ADLIan

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.

Hybrid flat roof is never a good idea. Depending upon calculation inputs it’s possible to show the design is ok but look at the input data very carefully!

EPS/XPS behind mech fixed plasterboard is acceptable (if not ideal). Marmox is XPS with thin (2mm?) cement facing. Mech fix this to rafters and it will melt (perhaps as low as 120 deg C) leading to product failure, showering anyone below with molten/burning polystyrene!! Having seen fire test of this it’s not nice and you do not want to be in this sort of room fire.

I agree the burning furniture is the issue but the structure has to be safe too. Fire fighters have a tough enough job and deserve medals as big as bin lids - if persons are reported in the building there is the further risk of molten/burning ceilings falling on them!

In respect of U-values the 'Regulations' do not drill down into detail hence the guidance in the Approved Documents. Unless otherwise stated the maximum U-values for floors, walls and roof is an area weighted average. This gives some flexibility but note however there is a back stop for roofs of 0.35 W/m2K. @Nickfromwales I have read the OPs post and pretty sure I understand what a 'chalet bungalow', 'ceiling' and' rafters' are. I stand by my original point that a combustible XPS ceiling to is not fire safe! The Building Regs/ADs on fire and fire safety would not allow it. On a more general note whilst this work does fall under the Regs unless you tell them I'm not sure how any Building Control Body can enforce and police such upgrade works.

I would question the fire safety implications of a Marmox or Jacoboard ceiling and doubt the manufacturer would promote this application.

'Non-combustible' is normally defined as a reaction to fire of Euroclass 'A1' (to BS EN 13501-1) so this gives the starting point. Check MIs - they will state minimum distance to combustible materials and may give a reduced distance if shielded by a non-combustible product and an airspace.

Under current Part L in England ALL dwellings should be air pressure tested (was a %age in past).