Internal wall insulation – Passivhaus

Internal wall insulation - Passivhaus

How can you express your undying devotion for the building in your life this Valentine’s week?

Whilst some will opt for showy expressions of affection, such as a new extension or bathroom refit, if you really want to make a lasting impact the best option may be a Passivhaus refurbishment.

The Passivhaus Standard is widely recognised as one of the top energy performance standards in the world. It sets clear and demanding requirements for both the thermal performance and airtightness of the building envelope. This fabric-focused approach offers an opportunity to view the full potential of internal wall insulation (IWI) applications.

Lena Gardens

Lena Gardens is a truly ground-breaking project and was only the second UK retrofit to be successfully certified to the full Passivhaus standard. The three-storey Victorian terrace presented a wide range of challenges for designers – Princedale Homes and Green Tomato Energy. The solid walled building is located within a Hammersmith conservation area meaning all air-tightening measures and insulation had to be fitted internally. As with many older properties, the building also features an irregular internal layout further complicating the installation.

To overcome these obstacles the design team developed many novel approaches and systems, paying considerable attention to the detailing and installation of the insulation and airtightness layers.


One of the most radical changes to the property was the decision to rehang all floor joists inside the thermal envelope. This allowed the IWI to be fitted against the external walls in a continuous layer, effectively eliminating a potential source of air loss and thermal bridging.

On the external walls 130 mm and 50 mm thicknesses of premium performance phenolic insulation framing board were fitted, achieving an outstanding U-value of 0.10 W/m2.K.  A further 50 mm layer of phenolic framing board was also installed on all party walls to prevent heat loss through to neighbouring properties.

To keep air-leakage levels within the Passivhaus limit (0.6 m3/hr/m2 at 50 Pa) a continuous layer of oriented strand board (OSB) was installed on all external and party walls, along with the roof and lower floor. Airtight tape was then applied to all junctions to seal the envelope.

Triple glazed sash imitation windows were used to replace the original windows. The design takes advantage of the period sash boxes, originally used to conceal the counterweights for the sash windows, by filling them with phenolic framing board – minimising thermal bridging. The window frames were also taped to the OSB layer to ensure an airtight seal.


The property features a pitched roof over the main terrace house and flat roofing over the rear kitchen extension. 130 mm of phenolic pitched roof board was installed between the rafters of each board, followed by the OSB layer and a second 50 mm layer of phenolic insulation.  The finished roofs achieved U-values of 0.14 W/m2.K.


Three different build-ups were used to reduce heat-loss through the ground and basement floors.

Under the basement floor a ground-to-air heat exchanger was installed. 150 mm of high performance PIR floor insulation was then installed over the heat exchanger and beneath a floating plywood floor, achieving a U-value of 0.14 W/m2.K

For the suspended floor alongside the basement, the floor joists were lowered to allow the installation of 200 mm PIR floor insulation above the joists.

In the ground floor kitchen, the original concrete floor slab was removed and 150 mm of PIR floor insulation was installed over the compacted hardcore.  A further 150 mm concrete slab was then installed above this. The latter two constructions both achieved U-values of 0.11 W/m2.K


The high level of airtightness within Passivhaus buildings necessitates the use of mechanical ventilation. A Genex Combi 185L unit, featuring a heat recovery efficiency of 76%, was installed within the basement. A ductwork system then provided distribution. Internal intrusion within the house from the ductwork system was minimised by using floor and ceiling grids and keeping the ductwork within floor voids and stud walls, where possible.


Initial performance testing showed that metered energy consumption had fallen to  around 10% of the previous level. The performance test values suggest that the actual specific primary energy demand is nearer two-thirds of the predicted demand (54.8 kWh/m2a compared with the predicted 72 kWh/m2a). Gas usage is now reduced to insignificant amounts used solely for cooking, and the house now needs less than 15 kWh per m2 of energy to be heated annually, far lower than the UK single family home average of 130 kWh per m2.

There are many benefits to upgrading the thermal performance of buildings, and the success of the Lena Gardens project demonstrates that with careful planning and detailing, it is possible to transform the energy performance of even the most challenging properties.

Find out more about the Lena Gardens Passivhaus project here.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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