• Air Tightness_01

Feilden+Mawson are currently engaged in the refurbishment of six listed Georgian townhouses to provide a new hotel for a joint venture between The Portman Estate who own the buildings and Z Hotels who will manage them.

An important aspect of the brief we have received from Portman is that the refurbished buildings should provide a substantial improvement on air tightness compared to elsewhere on the Estate.

Air tightness is an important component of improving the energy efficiency performance of a building. F+M are working with Sturgis Carbon Profiling to achieve this desired improvement.

Air tightness testing was carried out on a typical un-refurbished Georgian townhouse and it was discovered to allow some 50m3 per sq m per hour (50m3/m2/hr) of air to pass through its fabric. In energy conservation terms it was essentially performing as effectively as a sieve. The deemed to satisfy figure for Building Regulations approval is 10m3/m2/hr.

The problem with much Georgian construction is that it is substantially loose fit with junctions covered by cornices, architraves and skirtings. There are a number of typical problem areas –

  • Line of air tightness - Essentially a strategy has to be established to determine where the line of air tightness will be established.
  • The external brickwork - is often full of air passages due to low quality mortar fill to the joints. To counter this it is advisable to plaster or render the internal face of the external walls to seal these gaps. This can't always be achieved as often the existing plaster is of historic significance and has to be retained. Repairs can be a patchwork. Often on principal floors the plaster is on laths fixed to studwork leaving a gap to the external wall to enhance thermal performance, so there is no access to the external wall.
  • New work – new partitions, plaster board, floor and ceiling finishes, should be taped using a special vapour permeable tape to ensure vapour is not trapped in the building leading to rot and decay.
  • Service risers - A major risk to achieving desired levels of air tightness occurs at links at each floor to service risers and also where service risers exit through the roof. All these services will need to be sealed to their risers with tape.
  • Windows – are an opportunity for air to leak out of the building. Sash windows by their nature do not make an airtight seal. Secondary glazing is specified anyway for acoustic attenuation, and the secondary glazing frames are taped to the surrounding wall surfaces prior to the finishes being applied to achieve an enhanced level of air tightness. This all accords more closely with the requirements of the local planning authority who are anxious that the heritage value of the buildings is not eroded by the upgrade.
  • Roofs – the existing roofs often need substantial work to achieve appropriate levels of air tightness. This will generally involve inserting a membrane that is taped into the construction and which must not be punctured during the construction process.

  • Testing - To assess how successfully the air tightness is being carried out it is necessary to carry out air testing at key moments during the programme. This will help keep a check on build quality but also help inform the contractor on procedures in what is often a steep learning curve.
  • Programme sequencing - All of these impact on the conventional way the contractor orders his trades and work sequences. This therefore requires a strategic reappraisal by the project team to understand where out of sequence working will be necessary and how this will impact of the overall programme. We have discovered that initial estimates are that 8-10% can be added to the build programme, and a consequential uplift in costs might well follow.
  • Historic state of buildings - A consideration is that because these buildings have been effectively full of holes for the last 200 years they are naturally ventilated which has the benefit of controlling build up of moisture. Their loose fit design inherently looks after itself. Under the air tightness improvements we are sealing the building up, so introduce issues of stale air and its impact on occupants. As part of the hotels spec the bathroom extracts are extracting air continuously. Fresh is being introduced by ducting to each of the bedrooms. There are opportunities for heating the fresh air with the stale air thus conserving energy.

The aspiration – is to achieve as good a possible air leakage rate as is reasonably achievable given the constraints of the listed buildings and the budgets that were set for the project.

This is a work in progress and as yet we do not know how good the results will be. A standard refurbishment without any air tightness improvements tends to achieve a figure around 10 ie close to the building regs requirement. With air tightness improvements included it is hoped we will achieve a figure below 5, or perhaps nearer 3. Either way this is a prototype project for The Portman Estate and will help inform approaches on a range of future projects across the Estate.