ADLIan

SAP assessor may not have taken account of low emissivity foil on the PUR. As above, manufacturer figures can be used provided you have proof of the numbers

standard block outer, aircrete block inner, plasterboard on dab and 75mm PUR partial fill should give about 0.20 W/m2K (25mm or 50mm cavity makes no difference to U-value)

'Wind wash' is not normally an issue with loft insulation as the wind speed over the insulation is insignificant. From memory BBA and/or BRE have done work on this

Mass air movement into the loft also needs to be considered. Seal any gaps in the ceilings - around pipe and cable penetrations, light fittings etc. Not sure that emulsion paint will create much of a vcl as its moisture vapour resistivity is only about 0.5 MNs/g (polythene is 250 MNs/g or more)

Both systems work and problems are more down to workmanship or wall not being suitable for injection. More problems reported with mineral wool simply because this has the biggest part of the market, especially refurb. injected cavity insulation will also reduce air movement in the cavity so getting the best from the EWI. The EWI will mitigate potential penetrating damp across the wall.

But the sap assessor must have this info to sign off this part of the works. Rock and a hard place come to mind!! I give my clients a full design stage report with recommendations/requirements for building reg compliance (or more), not really my issue if Building Control do not take this into consideration. SAP assessor is not responsible for policing this part of the Regs

Totally agree. But checking BR compliance is a job for Building Control not a SAP assessor who will never visit the site.

I regularly see those U-values under current Regs, walls perhaps slightly higher at 0.20-0.22. They are only slightly better than the current Notional Building.

All On 15 Dec Govt issued the new Approved Doc L for England. https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l Also new version of SAP, Part F (Ventilation) & Part O (Overheating). All becoming effective in June 2022. Does not look to be a big change in U-values apart from tightening of worst case values. Air tightness tightened and use of solar PV used in Notional Dwelling. I am waiting for the new software from Elmhurst to assess the impact in more detail. One good point is plenty of practical guidance - not read it all so not sure how relevant. One big change is information gathering - documentary and photographic evidence required of almost all construction stages (insulation being fitted, junctions, windows, boilers, controls etc, etc....... ). These will be required by Building Control and the SAP energy assessor to assess the 'as built' dwelling. I can see this increase in red tape increasing the cost of assessments considerably. Personally I do want to spend the majority of my time chasing this data knowing its probably not available (I tend to deal with self builders and small developers) so will cease to be a SAP assessor for the new version of the Regs. I'll switch to 2D thermal modelling of junctions for bespoke psi-values. Plenty to read in the new ADs and SAP - perhaps 500 pages! Get your Building Reg application in before next June!

They are the limiting, backstop, values - in practice your u-vals will need to be much better. Your sap report contains the current values, the lower numbers are in the new AD L effective from June 2022

Incorrect install of plasterboard on dabs is another matter. Mineral wool cavity batt normally quite forgiving to quality of workmanship. Blown in open to problems if all cavity/voids not fully filled which is very difficulty to verify.

Mineral wool cavity batt /blown in at a density of at least 18 kg/m3 is normally enough to prevent air movement in a fully filled cavity.

Appr Doc Q only applies to new dwellings (including change of use).

To be expected. House drying out, ceiling not finished, no plasterboard, no AVCL…….It’ll be ok once ceiling etc finished correctly.

+1

May be……may be not???!!!

Read BS 5250. I’m not wasting any more of my time on this subject

@Dave Jones you are confusing ventilation of the whole house with ventilation of roofs. Please refer to BS 5250 (yes I’m using the 2021 version, at great expense! The information on the use AVCLs has not changed). An AVCL is required otherwise you are allowing lots of moisture vapour into the construction where it will condense on the next cold, impermeable surface above the insulation.

Values for the ply deck and the roofing membrane both look low. Nearer 1000 MNs/gm for ply and 800 MNs/g for the single ply (it's a continuous membrane with all joints very well sealed). These may be sufficient to alter the CRA. Problem with WUFI as I understand is that it is incredibly complex and there are no conventions for its use.

Hybrid roofs are generally discouraged. A hybrid pitched roof is often more forgiving as a breather type membrane is used. A hybrid flat roof is not so forgiving as the waterproof layer and ply/osb deck are both impermeable. To work there must be meticulous attention to detail in installing the AVCL ensuring all laps/joints/penetrations are very well sealed. Any imperfections may lead to problems in the longer term - moisture vapour may get through the AVCL but liquid water is effectively trapped. BS 6229 gives more advice on flat roofs. In a condensation assessment a lot will depend upon the vapour resistance value used for AVCL - does it assume the vapour resistance of the membrane, (which may be massively high if it includes a metal foil) or does the value reflect a more realistic 'in use' value including imperfections? As mentioned before its possible to get the CRA to say anything you want - it's knowing if correct and realistic values have been used. Not sure an intelligent membrane would work - best check with the manufacturer and any independent certification they may have

Please read BS 5250 . Lack of an AVCL will allow moisture vapour from inside the house to pass through the insulation (or gaps if foil faced) and condense on any impermeable layer above the insulation. Even if you use a breather type membrane it may not be able to get rid of all the moisture vapour. I have seen a layer of condensation form on the underside of a breather membrane which then effectively renders it non breathable. The problem with using calculators like the one referenced is that you can get them to give any result that suits you!!

Please read BS 5250.

Cold roofs should incorporate an AVCL to help prevent/minimise air movement and moisture vapour transfer through the system - what does get through can then removed by the ventilation above the insulation.

As ProDave & Nod, staggered stud with mineral wool (no need for high density! 100 kg/m3????) between and 2 or more layers of high density plasterboard, ideally different thicknesses. PUR will do nothing acoustically.

As above, not normally good practice to have insulation at different levels in a ‘warm’ roof. Check with insulation manufacturer and ask for condensation calculation. With a correctly detailed vcl it may work. Also check MIs and BBA certificate, 38mm min thick counter battens normally required