ADLIan

This cavity slab from URSA available up to 175mm & 200mm.

Built in mineral wool slabs. 150mm now a standard thickness

The failure mode with this form of construction (render/insulation/sheathing(?)/timber frame) was well documented following failures many years ago in Canada. It was the same form of failure identified in the 2 houses I had dealings with last year. The information that I have to hand including Building Reg Approved Documents, warranty providers requirements, BRE and TRADA all show a render board (or other cladding) with ventilated airspace behind. @ProDave had an issue with similar construction but I'm not sure he ever got to the bottom of it. He chose the system as it had BBA Certification from the wood fibre manufacturer (not sure which). Strangely this Certificate was withdrawn shortly after he completed the work - I suspect because the 'system' was inherently unsafe! Without some form of 3rd party Certification/Assessment I doubt any BCO would accept the system. Sorry but this shows a lack of understanding of this particular problem. If your company is supplying product for this render system into the UK it must be suitable (robust, inherently safe etc) in all locations, from the bottom of Cornwall to the norther tip of Scotland and from east to west. Does your company undertake some form of 'assessment' to assess suitability for each project in its particular microclimate? Can you please explain how the product can be hydrophobic yet can still wick moisture away?

Do not render direct onto (any) insulation onto timber (or steel) frame construction!! It's not accepted good practice and lots of failures reported. About this time last year I was dealing with an architect looking at remedial works on this form of construction - sole plates and base of studs all rotted on 2 houses built approx 15 years ago. Does PUR really form a VCL if the board joints are not sealed/taped in any way? Moisture vapour is a gas so will readily move through the joints. Discuss.......

Congrats! From past experience. The mad/sad thing is no one from the planning dept will actually check on completion that you built your house to the upgraded spec!!! BCO won’t be interested either.

The EPC should give the air pressure test result - either ‘as measured’ in this house or based on site average value.

It's not a very good advert for an insulation manufacturer when they don't know the difference between a high and a low U-value!

Not just an energy statement. It’s imposing a condition that 20% of the CO2 target emission is from LZCT. From past experience it will never be checked.

Nothing to do with SAP - its a perfectly acceptable inclusion in the U-value calculation and is covered in the U-value conventions document that supports the Bldg Regs/Appr Docs. Done correctly, to MIs, mass air movement behind the plasterboard will not occur.

Aircrete block will generally give a much better psi-value than a medium or dense block (in the same detail). In a typical ground floor/wall junction expect the psi-value with dense block to be approx 3 times higher than with an aircrete block.

Yup. But check with your Building Control Body as it it responsible for enforcement

Vertical flashing and soakers looking at tile type?

This work would fall under the Building Reg requirements (assuming England) to upgrade the roof insulation. Generally max Uval of 0.18 W/m2K (IIRC) above conditioned space or max 0.35 W/m2K above non-conditioned space. Building Control needs to be involved.

Doesn’t read well when they knock the competition, by name/type so much.

It's all in Appr Doc F in England. Do you currently have background (trickle) vents and intermittent extract fans to 'wet' rooms? If your air pressure test comes in at less than 3 you will need to upgrade this to continuous extract or a mech vent system. Get the test done and see what the result is. If you get less than 3 I am sure there 'ways' of increasing this to above 3 (open a trickle vent???). Your testing guy may be able to offer some simple 'advice'!.

Those numbers look to be the standard recommendations the SAP software generates and they are absolute rubbish - do you have a copy of the SAP report you could post? A good consultant should have the knowledge of construction technology, sensitivities of the SAP calculation and suitable cost effective upgrades.

PUR, PIR & polystyrene have no acoustic properties. Mineral wool should help here but if this is a beam supporting a wall above and bearing on another wall below flanking transmission may be in issue.

The exception is for masonry walls only - 2 leaves at least 75mm thick. Note this is only the English Regs - there is no similar exemption in Scotland, Wales or Ireland that I am aware of. Timber or steel framed construction is NOT included. The issue with a fully filled cavity (hence no cavity!) is covered by not having to seal the top of the cavity. I'm sure this guidance is based on risk and the fire 'engineering' related to single houses and fire/smoke in masonry cavity walls. I wonder if the above diagram may have been better if it called them 'cavity closers' with a nod to the thermal requirements of AD L. I note it does not use the phrase 'cavity barriers'.

AD B gives definitions of 'cavity barrier' and 'fire stopping' but not 'cavity closer'! Diag 5.3, Note 1, in the current AD B is quite specific about there being no particular fire performance requirement for the materials used to close the cavity. This note was included only in recent versions of AD B (post 2019?) and is not in the technical guidance for Wales, Scotland & Ireland. I have own my thoughts on why this was changed!! There is also no restriction on the fire performance of materials used in the cavity - mineral wool will be non-combustible, polystyrene and PUR/PIR will be classed as combustible.

If this is the case why are there so many products on the market for use around door and window openings that are based on polystyrene or PUR insulation that have no fire resistance properties quoted.

@Jammy5 you will need photo evidence of this junction. Be very careful of using an alternative (though outwardly similar) detail!!! @ETC if the detail complies with Diagram 5.3 above there is no fire resistance requirement for the cavity closer around openings so no need for CS board.

That’s the one!

SAP is not particularly sensitive to elemental U-values so hopefully OK - only your SAP assessor can give a definitive answer

The SAP assessor will rerun the calculation with the revised insulation details. Hopefully all will be OK if some flexibility built into the design otherwise they should advise on options to get a pass (improved air test number?). I don't think photo evidence was required under the old, 2013, version of Appr Doc L.

Since retiring I no longer have the inclination to write lengthy reports on this issue. Headlines are; A masonry cavity wall with cavity insulation only (full or partial fill) is normally very safe from harmful condensation. Adding a layer of internal insulation to the above may move the dew point too far inwards. A view must then be taken if any potential condensation risk is is acceptable or if the design needs to be changed. Building material manufacturers will be able to provide calculations. I think we are in agreement here. My other comment was in reply to your assertion, gleaned from cavity wall insulation installers, that insulation must not allow any condensate to ‘wick’ across the insulation. The amount of liquid water from condensation is negligible compared to that from wind driven rain and this latter point is the basis for testing on full size representative walls (not computer modelling/simulation). It is also the reason that the Approved Documents, NHBC guidance, BRE Report 262 etc give guidance on maximum exposure zones with full fill cavity insulation and measures (including external render) that can be adopted to reduce the risk.