Andy Swift, national sales manager for window and door foam sealant specialists ISO-CHEMIE considers the trade-off between airtightness and ventilation in sealing products…

Air quality within a building is a big issue. 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 – it is often argued that if by reducing air loss you then need to build-in designed ventilation, then 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) @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 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.

The industry is starting to understand that airtightness is only half the story for the sealants. That 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. 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 an airtight seal for this type of application, consideration must be given to several factors to assess if it’s correct for the joint-type.

ISO-CHEMIE can undertake an onsite CPD.