ADLIan

Those numbers look to be the standard recommendations the SAP software generates and they are absolute rubbish - do you have a copy of the SAP report you could post? A good consultant should have the knowledge of construction technology, sensitivities of the SAP calculation and suitable cost effective upgrades.

PUR, PIR & polystyrene have no acoustic properties. Mineral wool should help here but if this is a beam supporting a wall above and bearing on another wall below flanking transmission may be in issue.

The exception is for masonry walls only - 2 leaves at least 75mm thick. Note this is only the English Regs - there is no similar exemption in Scotland, Wales or Ireland that I am aware of. Timber or steel framed construction is NOT included. The issue with a fully filled cavity (hence no cavity!) is covered by not having to seal the top of the cavity. I'm sure this guidance is based on risk and the fire 'engineering' related to single houses and fire/smoke in masonry cavity walls. I wonder if the above diagram may have been better if it called them 'cavity closers' with a nod to the thermal requirements of AD L. I note it does not use the phrase 'cavity barriers'.

AD B gives definitions of 'cavity barrier' and 'fire stopping' but not 'cavity closer'! Diag 5.3, Note 1, in the current AD B is quite specific about there being no particular fire performance requirement for the materials used to close the cavity. This note was included only in recent versions of AD B (post 2019?) and is not in the technical guidance for Wales, Scotland & Ireland. I have own my thoughts on why this was changed!! There is also no restriction on the fire performance of materials used in the cavity - mineral wool will be non-combustible, polystyrene and PUR/PIR will be classed as combustible.

If this is the case why are there so many products on the market for use around door and window openings that are based on polystyrene or PUR insulation that have no fire resistance properties quoted.

@Jammy5 you will need photo evidence of this junction. Be very careful of using an alternative (though outwardly similar) detail!!! @ETC if the detail complies with Diagram 5.3 above there is no fire resistance requirement for the cavity closer around openings so no need for CS board.

That’s the one!

SAP is not particularly sensitive to elemental U-values so hopefully OK - only your SAP assessor can give a definitive answer

The SAP assessor will rerun the calculation with the revised insulation details. Hopefully all will be OK if some flexibility built into the design otherwise they should advise on options to get a pass (improved air test number?). I don't think photo evidence was required under the old, 2013, version of Appr Doc L.

Since retiring I no longer have the inclination to write lengthy reports on this issue. Headlines are; A masonry cavity wall with cavity insulation only (full or partial fill) is normally very safe from harmful condensation. Adding a layer of internal insulation to the above may move the dew point too far inwards. A view must then be taken if any potential condensation risk is is acceptable or if the design needs to be changed. Building material manufacturers will be able to provide calculations. I think we are in agreement here. My other comment was in reply to your assertion, gleaned from cavity wall insulation installers, that insulation must not allow any condensate to ‘wick’ across the insulation. The amount of liquid water from condensation is negligible compared to that from wind driven rain and this latter point is the basis for testing on full size representative walls (not computer modelling/simulation). It is also the reason that the Approved Documents, NHBC guidance, BRE Report 262 etc give guidance on maximum exposure zones with full fill cavity insulation and measures (including external render) that can be adopted to reduce the risk.

@Gus Potter I’ve spent a long time testing and designing cavity wall systems and know the difference between the issues with moisture from condensation and/or wind driven rain. When I have referred to the ‘cavity’ face of the outer leaf I assumed this would be understood to be the inner surface irrespective of a cavity or cavity insulation being present. Condensation may be predicted in a standard insulated cavity wall however it be at the internal face of the outer leaf - it may be considered as inconsequential as there is only masonry in the general wall area, the amount of condensate will be minimal, the outer leaf will be saturated at times following periods of wind driven rain. Adding further internal insulation to an already insulated cavity wall may move the dew point inwards depending upon material used and thickness. If steel/timber studs are used and condensation is predicted adjacent to them then a review of the design is needed. I stand by my comments. See BS 5250, insulation manufacturer details & BBA certificates. The BRE did work many years on ‘reverse condensation’ and it was considered not to be an issue.

There are exemptions for the cavity barrier requirement/performance - please see the section on ‘compartmentation’ in Approved Doc B

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.