First a bit of scene-setting on why airtightness matters
Airtight buildings are more energy-efficient, they save money:
Achieving a reasonable level of airtightness is important for the energy efficiency of dwellings and the comfort of occupants. The benefits of improved insulation levels and more energy-efficient heating systems are lost if warm air can leak out of a building and cold air can leak in energy, leaking energy is leaking money.
It’s the law:
A mandatory requirement for airtightness has been set by the Building Regulations (for England and Wales, and Northern Ireland) to ensure that reasonable standards are being achieved, and it is compulsory to subject samples of newly built dwellings to a pressure test in order to measure and confirm their airtightness on completion.
Current Building Regulations require that all new dwellings achieve an air leakage of less than 10m3/hm2. That is the air leakage rate per hour per square meter of envelope area.
The m3/hm2 figure is the headline ‘pass or fail’ number produced by the air testing engineer and demonstrates how much air (typically by depressurising) is being sucked into the building through ‘leakage’ when the fan is operating at 50 Pascal’s.
Uncontrolled leakage is the key:
The significant point to note is that this test measures uncontrolled air leakage. It is not concerned with trickle vents, extract fans or ventilation systems. Controlled ventilation sources will simply be taped up or sealed prior to testing. Therefore air permeability testing is looking for gaps and cracks in the fabric of the building. The most common ones occur in the same well-known locations in all builds it is these identified 'weak spots' that a good airtight design resolves. Most of the big issues are around connections, penetrations and junctions areas firmly in the scope of on-site trades.
Comfort levels increase:
Ventilation specialists will generally quote a ‘maximum’ airtightness of 4-5m3/hm2 as a healthy rate for a naturally ventilated house. That is, ventilated only with extract fans, trickle vents and windows. Anything tighter and some form of forced ventilation will be required, i.e. a mechanical ventilation system, additionally some mechanical vent systems have a heat recovery function (MVHR) which extracts (recycles) heat from exiting air to warm the incoming fresh air and recycles this would be the norm for high levels of airtightness such as Passive certified buildings.
It’s a team thing:
One overriding aspect is clear if everyone delivering the project ‘buys into’ the airtight theme, with good practices from the start, problems can be avoided later on: by problems we mean failing the airtightness test.
Mistakes cost money, sometimes a great deal of money:
Don’t forget that a new build that fails an airtightness test immediately becomes a refurb project and retrospectively correcting problems is more complicated and more expensive than getting it right the first time.
Avoiding problems involves all the trades:
Airtightness is a holistic process, it requires the linking together of a range of trades over different stages of the build, this begins with the architect’s drawings which are relied upon by all the participants to complete their allotted tasks. Ensuring that a secure airtight layer is not compromised on-site by a following trade is key. If all trades are aware of what is aimed for and understand the most common areas for failure they are less likely to compromise the airtight layer even inadvertently, it can also help identify weak spots or problems as they arise.
All too often trades are not given enough detail about airtightness planning for the build they are working on, they need to know, also watch out for ad-hoc trades or specialists who are not part of the resident team whose work will involve penetrating the building shell; the satellite installer, rainwater harvesting installer, the underfloor heating fitters, the sun pipe installer or the telecoms engineer, they also need to be made aware of what’s going on and be equipped to maintain the airtight layer.
What to watch
Below is a brief synopsis of things to do and to watch that are commonly identified by airtightness testers as elements that (if ignored) contribute to a building failing an airtightness test, applying the holistic approach, this is relevant to everyone on site.
- Design in airtightness ‘have a plan’
- “Set your target before you build, ensure all the trades know that this is going to be an airtight build.
- Clearly define the airtight layer on all drawings, design out problems from the start.
- Actively manage the plan to coordinate between all consultants and trades”.
- Airtightness involves everyone on site, and we mean everyone
- Delivering airtightness is a team game delivered to a design shown on a plan, no one should be allowed to work on the building unless they are up to speed on the airtightness ambitions and plan.
Check and check again
Have an inspection plan during construction – ensure that the airtight layer is not compromised by shoddy workmanship, possibly the most common cause of failure. Failures found during testing can lead to expensive remedial work.
Beware of dry lining
Dot and dab plasterboard can lead to problems, it is NOT airtight. The supporting block-work provides a corridor or path in the gaps between the dots for air to move critically always seal ceiling to wall junctions and behind skirting. Directly applied plaster to block work over 5mm thick is considered airtight, but beware of potential movement or shrinkage cracks, a belt and braces approach is to seal these cracks before plastering
Seal structural connections
Where elements connect there is always the potential for leakage, floor/wall, wall/ ceiling, other potential problem areas to ensure are sealed are suspended floors, joists between floors and ground floor screeds and slabs. Areas that require proper detailing and careful application are floor joists and roof wall connections. Remember' all the structural elements will be built over so rectifying mistakes later will be costly and disruptive, the motto is "do it once, do it right"
Seal waste and supply pipework
Ensure all waste and supply pipework is sealed where it penetrates walls and floors. Do not use expanding foam – it shrinks and breaks the seal very quickly. Holes for pipes and penetrations are often cut too large, make sure that a tight seal is possible around the penetration. There are plenty of 'designed' solutions to make these penetrations airtight, make sure that spray foam stays off-site and a suitable proven option is easily available - see list below
Seal Windows and doors
Pretty much all windows and doors could be fitted better, they are a common cause of failure. Seal all jambs, sills and lintels. Specialist sealing tapes can be over plastered or boarded over so the functional airtight layer remains hidden. Additionally, it is normally recommended that tapes are also on the exterior to 'windproof' the door or window.
Heating pipes manifolds
Manifolds from underfloor systems, electric conduits and radiator pipes are often not sealed into the floor screed – this is then covered up with flooring or carpet. The picture shows pipes sealed with Blowerproof Liquid Brush a paint-on airtight membrane to guarantee an airtight seal. Also many radiator pipes penetrate the wall behind a radiator which again is very hard to spot unless you know about it.
Don’t forget hidden areas, eaves cupboards, loft hatches and bath panels
Hidden penetrations are a common cause of failure they need sealing, watch out for air leakage through eaves cupboard doors and loft hatches especially in vaulted roofs or ‘room in a roof’ .
Seal around light fittings switches and sockets
Don’t ignore these they add up, make sure that you seal holes around light fittings, downlights, pull cords, switches and sockets.
Seal around services, water, waste gas etc.
All services enter the house from outside so all WILL penetrate the airtight layer, make sure they are sealed. Water, drainage, soil pipes, gas pipes, boiler flues and electricity cables are typically very leaky – watch out for meter boxes and around consumer units.
Test during the build
Find out if you have a problem during the build then you can fix before moving on, leaks can be spotted by using, anemometers or smoke detectors, even very small leaks can be detected with the back of your hand, whatever you use to identify the leaks this is a wise and sensible precaution.
Always keep a selection of products that can solve problems on-site. A small selection of tapes, sealants and membranes kept on sire will save costly delays and remove any excuse for not solving a problem using specifically designed materials. Problems WILL occur if the wrong product is used for the job.
Products that should never be used to try and fix leaks are
- PU Spray foam – unless is specifically designed as an airtight non-shrink foam, you’ll know if you have suitable foam because of the price!! Around 3 to 4 times standard foams and you will have to buy from a specialist supplier.
- Silicone sealant – it shrinks, it’s NOT airtight
- Decorators caulk and fillers – they are NOT airtight
- Acrylic fillers are NOT airtight even if they are called non-shrink
- Grab adhesives – they are NOT airtight
- PU foam strip – they are NOT airtight
- General construction tapes – they do not last and will fail due to the glue and fabric, they are not designed to be airtight.
- Acrylic putty is NOT airtight
- Multi-purpose fillers (premixed, cartridge or tub) - they are NOT airtight
- Cement-based compounds are NOT airtight unless sealed over
Get out of jail FREE- keep these on-site for leak sealing
- Brush applied liquid membrane: Blowerproof 5kg tub and 300g cartridge super easy and perfect for pipes penetrations, connections and difficult to treat areas, does not need a primer
- General-purpose airtight sealing tape Tescon Vana 60mm, used for sealing boards, membranes and rigid insulation boards and the 150mm which is perfect for making pre-formed corners
- Plaster in airtight tape Contega Solido SL 80mm, around windows and doors, connections and joints wherever a plaster finish is used
- Window and door sealing tape Tescon Profil, does what it says
- Adhesive - Orcon F, glues tapes, membranes and foils to each other and almost anything else, used to seal and stick it’s NOT a sealant.
- Tescon Primer – seals porous surfaces before applying tape
- Pro Clima grommets for pipes and cables– especially for penetrations through membranes and boards
- Blowerproof non shrink gap filling mortar - this is NOT airtight but is used to close gaps before the application of Blowerproof Liquid Brush or a suitable tape or grommet - see above. Non-toxic, super secure and rapid setting this is infinitely superior to any mastic or foam solution.
Most of the list above is made up of common reasons for failure, it will be obvious to all that the way to avoid this is by getting it right first time, for trades and site managers this means attention to detail but at the heart of this is design there is absolutely no substitute for good airtightness design starting on the architect’s drawing table.