ADLIan

@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.

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!!

There should be a cavity tray at this point. Perhaps it’s not specific in the Approved Document but the Building Regulations require that a wall resists rainwater penetration. In this instance the external wall becomes an internal wall below the roof and under severe wind driven rain the internal wall may become saturated if no cavity tray. I would imagine BCO will insist on one but you need to check as their interpretation & enforcement may differ.

I've worked on the testing of cavity walls and cavity insulation to resist wind driven rain. Once the outer leaf brick is saturated the rain will drain down the inner face - add the effect of increasing wind speed and the water can actually 'spray' across empty cavities (or attempt to through the insulation and any joints).

Do not add insulation in the roof void under a warm flat roof - it creates a hybrid flat roof that will most likely have condensation problems. By all means pack the roof edges with mineral wool to both minimise air leakage and ‘link’ the wall and roof insulation to minimise thermal bridging (& in a fire safe way).

Ask them for the exact wording/clause number for this. As we agree there is no commissioning certificate for the use of individual extract fans. There will be Building Reg requirement for extract rates from each 'wet' room but this should be in the spec for the fans

Be aware that retrofit cavity wall insulation is notifiable work under the Building Regs. Product must have BBA, or similar, 3rd party certification that normally covers both the product and its installation.

Aircrete blocks in external walls to help achieve Uvalue. Check construction, insulation and required Uvalue. Cheap & cheerful standard concrete block for inner walls, density helps with acoustics.

The Regulation, direct guidance and SAP guidance are all clear and consistent on current air pressure testing requirements.

Under the current Regs (after June 2022) ALL new dwellings must have an air pressure test done and this value is used to assess BR compliance and in the EPC. Under the previous version of the Regs it was possible to avoid the test for developments of no more than 2 dwellings by using a default air permeability value of 15 m3/(h.m2) at 50 Pa and this value is used to assess BR compliance and in the EPC. More often than not this will result in a BR failure. If an air pressure test is done the measured value is used to assess BR compliance and in the EPC. Both are Building Control issues and your SAP assessor should be aware.