I keep finding myself banging on about the ‘fabric-first’ approach and how it could well be the saviour of all humanity. I am always prone to a bit of over exaggeration, but super insulation, airtightness, mechanical ventilation heat recovery (MVHR) systems and good windows are the key to low-energy sustainable housing. The insulation thing is fairly self-explanatory — but what is all this about airtightness?
The logic is that once you have a super-insulated building (U values below 0.15kWh/m²) the main sources of heat loss are draughts and ventilating the house. Put simply, the draughts can be removed by creating a completely airtight layer in the external structure that is attached to airtight windows and doors — although it is a lot harder than it sounds. Once that has been achieved, you need to ventilate the house as people have a habit of breathing — and this is where MVHR comes in. This stops the loss of precious heat through open windows and trickle vents.
The savings are not just due to reduced heat loss. If you have a very airtight house with triple-glazed windows, you will have no internal draughts or convection currents — in other words, very still air. With still air, 19°C is the perfect internal temperature for most people. If there are convection currents or draughts, then 21°C is required. Remember the Government advert telling us to turn down the thermostat by 1°C and save lots of energy? Clearly, 2°C lower for the same comfort level has a big impact on energy consumption over the lifetime of a house. Combined with triple glazing, good airtightness is also fantastic at reducing external noise.
When you start to understand the significant benefits of airtightness, it is disheartening to realise that over the last 40 years we have actually gone backwards in this area. Dot and dab plasterboard is the real problem because brick or block with dot and dab is about as airtight as Shredded Wheat. So while the double-glazed windows aren’t letting in a breeze, the walls are leaking precious heat. New builds are very hard to retrofit for airtightness, whereas the walls in older properties with plaster on stone or brick are – bar the cracks – airtight.
Airtight construction has many benefits. It can help you reach the dream of a comfortable, quiet home with low heating bills — but it is not easy to achieve. I think it will be one of the biggest challenges facing builders and designers over the next few years, but it is absolutely critical for low-energy housing.
Here are my dos and don’ts for achieving an airtight house:
1. Airtightness doesn’t just happen — you need a very clear airtightness plan early in the design process, and you need to stick to it.
2. If you are serious about airtightness, work with builders and architects who know about it, or who at least want to find out.
3. If you are retrofitting your home, get an airtightness test before you start any work so that you understand where your leaks are right from the beginning.
4. Never rely on just one airtightness test — you will need at least two, possibly more. Make sure you get one before you cover up all your membranes and tapes prior to skimming plasterboard, so you can fix any leaks without undoing too much work.
5. While wet plaster on block/brick is airtight, the majority of airtightness is achieved using membranes and tapes. Make sure you use the right ones; they are expensive, but cheaper tapes go hard and lose stick in a matter of years, leaving your home leakier than a sieve. Mastic and silicon also go hard and crack so don’t rely on them. Non-hardening adhesives and tapes are available and, while expensive, are far cheaper than larger heating bills, potential rot and moisture damage.
6. Never forget to properly detail the junction between the walls and floors. I did this once, and it was a painful lesson.
7. Make sure everyone working on the site understands your airtightness strategy. A happy contractor with a drill can do a lot of damage in a few minutes.
8. Older building materials are vapour permeable: they allow moisture to move in and out. Any refurbishment or airtightness layer must allow these materials to continue to breathe — seal them up at your peril.
9. Woodburning stoves need direct (tertiary) air supplies from the outside in airtight buildings. Forget about open fires.
10. It is worth noting that while the Building Regulations set a maximum air leakage of 10m3/hr/m²@50pascals (pa), MVHR only becomes efficient at below 1.5m³/hr/m²@50pa — PassivHaus standards set a maximum of 0.6m³/hr/m²@50pa!