External Wall Insulation on a cavity block wall

https://www.gov.ie/en/publication/d82ea-technical-guidance-document-l-conservation-of-fuel-and-energy-dwellings/

when you externally insulate cavity blocks you must make sure that the soffits are removed and the insulation is carried up through them to meet the the roof insulation which must over lap the wall if you scroll down the above link you will find the Acceptable Construction Details TGD PartL Building Regulations 2021- External Wall Insulation

S.R.54 highlights that cavity block walls aren't advised for fitting of EWI, I believe/recal the reason is that it wouldn't be possible to fill the cavities with beads easily or completely, and/or that it couldnt be confirmed and likely then couldnt be fixed.

I believe the 2 main reasons EWI aren't advised is that, the potential for interstitial condensation exists if the temperature in the cavity fell below the dew point hence mould could occur (i.e. Not enough EWI thickness) AND thermal looping within the cavity spaces and thermal bypass if the cavity isn't capped.

I consider that the worst case is being catered for in S.R.54 and possibly Part L), and rather than suggest solutions, it is now written off. ie you probably won't get a grant for EWI if you have a block cavity wall, or possibly not for that section. (I think you definitely won't but I can't say for sure 100%). I feel if a calculation is carried out to ensure that sufficient EWI is fitted to prevent the dew point being encountered, then it won't be a problem,

BUT, PU foam insulation is available now that is applied as a liquid like other spray foams are applied in ceilings (which I don't agree with in ceilings). It appears to me that PU should be better in thermal performance than beads airblown in any cavity, because PU has a better lambda value. That said, if there are any issues with PU, you won't be getting it out of a cavity, so if considering it, I'd look in detail and ensure the business has a good reputation. A calculation to assess the correct EWI should eliminate the need to fill a block cavity, but you then have the wasted space that could be utilised as an insulation layer. I have thought about this in the past, and I consider there are some shortcomings in S.R.54 (overall it is fantastic and full of information, along with the building regs). I'm guessing because PU may not have been certified for that specific use, documents haven't and won't be updated to reflect that until a certification is in place, and I have no idea if that will ever be the case or if it is even being considered.

Another consideration is, how does PU foam if used affect any polystyrene type insulation that is already fitted, there may be no reaction, but that would need to be checked, because I think polystyrene/styrofoam types can start to dissolve if they have certain chemicals applied (possibly solvents or chemicals that may be part of PU or used making or applying it). One way of testing this could be to get some insulation and have prospective (PU) installers apply some spare product on polystyrene insulation foam board to test for any reactions.

Overall I think PU could be a solution, but so also is calculating the heat loss across the wall with the desired EWI thickness and determining if the dew point is hit at various temperatures and relative humidity values.

Another thing to note is that, an internally fitted airtightness layer is an important aspect/element of fitting insulation and that would also prevent moisture getting into the cavity space, as would be adequate ventilation. Depending on how comprehensive the EWI or cavity fills are, insulation products may assist with airtightness. A liquid applied membrane may be the solution or part of the solution for airtightness.

Agree with the above, EWI is commonly applied to hollow block walls, there's no way to fill the cavity. I'm not sure what common practice is/was for topping off hollow block walls, possibly a solid block laid flat? I can't imagine that wall plates are bedded directly on top of hollow blocks, so the tops of the hollow block walls could be somehow closed up first. It might be possible to see from the attic.

My house is constructed mainly from hollow blocks. On the upper part of the upstairs walls, there is a poured concrete ring beam, which effectively caps the top of the hollow block walls, so this will help to prevent heat being lost from the cavity, however the EWI should extend past the soffit and meet the attic insulation as already stated.

I'll be getting EWI installed in the near future. I'll be insisting that soffits are removed and the EWI extends up to the loft, for 2 reasons. My soffits are quite low, so if the EWI doesn't extend past the soffit, I'd be left with at least 6 inches of uninsulated concrete on the top of the upstairs walls. Also, if the EWI is applied without removing the soffits, it'll be a nightmare to replace the soffits in future, if needs be.

They'd most likely jog on to the next job. There's alot of poor quality installations going on where realistically attention to detail is missing.

There's newer boxes available that solve this.

You are correct, I made a mistake and must have misremembered what I read or was tired and trying to say something else. I think I read the highlighted part below and recalled it incorrectly, not 100% sure. I appreciate the correction.

What I don't understand about the below section (which I am sure I have read contradictory information, I believe in SR54 elsewhere). This section (which I actually read previously as I recalled the section when you highlighted it), is that it says cavity block walls should not be filled, and then later (see below) it says fill with a foam to prevent thermal bypass. While I believe the comment to fill the cavity likely means just at the top to prevent thermal bypass, I dont see why they suggest not to fill the cavity as if a layer of EWI is fitted, I think this should tend to limit what the section says about cold bridging. A PU foam applied as a liquid cant be used as insulation for cavity blocks?? BUT SR54 suggest it to prevent thermal bypass as per the lower image.

The only reason I think the cavity may be difficult to fill is the interconnected web of cavity blocks might make it difficult to implement completely and effectively as a blown bead option, but I wonder would a liquid filled PU foam be better/easier? maybe it's not worth the hassle and uncertainty that such a job is done thoroughly and completely and instead just add more EWI if possible.

they're talking about the gable wall only - cold bridging is unavoidable on the gable as the top of the wall is way above the level of the attic insulation so you can't bring the EWI over to meet the attic insulation. Also apparently the columns are not generally capped on the gable which means you'll get warm air rising through the columns and escaping out the top (I can confirm on my house, the gable wall is basically a big anti-radiator sucking heat out of the rooms on that side).

What they seem to be suggesting there is effectively capping the columns at attic floor level using foam or whatever to minimise this; but again, I'd assume no EWI contractor actually does this. Filling the columns entirely with foam is a thing some companies offer but from what I've read is a waste of time and money.

It does sound like overkill TBH, if you're going to insulate internally that's probably going to give you 95% of the benefit straight away.