ADLIan

Condensation occurring on the cavity face of the outer masonry leaf is irrelevant. There is a much greater amount of liquid water at this point following periods of wind driven rain. As part of the certification process (BBA for example) cavity wall insulation, built-in or retrofit, is tested to ensure it does not allow water from wind driven rain to cross the cavity, this water should still drain down the ‘cavity’ face of the outer masonry. Again not condensation related. Adding cavity insulation does not move the dew point ‘inwards’. Condensation, if it does occur, will still be on the inner face of the outer masonry leaf and may be considered as inconsequential.

BR standard for conversion work is lower than for new build plus relaxations outlined above. Get the conservation officer on your side I believe he can trump building control if building is listed, historically important or in conservation area. I’d have a lot more confidence in these companies if they knew the difference between a high and a low U-value.

@Big Jimbo the detail is a pitched roof gable as viewed looking up the roof slope. Not applicable to flat roofs. @Jammy5 the psi-value is based on the exact construction shown so should be built strictly in accordance with it. If required for clarity and confirmation more photos should be taken. Both BCO and SAP assessor will need this info.

Agree not the clearest advice in Appr Doc B! My view is that it refers to any part of the upper floor being above 4.5m and it’s not related to the floor height. To not fall foul of this requirement means the ceiling of the upper floor should be no more than 4.5m above GL. Next issue is any adjacent decking. This may have to be ignored as height looks to be related to ground level (the decking may be removed at some point in the future!). If you BCO will not offer guidance you may have to submit drawings and await comments.

The effect of the graphite is accounted for in the lower thermal conductivity compared to standard EPS so not dependant on application.

A few comments. It’s not good practice to add further insulation between the roof joists in a warm flat roof. This creates a hybrid roof that will be prone to condensation issues. The choice of VCL will depend upon how the roof is being fixed - either fully bonded or mechanically fixed. The insulation manufacturer’s advice should be sought on both of the above. The thermal bridge can be minimised by packing the roof edge, between top of wall and underside of the deck, with mineral wool insulation- a section perhaps 300mm wide. Tapered roof boards on such a small and simple roof will be expensive. Simpler & cheaper to create the falls in the roof structure

Standard cavity stop socks (mineral wool encapsulated in polythene sleeve, perhaps colour coded) do not incorporate an intumescent strip. They must be sized correctly, installed in continuous runs and be a tight friction fit in the cavity in order to function correctly - check MIs carefully. Post Grenfell make sure install is correct.

OP states this is a ‘new block of flats’. If this ‘new build’ then surely all design, method statements, competencies etc, etc should be sorted. Not the sort of question to be thrown out to a load of strangers on the internet.

So no technical justification for the numbers just one manufacturer’s sales line - I’ve never seen those definitions or limiting values used elsewhere. I’ll stick with the info in relevant British Standards.

What is the ‘sd’ value of a vapour control layer? Also where are these numbers defined?

That consultation is not the BR it is for PAS 9980 on fire assessment/remediation of existing blocks of flats.

Can you please explain the difference between a vapour barrier and vapour control layer and why one gives a higher condensation risk?

Yes, not banned as such. Massively difficult and expensive to “engineer” a solution that will ensure compliance with the Regulation for all the above reasons.

It’s covered in Approved Doc B, Section B4 and yes does look to be impossible! There is a guidance document on green walls but it is old, well before Grenfell. Regulation 7(2) would apply and overrides this guidance.

That section of wall is effectively an ‘external’ wall, with some shelter from the loft space the other side. Do I see the pattern of blocks in the thermal pic? Would indicate no insulation in this wall (single skin of 100mm block?). Add thermal bridging and thermal bypass and you have a huge heat loss here!!