The Importance of Breathability

The phrase “Breathability” is often referred to in relation to construction, and in particular solid wall construction, and in Scotland referring to traditional stone masonry construction.

Traditional solid walls generally comprise a lath and plaster inner layer, comprising timber laths, fixed to vertical timber battens, themselves fixed to the masonry walls, usually by timber “dooks”. The inner face of the laths are covered with lime plaster. Generally there is a void at the rear of the lathwork, to the internal face of the masonry. In Scotland the rest of the wall comprises a mass masonry construction, sometimes comprising masonry the whole way through, but more often or not an inner section of rubble masonry, comprising small sections of rough stone, set in lime mortar. Generally the external 100 to 150mm of the wall comprises stone with a dressed external face, these stones bedded in lime mortar.

The whole wall construction is described as being “breathable”, although is a bit of an misnomer, as really what we are referring to is the ability of water, and water vapour to move through the structure (the external wall), and not the ability of air to move through the structure, which is what “breathability” is often misinterpreted as.

Traditional mass masonry walls function in a different manner to more modern construction techniques, which usually require water to be repelled by roughcast or facing brick, or some other rain screen cladding. Mass masonry walls can absorb moisture, eg from rain, and then release it over a longer period of time by evaporation. This requires the masonry to remain “breathable”, that is able to allow the movement of water vapour in and out of the masonry.

When considering upgrading the thermal performance of mass masonry walls, it is important not only to consider the type of insulation to be used, and its position, but also the question of air tightness of the construction, as mass masonry walls were not constructed to achieve airtightness. Airtightness can be thought of as uncontrolled ventilation of the building, for example drafts from the sub floor entering the interior of the property, movement of air around window and door openings. It can be defined as – the uncontrolled inward and outward leakage of outdoor air through cracks, interstices or other unintentional openings of a building, caused by pressure effects of the wind and/or stack effect.

The construction of traditional masonry walls paid little attention to the question of airtightness, and indeed air movement was encouraged via chimneys and flues to remove noxious fumes, dampness etc. However uncontrolled air movement leads to increased heating requirements and costs, accordingly, the reduction of air infiltration is essential in the consideration of any proposals to upgrade the thermal performance of mass masonry walls.

Infiltration

Infiltration is often quoted in air changes per hour (ACH) at a pressure difference between internal and external. As it can be difficult to measure it is usual to use estimated figures, but these figures do not necessarily reflect individual cases. For example in a recent project the ACH was measured at 15ACH, whereas design tables and guidance indicate an ACH of 2, leading to a heat loss due to ACH being much greater than was predicted.

Airtightness

In addition, poor airtightness design and practice in a building can render the insulation less effective than its design suggests it should be, leading to greater heat loss.

Airtightness is also an important consideration when considering vapour movement in wall constructions, so that unnecessary restrictions on vapour movement are not introduced.

It is therefore essential to consider each case on its own, as it is seldom that individual cases are identical.

Accordingly, what is required for the thermal upgrade of mass masonry walls is not only the use of an appropriate type of breathable insulation, but also a methodology of decreasing the uncontrolled ventilation (airtightness) if this is possible.

It may not always be the case that airtightness can be improved, but it is prudent to consider options to reducing air infiltration losses that are identified, if possible. Each instance has to be considered on its own and the necessary analysis carried out to ensure that problems from condensation, possibly within the structure are not being created by the proposed works.

The information provided is by necessity brief, and hopefully serves as an introduction to the importance of maintaining the breathability of traditional masonry walls when considering the upgrading of the fabric of the walls to improve their energy efficiency.