he legal requirement to consider airtightness in building designs has been around since the 2006 changes to UK Building Regulations. This is the same for every version of the regulations that apply to all regions of the UK, whether it be England, Scotland, Wales or Northern Ireland.
The measurement for calculating air loss out of, or ingress into, a building is measured in cubic metres (m³), per hour (h), per square metre of building envelope (m²) and at a differential pressure of 50 Pascals (Pa) between internal and external air pressure. The backstop maximum leakage is 10, equivalent to 10m³ (h.m²) at 50Pa.
When designing to Passivhaus standards, this gets a little more complicated as they refer to airtightness as so many changes of air per hour. This relates more to the cubic capacity of the building or room, rather than the surface area of the envelope of the building. Notwithstanding this, to give some comparison, it is generally agreed that Passivhaus is looking for a goal of 0.6m³ (h.m²) at 50Pa, or less.
Initially, not too much notice was given to the requirement to have a maximum air leakage, as in reality an air loss of 10m³ (h.m²) at 50Pa could not really be called an airtight building, so the designs and detailing did not have to change radically to achieve it.
However, building standards have progressed with the requirement to conserve more and more energy. Although the legal backstop of 10 has not changed the target air leakage for SAP, separate target values set by the house-builders themselves have both significantly reduced. This helps to save heat loss out of the building, thus requiring less use of heating systems to maintain a comfortable temperature within the building, and in turn leading to greater energy efficiency and lowering CO2 emissions.
Build tight, vent right
This brings us to the question about air quality within the building. The more airtight we make our buildings the less fresh air we get into them through random unwanted gaps within the building fabric. Such problems can cause other issues for people with respiratory problems or young children, so proper ventilation is crucial.
‘Build tight, vent right’ has been a popular mantra relating to building design, particularly to those concerned with the ‘fabric first’ strategy, rather than just throwing ‘eco bling’ at a building design to achieve the relevant Standard Assessment Procedure credentials. It is often argued that if by reducing air loss, you then need to build in designed ventilation, so why bother in the first place?
The answer, apart from the fact that legislation must be complied with, can be summed up in one word – control. It is the difference between controlled air loss, or ventilation, and uncontrolled air loss and draughts, which is the nub of the whole issue.
How is ‘Build tight, vent right’ achieved? With regards to ventilation, this generally falls into two major camps – natural ventilation and mechanical ventilation. If you achieve an air loss of 3m³ (h.m²) at 50Pa or less, then it is a necessity to use mechanical ventilation. With an air loss of more than 3m³ (h.m²) it is generally considered that well-designed natural ventilation is sufficient for most domestic dwellings.
For unwanted air loss, it is generally acknowledged that the air loss between the junctions of windows and doors to walls, walls to floors and walls to ceilings accounts for more than 50% of the air loss in a house. Other common factors of air loss relate to the fabric of the building itself and the various holes knocked through the fabric for an assortment of reasons such as drainage, heating flues, electrical cables etc.
There are many products on the market claiming to be airtight seals. But, the key is to use the correct products in the correct environment and the extent to which they degrade over time. In addition, if one of the major requirements of airtightness is to reduce heat loss out of the building, then thermal resistance to these air leak sources surely needs to be considered as well.
Long-term solutions
Certainly just being airtight will reduce or stop heat loss by convection, but what about heat loss by conduction – often referred to as non-repeating thermal bridges?
The industry is starting to understand that airtightness is only half the story for the sealants. This is why the thermal insulation of joints between windows, doors and walls is now included in the latest SAP calculations, where previously it had simply been ignored.
When choosing a suitable airtight seal, the full requirements necessary to create a long-term solution must be considered. If it is simply to seal the edges or overlaps of an internal airtight membrane, then movement and thermal insulation are unlikely to be major factors to consider as any number of various stick-on adhesive strip tapes are likely to be suitable. As there are technical differences between them, some specialist knowledge or advice is always helpful.
However, when sealing an actual construction joint between similar or different materials, it is a different story. An illustrative example is the movement between joints created by the junction of different construction materials, such as windows to walls or walls to roof etc. Even joints between the same materials can move, such as brick or concrete expansion joints. The initial drying out of the building must also be considered.
In this respect, the differential movement between timber frame buildings and the external masonry facades is well-researched and documented. But the different coefficient of expansion factors of different materials is less well-known.
When selecting a suitable airtight seal for this type of application, installers must consider several factors to assess if it is correct for the type of joint to be sealed, based on:
- The need to accommodate movement.
- Any need to allow for conduction heat loss, to maintain the thermal integrity of the abutting materials.
- The suitability of the active adhesive system for both initial application and long-term use on the substrate.
- The potential for harmful emissions from the sealing materials and adhesives, and their effect on internal air quality must also be assessed.
An airtight seal therefore needs to be more than just an adhesive tape, but we have seen a bewildering range of options on the market. At first glance, they all look like they perform the same or a similar function. Whilst this is true to a certain extent, there are many performance variables meaning some products are more suited than others – in the same way as timber frame construction methods are all not identical or suitable for certain situations.
