Witch Hazel, Bristol – Passivhaus Design and Build & Affordable Housing

The Passivhaus homes were designed to form a two-storey apartment block in compact form with independent access to all apartments and no communal accommodation. The block was designed to ensure the main living spaces and bedrooms took advantage of large south facing windows which help to provide a significant proportion of the heating requirements.

The bathroom and kitchen were located to the north side where the services requirements could be close together. The Passivhaus design method considers many detailed elements, including the length of hot water pipes to make sure that the performance of a building is predicted accurately at the design stage.

Quattro Design modelled the scheme using the Passivhaus PHPP software to ensure that it would meet the required standards. To reduce risks, as part of this design process, we engaged specialist Passivhaus certifiers ‘WARM’, to undertake quality assurance work prior to their final evaluation and checks at the end of the scheme to enable certification.

Timber frame construction was used for the standard housing and was considered the preferred choice for the Passivhaus homes. This enabled the supply chain and construction methods to remain as co-ordinated as possible across all the schemes. Working with the Contractor, Quattro developed specific enhanced details to meet the Passivhaus criteria.

This involved:

• The use of thermal blocks around the ground floor
• Increased thickness of EPS insulation above the beam and block floor
• Additional insulation to the inside of the timber framing with the airtight barrier protected behind
• A significantly higher eaves to avoid reducing the insulation at the top of the wall

Fundamental to Passivhaus standards and low energy designs more broadly, is the careful integration of the MVHR (mechanical ventilation with heat recovery) system that helps to reduce energy use considerably.