Adding EWI on to a cavity wall only works if the cavity is filled fully and without any air flow through it. Not that easy to achieve.

Adding 50mm of EPS EWI will improves your wall from a u value of 0.5 to 0.3 Putting on 100mm EWI improves it to 0.21 Guesstimate based on a brick both sides cavity wall with 50mm EPS inside. Your original wall structure may differ from my assumption.

The difference in cost between 50mm and 100mm EWI is just the extra 50mm of EPS, other costs remain the same.

You can play around with different insulation options at
https://www.ubakus.de/en/r-value-calculator/?
Use the demo version.

You can put EWI on to pebbledash without a 'smoothing coat' if you attach the EPS with a fillet of adhesive all the way around the edge of the EPS sheet and 4 dollops in the centre. The EPS sheet is then pushed on to the wall firmly to seat the adhesive. This method uses a bit more adhesive but saves the cost of a smoothing render coat. Of course the pebbledash needs to be in good condition to take the EWI with no hollow patches. (if there are a few hollow patches then knock off the pebbledash and attach the EPS as above with perhaps a thicker fillet to account for no pebbledash)

To get a dust free pebbledash surface for EWI a pressure washer is a good option.

Osprey - I too considered EWI on our cavity walls but for the same reason Peter gave above, i.e. air movement within the cavity, I decided against it. It is one of those imponderables, very difficult to prove or disprove.

Posted By: OspreyJeff
I am pretty sure bonded eps beads injected into the cavity would pretty much eliminate air movement there, or am I missing something?

Depends totally upon the workmanship of the person doing the injection and unfortunately there is no easy way to check the results of the work.

Posted By: OspreyI appreciate 100mm ewi would be better than 50 (and 150mm better still) but there are some practicalities that may not allow it.

Can you outline those practicalities? might make a difference. On a project with 200EPS EWI on cavity walls with unknown-quality previous cavity fill, it's made a huge difference, as you'd hope with 200 EWI. No attempt to seal top of cavity wall.

Sure, unfilled cavities allow lateral convection from warmer spots on the cavity face of the inner skin to cooler spots on the cavity face of the outer skin (aka bypass), but do they really introduce outside air to inboard of the EWI? Where does the outside air come in?

Yes, you can't rely on rendered EWI as 'airtight layer' but it's still quite good at it. If that EWI is applied on extg render, then that's also quite good at it, as long as you fill render cracks before EWIing. Ditto internal plaster, as long as you deal with gaps in it - inter-floors, elect boxes etc.

Can add up to a fair wind-barrier, even if there's no formal 'airtight layer'. So where's the big 'draw' of outside air?

<blockquote><cite>Posted By: fostertom</cite>Sure, unfilled cavities allow lateral convection from warmer spots on the cavity face of the inner skin to cooler spots on the cavity face of the outer skin (aka bypass), but do they really introduce outside air to inboard of the EWI? Where does the outside air come in?
</blockquote>
My cavities are vented to the underfloor air bricks and and also have a couplemof their own airbricks.

Posted By: OspreyMy cavities are vented to the underfloor air bricks and and also have a couplemof their own airbricks.

Posted By: OspreyMy house cavity tops are accessible from the loft. If fact they provide the sole loft ventilation, which will have to be addressed when I do the walls..

How wonderful - outside air flowing through the cavity to ventilate your loft with air that will have been warmed by the inner leaf of the cavity wall-----------Hmmm

BTW if you select the Html button at the bottom of the comments box then quotes come out in blue, which makes the post easier to read.

Posted By: Peter_in_Hungary

Posted By: OspreyMy cavities are vented to the underfloor air bricks and and also have a couplemof their own airbricks.

Posted By: OspreyMy house cavity tops are accessible from the loft. If fact they provide the sole loft ventilation, which will have to be addressed when I do the walls..

How wonderful - outside air flowing through the cavity to ventilate your loft with air that will have been warmed by the inner leaf of the cavity wall-----------Hmmm

BTW if you select the Html button at the bottom of the comments box then quotes come out in blue, which makes the post easier to read.

Agreed, but loft, and everywhere else is very dry, and in good condition for 100year old house. Not the warmest though.
(Thanks for html hint)

Posted By: OspreyStill needed for underfloor, imo

AFAIK, the consensus here was no, an old myth, not needed, provided any water entering from outside has been fixed.

Do these comments help allay worries of outside air blowing around in the cavity? What you say, Peter, Rob and Jeff?

I too have EWI (160-200mm woodfibre) outside a 55mm cavity, filled with blown fibreglass. It works very well indeed, but like RobL (very similar house) I've spent a long time finding and bunging up holes into the cavity, particularly between floors (lots of bricks missing for wiring and I taped round all the joists), and around windows. The top of the cavity has been sealed on half the house, but not the other side (yet). That was done later, when scaffolding was available.

So yes, as others have said, 50mm EWI is a very good idea, but only if you also fill the cavity. Filling it in more airtight ways will likely work better. And sealing off as many holes as you can find will also improve it. All of this is quite hard to quantify (i.e just filling the cavity will probably allow the EWI to work 'adequately', but the more hole-bunging the better, espescially the bottom-vents as those will generate significant stack ventilation).

In general the continuous EWI layer will be better insulation than the cavity layer as that often ends up with gaps and is bridged by wall ties and cement spillage and usually about 300mm of crap at the bottom. So I'd expect it to be _more_ than twice as good as just filling the cavity. You could also use PIR insulation for better U-values, given that your thickness is restricted to 50mm. There is a system for that (weber maybe?)

I would strongly advise spending the extra time/money to move your drainage goods outside the EWI so as to have a continuous layer, and it just looks way better. Yes, to some degree the fact that you also have cavity insulation means you don't _need_ to do this from a thermal point of view, but it still looks cheap if the insulation just stops around plumbing goods, and it makes rendering difficult and is likely to let water in.

Agreed. But 50mm EWI plus 50mm of cavity is 'not terrible'. U=0.27

One approach where 'narrow driveway/passage' is the problem is to step it out above head height so the mediocre area is minimised. And the option remains for some IWI too one day to compensate.

have any of you used kingspan k5 phenolic boards their Thermal Conductivity: 0.021W/mK

The proposal would comply with the building regulations limits for renovated walls (U=0.3), no problems there.

There is a further get-out that insulation should be "technically and functionally feasible and have a simple payback no more than 15 years", in which case U= 0.7 would also be acceptable.