ADLIan

What is the span of the roof? More than 5m creates a problem. Do the proposed vents allow cross ventilation to each roof joist void? They appear to comply with the British Standard and Building Regs for open area. Can the vent manufacturer help? On a general note cold flat roofs should not be used with current levels of insulation. Relying on unknown and unquantifiable ventilation levels leaves a big question mark over the long term performance. Perhaps what the BCO is alluding to.

As the name suggests Koolduct is for insulation of A/C ducts and not for use in floors, walls or roofs. Not sure who would risk that in a floor under the slab or screed!

If you are just completing now you may only have the 'as designed' SAP. On completion this is updated to the 'as built' to include the air test result, actual insulation/thickness installed, make and model of heating system, solar PV etc, etc. At this point the EPC can be lodged too. If you can get the 'as designed' SAP file from the original assessor you may be able to pass this to a new assessor to do the 'as built' assessment. Check the EPC register as you first port of call.

They're 'seconds' for a reason - they've failed quality control procedures. May not have correct compressive strength or thermal performance as well as dimension problems. May not comply with relevant manufacturing Standards. With no label you don't know what you're buying.

100mm mineral wool insulation is often used between joist for acoustic insulation in intermediate floors within dwellings - see Appr Doc E. Some plasterboard & I-joist manufacturers have systems that comply with the Regs without the need for this insulation. The insulation will also have thermal properties but this is irrelevant here. An electrician will be able to advise but I believe good practice is to clip the cable to the sides of the joist above the insulation. Cables under the insulation may need de-rating but this may not be too much of an issue with lightly loaded lighting circuit cables.

SAP assumes the main heating system, gas boiler in your case, works and supplies the heating and hot water even if broken. If you choose alternative space/water heating that’s your call and not SAP.

Here are 3 examples one from the now out of date Accredited Construction Details (so been good practice for some time) and one from URSA and Knauf current publication. There are many more.

With insulation at horizontal ceiling level the gable cavity insulation should extend something like 300mm above the top level of the loft insulation. Should also use a cavity tray to protect the top edge of the cavity insulation and change of construction (in practice I’m sure this is rarely done!) Alternatively insulate all of gable.

And not choice of insulation type! Can we put 'sweating' in the same bin as 'thermal mass'?

Rash statement as you know nothing about me, my background, knowledge or experience.

Not to the Psi-value (the heat loss at a junction) which is all the RCDs are looking at. They cannot cover every scenario of floor level, external levels, DPM/DPC positions etc

If by 'sweating' you mean prone to condensation this statement is rubbish. Bad roof design causes condensation problems not the type of insulation.

I wouldn't get too hung up on the position of DPC/DPM in these details. Their primary function is to show the geometry of junctions (with all adjacent materials and physical properties) in order to arrive at the Psi-value - the DPM/DPC has no effect here.

The 15% thermal bridging with timber frame means you don't get the benefit of a 32 product between the studs but there is a big cost associated with it. A 35 product will give basically the same U-value and is more cost effective.

If you fully fill the rafter with insulation you then need to counter-batten and batten for the tiles/slates.