ADLIan

Look very carefully at your detail - there are a couple of timber elements missing that are needed to make this detail 'buildable' and they will impact the psi-value. Also regarding the general detailing of an I-joist corner - is your detail from an approved source/manufacturer? I'm thinking more from a structural connection perspective here. I do not know how psiclops works so cannot comment if the above are down to user input or how psicops handles the data.

'Cavity barriers' and 'fire stopping' are totally different items and are noted as such in Appr Doc B. Fire stopping is normally related to compartment walls and floors found in blocks of flats, semi detached and terraced houses. Even the BCO is confused!

Just a couple of comments on the attachment after a quick look through. Standard external temp is normally 0 Deg C rather than -5 though this probably makes no difference. Also there are timber elements missing from the junction detail.

So nothing stopping manufacturers (block, insulation etc, etc) generated psi-values being used. Until recently I was Elmhurst accredited so know the process, been through the program, trained in 2D & 3D calcs along with consultancy for insulation manufacturers. If I received a set of psi-values from a self builder and random website I'd be back to using the SAP defaults to protect myself. Manufacturers have normally invested in their technical people to undertake these calculations. Sorry Robin but please check SAP and supporting documents for thermal bridging.

The floor edges are dealt with as a linear thermal bridge separate from the insulation (& Uvalue) in the general floor area. The ground floor/wall junction is a difficult one when preventing/minimising the thermal bridge. This is more difficult with a suspended concrete slab that bears on inner masonry - 'overlapping' the cavity wall and floor insulation is one option. This is further complicated with beam and block. Relatively simple where the beams run parallel to the wall (2D thermal analysis) but probably requiring a 3D thermal analysis where the beams run into and bear on the wall. Again extending any cavity insulation to well below the floor insulation will help.

I still cannot find where SAP 10.3 does not allow manufacturer psi-values. Is there another guidance doc that you are referring to? Can you please post here?

Agree with comments re the OP. If the FEM Uvalue of an I-joist wall is so much better than that from the BS EN 6946 method I would suggest one result is incorrect. The combined method gives accurate results provided the I joists are modelled correctly. Therm output is not very intuitive!! I’ve used both methods on a wide range of wall details and they tend to agree. A quick read of SAP 10.2 and 10.3 appears to have identical guidance on dealing with linear thermal bridges. Where do you see any differences?

Do you already have Building Reg approval for this house? SAP 10.3 is in support of the next Appr Doc L1 (2026) effective from March next year. Using both SAP 10.2 and SAP 10.3 is just extra work for no real benefit. As I understand it both versions deal with linear thermal bridges in the same manner and manufacturers of insulation/blocks/TF etc (as competent persons) the psi-values are acceptable. Not much background info on who or what is behind psiclops - tread carefully!

I don't understand how this is suddenly an issue now the work is underway as this should have been sorted as part of the design stage SAP calculation and your Building Reg application. Has the SAP assessor taken the lazy option and used the default (worst case) thermal bridge numbers? Most insulation, block & lintel manufacturers have their own sets of psi-values that can be used and these will be a lot better than the default values. The TF manufacturer/designer should also have bespoke psi-values. Producing bespoke psi-values is very time consuming - if there are a lot of junctions to assess costs could easily be £2k plus!

Do not confuse wood wool and wood fibre boards - they are very different materials.

Please do not omit the fixing(s) - it's only one or 2 per board (check MIs). In a fire any plastic insulation attached to the plasterboard will melt or soften and the plasterboard may fall off the wall. This exposes more fuel to a fire (combustible insulation) and blocks possible escape routes. Additionally it could be a major hazard for fire fighters if they have to enter the building if people reported in there.

I believe approved contractors (to PAS????) can self certify

I’m sure PUR is ok for with render. Does this product have a foil face? This may be the problem as render will not stick, product for direct render normally has a glass tissue facing

Ask him what part of Approved Doc H (assuming England) applies. New one to me.

The BS that covers the calculation of ground floor U-values includes an assessment of the sub floor ventilation in suspended floors

For floors with the same footprint, edge conditions and say 50mm of PUR the solid floor will offer a slightly better U-value, but perhaps only by 0.01 W/m2K, compared to B&B suspended floor. With 150mm+ of PUR there's even less of a difference - unless you start looking at the 3rd decimal place or beyond.

Not so. Current version of BS does not use this number. It uses monthly weather data based on the building location

Condensation is often predicted with cavity walls. That’s the problem with taking the calculation at face value as in this instance any condensation risk is probably irrelevant.

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!