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WUFI Verified Retrofit (Guest Post) L#1601

By August 23, 2026August 24th, 2026One Comment

WUFI Verified Retrofit (Guest Post)

GBL > Guest Posts > WUFI verified Retrofit > L#1601

About:

A 30-Year WUFI Simulation, End to End

  • Most internal wall insulation in this country is specified from a catalogue.
  • A board thickness comes out of a U-value target, a vapour control layer goes in because the datasheet says so, and nobody asks what the wall will be doing in year twelve.
  • Inspections of the government’s own schemes have found defective installs in more than 30,000 homes.
  • A fair share of the failed jobs we open up and repair were built exactly to their specification.
  • The specification was the problem.
  • Our company does it differently, and this article is the write-up I promised: one hygrothermal simulation, end to end.
  • What goes in, what comes out, how we read it, and what got built because of it.
  • The example is a representative one, a typical Glasgow tenement flat of the kind we insulate week in, week out, rather than a single named job.
  • Every step below is the actual process.
  • The simulations are run in WUFI with a building physicist who co-authors PAS standards.
  • I do the surveying and the building.
  • He does the physics.
  • Neither of us signs off a wall the other hasn’t seen.

Editorial Comment:

  • As a prior specification consultant it pains me, but I am not surprised, so little is taught on specification.
  • As a prior educator it pains me that materials science, building physics are not taught adequately.

The wall

  • Take a typical pre-1919 Glasgow tenement flat.
  • The external walls are solid sandstone rubble, commonly half a metre thick or more, plastered on lath internally with an air gap behind, lime-bedded, no cavity, no damp-proof course.
  • Suspended timber floors with joist ends built into the masonry.
  • Original air bricks ventilating the sub-floor void.
  • The wall has kept people warm and dry for a century because it breathes: it takes on rain, holds it, and dries out again in both directions.
  • That drying ability is the thing every simulation is really testing.
  • Add the wrong layer on the inside and you haven’t insulated the wall, you’ve plugged it.

Editorial Comment:

  • The week of COP in Glasgow we were invited to Edinburgh to present Green Retrofit Calculator (GRC) to their event
  • I was handed a copy of Guide to Energy Retrofit of Traditional Buildings book as a thank you
  • Section 7 Ventilation pointed out that the suspended ground floor ventilation zone is linked all the way up to the attic via the void behind lath and plaster of external walls
  • Wow, that could be a 7 storey cavity, I never knew, now I do.

What goes into the model

  • A WUFI run is only as honest as its inputs.
  • Ours are built from four things.

The construction, as found.

  • Not as assumed.
  • We survey the actual wall: thickness, stone type, condition of the pointing, the internal finish and what is behind it, moisture readings top and bottom.
  • Where the lath and plaster comes off, we record what the strip-out shows.
  • A rubble wall with an open, porous core behaves differently from coursed ashlar, and the material data in the model has to match the wall in front of you, not the nearest library entry.
  • Where there is doubt, the physicist runs the pessimistic case.

Editorial Comment:

  • If an incompetent insulation system exists, it must be removed before the wall survey commences.

The climate file.

  • WUFI works through a full year of real hourly weather data for the location, cycled for the length of the simulation: temperature, humidity, solar gain, and critically for us, wind-driven rain.
  • Western Scotland takes some of the heaviest driven rain exposure in Europe.
  • A build-up that models comfortably in the south of England can fail on paper the moment you load a Glasgow climate file against a west-facing elevation.
  • Same wall, same insulation, different sky.

Orientation and exposure.

  • Each elevation is modelled separately.
  • A sheltered east wall facing a tenement across the street and a west gable taking the full prevailing weather are different problems, and the model treats them that way.
  • Rain absorption at the outer face is set from the exposure and the condition of the stone and pointing.

Internal loads.

  • People make moisture. Cooking, washing, drying clothes, breathing.
  • The model runs a realistic internal humidity profile for an occupied flat, not an empty laboratory box.
  • A build-up that survives a dry, well-ventilated household can fail under a family of five with the windows painted shut, so we model the humid household.
  • Then it runs. Hour by hour, layer by layer, for a simulated 30 years.

What comes out, and what it means

  • The output isn’t a pass or fail stamp.
  • It’s a set of curves, and three of them decide the specification.

Total water content in the masonry.

  • The wall enters the simulation with a realistic moisture load and lives through 30 years of weather.
  • The question is whether it finds an equilibrium.
  • A healthy result oscillates with the seasons around a stable line.
  • A failing result climbs, year on year, because the build-up has cut off more drying than the weather puts in.
  • Once that line is rising, nothing downstream matters.
  • The wall is accumulating.

Interstitial condensation risk at the interfaces.

  • The dangerous plane is where the new build-up meets the old wall, because insulation makes the masonry behind it colder, and cold surfaces are where vapour condenses.
  • The model tracks relative humidity at that interface through every hour of the run.
  • What we look for is how often it sits above the levels associated with mould growth and timber decay, and for how long at a time.
  • Brief winter peaks that dry off in spring are the wall behaving like a wall.
  • Sustained high humidity at the interface, month after month, is a rot forecast.

Conditions at the joist ends.

  • The joists carrying the floor are buried in the same masonry we’re about to make colder.
  • If the moisture regime at the bearing ends deteriorates, the insulation is quietly eating the structure.
  • This curve overrules everything else.
  • No energy saving justifies losing the floor.

 

  • Run the standard catalogue answer for our representative tenement wall, foil-faced rigid board with a vapour control layer, against a west-facing Glasgow elevation, and the model tells a consistent story:
    • inward drying is gone, the masonry behind the insulation runs colder and wetter, water content trends upwards instead of settling, and the interface and joist ends spend long stretches of the winter in the danger zone.
  • The Glaser-method check on the same build-up passes it, because Glaser can’t see rain.
  • That gap between the two methods is, in one sentence, why the strip-out side of our business exists.

The build-up the model forced

  • So the model overrules the catalogue, and what gets built is this.
  • A lime parge coat straight onto the stone after the strip-out, closing off the old air gap, then wood fibre board bedded in lime, finished in lime plaster.
  • No vapour control layer, no foil, no plastic.
  • Every layer vapour-open, so inward drying continues through the new build-up rather than stopping dead behind it.
  • Wood fibre also buffers moisture, absorbing the winter peaks and releasing them when conditions allow, exactly the behaviour the curves reward.

 

  • Thickness comes from the model too, and this is the part specifiers resist:
    • beyond a certain point, thicker insulation buys almost no extra energy saving while pushing the masonry colder and wetter.
  • The model shows where that point sits for each elevation.
  • On an exposed gable the honest answer is sometimes a thinner board than the client expected, or none on that wall at all.

Junctions get the same discipline.

  • Window reveals and lintels are insulated with thinner board so they do not become the new condensation focus.
  • Joist ends are detailed to stay ventilated where the model asks for it.
  • And the original ventilation stays: air bricks are preserved and kept clear, because the sub-floor void needs its airflow more after insulation, not less.
  • Blocked air bricks turn up in a remarkable number of the failed jobs we strip out.

The point

  • The simulation takes days, not hours, which is why the volume schemes rarely commission one.
  • Against the cost of remediating one saturated elevation, it is not a serious expense.
  • Every install we do, grant-funded through Home Energy Scotland or private, gets its model before it gets its specification, because the alternative is finding out in year twelve with a crowbar.
  • The wall spent a hundred years proving it can dry.
  • The least we can do is prove, in advance, that it still can.

© Calum Farquhar 19th August 2026

Guest Writer: Calum Farquhar (Guest Writer) L#1610

Editorial Comment: BrianSpecMan

© Calum Farquhar 19th August 2026
© GBE GBC GRC GIC GGC GBL NGS ASWS Brian Murphy aka BrianSpecMan ******
23rd August 2026 – 24th August 2026

Images:

GBE CPD WUFI Verified Retrofit T1

[FIG 1: strip-out of a failed vapour-closed install on a Glasgow tenement wall, saturated masonry exposed]

[FIG 2: wood fibre boards bedded in lime adhesive on the prepared stone wall]

[FIG 3: lime plaster finish going on over the wood fibre]

[FIG 4: original air brick preserved and clear at completion]

Guest Writers:

Photo

Author: Calum Farquhar

Brian Murphy Presenting CAPEM Compass at Avnir 2011 Lille France BRM 1024x577

Editor: Brian Murphy

BrianSpecMan

Guest Posts on GBL

    • WUFI Verified Retrofit (Guest Post) L#1601 (this page)

© GBE GBC GRC GIC GGC GBL NGS ASWS Brian Murphy aka BrianSpecMan ******
23rd August 2026 – 24th August 2026

See Also:

GBL Case Study

GBL Evidence Based Case

  • WUFI Verified Retrofit (Guest Post) L#1601 (this page)

GBL Guest Posts

  • WUFI Verified Retrofit (Guest Post) L#1601 (this post)

GBL Guest Writer

GBE CPD

GBE CPD Topic

  • Retrofit

GBC Calculators

  • WUFI

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  • BSI PAS 2030
    DECC’s Handbook for the Green Deal developed with BSI under the Publicly Available Standard scheme
    The BSI Publicly Available Standard for the implementation of the GreenDeal funding mechanism.
    Not obviously available from the BSI website.
    With the demise of GreenDeal due to high risk and interest rates on embedded loans, and the subsequent damage done to properties, damage still continuing within ECO schemes; PAS 2030 (installation) was updated and PAS 2035 (design) was created and continues to be updated.
    Effectively a Code of Practice for Installation for retrofit works under ECO and GreenDeal funding

    Early versions had chasms of effectively bad applications and erroneous requirements
    The 2019 Edition coordinates with PAS 2035:2019 to radically improve the competency of all parties involved in design requirements and installation and improve the outputs
    See Also: GreenDeal, PAS, PAS 2030, PAS 2035 (NGS BRM ’11 – GBE BRM ‘26)
  • BSI PAS 2030 2035: 2019
    The BSI Publicly Available Standard for the refurbishment of UK housing.
    With the demise of GreenDeal due to high risk and interest rates on embedded loans, and the subsequent damage done to properties, damage still continuing within ECO schemes; PAS 2030 (installation) was updated and PAS 2035 (design) was created and continues to be updated.
    Robust procedures to prevent damage to historic housing by adding surveys, risk assessments and requiring competent designers and installers to develop and install competent proposals.
    The 2019 Edition coordinates with PAS 2030:2019 to radically improve the competency of all parties involved in design and installation and improve the outputsEffectively a Code of Practice for Design for retrofit works under ECO and GreenDeal funding
    See Also: GreenDeal, PAS, PAS 2030 2035, ECO
    (GBE BRM ‘26)
  • ECO    See Also: ENERGY COMPANY OBLIGATION
  • ECO 4 See Also: ENERGY COMPANY OBLIGATION

  • ENERGY COMPANY OBLIGATION (ECO)
    UK Government’s principal tools for driving uptake of energy efficiency measures were the energy company obligations: Carbon Emissions Reductions Target (CERT) and Community Energy Saving Programme (CESP).
    The government has committed to replacing these programmes when they expired at the end of 2012 with a new ECO.
    The obligation will be restructured to bring it up to date and enable it to function alongside the then Green Deal finance market. (Green Deal is no longer running after a poor start)
    The ECO is focused on those households who need support over and above the Green Deal and in the absense of Green Deal.
    It will provide measures which help the most vulnerable low income households and also offer support to the expensive to treat properties, such as those with solid walls.
    The Green Deal was the UK government’s initiative to kick-start the green refurbishment revolution.
    The program was to encourage both homeowners and businesses to upgrade the energy efficiency of their buildings at no up-front cost.
    These improvements were to be offered by private firms who would then recoup the capital cost, together with an interest charge, over a period of time through the resulting savings on the occupant’s energy bills.
    (2degrees ’11 – GBE BRM ’22)
    Government driving uptake of energy efficiency measures:
    Carbon Emissions Reductions Target (CERT) and Community Energy Saving Programme (CESP) expire at the end of 2012, replace by ECO.
    Restructured: up to date and work with Green Deal.
    ECO focuses on households who need financial support
    Fuel Poverty (Fuel costs >10% of income)
    See Also: CERT, CESP, GreenDeal, Fuel Poverty,
    (NGS BRM ’11 – ’12)
    The Energy Company Obligation provides financial support for energy efficiency measures as part of the Green Deal, and aims to fill gaps in Green Deal funding.
    For example by funding measures that would not normally meet the ‘golden rule’ such as solid wall insulation.
    (STBA RRGW ’21)

  • ENERGY COMPANY OBLIGATION (ECO4)
    Consists of one distinction called the Home Heating Cost Reduction Obligation (HHCRO) or “Affordable Warmth” scheme.
    ECO 4 scheme is a government energy efficiency scheme introduced in April 2022 to tackle fuel poverty and help reduce carbon emissions.
    Under the ECO scheme, leading energy suppliers like British Gas, EDF Energy, EON, OVO, Scottish Power, and SSE are obliged to help UK households with energy efficiency measures such as Thermal Insulation, Heat pumps and Solar Panels.
    This Scheme aims to help eligible people reduce the cost of electricity and heating by installing energy-efficient measures.
    To improve energy efficiency across the country, the UK government has allocated £4 billion to install solar panels and heat pumps.
    (GBE BRM ’22 – GBE BRM ’26)

  • PAS      See Also: PUBLICLY AVAILABLE STANDARD
  • PUBLICLY AVAILABLE SPECIFICATION 
(PAS)
    (BSI PAS 1192-2 ’13)

  • WUFI  See Also: WÄRME UND FEUCHTE INSTATIONÄR

  • WÄRME UND FEUCHTE INSTATIONÄR (WUFI)
  • A modelling tool for calculating hydrothermal performance in structures
  • Computer assisted simulation programme for heat and humidity transports (dynamic) to BS EN 15026
  • www.wufi.de
  • See Also: Delphin
  • (NGS BRM ’09 – ’12)
  • A modelling tool for calculating hydrothermal performance in structures
  • (HES Interreg Energy Pathfinder ’22)
  • See Also: GBC, Green Building Calculator, GRC Green Retrofit Calculator, Delphin,
  • (GBC BRM ’23)

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23rd August 2026 – 24th August 2026

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