Inter-rafter insulation is the most common and economical type of thermal insulation of a pitched roof, in which the spaces between the rafters are filled with suitable damping material.
The height of the rafters and the inter-rafter insulation is often not determined by the statically required height of the rafters, but by the required insulation thickness of the inter-rafter insulation. A combination of inter-rafter insulation with additional above-rafter insulation is also possible and widespread.
Inter-rafter insulation is either clamped between the rafters as panels or mats, or blown into the cavity in the form of flakes. The choice of an insulation material has a considerable influence on thermal insulation in winter, summer thermal insulation and sound insulation.
In line with current developments in energy-efficient construction, roof insulation with ever greater material thicknesses is being installed. It is not uncommon for the determination of the insulation thickness to determine the roof cross-section instead of the static specifications. In addition to the actual requirements for thermal insulation, the choice of the right insulation material also has a major impact on thermal insulation and sound insulation in summer. In the planning, it is therefore important to ensure that all essential criteria are taken into account. This also includes the ecological and economic differences of the individual insulation materials. An appropriate weighing of all properties determines the quality of a roof and the satisfaction of the users in the planning phase.
Area of application: When choosing an insulation material, the area of application according to DIN 4108-10 must be taken into account, for which the insulation material must be approved. In this case, this is the application "Inter-rafter insulation" with the abbreviation "DZ".
Insulation between rafters and wooden components: In accordance with DIN 68800-2, only the following insulation materials may be installed in accordance with DIN 68800-2 so that the wooden components can be classified in hazard class 0 (GK0) and thus installed without further wood protection measures:
Most common insulation materials have corresponding building authority usability certificates. However, in the case of inter-rafter insulation that is not mineral fibre insulation materials according to DIN EN 13162 or wood fibre insulation boards according to DIN EN 13171, it must always be checked whether this proof of usability is available.
Vergation in detail: To avoid thermal bridges, the connection details must be planned in accordance with DIN 4108, Supplement 2 or kept as individual verifications. Supplement 2 contains planning and implementation examples for the reduction of thermal bridges. The design recommendations establish reference levels for the quality of connections. If the details are designed according to these specifications or if proof of equivalence can be provided, the flat-rate thermal bridge surcharge of 0.05 W/m²K can also be applied in the mathematical verifications according to EnEV. The following detail is recommended for the formation of the verge of a roof with inter-rafter insulation:
Sound insulation: In addition to summer and winter thermal insulation, sound insulation is also an important criterion when planning a roof structure. Requirements for sound insulation are laid down in DIN 4109 and, for apartments, also in VDI 4100. In addition to airborne sound sources, e.g. road and aircraft noise, structure-borne sound sources, e.g. precipitation, must also be taken into account when it comes to roofs. Double-shell constructions and a rear ventilation level have a positive effect on sound insulation. However, the type of insulation material also influences sound transmission: materials with a high bulk density, such as glass wool, wood fibres and cellulose, have a better sound-absorbing effect than insulation materials with less mass, such as B. EPS.
Moisture protection: Insulation materials must be particularly protected against moisture, as this impairs the insulating effect and can lead to structural damage (e.g. mould) in the long term. Protection against moisture from the outside is guaranteed by the covering in conjunction with the underlay.
However, water vapour from the inside can also cause moisture in the roof structure, which can subsequently lead to condensation water precipitation. Water vapour flowing through it does not harm vapour-permeable insulation materials as long as no condensation precipitates. To avoid this, practically every pitched roof structure requires a special diffusion-inhibiting layer (vapour barrier) on the inside of the roof structure, which must be matched to the respective roof structure. In the case of a diffusion-open roof structure, water vapour from the room air can diffuse to the outside through the insulation. In the case of a vapour-tight roof cladding without rear ventilation, a vapour barrier or a moisture-variable vapour barrier must be installed to prevent damage to the insulation level.
Mathematical proof of condensation: To prevent condensation from precipitating during the transport of water vapour due to the sequence of layers of the roof structure, the diffusion resistance of the materials must decrease from the inside to the outside. The outside of the roof must therefore be more diffusion-open than the inside of the roof.
DIN 4108-3 specifies the assignment of the Sd values for the outer and room layers. If this sequence of layers is adhered to, there is no need to provide a mathematical proof of condensation. For all other exterior components, mathematical proof of condensation water safety should be provided.
Condensation due to water vapour convection: In order to avoid internal condensation due to water vapour convection, an intact air seal layer must be guaranteed. Particularly small leaks on diffusion-tight membranes (vapour barriers) can cause severe moisture damage, as water vapour convection can transport significantly more moisture than diffusion.
Rear ventilation: Since vapour-tight underlays were predominantly used in the past, it was common to choose an insulation thickness that was lower than the rafter thickness and thus to obtain a ventilation level connected to the outside air directly above the insulation. This is referred to as a ventilated roof. This allowed the moisture to air out under the vapour-tight underlay. Since nowadays the insulation thicknesses are greater, the space between the rafters must be fully used and on-roof insulation is often used, diffusion-open sub-covers are usually installed and the rear ventilation level is only placed under the roof covering. It ensures that moisture is ventilated and improves the thermal insulation of the attic in summer.
U-value calculation: In the case of inter-rafter insulation, the proportion of rafters in the roof must also be taken into account when calculating the U-value for the roof. The impact can be reduced by additionally attaching an on-roof insulation board or by using rafters with very narrow cross-sections, such as double web girders.
panel/mat insulation materials: Many insulation materials for inter-rafter insulation are available as panels or mats. The installation is carried out from the inside after the roof membrane has been sealed. The insulation panels/mats are clamped between the rafters with a little extra width and do not have to be mechanically fastened any further. Mats are usually supplied in rolls, which are cut to the desired length for installation. Plates are butt-butted or installed in tongue and groove. The advantage of insulation boards is that they are quickly installed, but special attention must be paid to thermal bridges, as the panels, some of which are somewhat stiff, may not fill all the bottlenecks or leave joints between the panels open. Joints and bottlenecks must be filled with insulating wool.
Installation of blown-in insulation: Insulation materials in the form of flakes are blown in between the rafters. For this purpose, the rafter layer is planked on both sides, usually by means of a formwork or sub-roof panel on the upper side and by the inner cladding on the underside, a vapour barrier membrane or wood-based panels. The resulting cavity is opened at one point per rafter spacing. The blowing hose of a blowing machine is passed through this opening and the flakes are blown in under pressure. The quantity to be filled is calculated from the size of the cavity and the blow-in density specified by the manufacturer. The blow-in opening is then professionally closed again. A common shortcoming of this insulation technology is that too little insulation material has been blown in with too little pressure. To check this, thermal images of the insulated components should be taken, which make cavities in the insulation plane visible. The installation of blown-in insulation may only be carried out by trained skilled workers from licensed companies. Blowing in can lead to high particulate matter development, which is why respiratory protection is recommended for processing.
Dismantling mineral wool
Specialprecautions must be taken when removing old mineral wool insulation. All mineral wool insulation produced before 1996 is generally considered carcinogenic, and since June 2000 it has been forbidden to install insulating materials suspected of being carcinogenic. In addition to respiratory protection, the protective measures also include protective clothing and other hygienic and organizational measures, which can be found in the ►Hazardous Substances Information System of the German Employers' Liability Insurance Association for the Construction Industry and in the BG Technical Info D235. In the meantime, changes have been made to the fibers, which means that carcinogenic, respirable fiber dusts are no longer produced.
Note: DIN 4108-1 (Thermal insulation in building construction; Sizes and units) has been withdrawn and replaced by DIN EN ISO 7345
DIN 4108 Supplement 2, Thermal insulation and energy saving in buildings - Thermal bridges - Planning and execution examples
DIN 4108-2, Thermal insulation and energy saving in buildings - Part 2: Minimum requirements for thermal insulation
DIN 4108-10, Thermal insulation and energy saving in buildings - Part 10: Application-related requirements for thermal insulation materials - Factory-made thermal insulation materials
DIN 68800-2, Wood protection - Part 2: Preventive structural measures in building construction
DIN 18334, VOB Part C, General Technical Contract Conditions for Construction Services (ATV) Carpentry and timber construction work
DIN EN 13162, Thermal Insulation Products for Buildings - Factory Made Products of Mineral Wool (MW) - Specification
DIN EN 13163, Thermal Insulation Products for Buildings - Factory Made Products of Expanded Polystyrene (EPS) - Specification
DIN EN 13171, Thermal Insulation Products for Buildings - Factory Made Products of Wood Fibres (WF) - Specification
DIN EN ISO 7345 Thermal insulation - Physical quantities and definitions
EnEV - Energy Saving Ordinance for Buildings, Ordinance on Energy-Saving Thermal Insulation and Energy-Saving System Technology for Buildings
German Roofing Trade: Regulations, published by the Central Association of the German Roofing Trade (ZVDH)
Source: bauwion
