Glossary

Above-rafter insulation
Highly effective method of insulation in which the insulation layer is installed over the rafters. This method creates a continuous thermal insulation covering that protects both the living space and the roof structure.

Vapour barrier/vapour check
Layer in the roof system that either completely (vapour barrier) or largely (vapour brake) prevents moisture from penetrating into the thermal insulation. When a layer of insulation becomes damp it becomes ineffective.

Diffusion
This is the phenomenon of water vapour passing through structural components (e.g. timber or stone, but not metal). In winter, the high air temperature and humidity within the house results in a high water vapour pressure. The water molecules "press" outwards without any need for air movement (in contrast with convection).

Energy saving regulations
Since 1st February 2002 the energy saving regulations (ENEV) have been in force in Germany. They specify detailed requirements for thermal insulation in new builds, and also require specific measures when installing, converting, replacing or renewing external structural components.

k-value
see U-value

Convection
Movement of air brought about by differences in air pressure. At
joins between structural components that are not airtight, convection can lead to considerable heat loss and also unwanted movement of moisture. Convection can cause up to a thousand times more humidity to penetrate the structure than diffusion.

Polyurethane (PIR)
Rigid polyurethane foams are ideal for use as thermal insulation materials because of their extremely low thermal conductivity and their particular material properties (rigid, biologically neutral).

Reference roof pitch
The shallowest roof pitch at which a particular roof cladding proved in practice to be sufficiently rain-resistant.

U-value
The U-value (thermal transmittance, formerly known as k-value) describes the rate of heat transfer through structural components. The higher the thermal insulation efficiency of a structural component, the lower its U-value will be. It is expressed in W/m²K, so it defines the energy loss per square metre of surface area with a temperature difference of 1°C.

Subroof membrane
The most reliable protection category of the second water-draining level below the roof cladding, in the form of a waterproof or rain-resistant subroof membrane.

Underlay
Medium protection category of the second water-draining level, in the form of either a bonded or overlapped underlay, according to requirements.

Sarking membrane
Simplest form of the second water-draining level below the roof cladding, which is freely stretched over the rafters.

Below-rafter insulation
Insulation method under the rafters: has the drawback of heat losses through the walls and a loss of room space.

Between-rafter insulation
Conventional method of insulation between the rafters, which can be done either with a ventilation gap or as full-rafter insulation (i.e. filling the full rafter depth). To comply with the energy saving regulations, this method of insulation generally requires a larger rafter height than would be necessary from a purely structural point of view. Furthermore, undesirable thermal bridges are created by the rafters and all partition walls.