External cavity wall - 140-150mm cavity?

[PeterW]

The drill pattern that they use is quite regimented and they fill bottom up and don’t rely on it being blown a significant distance although from memory the installers did say it falls and compacts under gravity for about 5m so they could do it all from the first floor of a standard house. The thing to note - and to tell them - is where any cavity trays are and where they may extend either side of openings, and if there are any for abutment walls or other details where beads can’t easily flow. 

[Mr Punter]

13 hours ago, Moggaman said:

Would 200mm blown beads bring you under the 0.15 passive level?

 

With brick outer, 200mm of the premium beads and block interior you get 0.1539.  Do they round down?  You could use AAC block for internal and it would be even lower.  On lots of the calcs they do plasterboard on dabs, which helps the u value a bit.  I prefer direct plaster.  More chance of airtightness.

[Oz07]

Them beads go everywhere providing you you a decent width cavity and it's been kept clear I'd be more than confident that there would be no voids 

[Iceverge]

+1 no voids. I drilled trial holes in a awkward spots afterwards to check. Also the DPC we used to close the cavity was bulging up everywhere. 

 

They may not be 100% bonded thought so any cores should be done prior to pumping. 

[patp]

On 15/09/2020 at 14:38, Moonshine said:

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

We were told that dot and dab plasterboard on blockwork negates any insulation. The advice we were given is to wet plaster internal walls.

[PeterW]

52 minutes ago, patp said:

We were told that dot and dab plasterboard on blockwork negates any insulation. The advice we were given is to wet plaster internal walls.

By whom..??!! That’s entirely incorrect .. if done correctly with continuous band top and bottom and around sockets etc you have fewer issues but in no way does it negate the insulation.  

[Oz07]

It improves the sap score if detailed right. 

[Mr Punter]

It is quite common on developer houses to have partial fill insulated cavity walls with dot and dab plasterboard, where outside air freely circulates around both cavities.  I like wet plaster as an airtightness barrier.

 

I have been looking at poly bead fill but I am moving towards premium mineral wool.  It has the same R value and none of the fire risk.

 

Flammable insulation may be going rapidly out of fashion, especially in walls.

 

https://www.insidehousing.co.uk/news/widely-used-polystyrene-insulation-system-shown-to-be-similar-to-grenfell-style-cladding-in-australian-fire-test-69005

[oldkettle]

25 minutes ago, Mr Punter said:

It is quite common on developer houses to have partial fill insulated cavity walls with dot and dab plasterboard, where outside air freely circulates around both cavities.  I like wet plaster as an airtightness barrier.

 

I have been looking at poly bead fill but I am moving towards premium mineral wool.  It has the same R value and none of the fire risk.

 

Flammable insulation may be going rapidly out of fashion, especially in walls.

 

https://www.insidehousing.co.uk/news/widely-used-polystyrene-insulation-system-shown-to-be-similar-to-grenfell-style-cladding-in-australian-fire-test-69005

 

This does not look like a full fill insulation and not in a masonry wall cavity? 

[Mr Punter]

5 minutes ago, oldkettle said:

This does not look like a full fill insulation and not in a masonry wall cavity? 

 

No, it isn't.  The Inside Housing article was about polystyrene external wall insulation being flammable in high rise flats.  It may be that in the future any flavour of flammable wall insulation is unacceptable in any residential building.

[oldkettle]

30 minutes ago, Mr Punter said:

 

No, it isn't.  The Inside Housing article was about polystyrene external wall insulation being flammable in high rise flats.  It may be that in the future any flavour of flammable wall insulation is unacceptable in any residential building.

 

I'd hope not based on the fact that there is a massive difference in conditions between EWI and a full fill cavity and between a high rise building and a two storey house. 

 

Edited to add: I wonder where it leaves ICFs which in many cases use EPS forms. 

Edited December 12, 2020 by oldkettle

[Oz07]

I think big difference between multi occupancy buildings and single residential houses 

[willbish]

From what I've read the inclusion or exclusion of a 5mm or 10mm cement render board can make all the difference to these systems fire rating.

 

16 hours ago, oldkettle said:

: I wonder where it leaves ICFs which in many cases use EPS forms. 

With 6 inch of solid concrete between any external fire and habitable area, it leaves them very safe

[Mr Punter]

52 minutes ago, willbish said:

From what I've read the inclusion or exclusion of a 5mm or 10mm cement render board can make all the difference to these systems fire rating.

 

I understand that the Grenfell system was tested with cement board and failed, so was tweaked until it passed.  Agreed there is a world of difference between a high rise strongly vented cavity and a low rise semi closed cavity.

[patp]

23 hours ago, PeterW said:

By whom..??!! That’s entirely incorrect .. if done correctly with continuous band top and bottom and around sockets etc you have fewer issues but in no way does it negate the insulation.  

The lecturer at Potton homes self building course.

[patp]

1 hour ago, patp said:

The lecturer at Potton homes self building course.

I have just been corrected! It is not that it negates the insulation but that wet plaster is better than dot and dab for heat retention.

[A_L]

2 hours ago, patp said:

but that wet plaster is better than dot and dab for heat retention

 

The thermal resistance of 13mm of even insulating plaster is 0.07m²K/W and 13mm of plasterboard is 0.05m²K/W. The difference, 0.02m²K/W, is insignificant when you consider that the minimum thermal resistance for a new build wall is 3.33m²K/W and most builds on here are greater than 5.0m²K/W

[ADLIan]

With plasterboard on dab you can add the thermal resistance of the airspace (& dabs) too so will be better than wet plaster. Where wet plaster may offer an advantage is in improving the air tightness of the wall.

[A_L]

19 minutes ago, ADLIan said:

you can add the thermal resistance of the airspace (& dabs)

 

Agreed, if the airspace is about 15mm then can add about 0.15m²K/W but only if airspace unventilated. Just trying to illustrate small difference between options.

[oldkettle]

On 13/12/2020 at 10:22, willbish said:

With 6 inch of solid concrete between any external fire and habitable area, it leaves them very safe

 

I hope so, but ultimately if the wall outside is on fire then windows are at risk. Again, hopefully, only a potential problem for very tall buildings.