Expert knowledge

Above-rafter insulation

In the case of above-rafter insulation, also known as on-roof insulation, the insulation is above the rafters, usually with an inner formwork as an inwardly visible room closure.

For example, a continuous insulation shell lies on the roof structure. The above-rafter insulation is used both in conjunction with a visible roof truss and with inter-rafter insulation. However, it can also be used for energy-efficient renovation, e.g. to avoid structural interventions from the inside in an already converted attic space.

More about above-rafter insulation

The roof is a highly stressed component due to the effect of large temperature differences, sun, snow and hail. Rafter insulation must therefore be dimensionally stable, as it is exposed to large temperature fluctuations and would result in thermal bridge joints due to changes in shape as a result of heat or cold. 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. Top-rafter insulation is often used in combination with inter-rafter insulation in order to achieve the required insulation values without unnecessarily increasing the rafter cross-sections. Rafter insulation is laid as panels in bonds, multi-layer and butt-offset or with tongue and groove.

Notes on renovation planning

: In the energy-efficient renovation of existing buildings, above-rafter insulation is often advantageous, as the converted roof space remains habitable during the insulation work and the existing rafter heights are often too low to achieve sufficient insulation values by inter-rafter insulation alone. Above-rafter insulation is somewhat more complex than inter-rafter insulation, and all roof connections, skylights and gutters must be renewed. The building volume changed by the additional roof structure must also be examined under building law, e.g. with regard to increased distance areas. In the case of non-detached houses, the changed connection heights to the neighbouring buildings must also be taken into account in good time during the planning.

Additional functions: Many manufacturers offer insulation boards that immediately serve as a backdrop. Either they have a covering film with overlap attached to the top for bonding (usually in the case of products made of PUR or rock wool) or the board itself is suitable as a covering due to its surface and a tongue and groove system (in the case of wood fibre panels). Plates are also available for which an additional vapour barrier layer is superfluous and which are also airtight or windproof thanks to adhesive tapes. Some panels can also further improve sound or fire protection by containing layers of other materials. In addition, there are insulation boards with integrated battens on which the roof covering can be mounted directly.

Areas of application: When choosing an insulation materialthe permissible area of application according to DIN 4108-10 must be taken into account. In the case of above-rafter insulation, this is the "DAD" designation for "external insulation of roof, protected from weathering, insulation under coverings". In addition, in the case of above-rafter insulation, the property in relation to the compressive strength is still specified. A distinction is made between "dk" (no compressive load capacity), "dg" (low compressive load capacity), "dm" (medium compressive load capacity) and "dh" (high compressive load capacity).

Sound insulation: In addition to summer and winter thermal insulationsound 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

In the case of diffusion-open roof structures with above-rafter insulation, rear ventilation under the roof covering must be planned. This ensures that moisture is ventilated and improves the thermal insulation of the attic in summer.

Is the retrofitting of above-rafter insulation of an existing roof subject to approval? The individual state building codes contain different requirements for this purpose. For example, in Bavaria, "roofing including thermal insulation measures except for high-rise buildings" is generally exempt from proceedings (Bavarian Building Code Art. 57 para. 1 no. 11f).

In any case, however, compliance with substantive requirements (e.g. compliance with distance areas, extent of structural use) must be observed. These result in particular from the state building code and from the development plan, if available.

The increased building volume due to the subsequent above-rafter insulation and the changed position of the roof edges should therefore always be checked by an expert, e.g. an architect, with regard to building law implications. Depending on the facts, this may require the building supervisory approval of a deviation from the state building code and/or an exception or exemption from the development plan.

Notes on construction Installation

: In the case of insulation boards with straight edges, the panels must be laid in a composite and without joints. By dividing the intended insulation thickness into two layers with offset installation, thermal bridges at the panel joints are avoided, but the use of single-layer insulation with tongue and groove joints is usually more economical.

Fastening: When installing above-rafter insulation, the compressive strength of the insulation boards (according to the product data sheet of the insulation manufacturer) must be observed. In the case of panels with a compressive strength < 50 kPa, the compressive loads from the roofing must be transferred through the fasteners (e.g. by double-threaded screws) or pressure-resistant insulation strips must be installed. In order to dissipate the thrust force of the roof covering into the construction, the insulation boards are either attached to the roof structure by special screws or nails through the counter battens or a so-called push plank is installed in the eaves area, which prevents the insulation boards from slipping. Most manufacturers specify precise assembly guidelines and type statics for their products.

Standards and literature

NoteDIN 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 products - Factory-produced thermal insulation materials

DIN 18334, VOB Part C, General Technical Contract Conditions for Construction Services (ATV) Carpentry and timber construction work

DIN EN 13162, Thermal insulation materials for buildings - Factory-made products made of mineral wool (MW) - Specification

DIN EN 13163, Thermal insulation products for buildings - Factory-made products made of expanded polystyrene (EPS) - Specification

DIN EN 13165, Thermal insulation products for buildings - Factory-made products made of rigid polyurethane foam (PUR) - Specification

DIN EN 13166, Thermal insulation products for buildings - Factory-made products made of phenolic resin foam (PF) - Specification

DIN EN 13171, Thermal insulation materials for buildings - Factory-made products made of wood fibres (WF) - Specification

DIN EN ISO 7345 Thermal insulation - Physical quantities and definitions

VDI 4100, Technical rule, Sound insulation in building construction - Apartments - Assessment and proposals for increased sound insulation

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)

... read more on bauwion - BauWissenOnline

Source: bauwion