Whinfell: retrofitting non-standard timber-frame housing for Gentoo Housing

Greener Solutions Group was appointed by Regen Group in September 2021 to provide retrofit assessment, energy improvement modelling, retrofit coordination and retrofit design for the Whinfell retrofit and decarbonisation project, delivered on behalf of Gentoo Housing under the Social Housing Decarbonisation Fund. The properties were non-standard, system-built and timber-frame construction with little reliable information about the original wall build-up. Some dwellings had an initial calculated space-heating demand above 200 kWh/m²/year against an SHDF target of approximately 90 kWh/m²/year and EPC Band C. The final solution combined upgraded timber-frame insulation, external wall insulation, high-performance windows and doors, insulated flat roofs and a humidity-controlled underfloor ventilation strategy.

Project at a glance

• Client: Gentoo Housing

• Principal contractor: Regen Group

• Specialist retrofit consultant: Greener Solutions Group

• Initial appointment: September 2021

• Funding programme: Social Housing Decarbonisation Fund (SHDF)

• Construction type: non-standard, system-built and timber-frame

• Initial space-heating demand: in excess of 200 kWh/m²/year on some dwellings

• Target space-heating demand: approximately 90 kWh/m²/year

• Target rating: EPC Band C

• Services: retrofit assessment, energy improvement modelling, retrofit coordination, retrofit design, ventilation strategy, technical support

  
  

What made Whinfell a complex retrofit project?

The properties comprised non-standard, system-built and timber-frame construction, with limited reliable information available on the original build-up of the external walls, existing insulation and external finishes. At the outset it was unclear whether the external envelope was rendered masonry, render applied over mesh and timber panels, or a combination of methods across the development. Establishing the existing construction and thermal performance was therefore fundamental before any retrofit strategy could be developed.

The bungalows and end-terrace properties presented the greatest difficulty. Bungalows lost significant heat through their existing flat roofs and suspended timber floors, while end-terrace properties were additionally affected by heat loss through exposed gable walls. Set against the relatively poor thermal performance of the existing timber-frame walls, reaching the required improvement was a substantial technical challenge.

The programme was also exceptionally compressed. Following initial discussions between Regen and Greener Solutions Group, our surveying teams mobilised and began collecting property data the following week. That information then had to be assessed, modelled and translated into initial retrofit proposals and draft designs for submission roughly a week later.

How were the improvement options assessed and modelled?

Greener Solutions Group undertook detailed retrofit assessments and energy modelling to understand both the existing performance of the properties and the combination of measures needed to meet the project's objectives. The strategies investigated included:

• Roof insulation upgrades

• Internal wall insulation

• External wall insulation

• Suspended floor insulation

• Replacement high-performance windows and doors

• Improved ventilation

• Improvements to the existing timber-frame insulation

  
  

Different combinations were modelled and reviewed for their effect on EPC performance, space-heating demand, technical risk and practicality. Considerable design development followed between Greener Solutions Group, Regen and the wider project team, working to establish what could realistically be achieved within the properties while meeting as many of the funding KPIs as possible. Despite the tight programme, the assessment, modelling and initial design information was delivered in time to support the funding submission.

How was moisture and ventilation risk managed?

Improving airtightness and thermal performance can introduce condensation and moisture problems if ventilation is not addressed alongside. Greener Solutions Group therefore treated ventilation as an integral part of the whole-house retrofit strategy rather than considering insulation measures in isolation.

The proposals included ventilation system upgrades, with low-rate continuous ventilation incorporated where appropriate to control internal humidity, manage moisture and reduce the risk of surface and interstitial condensation once the building fabric was improved. This mattered particularly given the timber-frame construction and the scale of the changes proposed to the thermal envelope.

What did the intrusive surveys reveal?

Once funding was confirmed, intrusive surveys gave a much clearer picture of the existing construction. As is common on complex retrofit projects involving non-standard construction, the findings changed the proposed scope. The surveys identified additional constraints, but they also opened up opportunities to refine the specification and develop a more effective solution.

The final agreed wall strategy involved removing or upgrading the ageing insulation between the existing timber studs and replacing it with thicker, higher-performance modern insulation board. The timber-frame wall construction was then further enhanced with an external wall insulation system and rendered finish, substantially improving the thermal performance of the external envelope.

How was heat loss through suspended floors addressed?

The suspended timber floors produced one of the most significant design developments on the project. Conventional improvement would have required insulation beneath or between the floor structure. That was technically achievable, but it would have created considerable disruption for residents and added complexity, cost and programme risk.

Through careful assessment of the building physics and the use of new technology, the project team developed an alternative incorporating humidity-controlled underfloor ventilation. This allowed the moisture risk associated with the suspended floor void to be actively managed and enabled the project to avoid extensive underfloor insulation works.

The solution struck a balance between energy performance, moisture management, technical risk and resident disruption. It illustrates the value of treating a dwelling as an interconnected system rather than specifying individual retrofit measures in isolation.

The final retrofit solution

The whole-house solution combined fabric and ventilation improvements:

• Upgraded insulation within the existing timber-frame wall structure

• External wall insulation with a new rendered finish

• High-performance replacement windows

• High-performance replacement external doors

• Highly insulated replacement or upgraded flat roofs

• Improved ventilation provision

• Low-rate continuous ventilation where required

• Humidity-controlled suspended floor ventilation

• Detailed consideration of condensation and moisture risks throughout the design

Greener Solutions Group's role

Retrofit assessment

Detailed assessment of the existing dwellings, construction, condition, ventilation and energy performance.

Energy improvement modelling

Modelling different combinations of measures to determine how the properties could move towards the required SHDF energy-performance targets and EPC Band C.

Retrofit coordination

Coordinating development of the whole-house retrofit strategy and ensuring individual measures were considered collectively in line with the principles of PAS 2035.

Retrofit design

Developing and supporting technically appropriate retrofit solutions for the non-standard timber-frame and system-built properties.

Ventilation and moisture strategy

Assessing the implications of increased insulation and airtightness, and incorporating ventilation measures to manage humidity and condensation risk.

Technical design development

Working collaboratively with Regen and the wider project team as intrusive surveys revealed additional information and the specification evolved.

Project outcome

Whinfell shows why detailed assessment, energy modelling and specialist retrofit design matter when improving non-traditional and system-built housing. The project began with substantial uncertainty about the existing construction, properties with space-heating demands above 200 kWh/m²/year, demanding SHDF performance criteria and an exceptionally compressed initial design programme.

Through detailed assessment, iterative modelling, intrusive investigation and collaborative design development, Greener Solutions Group supported Regen in developing a practical whole-house solution capable of delivering substantial improvements to the properties.

It is also a strong example of retrofit design extending well beyond adding insulation. The relationship between walls, roof, floors, windows, doors, airtightness and ventilation had to be considered collectively, so that energy performance improved without creating unintended consequences elsewhere in the building. Combining upgraded timber-frame insulation, external wall insulation, high-performance windows and doors, improved flat roofs and humidity-controlled underfloor ventilation delivered significant improvements while reducing disruption to residents.

Frequently asked questions

What is non-traditional or system-built housing?

Non-traditional or system-built housing refers to homes constructed using methods other than conventional masonry cavity walls, including timber frame, concrete panel and steel frame systems. Much of this stock was built in the post-war period. It presents particular retrofit challenges because the original construction is often poorly documented and standard insulation approaches may not be suitable.

What is space-heating demand measured in kWh/m²/year?

Space-heating demand expresses the energy required to heat a dwelling per square metre of floor area each year. It is used as a performance target on funded retrofit programmes because, unlike an EPC band, it measures the actual thermal performance of the building fabric rather than the cost of running it. Lower figures indicate a better-performing envelope.

Why does timber-frame construction need special consideration in retrofit?

Timber-frame walls are more vulnerable to moisture accumulation than masonry, because trapped moisture can lead to decay within the structure. Changes to insulation and airtightness alter how moisture moves through the wall, so interstitial condensation risk has to be assessed carefully before measures are specified.

What is interstitial condensation?

Interstitial condensation occurs when warm, moist air penetrates a building element and condenses within the construction rather than on its internal surface. It is a particular concern in retrofit because it is hidden from view and can cause damage to insulation and structural timber over time.

What is humidity-controlled underfloor ventilation?

Humidity-controlled underfloor ventilation regulates airflow through a suspended floor void in response to moisture conditions, rather than relying on fixed permanent openings. At Whinfell it allowed moisture risk in the floor void to be actively managed, avoiding the disruption and cost of extensive underfloor insulation works.

Talk to us about non-standard construction retrofit

Greener Solutions Group provides retrofit assessment, energy modelling, coordination and design for technically challenging and non-traditional housing stock across the UK. Get in touch to discuss your project.