The original concept of a breathing wall was of internal air flowing out through the walls to reduce the concentration of indoor air pollutants. In winter, natural ventilation levels are typically low and dwellings are at positive pressure, so this breathing could have been passively induced. However, research showed the approach to be ineffective, and the concept of a breathing wall now refers to diffusion of water vapour, not air, through the fabric. (A related concept of air-breathing, still being explored, is dynamic insulation - aj 31.7/7.8.97.)
What, then, is a mainstream breathing timber-frame wall? The answer is simple. The breathing aspect of any timber-frame wall is about eliminating the risk of interstitial condensation. This is done by controlling the flow of water vapour from the high-humidity interior of the building to the lower-humidity exterior such that water vapour can escape through the outer layers of the wall more quickly than it can enter through the inner layers.
There is a rule of thumb for this which ensures all timber-frame walls breathe. The inner layers, on the warm side of the insulation, should have a vapour resistance at least five times greater than the layers on the cold side of the insulation. All timber-frame walls are made to breathe in this way or they do not work!
If it is that simple, why are there frequent enquiries to trada Technology? In these conversations there is often a veiled reference to conventional timber-frame buildings being unhealthy because the occupants are living in a sealed plastic bag.
What enquirers often mean by a breathing wall is a specific construction. From the inside, this wall consists of Gyproc Duplex plasterboard, Masonite I-beam studs with Warmcel cellulose fibre (recycled newsprint) insulation between studs and either Bitvent (impregnated softboard) or Panelvent (medium board) as the outside sheathing.
The Duplex board has a metallised polymer backing and has a vapour resistance of around 60MNs/g. On the cold side, softboard has a vapour resistance of 0.4MNs/g and medium board of around 1.5MNs/g. Thus the vapour resistance ratios are 150:1 and 40:1 respectively, both comfortably in excess of the 5:1 ratio, and the wall breathes freely.
Conventional uk timber frame is, from the inside, plasterboard, a 120 micron polyethylene sheet vapour-control layer on solid timber studs with mineral wool (glass or rock) insulation between the studs, an oriented strand board (osb) outside sheathing and a breather membrane. The warm side vapour resistance is around 250MNs/g while that on the cold side is about 5Mns/g, a vapour resistance ratio of 50:1. Conventional timber frame is moving towards metallised polymer-backed plasterboard rather than a separate polyethylene sheet, which would give a resistance ratio of 12:1 - again, an adequate breathing wall.
trada Technology has been involved in developing alternative timber-frame walls for many years. The latest is the 'reverse wall'. From the inside, it consists of plasterboard on an osb sheathing on timber studs with glass wool between the studs and an outside sheathing of moisture-resistant plasterboard. In this wall the osb is the principal vapour-control layer and the inside vapour resistance is about 5MNs/g. The cold side resistance is 0.75MNs/g. The vapour resistance ratio is about 7:1.
In the Duplex/Warmcel/Bitvent wall noted earlier, the osb sheathing has as acceptably low vapour resistance as the outer layer, which allows water vapour to pass into the outer cavity. In the reverse wall, the osb is moved to the inside and becomes a vapour-control layer with sufficiently high vapour resistance to restrict the flow of water vapour into the wall. The vapour resistance ratio of layers to control vapour flow ensures that these and all timber-frame walls breathe.
Architects simply need to specify wall constructions based on anunderstanding of their performance characteristics - one of which is breathability.
Geoffrey Pitts is chief architect at trada Technology
edited by Barrie Evans