F: Air tightness

I think air tightness is the also ran of energy efficiency when actually it should have at least equal standing to insulation measures, and this observation is particularly relevant to older homes. Therefore it would be good if you read this page in conjunction with Section E

Thanks also to Ecological Building Systems who supply the pro clima products used here for tweeting this page and sharing what's going on. You can follow them on Twitter for case studies etc (username @ecologicalbuild) or visit www.ecologicalbuildingsystems.com

85% lower air leakage!
Air permeability test result. Scroll down to find out more!


In Section E we explored the idea of environmental comfort and the role insulation has to play in achieving it in an older home, the lower-energy-terraced-house being used as a case study. Here, we consider air tightness, which is best explained visually. An airtight house might be visualised as a sealed box impermeable to air entering or leaving unless the occupiers intend it (see Section G: Ventilation and below); whereas UK homes (old and relatively new) are "notoriously leaky", (RIBA, undated). Warm (heated) air will find it's way out and be replaced by draughts of colder air from outside. This in effect is expensive warm air (heated at occupier expense) being constantly cooled by incoming air. In particular warm air rises sucking in cold air elsewhere (Hall, K., et al, 2009) and can end up in the loft space for example. Meanwhile home owners up and down the land are sitting feeling smug with their energy company funded loft insulation, while warm air escapes through gaps in and around it having first bypassed ceiling cracks or holes for lighting cables or commonly the ill fitting and unsealed/ uninsulated loft hatch. The list of escape routes is almost to infinity, (well to 16 according to the Energy Saving Trust (2010a) including through gaps around floors, around window and door openings, chimneys, pipe and cable entry/ exit points etc). According to Greenspec (2010) who list 36 escape routes, up to 20% of heat energy is lost from older houses in this way. Of even more interest perhaps is the proportion of heat energy lost which results from air leakage. As insulation standards have improved air leakage heat losses have risen from c.30% to perhaps 66% or more today, reducing the effectiveness of insulation meausres already taken, (Hall, K., et al, 2009). I rest my case, (see statement at the top of this page).

So, air leakage can be defined as air movement through a buildings fabric which is unplanned and uncontrolled, (Energy Saving Trust; 1997, reprinted 2006). It can be quantified by measuring the rate of air leakage in/ out of the building fabric, expressed as m3/h/m2, (ibid)*. This measure is known as air permeability and is currently the focus of considerable effort in new homes (albeit with mixed results, (RIBA, undated) and increasingly in the refurbishment of older homes. The development of the PassivHaus refurbishment standard due to be launched in the summer or 2011 (Taylor, M., presentation at EcoBuild, March 2011) is attracting attention to the subject; and the Energy Saving Trust have issued advice on the subject for almost a decade, (EST, 2004, reprinted 2005). 

The reduction in internal temperature that results from air leakage means that the heating system has to work harder than might otherwise be necessary, costing the occupier money at the same time as making them feel less comfortable. It's reasonable to assume that heating systems (boilers and radiators) are oversized and cost more to buy therefore because they are intended to heat a kitchen colander. There are also structural risks in draughty buildings because leaking air can be damp, and damp air might damage the building fabric and insulation materials, (EST; 1997, reprinted 2006). It's a no-brainer therefore to seal up wherever possible any gaps, cracks and holes in the building fabric. However, it must be remembered that if you were to take this idea to the extreme then there might be insufficient fresh air for occupiers to breath with subsequent consequences to health! It is important, nay essential to plan for fresh air! The way that this has typically been done in the past is to allow the gaps, cracks and holes to do it, combined with opening of doors and windows, and operation of extraction fans. In an airtight house the eradication of UNplanned and UNcontrolled movement creates a requirement for planned and controlled air to be introduced and removed from the building. This is known as ventilation and you can read more about it and how it will be managed in the lower-energy-terraced-house in Section G. How air leakage is tackled in the house is explored below.


Just about nothing in the design and construction of the lower-energy-terraced-house can be considered useful in a modern context in minimising or limiting heat loss to maintain thermal comfort and minimise fuel bills; and consequently minimise carbon emissions resulting from energy use. The house is a century old Edwardian mid terrace, designed over 2 storeys with a 2 storey outrigger to the rear, and when bought was super leaky, to the extent of having holes in the suspended ground floor large enough to fall through (and I did). Measuring the air tightness at time of purchase was thereby almost pointless but when complete it is planned to measure a similar property in 'typical' condition for it's type. Sadly the owner doesn't know this yet! the results however can be compared with the house as refurbished and the benchmarks below used as a guide. Meanwhile as a guide consider the Sheffield Eco Terrrace (joint Energy Saving Trust/ Sheffield City Council project which measured pre-refurb air permeability at 21.7m3/h/m2 (Energy Saving Trust, 2010b) and compare with the new build standards below.

 House typeAccepted practice (m3/h/m2) Good practice (m3/h/m2) Best practice (m3/h/m2)Reference
New homes (typically referring to traditional build brick and block method) Less than 10.0 5.0 or less Minimum requirement laid down in the Building Regulations Approved Document Part L (2006, updated 2010), (Evans, H.M.A., 2010), and based upon CIBSE TM23 2000 (Hall, K., et al, 2009).
All homes recommended standard according to CIBSE technical memorandum 23   8.0 for natural ventilation/ 4.0 for mechanical ventilationRIBA Principles of Low Carbon DDesign and Refurbishment (undated)
New homes with natural or mechanical ventilation 4.0 - 7.0 (i) Less than 3.0 (ii) (i) Energy Saving Trust, (1997, reprinted 2006)/ (ii) Ecological Building Systems, 2010
New homes built to the AECB CarbonLite standard3.0 or less with mechanical extract ventilation/ 1.5 or less with mechanical heat recovery ventilation (AECB Silver standard) 0.75 (AECB Passivhaus standard for UK) AECB CarbonLite Programme, (2011)
New homes built to the PassivHaus standard
  0.6 or lessEcological Building Systems, 2010

Other than the CIBSE standard expressed above specific standards for refurbishment of UK residential homes are hard to come by, but when complete the lower-energy-terraced-house will get tested*. I think if I got to 7.0 I'd be satisfied, and to 5.0 very pleased: 
  • Perhaps the best example is to compare with other projects, for which air tightness figures are not that easy to find. For starters until I come up with more the Sheffield Eco Terrace mentioned above achieved 6.7m3/h/m2 (Energy Saving Trust, 2010b) which I think was very decent taken taken in the light of the table above. 
  • The long awaited Building Research Establishment replacement to the Ecohomes Standard called "BREEAM Domestic Refurbishment" may set new standards when launched in the summer of 2011.
  • But most scary of all my friends at Ecological Building Supplies (Ireland and UK distributors of the air tightness system being used at the lower-energy-terraced-house) have thrown down the gauntlet and suggested 3.0 m3/h/m2 might be achieved. If that happens I'll swim the Irish Sea with the results to show them....
* The measurement of air permeability is undertaken with an air leakage test which involves pressurising the building using a large fan, usually placed within a large seal placed over the open main entrance door. Air is moved in/ out of the building through the fan and the flow rate (m3/h) divided by the internal surface area (m2) at a maintained pressure difference between inside and outside, 50 Pascals.


Air tightness in refurb is a labour of love (nee obsession). It is difficult whether gutting an empty property or whilst the property is occupied, perhaps more difficult in the case of the latter. There is plenty of advice for the latter known commonly as sealing (eg Nicholls, R., et al , 2009) which tend to involve silicone and sealant guns while on your knees around skirting boards and behind the washing machine! But here I must concentrate on the case study. The lower-energy-terraced-house was stripped back to a shell. Even much of the plaster was removed, and of course the ground floor was replaced, and first floor ceilings will be. The property was effectively uninhabitable at the start of the project.

The key issue was to set a perimeter fence. The fence would be the airtightness barrier through which no air was to escape (or break in) unless planned to do so for occupant ventilation. This was established as the suspended ground floor perimeter and party walls and first floor ceiling level. Two methods are being used:
  1. Rendering of brick walls (party and internal dividing) to seal up gaps, cracks and holes. 
  2. Application of air tightness/ vapour membranes and/ or tapes to all perimeter surfaces 
As for specifics:

 Measure undertaken Illustration Why? What more might have been done?
Rendering of brick wall surfaces. Rendered surfaces will be plastered except where cold bridging is a concern where insulation backed plasterboard will be fixed over.. 

To seal up gaps, cracks and holes. Rendering or parging of external facing walls would have been an extra barrier layer to air, and would have made insulation measures easier. Some areas of brickwork were very hard to reach at the junction between ground and first floor, and along staircase join to party wall. Removing more obstacles would have been expensive but improved the result. As it was first floor timbers were cut away. Void between stairs and wall sealed with foam.
New suspended ground floor was taped at the perimeter and across all board joints prior to the insulation slab and top floor deck being installed. Grommets were applied at pipe entry points. 
To prevent cold damp air infiltration from below the suspended floor from entering the living space.Some might argue a concrete floor might have been better but was not practical in this case for various reasons, not only the embodied energy inappropriate to a low energy experiment, but because of the depth (14m) to the earth below. Rather than taping a membrane could have been laid.

And yes, I got the detailing wrong on the intersection of the different tapes in this picture but I'm man enough to show it anyway!
Membrane applied to EcoStud insulation system on external facing walls 
 Ongoing, detail to follow 
Membrane applied to first floor ceiling joists

 Ongoing, detail to follow 

A word (or more) on chimney stacks...

I'll start with a confession. I should have thought much harder much sooner about the chimney stacks and the common held wisdom that stacks should not only be ventilated passively (ie from the bottom of each flue but with heated 'house' air that can keep the stack from moisture and the build up of silicate deposits which might otherwise damage the chimney structure and inward (room) facing surfaces. I considered venting from underneath the suspended ground floor, and infact I very recently located some Energy Saving Trust advice to that effect somewhere and would reference it if I could remember where! But surely the chimney would then be vented with moist and oft damp air, albeit fresh and moving? Anyway, too late now, and so although seal-able for the purposes of the air tightness test that will be undertaken post completion of the refurb I'm still pretty chewed off about it. Not that the budget constraints would necessarily have allowed for the removal of stacks but in pursuit of air tightness old chimneys, prevalent in leaky old homes are a challenge. To cap it all I met a PassviHaus architect at EcoBuild who simply sealed chimney stacks (no ventilation) to help achieve his air tightness objectives. 

It is important to consider all these actions in relation to air tightness, see Section F.


One day when time was short I had some labouring help to move a whole load of plasterboard into the house and up the stairs. Admittedly I didn't brief the labourer very well. Perhaps this is typical however of the sort of thing that happens when casual labour is used in on-site building and refurbishment? As we seek higher building standards in pursuit of more efficient homes we will be trying to apply factory (off-site) type controls to a on-site construction situations. Pause for thought...

I've also been suing a hygrometer to review moisture levels post commencement of plastering/ skimming work. The airtightness membrane has a repetitive message 'plastered' all over it (pardon the pun) warning of the prospects of heightened drying out times as a consequence of great air tightness. So really doors and windows all have to be open as much of the time as possible. This of course in not always practical when security is taken into account also. More on this and the observations of moisture levels when the long awaited Ventilation Section G is launched.

Air tightness/ permability test result and commentary

An air permeability test of the completed structural envelope has produced a result of 1.7 m3/h-1/m-2 @ 50Pa, (Table 1). This compares well with the benchmark property and best practice for retrofit (excluding Passivhaus Retrofit) varying between 3.0 and 4.0 m3/h-1/m-2 (RIBA, 2009). Air tightness in the case of the retrofit project reduces forecast annual energy demand by 0.8 MWh (6.8%) and carbon emissions by 0.2 tonnes (5.0%) compared to the notion of undertaking all of the insulation measures without improving air tightness. Further testing was also conducted to identify air permeability for the dwelling in occupation, meaning account was taken of permanent openings used for chimney ventilation, and to review other shortcomings in the following areas:

External facing walls/ openings 

This was in the form of a membrane stapled to the IWIS studwork prior to installation of plasterboard. At perimeter edges the membrane was fixed with a flexible airtightness adhesive. No electrical, gas (except the existing mains supply) or water service entry points were located in the perimeter walls to minimise the potential for air leakage. Joins in the membrane were overlapped and sealed with tape. The membrane was run continuously between the ground and first floor and sealed around first floor joists. Pro Clima membrane was extended into window and door reveals and taped to the edges. No weaknesses were identified in air permeability testing indicating that the system is highly effective if carefully installed. Concerns remain re. the longevity of membranes and tapes given the potential for later property modifications to result in their damage. Shortcomings were identified in the specification of window openings, all of which were modified prior to air permeability testing to enhance the draught seal around openers. This illustrates well the importance of air tight sealing of insulation to prevent heat bypassing thermal barriers rendering them less effective.

Ground floor

Membrane on the IWIS was taped to the upper floor deck, and joins in the deck between flooring boards taped using Pro Clima air tightness tapes. Pipe entry points were sealed with flexible grommets, which however failed when pipes were subsequently handled by trades and dislodged. This resulted from the use of flexible piping whereas better practice would involve using copper pipe fixed in position so that the grommet seal remains undisturbed. It is also important to ensure good connectivity between floor deck and walls to limit potential air leakage paths. Weaknesses were identified and sealed on two internal wall perimeter points where cables entered the envelope from the sub-floor. The handling of pipes and cables requires particular care.

Loft space

Due to the potential for air to escape between plasterboard sheets, and for moisture to find its way to the loft space, the underside of first floor ceiling joists had Pro Clima airtightness membrane applied prior to over-boarding. Cable entry points were sealed with both grommets and air tightness sealant which proved effective. However the loft hatch which was designed to comply with BS 9250 (code of practice for air tightness) proved ineffective and was responsible for 6% of air flow. This was only reduced to 3% with on-site modification, suggesting that the hatch should be replaced.

Chimney breasts

I confessed further up this page over the matter of chimney breasts which I retained with four open fire places sealed and vented. The vents are sealed for air tightness testing. The impact of their inclusion to assess their impact on air flow is to increase the whole house infiltration/ exfiltration by 17%. Retained chimney breasts are completely counter intuitive to the concept of air tightness, providing also a thermal bridge to colder air within the flue. Given time over the plan would be to remove them completely making both a contribution to thermal performance through air tightness and adding 2.38m2 to habitable floor space, (happily offsetting  three quarters of space given over to the IWIS).

Further reading

There's lots of stuff, try these and also some attachments below:

  • Hall, K., et al, (2009). Green Building Bible, Volume 1.  Green Building Press, Llandysul.
  • Nicholls, R., et al, (2009). Green Building Bible, Volume 2.  Green Building Press, Llandysul.
  • Greenspec, (2011). Refurb-airtightness. Available at: http://www.greenspec.co.uk/refurb-airtightness.php
  • AECB, (2011). CarbonLite Programme. Available at: http://www.aecb.net/
  • Evans, H.M.A., (2010). Guide to the Building Regulations 2011 Edition. RIBA Publishing, London.
  • Ecological Building Systems, (2010). pro clima Intelligent Airtight and Windtight Building Systems product portfolio. This and more available at: http://www.ecologicalbuildingsystems.com/
  • Energy Saving Trust, (1997, reprinted 2006). Improving air tightness in dwellings. Document GPG224 - THIS DOCUMENT CAN BE DOWNLOADED HERE
  • Energy Saving Trust, (2004, reprinted 2007). Energy Efficient Refurbishment of Existing Housing. Document CE83 - THIS DOCUMENT CAN BE DOWNLOADED HERE
  • Energy Saving Trust, (2010a). Sustainable refurbishment. Document CE309 - THIS DOCUMENT CAN BE DOWNLOADED HERE
  • Energy Saving Trust, (2010b). Sheffield Eco Terrace A Refurbishment Case Study. Document CE322 - THIS DOCUMENT CAN BE DOWNLOADED HERE
  • Royal Institute of British Architects, (undated). Principles of Low Carbon Design and Refurbishment. RIBA, London (ISBN 978 0 9561064 0 7)
  • Harland, E., 1993, (reprinted 2004). Eco-Renovation: the Ecological Home Improvement Guide. Green Books, Totnes.
Andrew Gill,
15 Apr 2011, 16:04
Andrew Gill,
10 Mar 2011, 18:18
Andrew Gill,
10 Mar 2011, 18:28