Cavity Wall Insulation on Extension

[Who Me]

I have some plans in for an extension on a 1960 built bungalow and my architect is proposing 100mm cavity with 90mm solid insulation. I am wanting to have 100mm rockwool type insulation but this wont reach the required U-Value.
She is unsure but says I may be able to get a SAP done for the extension where I can use the rockwool. Before I start sending plans to online companies who may or may not be able to get my required building regs has any of the PH massive come across this issue or indeed SAPped your way around solid insulation.
Im all for better insulation but the cost of solid for the extension whilst the rest of the house doesnt have any is not making sense to me, or will the SAP require me to do more to the existing house which just cost me more money in the long run.

[pghstochaj]

Who Me said:

I have some plans in for an extension on a 1960 built bungalow and my architect is proposing 100mm cavity with 90mm solid insulation. I am wanting to have 100mm rockwool type insulation but this wont reach the required U-Value.
She is unsure but says I may be able to get a SAP done for the extension where I can use the rockwool. Before I start sending plans to online companies who may or may not be able to get my required building regs has any of the PH massive come across this issue or indeed SAPped your way around solid insulation.
Im all for better insulation but the cost of solid for the extension whilst the rest of the house doesnt have any is not making sense to me, or will the SAP require me to do more to the existing house which just cost me more money in the long run.

I don't understand the logic there, your heat loss in that part of the house will be determined by the insulation in that part of the building, it has nothing to do with the rest of the house. What is the cost differential before you spend time doing a SAP calculation? You will be spending money on a calculation rather than on reducing the heat loss from the room.

Assuming typical wall build up (Brick, cavity, medium density block, dot and dab plasterboard), you will achieve 0.3 W/m2/K with your 100 mm of rockwool. With the 90 mm of PIR you will just about squeeze 0.18 (in fact, it might be just below and you will not meet building regs anyway). That means your walls will lose 18/30th of energy if you use PIR.

Unless the rules have changed, you can also do a weighted equivalent calculation where you over-specify other parts of the build (e.g. less glazing, triple glazing, more ceiling insulation, more ground insulation etc) to offset the non-compliance with the wall.

PIR is roughly half the heat loss for a given thickness, hence your 90 mm of PIR will be roughly double the performance of 100 mm of rockwool.

[reggie747]

Who Me said:

I am wanting to have 100mm rockwool type insulation.

Why do you want this ??

[barryrs]

You just need to adhere to the maximum u-values quoted for new fabric elements in existing dwellings.

Table 4.2 in Part L1

In this case it’s 0.18W/m2K

DriTherm 32 or CWS32 are the most common mineral batt products but to get the u-value above would require 150mm thick insulation.

[Hans_Gruber]

Can solid insulation create a moisture / thermal bridge? If so just a 10mm tolerance for the bricky might be a consideration

[andya7]

Just had a client pay £330(+vat) for the 'SAP10' calculations, where a new orangery is/was being built onto an existing (large) detached dwelling. This proved that even with the amount of glazing/rooflights (i.e. >25% of floor area) the 'whole house' was deemed acceptable.

When you say '90mm solid insulation', I think that will mean 90mm Celotex Thermaclass... double check as that stuff is expensive. Normally specified to retain a 100mm cavity, with no real thought as to cost.

The only way you will get away with 100mm full fill cavity is to have insulated plasterboard on the inside, which adds cost and the waste is painful to see and you have to pay to take plasterboard away... can't throw it in the bin/skip.

What is becoming the norm is a 150mm cavity with solid 'quilt insulation', however that also depends on the type (thermal efficiency) of the inner and outer blocks you use;

Brick outside, with 150mm Dritherm 37 Standard and a 'Celcon standard 3.6N block' with plasterboard on dabs will give you 0.2 W/m²K... the requirement for an extension is 0.18. However, if you use Dritherm 32 Ultimate you will get 0.18, but both are with a 150mm full fill cavity.

Even if you use 90mm Thermaclass (as your Architect has speficied) a brick outer and a 'Celcon standard 3.6N block' on the inside, with plasterboard on dabs will still only give you a value of 0.19 (according to my software)... which doesn't comply.

[Who Me]

PHers

Thanks for the quick replies and advice, in response to some of the questions

Pghstochaj, My logic was same as you mention some sort of weighted SAP for the whole building but likely not cost effective
Reggie, Cost of the solid insulation and potential wastage was reason why i was trying to avoid this route
Barryrs, Aware of the 0.18 just looking at ways to get there
Hans Gruber, very interesting point you make, yes the 10mm is supposed to the airgap but agree a builder with snots and cement may struggle with this 100% of the time
Andya7, Thanks for your time to reply and interesting that you say your calculations only get to 0.19 with 90mm solid as I plan to use a standard 3.6n celcon block and dot and dab inside. Certainly wouldnt want to be seeing the cost and waste of insulated plasterboard.

Guys again thanks for the response, I am leaning towards getting some calcs done for a 150mm cavity now as the small loss of room size doesnt affect things much for me and after reading your responses im not going to go wasting time and money sending stuff for SAP assessments that possibly cost me more in the long run. Im not against having a home insulated better but the farce of insulating a new extension to modern regs then installing trickle vents for ventilation along with the cost makes me want to find the most cost effective way of doing it.

[pghstochaj]

Who Me said:

PHers

Thanks for the quick replies and advice, in response to some of the questions

Pghstochaj, My logic was same as you mention some sort of weighted SAP for the whole building but likely not cost effective
Reggie, Cost of the solid insulation and potential wastage was reason why i was trying to avoid this route
Barryrs, Aware of the 0.18 just looking at ways to get there
Hans Gruber, very interesting point you make, yes the 10mm is supposed to the airgap but agree a builder with snots and cement may struggle with this 100% of the time
Andya7, Thanks for your time to reply and interesting that you say your calculations only get to 0.19 with 90mm solid as I plan to use a standard 3.6n celcon block and dot and dab inside. Certainly wouldnt want to be seeing the cost and waste of insulated plasterboard.

Guys again thanks for the response, I am leaning towards getting some calcs done for a 150mm cavity now as the small loss of room size doesnt affect things much for me and after reading your responses im not going to go wasting time and money sending stuff for SAP assessments that possibly cost me more in the long run. Im not against having a home insulated better but the farce of insulating a new extension to modern regs then installing trickle vents for ventilation along with the cost makes me want to find the most cost effective way of doing it.

You need the trickle vents (in most cases) whether you insulate the room well or less well anyway, it doesn't really change the performance of the extra insulation.

[bobtail4x4]

Im worried that your "Architect" cant do a simple heat loss calculation,

[OutInTheShed]

bobtail4x4 said:

Im worried that your "Architect" cant do a simple heat loss calculation,

Or even find the calculator on the interweb

https://celotex.co.uk/technical-services/tools/onl...

[pghstochaj]

bobtail4x4 said:

Im worried that your "Architect" cant do a simple heat loss calculation,

This is my experience too, though. On domestic projects, architects just state the standard build ups needed to achieve BR compliance and if you want to do anything more interesting, you normally go to one of these SAP specialists. On my extension I did an area weighted calculation (I am a chartered chemical engineer). I wouldn't have trusted my architect to do that, at all.

On non-domestic projects (which is my field), architects aren't allowed near anything like heat calcs (SBEM calcs are done by specialists).

[Chumley.mouse]

Don’t forget making the cavity 150mm will mean a wider footing so more cost digging out , waste removal, and concrete in the trench……..this might cost more than using insulated plasterboard on the inside.

[Aluminati]

Thought new regs now demand a 150 cavity ?

[037]

its much easier to just go with the 90mm PIR and forget about it. Think I paid around £12 per m2. 150 cavity looks stupid on an extension and insulated plasterboard is just as expensive and harder to use..

[Mark V GTD]

Interesting. So what would be the choice for a 300mm wall with brick outer leaf - 100mm Thermaclass?

Sorry I see they don’t do 100mm so presumably 115mm thickness would get your 0.18 u value with a wall thickness of 320mm (less internal finish)?

Edited by Mark V GTD on Thursday 28th August 00:00

[barryrs]

Unilin CavityTherm insulation can still get you a 0.18 u-value in a 300mm cavity wall but again its an expensive option.

[Mark V GTD]

Thanks for responding to that Barry. Product price isn’t an issue - just wanting to retain a 300mm structural cavity wall if poss.