Lauma Balina, Principal Engineer, Partner at Max Fordham, answers readers’ questions on Heritage buildings and Part L compliance.
The deep retrofit of Grade I listed New Court at Trinity College, Cambridge, by 5th Studio and Max Fordham included the use of internal wall insulation, and reduced window heat loss by 80 per cent (photo: Timothy Soar).
How stringent is Part L in respect of heritage buildings, and what are the principal issues involved in achieving compliance?
When it comes to refurbishing existing buildings, Part L wording is quite lenient, allowing specifiers to limit or omit improvements if they’re too expensive, complicated, or if the works could cause damage to the existing fabric. In my opinion, the current system makes it too easy to avoid fabric improvements, and Part L could be more ambitious in its guidance. It’s by no means easy, but we should all be rising to the challenge and finding ways to do the best we can for every project.
It’s important that fabric improvements are designed correctly to avoid excessive moisture build-up. When done badly, excessive moisture can damage the fabric through freeze-thaw cycles, cause damp and mould growth, or increase heat loss if insulation gets moist. Meeting the Part L target value for walls is challenging with internal wall insulation (IWI) without causing unintended consequences. External wall insulation (EWI) is theoretically easier, but can also be done badly, and with heritage buildings is unlikely to be an option. Get the right consultants who can do it well, and use them to their full potential!
What is the best way to approach Part L compliance when working on a heritage project?
The entire design team and client should be considerate of the building’s limitations – don’t force it to do something it can’t achieve. There’s likely limited space for M&E equipment, so designing out the need for active equipment by maximising passive measures will be key. Reduce heat loss and maximise natural ventilation for overheating to make the equipment smaller. But do remember that a well insulated airtight building will need mechanical ventilation to manage internal air quality and humidity. With existing buildings it’s also important to understand peak loads in order to minimise the equipment needed to unlock retrofit options.
What are the most effective but least invasive measures for boosting thermal performance?
Thermal performance is all about fabric performance and passive design measures. Sealing pathways that warm air is using to leave – and cold air to enter – the building, and reducing overheating will be most effective. Fixing window frames, repointing brickwork, filling in holes, applying window film to reduce solar gain, and facilitating controlled natural ventilation are all part of the solution.
Are Passivhaus design principles compatible with heritage buildings in terms of refurbishment or retrofit works?
We’re currently working on several higher education heritage buildings targeting EnerPHit, the Passivhaus retrofit standard. Designing to this standard is probably the best way to manage the risks associated with retrofit as it’s stringent on quality, thermal comfort and protection of the building fabric. While it is more expensive than a traditional design approach, projects like the Entopia Building in Cambridge by Architype, Feilden Mawson and Max Fordham, have shown that it doesn’t need to be excessively so.
How important is it to improve airtightness? And where can the biggest gains be made?
A large proportion of heat loss comes from poor airtightness in existing buildings. Some of the areas with the biggest impact are window frames; sealing between floor boards (make sure you seal behind the skirting boards as well); sealing gaps in the attic and around the attic hatch; and any other obvious holes in the façade.
What are the main problems associated with insulating solid walls on heritage buildings?
You need to consider what parts of the fabric are listed; you probably won’t be able to cover up panelling on the walls for example, but insulating the attic shouldn’t be a problem. Insulation needs to be designed in a way that avoids moisture-related issues. Low-cost options, such as mineral wool for IWI, are unlikely to achieve very good U-values with exposed brick or stone façades. Anything better will require more expensive materials. Achieving thermal continuity can be tricky and needs careful consideration.
What are the preferred methods for insulating solid walls?
Ideally it’s EWI, as this reduces the risks of moisture build-up in the fabric. However, with heritage buildings this is unlikely to be feasible. When using IWI, first make sure that the façade won’t let rain into the fabric. Then insulate with vapour and capillary-open insulation, such as woodfibre, lime and cork render, or calcium silicate.
What can and should be done to improve the thermal performance of existing windows?
This will depend on the condition of the existing windows and heritage considerations. If the existing frames are in a good enough condition and need to be retained, repairs can be made to minimise drafts. This would ideally be paired with secondary glazing to improve thermal performance. However, considerations should also be made for moisture build-up. This is assuming that the windows can’t be replaced.
For further information, please visit www.maxfordham.com
Source: Architecture Today
