Peter Thompson, General Manager of ISO-Chemie UK, gives insight into the trade-off between airtightness and ventilation in sealing products.
The legal requirement to consider airtightness in building designs has been around since the 2006 changes to UK Building Regulations.
The measurement for calculating air loss out of 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 back stop maximum leakage is 10, equivalent to 10m3 (h.m2) @50Pa.
Although the legal backstop of 10 has not changed, the target air leakage for SAP and separate target values set by house builders themselves have 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.
The more airtight we make our buildings the less fresh air we get into them through 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. How is ‘build tight, vent right’ achieved? With regards to ventilation, this generally falls into two major camps – natural and mechanical ventilation.
If you achieve an air loss of 3m3 (h.m2) @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 natural ventilation is sufficient.
For unwanted air loss, it is generally acknowledged that the air loss between the junctions of windows and doors to walls, walls to floors etc. account 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 holes knocked through the fabric for 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 know 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.
Certainly, just being airtight will reduce or stop heat loss by convection, but what about heat loss by conduction?
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 window, 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.
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. 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.
- The suitability of the active adhesive system for both initial application and long term use on the substrate.
An airtight seal therefore needs to be more than just an adhesive tape. ISO-Chemie has seen a bewildering range of options on the market. At first glance, they all look like they do 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.
