Breathing roofs

The majority of new houses in the UK are built with ventilated roof spaces. However, in recent years another method of controlling condensation has been introduced. This is known as a ‘breathing roof’. Its title is slightly misleading because the roof does not actively breathe, instead it allows vapour to escape from it via a vapour permeable underlay. Although some insulation manufacturers recommend breathing roofs, some tile manufacturers prefer ventilated roofs.

The features of a cold breathing roof are:

-   a vapour control layer at ceiling level to limit the amount of water vapour entering the loft
-   a breather membrane as the roof underlay, to allow dissipation of water vapour from the loft space
and, just as importantly, to act as a windproof layer
-   10–25mm deep counter-battens to provide an air space above the breather membrane for
improved ventilation (i.e. to aid dissipation of moisture vapour)
-   ceiling level insulation that is pushed up tight against the breather membrane to prevent air
leakage into the loft at eaves level.

A breathing roof provides a loft space which is clean and relatively dry. Additionally, the avoidance of draughts improves the energy efficiency of mineral wool insulation.

However, when considering breathing roofs the following should be taken into account:

-   they should only be used in dwellings with a simple rectangular plan: complex plans hinder the
degree of ventilation
-   achieving a convection-tight ceiling is quite difficult in practice
-   close-fitting tiles will require tile ventilators
-   the underlay should have taped joints
-  a breathing roof should never be used without first carrying out a condensation risk analysis
-   capital costs are higher than for ventilated roofs.
The drawing below shows typical construction for a cold breathing roof – in other words the insulation is at ceiling-joist level.

The construction is slightly different for rooms in the roof. In a ‘true’ warm, breathing roof, the insulation would be on top of the rafters. However, in practice, the required thickness of insulation would make the construction impractical. You are more likely to find the insulation in between the rafters or, if they are not very deep, in between and below the rafters. A vapour control layer is required on the warm side of the insulation. If the roof space is to be used for accommodation rather than storage, it would be normal practice to use the construction shown on the right.

This is a modern breathing roof – with rooms in the roof for accommodation. It is similar to the construction shown in the figures above.