Air Tightness

Air-tightness target exceeded
It was a considerable challenge to make the building air-tight, so we were especially pleased with the results of the independent air tightness testing carried out by Ian Hancock (pictured on right)Ian Hancock of HRSS of HRS Services  on December 18th. We exceeded our target which was the ‘best practice’ rating of the professional air-tightness testing body ATTMA. That allows an air permeability rate of 3 cubic metres per square meter per hour at a pressure difference between internal and external of 50 pascals (equating to a wind speed of around 32.5 kilometers/hour) .
For our two air-tightness zones (or envelopes), we achieved 2.25 for the main office area and 2.38 for the flat. If we’d had a single air-tightness zone for the whole building, Ian Hancock said we would have come in even lower without the internal air movements between the two zones he’d found. The smoke sticks he used to identify leaks showed air moving through the dividing wall between the disabled WC in the offices and the flat hallway. At the time of testing, this wall relied on plasterboard barriers, which are not air-tight. Once this wall is skimmed, we can expect an even better result for both zones.
Door fanShown alongside is the door fan which was used to pump air into the building at a differential pressure with the outside of 50 pascals. The rate of outflow of air is then measured and compared with the area and volume of air within the envelope to give a result in cubic metres per square metre.





Problems in making the building air-tight
What were the problems in making it air-tight? Primarily, it was the building’s unusual structure as it has been extended over the years. Its mixture of solid and cavity walls, and duo-pitch, mono-pitch and flat roofs, required a different solution for each surface. Secondly, there were many paths of air leakage to start with. The old lead light windows, together with the doors, junctions between walls and floors and above all the roofs. Plaster wasn’t always continuous, with gaps where first floor floorboards were laid against the external walls, for example.
Air in buildings leaks through solid brick, wall cavities, plasterboard, door and window surrounds, chimneys, junctions between walls and floors, inlets for water, electricity, gas and communications and most of all, through the roof. Historically, ventilation in Britain has relied on these sources of air leakage to provide ventilation, and home energy improvements such as replacements of traditional draughty windows by well fitting and sealed double glazed units, or blocking up chimneys, have often had the unintended consequence of causing condensation and mould.
Air leakage can halve the benefits of insulation, with a bigger effect in super-insulated buildings such as the Eco Offices. Eliminating uncontrolled air leakage is a key part of Passivhaus and other low-energy building designs. Cold draughts reduce occupant comfort, even if room temperatures are high.
How did we go about it?

Firstly, we read the case studies provided by the Energy Savings Trust on refurbishment projects such as Grove Cottage. We read air-tightness guidelines like those published by the Scottish Ecological Design Association . We listened to the experiences of local developers such as Martin Gladwin of the social landlord Plus Dane on the challenges they’d faced in making a solid wall Victorian end terrace in Broxton Street  air tight.
We took advice from Bill Butcher of the Green Building Company ,Bill Butcher who gave us a day’s training at their Huddersfield base on how they built Britain’s first cavity wall Passivhaus in Denby Dale, and supplied us with an excellent training video, now on their website . We learnt from Bill Butcher (shown alongside) and others the importance of carefully planning all penetrations into the building and not leaving it to the different trades making holes as and when they needed them, innocently but needlessly undoing the good work that others had achieved.
We passed on what we’d learnt to our contractor T.Sloyan and Sons, who brought their own experience and creative solutions to the task in hand. In particular, the site manager Terry Gray, the joiners Bob Nelson and Oein O’shea, in combination with Tom Farrell, our clerk of works, came up with solutions where they were required.
Defining the air tightness zones
Our architect Dan Smith decided where the air seal lines should go. All the existing plaster was stripped off. The ground floor was rendered air-tight by the floor screed, the external walls with sometimes three coats of new plaster. The flat roof was given an air-tightness membrane over the existing roof slab and beneath the insulation layer. Pitched roofs forming sloping ceilings within a room were clad with membranes attached to the rafters. The unused spaces within pitched roofs and above flat ceilings were left outside the air-tightness zone and air-tightness membranes were secured to the ceiling joists beneath the loft insulation.
Additional ‘belt-and-braces’ measures to provide backup to the primary air-tightness barriers were the old floor and flat roof slabs, expanding foam in junctions and the use of ribbons of adhesive behind the external insulation boards to prevent airflows between the old pebbledash walls and the insulation.
One or two zones?
Initially, we planned to have a single air tightness zone for the whole building, but once the office ceilings had been taken down to expose the first floor joists, Terry Gray suggested it would be relatively straightforward to install an air-tightness membrane to the joists, and we took the opportunity to do so, helping to separate the commercial and residential uses of the building.
All membranes were carefully glued, stapled and sealed with air-tightness mastics and tapes . The junctions of windows and doors with masonry or timber reveals used a split tape suitable for corners. Internal pipe runs for water, gas and power which passed outside the air-tightness zones or between zones were put through service voids then filled with expanding foam and secured at the air seal junctions with mastics and tapes.
The location of all service penetrations through the external brickwork and insulation, from power and data cables to boiler flues, overflows and waste water pipes, were planned in advance, and marked up for our core driller, Richard English pictured with his drill and  Richard Englishcompressor on right, and in action on left,Richard English in action to make the necessary holes. Air-tightness grommets  were installed on either side of the wall over the pipes or conduits inserted through the holes, before inside plastering and external insulation and rendering. For cables, plastic conduits were used through which cables could be easily inserted and replaced, and then sealed. A grommet sealing an 20mm diameter conduit ready for an external light is shown below, as is a 110mm ones for the soil pipe.

Soil-pipe grommet

Cable conduit grommet

Where external lights, alarm sounders and other powered equipment were used at the end of a cable passing through, plywood pattresses were later fastened around the pipes during external insulation, to hold the weight. 

So far, only a single mistake has been made in correctly placing the external holes, in this case the flue from the gas combination boiler in the flat. The measured height of the flue from the floor failed to take account of the depth of the Hush Panel  floating floor used in the flat, but that was solved by sacrificing one of the two window panes and constructing an insulated and air-tight box to fit snugly around the flue pipe.
Ensuring acceptable levels of air-tightness requires a lot of planning and considerable attention to detail in construction. It is obviously much easier to achieve in new-build than in a complicated retrofit. But hopefully, some of the Ullet Road Eco Office lessons will be picked up in the low carbon retrofits to come, just as we learnt from others.