batts, BBA certified as ok for full fill. My wall is brick not render and during the winter the West wall (facing the Atlantic) bricks got sodden and I had to core drill a hole for ASHP pipes, this enabled me to test if the insulation was wet Which it was not apart from the first inch touching the bricks. No idea what my U value is but it’s a warm house. Handling the insulation was ok, not like fibreglass at all.

Edited by joe90

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15 minutes ago, joe90 said:


my 200mm cavity is full filled with 2x 100, Dritherm 32 Batts, BBA certified as ok for full fill. My wall is brick not render and during the winter the West wall (facing the Atlantic) bricks got sodden and I had to core drill a hole for ASHP pipes, this enabled my to test if the insulation was wet Which it was not apart from the first inch touching the bricks. No idea what my U value is but it’s a warm house. Handling the insulation was ok, not like fibreglass at all.

 

Good to know, if it was thermalite block internally you are probably in the realm of a U value of 0.14.

 

i was looking at the Dritherm 32 as an alternative to the Superwall 32.

 

So here is a question, is the 50mm gap between PIR and external skin an actual set in stone requirement, as here is a product with a 10mm gap.  

 

https://www.insulationsuperstore.co.uk/product/recticel-eurowall-plus-full-fill-cavity-insulation-90mm-54m2-pack.html

 

could you not have a 125mm cavity, with 100mm PIR and 25mm gap to keep separation?

 

Edit, it seems that the BBA cert requires a design of 50mm residual cavity, but could be down to 25mm on site.

 

https://www.bbacerts.co.uk/uploads/files/CertificateFiles//53/5343ps4i1a03042019.pdf

 

Edited by Moonshine

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My inner skin is block as I hate thermalites (don’t ask). By brickie did not like full fill as he was “old school “ and believed an air gap was required but he accepted the BBA cert.

 

4 minutes ago, Moonshine said:

could you not have a 125mm cavity, with 100mm PIR and 25mm gap to keep seperation?


I guess so, my reason for dritherm was ease of installation as PIR needs to be fitted tightly or foamed together to avoid gaps. Read somewhere that small gaps in insulation drastically reduces its efficiency and batts can be “stuffed” into odd corners etc. Also PIR is a petrochemical product and I bought myself a hair shirt 🤣🤣🤣🤣

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2 minutes ago, joe90 said:

batts can be “stuffed” into odd corners etc. Also PIR is a petrochemical product and I bought myself a hair shirt 🤣🤣🤣🤣

 

i am sort of coming round to the full fill cavity, and the recycled element is a plus.

 

The main concern is the water ingress potential....maybe rendered, at 150mm thick, and water repellent it may solve that issue.

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I am considering blown EPS beads for a new build with brick outer, 140mm cavity and plastered blockwork inner.  Rigid board insulation is often prone to lashups, with gaps between boards, at corners and around openings, as well as damage on site.

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Dip an offcut of dritherm in a water butt or puddle. I've seen it sat in a puddle for days, lift it up and the water just repels off. Doesn't wick or soak up at all really.

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20 minutes ago, Mr Punter said:

Rigid board insulation is often prone to lashups, with gaps between boards, at corners and around openings, as well as damage on site.

Having nearly done half my house with ridged board PIR i realise that it must be nearly impossible to install it REALLY WELL  on a commercial basis, it just takes so much time and effort to get it 100% right. It’s so obvious that should you get it wrong it will DRAMATICALLY reduce in its performance, And should air be able to get round the back of the board / on the warm side And flow it would render the insulation virtually useless. I have gone for cutting, foaming  and taping in multiple layers and the result is fantastic ..... but I have gone seriously OCD And if I put a cost to my time it would be loony asylum stuff for me. 

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1 hour ago, Moonshine said:

 

i am sort of coming round to the full fill cavity, and the recycled element is a plus.

 

The main concern is the water ingress potential....maybe rendered, at 150mm thick, and water repellent it may solve that issue.


Yours is rendered block so should not be a problem, as I said mine is brick and will wick water but it got nowhere once in the dritherm. Blown beads was an alternative but as it’s always windy down here I am sure half of them would end up in Somerset 🤣🤣🤣🤣🤣.

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56 minutes ago, Oz07 said:

Dip an offcut of dritherm in a water butt or puddle. I've seen it sat in a puddle for days, lift it up and the water just repels off. Doesn't wick or soak up at all really.

 

I might have to give that a go for my own sanity.

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33 minutes ago, Cpd said:

Having nearly done half my house with ridged board PIR i realise that it must be nearly impossible to install it REALLY WELL  on a commercial basis

 

how about the ones that have tongue and groove fixings?

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Just now, Moonshine said:

 

how about the ones that have tongue and groove fixings?

 

I had some full fill Celotex (now withdrawn) boards fitted.  Some were upside down / cut badly round openings / back to front.  Bricklayers can be decent at brick / blockwork but mostly the insulation just gets in their way.

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As above, if it’s not you doing the work I would look at a solution that provides the least amount of opportunity to go wrong. Yes tongue and groove is good and will help but you still have so many locations where it could fail, top, bottom, windows, doors , corners, services.... the list  goes on and on. It’s just my personal view as I have read and seen so many installations done badly with no easy fix to solve it. It’s still my number one insulation and I will continue to use it but it’s all work I am doing myself so know I am getting a pukka job. 

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Blown beads all the way. 175mm gives you 0.165 which is what I would go for. 

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So fluffy insulation batts vs PIR

 

- Easier to install / less to go wrong

- similar u value as pir for same cavity thickness, fully filled cavity compared with a residual cavity of 50mm used for pir.

- Cheaper

- Easier to cut (?)

- uses recycled materials 

 

Downsides

 

- could be irritating to install / itchy, mitigated by weather correct protection 

- potential water issues, mitigated by water resistant covering and external skin

- doesn't let the building 'breathe'

 

Edited by Moonshine

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39 minutes ago, Moonshine said:

- could be irritating to install / itchy


No

 

40 minutes ago, Moonshine said:

potential water issues,


No (Apart from during the build. I covered all wall tops during inclement weather) see.....

2 hours ago, Oz07 said:

Dip an offcut of dritherm in a water butt or puddle. I've seen it sat in a puddle for days, lift it up and the water just repels off. Doesn't wick or soak up at all really.

 

40 minutes ago, Moonshine said:

doesn't let the building 'breathe'


yes it does it’s vapour permeable.

 

 

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Don't forget to air-tighten under that dot and dab...

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15 hours ago, Moonshine said:

So fluffy insulation batts vs PIR

 

- Easier to install / less to go wrong

- similar u value as pir for same cavity thickness, fully filled cavity compared with a residual cavity of 50mm used for pir.

- Cheaper

- Easier to cut (?)

- uses recycled materials 

 

Downsides

 

- could be irritating to install / itchy, mitigated by weather correct protection 

- potential water issues, mitigated by water resistant covering and external skin

- doesn't let the building 'breathe'

 

all the reasons and problems you mention is why i will be using isotex ICF wodcrete blocks

-may seen more expensive at first glance --but add up all the other things --i don,t think it is

 walls and insulation in one go -clad with what ever you like - outer brick wall -brick slips -render ,wood clad

 

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Generally BC will not allow full fill cavity insulation in Scotland.

 

Blown beads seem like they would be the most reliable fill option from my research.

Edited by AliG

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