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Airtight flooring key on Passivhaus retrofit

23 Jul 12 An airtight installation of the flooring insulation was fundamental to achieving Passivhaus retrofit certification on this 1950s Midlands home. Alex Smith reports.

A semi-detached house in Warwickshire has become the second building in the UK to secure the Passivhaus certification for a refurbishment known as the EnerPhit standard.

The design and specification of the flooring was key in achieving the standard. A combination of insulation and the installation of an airtight barrier helped cut the U-value of the flooring from around 2.5 to 0.1W/(m2K). Overall the refurbished home achieved an air leakage at +/- 50 Pa of just 0.57/hr, well within the 1/hr required under the EnerPhit standard.

The project in Wellesbourne was part of a pilot project looking at how the energy performance of a pair of 1955 Wimpey no-fines semi-detached homes could be improved through creating highly insulated, airtight buildings.

Number 58 Elliott Drive was refurbished to Passivhaus standards and had new insulation in the floor, walls and floor as well as a MVHR system, solar PV tiles, gas central heating and low energy appliances. The affordable green retrofit project next door just had external insulation installed and air tightness was improved without the use of membranes or specialist tape.

The foundations of the EnerPhit property were insulated both inside and out, with 200mm of insulation to reduce thermal bridging and thus heat loss through them.

Digging up original floor

This meant that the original concrete floor was dug up and 200mm of insulation installed under a new slab. It would have been cheaper to keep the original floor in place but the floor build-up would have resulted in an unacceptable reduction in ceiling height. No insulation is installed in the floor of the affordable retrofit.

“You have to take thermal bridging into consideration because it can negate the benefits of the insulation,” says Sarah Price, senior consultant at engineering consultant Encraft.

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“While the insulation helped improve the thermal performance, the installation of an airtight barrier ensured heat loss via air transfer was kept to a minimum. “The DPM formed the airtight barrier in the flooring, and special care was taken to ensure it was not penetrated during the services fix.”

While flooring performance in the EnerPhit property was vastly improved, there are drawbacks. As well as the extra cost of insulating the floor, the work is disruptive and can’t be done with tenants in situ. This is one of the main reasons why Orbit Heart of England Housing Association won’t be insulating floors on future low-energy refurbishments, for now at least.

Insulating the floor step by step

1. Cloaking the foundations. The foundations of all external walls were dug out and cloaked in 200mm of moisture-resistant Celotex PIR (polyisocyanurate) insulation internally and 200mm of XPS (extruded polystyrene) externally. Gaps between insulation boards filled with expandable foam (pictured). External insulation below ground level was protected by polythene sheets and cementicious board.

2. Insulating the interior floor. Insulation was placed inside the foundations once the concrete floor had been dug out. The space inside the foundations was filled with compressed hardcore and sand so as not to puncture the damp proof membrane (DPM) which acts as the airtight layer (pictured). Slabs of 200mm Celotex XR4000 insulation were fitted over the DPM.

3. Making penetrations airtight. All service penetrations were sealed with specialist air tightness tape and grommets. Rubber grommets were used to seal services pipes and cables penetrating the floor (pictured). Insulation was also fitted around the penetrations to avoid air gaps and heat loss.

4. Pouring the concrete. Once the penetrations had been sealed, 100mm of concrete was poured. 25mm insulation upstands were installed prior to pouring the concrete to prevent thermal bridging from the foundations (pictured). The DPM was taped to a mesh on the internal load-bearing wall prior to plastering.

Project team

  • Client: Orbit Heart of England Housing Association
  • Architect: ID Partnership
  • Building services: Encraft
  • Contractor: Property Matters

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