Why Walls Need To Breathe

We have all seen condensation running down windows or walls, especially in poorly insulated kitchens and bath­rooms. In well-insulated houses with double glazing, this surface condensation is less likely to occur, as the surfaces will be relatively warm. Instead, moist air will try to migrate slowly through the walls, and the danger here is that it may condense into liquid if it hits a relatively cold surface within the wall con­struction. This is known as inter­sti­tial con­den­sation, which can cause timber studs or rafters to rot.

The conventional solution to avoiding inter­stitial condensation within timber walls is to install a ‘vapour barrier’, or polythene sheet, on the inside of the insulation, behind the plasterboard. This will work provided the polythene is kept intact. In prac­tice, however, holes are made to get services through, or it is not properly sealed around openings, meaning moisture can then get into the wall cavity — a recipe for disaster!

Other, more reliable, solu­tions have recently become available which accept that some moisture will get into the wall, but which make it very easy for any moisture inside to migrate to the outside and evaporate. In this approach, a ‘vapour control layer’ (VCL) is used in place of the vapour barrier. This VCL is roughly five times more vapour resistant than the outer sheathing and could be ply, hardboard, foil-backed plaster­board, or a suitable building paper. The outer layer needs to be porous and ‘breathable’, such as bitumen impregnated fibreboard.

The outer sheathing can also provide some racking resistance for the timber frame and may eliminate the need for diagonal bracing. A ventilated space (min 25mm) should be left between the sheathing and the wall finish. A 25mm ‘services zone’ behind the plasterboard will help to ensure that the VCL stays intact. Ideally, the insula­tion material should be ‘hygro­scopic’, or able to regulate humidity levels by absorbing excess moisture. Examples are cellulose fibre, such as Warmcel 500, or sheep’s wool, such as ‘Thermafleece’.

Breathing roof constructions are important too. Most traditionally constructed ‘cold’ roofs with ventilated loft spaces are in no danger of harbouring condensation. There are now sev­er­al vapour-permeable sarking felts available (e.g. Klober ‘Permo’) and these should always be used on new or replace­ment roofs.

In spite of the title, the wall or roof does not actually ‘breathe’, as it is moisture rather than air which is passing through it. It is, however, similar to us wearing a ‘Goretex’-type coat, instead of an impermeable plastic coat which makes us sweaty because it will not allow moisture through.

LEFT: Timber frame 'breathing wall' with spaced studs, developed at the Centre for Alternative Techology; RIGHT: 'Breathing' roof systems utilise a vapour-permeable sarking felt to allow the passage of moisture.

Further reading:

• How to Insulate Old Houses
• Solutions for Insulating Walls