Paul Jennings from Aldas explains why airtightness is an important factor to grasp when constructing an energy efficient home. He also shares how forward planning, attention to detail and teamwork are imperative for achieving good airtightness results.
Interview with Paul Jennings
After doing an engineering design degree in the 1980s, Paul worked for an energy charity in London. This charity acquired some air testing equipment because fuel poverty was becoming more of an issue and Paul began testing various council flats and houses for airtightness, with a view to improving the stock.
Historically There Have Been Some Very Airtight Buildings
Paul says that one of the most airtight structures he's tested was part of Gloucester Cathedral. Despite being 600 years old, due to the nature of the construction with lime plaster over stone, it is pretty airtight. Conversely some modern housing – built by developers in the seventies, eighties and nineties – is not very airtight.
Poorly-Built Modern Housing Will Need to Be Demolished
Paul believes that it's much easier to improve the airtightness of a property from the Victorian and Edwardian era than it is to fix poorly-built modern housing. He says that there's too much to do and that we'll end up just knocking them down and starting again.
There Are Two Primary Measures of Airtightness
1. Air permeability is the amount of air which flows through the fabric – the walls, the roof and the floor – of a building per hour at a particular pressure. This pressure is usually 50 pascals (50 Pa), which is equivalent to 5 millimetres of water gauge pressure. This is not a huge pressure and Paul puts it into perspective by saying that you would apply 2000 Pa with your mouth when you drink a milkshake through a straw. Yet it doesn’t hurt you. That is air permeability and it's part of the (UK) building regulations for new builds.
2. Air changes per hour is a volumetric measure of how much air passes into or out of a building per hour at a given pressure (which is also usually 50 Pa). This is most appropriate for conditioned buildings that have heat recovery ventilation, where the warmed or cooled air passes through the ventilation system rather than leaking out around doors, windows, through cracks, etc. So the measure of air changes per hour is what is used for more airtight buildings, such as buildings that meet the Passivhaus standard.
The stringent airtightness target of a Passivhaus is 0.6 air changes per hour whereas UK building regulations require a maximum air permeability of 10. Therefore achieving the Passivhaus criteria is about 16 times more onerous. Also, the closer you get to the lower yield for which you are aiming, the harder it is. Getting airtightness down from 10 to 5 is easy, but getting it from 1 to 0.6 is generally very hard. Obviously this varies according to building design, building type, but above all it varies according to the quality of the builder. Paul stresses that the biggest issue we face with delivering airtightness is how we make our builders do what they need to do and work to these sufficiently high standards.
Good Design is Integral to Achieving an Airtight Building
The first part of the process is to look at the design of the building because there are things that can be done to make it easier to achieve airtightness. For example, in Germany it is common to build upper floors with concrete rather than suspended timber. This is because where there's suspended timber there are voids which air can pass through. Paul talks about doing tests on three storey town houses where the room thermostat on the internal wall of the mid-floor living room is leaking air. That's because when you’re testing it, air is coming up through it because it’s connecting through the voids in the walls and floors. Having solid upper floors would make a significant difference.
Also, warm roof construction (where the insulation follows the line of the roof) would be beneficial for airtightness. Paul says: “A cold roof often has a tricky detail at the eaves where you have to try and maintain airtightness but you also want ventilation into a cold loft space, so trying to have those two things in what is becoming a tight space in that angle in the eaves, often becomes difficult for people and one of the places it goes wrong.”
Most Building Materials Are Pretty Airtight
A lot of materials (including glass, plasterboard and solid timber) are pretty airtight. Blockwork, however, can vary a lot. Paul's seen walls that he can blow through!
“One of the things we say for new build [is] if you have blockwork coming off an external wall into the dwelling you have to make sure it is airtight for at least a metre into the dwelling. So you basically have to plaster, if you are doing a wet plaster system, on the return internal partition walls as well as on the inside of the external wall. Even then we’ve had cases where they are using recycled power station ash, fuel ash, in the blockwork, they can be very very porous and a metre is not enough. We’ve had air running through internal dividing walls by up to four metres, which goes beyond what you are expecting, it goes outside the design and then becomes a problem. So often with this it’s where you have variations, where people move a duct, people change a window size, people discover that blockwork is leaking where it is not, it goes wrong. If you are smart for airtightness you have a programme, because it doesn’t happen by accident.”
There Should be a Minimum Airtightness Standard of 3
The minimum airtightness standard that Paul tends to work to is 3 air changes per hour, which is the AECB Silver Standard, obviously going down to 0.6 for Passivhaus.
Delivering Airtightness is All About Planning
Specifying just the target or even the materials is not enough, it's about the process. Air barrier drawings must be created, showing in a red line what the airtightness is, where it is, with notes specifying the material, the package of works and who is doing it. Then there is the process of going through and checking, which is called a design review, making sure that it all joins up.
Mistakes During Construction Need to be Dealt with, Rather than Hidden
Paul says that everyone makes mistakes but that part of the problem with the [UK] construction industry is mistakes are hidden. On many occasions he's seen something that’s been put in the wrong place, like a hole for a boiler flue, a hole for a heat recovery duct or an extract fan. Instead of owning up to this, acknowledging it and dealing with it in the appropriate way it gets hidden. This can have ramifications for many years to come.
As Soon as You Have Voids, Air Can Go Anywhere
As soon as you have voids, such as a cavity wall, the air can go anywhere. Even if you can't see a hole on the outside of the building, if there is a hole in the inner leaf connecting to the cavity that’s as good as to outside. This is also applicable with old buildings. For example, you may think you have a solid stone wall of half a metre in a church, but actually it's stone on either side with a rubble-filled cavity. A lot of air will pass through those walls. As soon as it's lime plastered on the inside, the problem goes. People do not expect buildings to leak where they do leak.
Buildings Can be Tested for Airtightness with a Blower Door
In order to carry out this test a calibrated small, big or very large trailer fan is mounted in an open doorway or window. There are various adjustable door systems, too. Paul describes how it has varying ranges of how much air it will blow, from small to very large. Normally the process begins with sucking air out of the building. This is called depressurisation. With the building at a lower pressure the atmospheric pressure forces air back through the cracks, the gaps, the leaky dampers, the windows that the draught seals are not quite right, etc. Above 25 Pa a strong draught is created, which is maintained for the duration of the test and then the locations of these draughts can be established.
During Testing There are Numerous Ways to Detect Draughts
Often you can feel draughts with your hand. If you put your eyeball next to a draught it may be even easier to perceive because your eyes are even more sensitive. There are also different sorts of chemical smoke and stage smoke that can be used. Acoustic testing is another option. You can also put anemometers next to where you think the whole is.
Infrared cameras can help find leaks because, especially in the very airtight buildings, it can be the coldness where the air’s leaked that shows you where the leak is rather than feeling the leak itself. After ten minutes of cold air it starts to show up on a thermographic camera.
Builders Often Turn Air Tests into Leak Location Exercises
Under (UK) building regulations, construction testing is required for houses, albeit sample testing. However, Paul says: “One of the current issues is that the building regulations say if you do a test and it fails you not only have to fix it and retest it you have to test another one of the same type as well – the sample size goes up. Yet there have been no failures or almost no failures reported, and why is that? Well we know why that is because any builder, sensible builder, says actually let’s make this a leak location exercise not a test and we’ll fix it and then you can test it again, which is legal but is definitely against the spirit and we definitely, I think as a nation, are building in problems for the future with poorly performing houses where we have small sample sizes on large volumes, large numbers of houses and especially where they are timber frame.”
Research is Needed to Discover How Long Airtight Barriers will Last
While the suppliers of materials, especially the better airtightness materials, do accelerated ageing on their products claiming they will perhaps last for 50 years, they are not in a 50 year old building. So nobody really knows how long airtightness barriers can remain and that's why more research and monitoring is necessary. Most things are fairly stable, but it’s the joints, the interfaces and the penetrations that move. Very high quality mastics have a bit of give and allow the building to move, to shrink, to thermally change over time. At the other end of the scale, where builders have used very cheap caulk, cracks can begin to form within weeks of it being applied.
Vapour Open Materials Allow Moisture Through Without Letting Air Through
Vapour open materials will allow moisture through without letting air through. Moisture will wick through materials so will soak into a service, pass through the material and come out the other side. That doesn’t require a hole that will let air through – the building is vapour permeable. In the past they used to be called breathable but that’s a misnomer because it’s not. Recently they have created what they call ‘intelligent materials' which vary according to temperature and pressure in which direction they allow moisture to move, but again they are airtight and nobody should be under the illusion that if they want a vapour permeable building they have to make it airtight.
Extremes in Ventilation Often Occur in Traditionally-Built Buildings
Paul says at the moment most UK houses have too much ventilation when the wind blows and not enough ventilation if the wind doesn’t blow. That means we have the worst of both worlds. However, as we move to more airtight constructions or very airtight with Passivhaus, there must be ventilation. People worry about this but we don’t need very much movement of air to give us the oxygen we need to breath. It is reckoned to be about thirty litres per second per person for moisture control and for breathing, depending on exercise levels, two three or four. It’s an order of magnitude difference. Paul jokes: “You will be getting black mould growing out of your ears before you suffocate in an airtight house!”
Indoor Air Quality is Another Part of Airtightness
When considering airtightness it's important to remember there is less dilution of pollutants happening and that means avoiding paints that are going to give off volatile organics, for example. So, be careful about specification of paints, furniture and things like new computers that give off lots of toxic materials. All of these things become more of an issue in an airtight house, but of course the big one is moisture.
Choose Contractors with a Good Track Record of Achieving Airtightness
Paul recommends that when looking for contractors, you should find out what they've done elsewhere and what air results they have achieved. If you hire a bog standard builder without any knowledge of airtightness to carry out the work, then be aware that you will be paying for their learning curve. It does depend of the level of airtightness you are trying to achieve.
We Need to be Delivering Better Quality Airtightness, Particularly in Refurbishment
Paul reckons that when it comes to new build projects potentially we are not bad in terms of airtightness. In refurbishment he says we’ve [UK] got to learn a lot. The focus should be on improving the skills base and doing much more testing of refurbishment projects so that people understand the issues and can see the difference before and after. This is not only for energy efficiency but if we want buildings to last. Sustainable construction means construction that is not rotting from interstitial condensation 5,10 or 15 years down the line.
Take a Fabric First and Avoid the Eco Bling
While there is a place for solar panels and the like, our priority should be in getting the building fabric right first. Paul believes a lot of the construction industry should be taken to task for the poor quality they deliver in refurbishment.
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