ADLIan

Cavity barrier may be required to separate any wall cavity from any roof cavity even in this instance. BCO interprets and enforces Regs so his call here.

Units are wrong then. Should be W/mK.

Numbers look right for conductivity in W/mK.

U-vals are only quoted to 2 decimal places so still 0.18! Even if to 3 places a difference of 0.003 W/m2K is unlikely to impact overall energy assessment

Save yourself a lot of money and use 035 mineral wool within the timber frame. Probably gives the same u-value as 032 product at fraction of the cost (15% timber frame fraction ‘degrades’ thermal performance of this layer).

Psi-values are required as part of the SAP calculation input. The accredited construction details can no longer be used so option is to use default psi-values (which makes BR compliance very tough) or calculated psi-values (or mixture of both). There is lots of published data on masonry and timber frame psi-values - see trade associations, building product manufacturers. For non-standard construction such as SIP, insulated formwork, light steel frame etc bespoke calculations may be only option, they are complex and time consuming hence costly!

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.