Highest performance insulation material, plus can you trade higher values in one area for lower values in another

[Del-inquent]

We've got several area of ceiling down in our Chalet bungalow project so I'm looking at bringing it a bit more up to current spec whilst we're at it. I could have sworn I already asked on here but can't find it so...

Problem mostly is they're only 100mm rafters with old Sarking felt on top and don't have a lot of room for insulation. Doors, windows and other badly designed features mean that max I could put below rafters on any pitched section is 30mm (plus 12mm PB). In the flat roof dormer areas (cold roof), I could put potentially up to 75mm below rafters. The dormers form the greatest part of the roof area that is of concern, the pitched areas either side are tiny in comparison.

When calculating, is it a case of "all areas must meet minimum" or "the average over all areas must be"?

First thoughts are 50mm PIR between rafters to keep 50mm ventilation above and then whatever I can fit where it can be fitted. I could stuff the very top between ceiling and ridge with as much as I wanted, it's not a lot of use for anything else.

Is there any more advanced material I should consider that will give considerable increased performance with thinner materials? I contacted Superfoil but their figures were worse than PIR with 50mm between / 25mm PIR + PB under.

Any routes to help with calcs and optimising?

[Redbeard]

Phenolic is a gnat's whisker better (lambda value 0.019W/mK IIRC) than PIR (0.022) but I think quite a bit more expensive, so only use perhaps where things are really tight. Aerogel, of course, has lambda of 0.015, but it is costly.

 

It does not sound like you are re-roofing. If you were you could consider counter-battening and using a membrane which will allow full-fill. Proctors make one, I think.

 

From what you describe I think you'd struggle to get 0.16W/m2K even if you get really expensive, and it doesn't sound like you have any areas where you can go really thick to compensate.

[Nickfromwales]

Ditch the plasterboard and use Marmox / Jacoboard / other (cheaper but just as good) XPS boards, and plaster over those. 
 

Then you have 30+15mm of insulation then just skim. 

[Del-inquent]

1 hour ago, Redbeard said:

Phenolic is a gnat's whisker better (lambda value 0.019W/mK IIRC) than PIR (0.022) but I think quite a bit more expensive, so only use perhaps where things are really tight. Aerogel, of course, has lambda of 0.015, but it is costly.

 

It does not sound like you are re-roofing. If you were you could consider counter-battening and using a membrane which will allow full-fill. Proctors make one, I think.

 

From what you describe I think you'd struggle to get 0.16W/m2K even if you get really expensive, and it doesn't sound like you have any areas where you can go really thick to compensate.

We would like to re-roof really but it isn’t in budget, had a tonne of unforseen problems. Think I am a bit stuck really! 

[Del-inquent]

7 minutes ago, Nickfromwales said:

Ditch the plasterboard and use Marmox / Jacoboard / other (cheaper but just as good) XPS boards, and plaster over those. 
 

Then you have 30+15mm of insulation then just skim. 

Hadn’t thought of that, certainly a way to claw back a little.

[Nickfromwales]

2 minutes ago, Del-inquent said:

Hadn’t thought of that, certainly a way to claw back a little.

Yup. 👍 

[ADLIan]

9 hours ago, Nickfromwales said:

Ditch the plasterboard and use Marmox / Jacoboard / other (cheaper but just as good) XPS boards, and plaster over those. 
 

Then you have 30+15mm of insulation then just skim. 

 

I would question the fire safety implications of a Marmox or Jacoboard ceiling and doubt the manufacturer would promote this application.

[Nickfromwales]

12 hours ago, ADLIan said:

 

I would question the fire safety implications of a Marmox or Jacoboard ceiling and doubt the manufacturer would promote this application.

I’d ask the designer, first, and not make such assumptions, just saying. My comments were based on the information provided. 
 

Did you not read that these are ceilings at roof level? 
 

If this was a ceiling of the ground floor then it wouldn’t need insulation, but regarding fire regs I’m not sure this would be that much of a concern.

 

Building control would confirm, but at these levels I’m sure they’re not calling the shots….

 

Afaic, the carpet would pose more of a threat to the preservation of life here.

[Gus Potter]

On 10/11/2025 at 20:34, Del-inquent said:

When calculating, is it a case of "all areas must meet minimum" or "the average over all areas must be"?

First thoughts are 50mm PIR between rafters to keep 50mm ventilation above and then whatever I can fit where it can be fitted. I could stuff the very top between ceiling and ridge with as much as I wanted, it's not a lot of use for anything else.

Is there any more advanced material I should consider that will give considerable increased performance with thinner materials? I contacted Superfoil but their figures were worse than PIR with 50mm between / 25mm PIR + PB under.

Any routes to help with calcs and optimising?

Have a look at the building regs. On upgrades I often have to have a discussion with BC in terms of what is "reasonably prcticable" In your case as you don't seem to be increasing the building foot print or the volume then this supports the "in so far as reasonably practicable" approach.

 

The key to this is as you have clocked it is to maintain good ventilation, the more insulation the more ventilation you need, that's a rule of thumb. 

 

The other key is to tightly fit the insulation and use tapes to stop convection currents bypassing your hard work installing the insulation.

 

Last is to install a good vapour barrier on the warm side to stop water gas getting into the insulation layer in the first place. Moist air does liittle harm.. the harm arises when the gas temperature lowers and you get water condensing out, we call this the dew point. 

 

The best way to get increased performance is to use insulating plasterboard on the underside but this may reduce the ceiling height. This works as you don't have the repeating bridging effect of the joists. 

 

Below is a link to Kingspan. You can play about to see the difference between insulated plasterboard and insulation between the joists. 

 

https://u-valuecalculator.com/gb

 

Hope this helps and you have a bit of fun playing about with it. 

 

 

 

 

[saveasteading]

On 10/11/2025 at 21:34, Del-inquent said:

the average over all areas must

I'm not sure this has been gomoletelh answered.

As always, read thd regulations to grasp the full requirement of thd green bit.. ie the regulation itself.

For each area there are.ninum requirements but they can be averaged.

 

Eg on the steading we have a small area of wall in the original exposed stone. It has some insulating property being 600 thick, but of course it is poor. Extra elsewhere satisfies the regulation, and the bco agreed that the heritage was worth showing.

 

[saveasteading]

8 hours ago, saveasteading said:

gomoletelh

Today's word.

I must not use BH too late!

[ADLIan]

In respect of U-values the 'Regulations' do not drill down into detail hence the guidance in the Approved Documents. Unless otherwise stated the maximum U-values for floors, walls and roof is an area weighted average. This gives some flexibility but note however there is a back stop for roofs of 0.35 W/m²K.

 

@Nickfromwales I have read the OPs post and pretty sure I understand what a 'chalet bungalow', 'ceiling' and' rafters' are. I stand by my original point that a combustible XPS ceiling to is not fire safe! The Building Regs/ADs on fire and fire safety would not allow it.

 

On a more general note whilst this work does fall under the Regs unless you tell them I'm not sure how any Building Control Body can enforce and police such upgrade works.

[Nickfromwales]

2 hours ago, ADLIan said:

In respect of U-values the 'Regulations' do not drill down into detail hence the guidance in the Approved Documents. Unless otherwise stated the maximum U-values for floors, walls and roof is an area weighted average. This gives some flexibility but note however there is a back stop for roofs of 0.35 W/m²K.

 

@Nickfromwales I have read the OPs post and pretty sure I understand what a 'chalet bungalow', 'ceiling' and' rafters' are. I stand by my original point that a combustible XPS ceiling to is not fire safe! The Building Regs/ADs on fire and fire safety would not allow it.

 

On a more general note whilst this work does fall under the Regs unless you tell them I'm not sure how any Building Control Body can enforce and police such upgrade works.

Agreed. But the insulation values wouldn’t pass grade either so all likely to be ‘off the radar’.

 

Good point of course, and something further for the op to consider.

 

But all soft furnishings are combustible in that room, and the XPS would be behind the skim, so if an open flame was removing the skim to get to XPS a) you’d not be alive to watch in reality, or b) would be standing outside waiting for the fire brigade. 

[ADLIan]

I agree the burning furniture is the issue but the structure has to be safe too. Fire fighters have a tough enough job and deserve medals as big as bin lids - if persons are reported in the building there is the further risk of molten/burning ceilings falling on them!

[SteamyTea]

BS9991 was updated in November 2024.

https://ww3.rics.org/uk/en/journals/built-environment-journal/bs-9991-fire-safety-guidance.html

 

Quick overview here:

https://knowledge.bsigroup.com/products/fire-safety-in-the-design-management-and-use-of-residential-buildings-code-of-practice-2

 

[Nickfromwales]

4 hours ago, ADLIan said:

I agree the burning furniture is the issue but the structure has to be safe too. Fire fighters have a tough enough job and deserve medals as big as bin lids - if persons are reported in the building there is the further risk of molten/burning ceilings falling on them!

I’m in complete agreement. 

[saveasteading]

5 hours ago, ADLIan said:

Fire fighters have a tough enough job

Agreed.

I was fortunate to get a long discussion with a senior fire officer.

There had been arson, burning lots of very flammable stuff stacked against the warehouse we had newly handed over. I was there to see the damage.

 

The building was intact, it being metal cladding. The paint was gone as were the plastic screw heads. Otherwise intact.

The officer explained that they had rules about working too close to materials that could hurt them. 

 

Thus in this case they were very nervous about composite cladding falling off the wall. He couldn't explain why it might fall but they knew it happened.

So they had stood well back, and this had slowed their work.

 

Ours was built on site with metal panels  outside and in, and a fibreglass infill..  which we later saw had turned to sand.

 

He said he wasn't permitted to praise or condemn any product. He said our product was brilliant but he could not say that officially.

 

 

This makes sense esp after we see how  Grenfell had false fire tests from a cladding manufacturer.

 

Could a ceiling of eps behind plaster- board or plaster fall down through melting?  I think not, but who is qualified to say?

 

 

 

 

[ADLIan]

EPS/XPS behind mech fixed plasterboard is acceptable (if not ideal). Marmox is XPS with thin (2mm?) cement facing. Mech fix this to rafters and it will melt (perhaps as low as 120 deg C) leading to product failure, showering anyone below with molten/burning polystyrene!! Having seen fire test of this it’s not nice and you do not want to be in this sort of room fire.