ADLIan

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).

From memory it’s towards the end of AD L1, section on extensions. Max glazed area of extension is 25% of floor area of extension plus the m2 of any openings made redundant as a result of the extension. With roof lanterns/pyramids the glazed area is based on the area of each glazed section, not just the hole in the ceiling. With big sliding doors and a roof lantern you may fall foul of the ‘simple’ methods and need the SAP calculation route.

Actis require a 200mm airspace to any surface expected to be at 80 deg C or more. This probably conflicts with all of their roofing details!

A few comments: The DIN standard is specifically for Germany and perhaps linked with compliance with their Building Standards so may not be relevant here You need to use BS 5250 Above BS does not accurately model ventilated airspaces. Convention is to do 2 calcs, one including the ‘ventilated’ airspace and another ignoring all elements external to the ventilated airspace (zero moisture vapour resistance). Actual performance is somewhere between these extremes. Allows a comparison to be made to select safest option

Report this sort of product failure to the manufacturer. They may already be aware of the issue. Some form of compensation needed? (But not replacement Actis Hybris!!)

No cill section under door & frame?

No glue = cost saving!!

I used to be involved with injected cavity wall insulation as a system designer. The failure rate, measured by claims on the guarantee scheme was unbelievably low, less than 1%(?) of all installations. There were poor installs/poor surveys many years ago when lots of grants were available but still a very low failure percentage.

You will probably need both an energy assessor and a QS. The energy assessor can undertake the SAP assessment but may not be ‘qualified’ for the costing calculation, this where the QS steps in. Best get agreement from your BCO on the definition of ‘suitably qualified’ and who can be used.