ADLIan

Don’t like the idea of ventilating any cavity in a wall straight into the loft space due to the risk of fire spread

That membrane look to be for pitched roofs. Doubt it would keep the water out on a flat roof hence your problem

Link is to America. Check UK site

Knauf ecose is a binder not a blowing agent. It states ‘no added formaldehyde’ - does this mean totally formaldehyde free??

Kingspan and ‘higher fire safety performance’ - words I never thought I’d see in the same sentence following the Grenfell Tower fire. All PUR foams burn.

Spax do similar in warm, pitched roofs. Rather than fixing being only in bending, due to vertical load, through thickness of the insulation it’s also part in tension. Makes perfect sense. Good fixings.

Open perpends, mainly to drain the cavity, are not normally classed as 'ventilating' the cavity as they are too small to actively promote mass air movement

The bright foil face works both ways in improving the thermal resistance of an adjacent airspace. There is no difference in U-value if working from outside to in or inside to out. Black bodies (equivalent to most construction products) both absorb and emit 'large' amounts of energy. Reflective surfaces (such as these reflective AVCLs and breather membranes) do the opposite. The rate of heat transfer by radiation is proportional to the emissivity of the surface and the fourth power of the absolute temperate in kelvin. A temperature difference of say 5K across the cavity is insignificant compared to the absolute temp. Some very complex physics above my pay scale.

An unvented void without reflection is around 0.14 R value, without the reflective your U value moves from 0.13 to 0.14. But is also directional, so is reflecting heat from outside, so in summer useful (?), not sure about any real use in winter as it cannot reflect heat back in to the house because of it's position. Take the marketing blurb with a pinch of salt. Standard unvent cavity has R of 0.18 m2K/W can be as high as 0.80 (ish) with highly reflective surface. Product is directional in that only bright finish on one side, MIs show product internal to insulation with reflective face onto airspace, in this case works as stated.

Too many years in the industry! Timber/steel frame good practice guides & MIs. Building Regs, Appr Doc C (or equivalent)

The VC Foil only has a thermal performance if there is an airspace present next to it - if sandwiched between other products it adds nothing thermally. In timber frame construction with masonry outer full fill insulation should NOT be used, a 50mm clear cavity needs to be maintained behind the masonry leaf.

Render direct onto insulation direct onto timber frame is not accepted practice - see Building Regs and any good guidance on the design of TF. I believe this is a result of failures observed in the USA and Canada that took many years to become apparent. Should be a rainscreen system using render board with ventilated cavity behind. Not sure I’d want polystyrene in this system however due to fire risk.

Cold flat roofs are frowned upon now. Not sure how you can cross vent your roof. Warm roof is safer option

Won’t comply with relevant BSs or Building Regs. Architect should know this. In a cold roof 50mm cross ventilated void is required above the insulation, can’t see how this can done here. Warm roof is the best (only) option.

OP is in Preston.