Rafter insulation - complex, but particularly effective

From the point of view of building physics, rafter insulation is an optimal, but also the most costly, method of insulating a roof. It allows continuous, seamless thermal insulation without reducing the usable space of the attic. Because of the construction costs and the comparatively high costs, a rafter insulation is particularly suitable for new buildings as well as for construction projects, within the framework of which the entire roof is also to be renewed.

Construction experts consider the rafter insulation to be the most efficient, cleanest and also the most space-saving variant of the roof insulation. However, a new covering of the roof is necessary for this form of thermal insulation. Their energy efficiency results, in particular, from the fact that thermal bridges are ruled out when properly performed. The insulation forms its own, continuous level of the roof structure, which has no leaky transitions. In renovation projects, the attic remains habitable throughout the construction work, as the roof grows "outwards". The installation of a rafter insulation is always in the hands of a specialist, because it requires complex practical and building physics knowledge. The cost of a rafter insulation amounts to 150 to 200 euros per m2.

Table 1: Average m2 cost of a roof construction with rafter insulation

substructurevapor barrierinsulationshortfallbattensroofingAccessory / connection material
22 EUR12 EUR38 EUR14 EUR16 EUR21 EUR12 EUR

What is a rafter insulation?

In the case of rafter insulation, the insulating layer is applied to the rafters. It is particularly suitable for the thermal insulation of large roof areas with simple roof geometries and without sides or pointed floor surfaces. A Aufsparrendämmung is an external insulation and is thus exposed to high levels of weather and possibly also mechanical stress. The installed insulating material must therefore be weather and dimensionally stable.

Auflattdämmungen

Slatted insulation is a special variant or a further development of the rafter insulation. They are not only mounted on the rafters, but also on the roof battens. They provide essentially the same advantages as classic insulation on rafters: energy efficiency, exclusion of thermal bridges, freedom of design and room gain in the attic.

Construction of a rafter insulation

A rafter insulation requires a special structure, which must be adhered to in principle. From the inside to the outside, the following components are required for such a construction:

  • Substructure / exposed sheathing
  • Steam brake: For rafter insulation, a vapor barrier or vapor barrier is usually required to prevent moisture penetration of the insulation layer and the roof structure due to condensation. Plastic insulations - for example made of XPS - already have this effect due to their material structure. In addition, however - while maintaining the airtightness of the roof construction - also vapor-open Aufsparrendämmungen possible.
  • insulation boards
  • Underfloor panels or sub-panels: If necessary, this layer of the roof construction serves to waterproof the insulation layer, provided that the insulation itself does not have sufficient water-repellent properties. If the insulation material is inherently hydrophobic, this layer increases the overall weather resistance of the construction and precludes moisture penetration into joints or microcracks. For pitched roofs, the undercoverage is pulled over the insulation layer into the roof overhang / eaves area as so-called eaves detail of the rafter insulation.
  • Holding and counter battens for roofing: With regard to the order of the layers of such a roof, the already mentioned battens are a special case since they lie on the battens of the roof. A possibly required vapor barrier or barrier as well as the hydrophobization protection in this case naturally also lies above the battens.
  • Roofing.

Which insulating materials are suitable for insulation on rafters?

In a Aufsparrendämmung it depends on high quality insulation materials. The insulation material is used from the outside and is therefore exposed to weather and temperature fluctuations to a high degree. Low-quality materials are likely to crack, which negatively impacts both the efficiency and the life of the roof insulation. Insulating boards are usually used for laying a rafter insulation.

Uncomplicated and safe installation by insulation systems

Prefabricated insulation systems are offered in the trade, which are equipped with Pfalzschienen at the edges and facilitate the complete laying of the insulation boards.

Insulation materials for a rafter insulation

Most of the rafter insulation is made - partly for economic reasons - with established insulation materials such as mineral wool or plastics (EPS / Styrofoam, XPS, PUR / PIR). In particular, XPS and PUR / PIR insulation is extremely hard-wearing, not only due to weather conditions but also due to mechanical influences. However, it is also possible to use insulation mats with natural insulating materials, provided that they are available in sheet form and are sufficiently robust, pressure-resistant and weather-resistant. For example, wood fiber or hemp insulation boards are also used in this area of ​​thermal insulation. Apart from PUR / PIR with not fundamentally different thermal insulation performance, both offer advantages.

Sound and heat protection

Not only are there high demands on the roof with regard to thermal insulation, but also with regard to efficient sound and heat insulation. Most natural insulating materials are at least tendency to mineral wool and plastics in terms of their sound and heat protection properties.

particularities

The insulation thickness of a rafter insulation is usually limited to 28 cm, otherwise the roof construction of rafters, insulation, battens and roofing becomes too high. This special feature may have an influence on the choice of insulating material, especially as many builders deliberately exceed the minimum insulation thicknesses required by the 2014 Energy Saving Ordinance (EnEV) in order to achieve particularly efficient thermal insulation. In general, it applies here that an insulating material with the lowest possible thermal conductivity requires lower insulation thicknesses and is advantageous for a rafter insulation - and also for the other forms of roof insulation (uprising and intermediate rafter insulation). Above all plastic insulation proves their efficiency in this area.

Table 2: Thermal conductivity, minimum insulation thicknesses and prices of selected insulation materials

insulationglass woolEPS / StyrofoamXPSPur / Pirwood fiberhemp
Thermal conductivity (W / (mK))0,032 – 0,0400,035 – 0,0450,035 – 0,0450,02 – 0,0250,04 – 0,0550,04 – 0,045
Minimum insulation thickness (cm)141414101816
m2 price (EUR)10 – 205 – 2018 – 3010 – 2040 – 5010 – 27

Tips & Tricks

A rafter insulation is the building physics optimal thermal insulation for the roof. It is particularly suitable for new buildings and renovations, where the roof can be completely renewed, as this type of insulation is always associated with a new roofing of the roof.

Insulation combinations for the roof

Especially in old building renovations, but also in new buildings are often suitable for optimal insulation combinations, for example:

  • On and Zwischensparranendämmung
  • Intermediate and sub rafter insulation
  • On, intermediate and sub rafter insulation.

For example, in such designs, inter- and sub-rafter insulation also reduces the risk of thermal bridges. In addition, the entire rafters space for the insulation and an effective sound and heat protection is used. There is also a much wider range of insulating materials available for the insulation on the inside of the rafter.

Diffusion-open rafter insulation?

The trend of modern thermal insulation is increasingly turning into vapor-permeable insulation solutions that support optimal moisture exchange. A vapor-permeable structure and corresponding insulation materials increase the fault tolerance in the case of rafter insulation, since they are able to largely compensate for the consequences of leaks in the insulation-for example due to microcracks. In general, the integration of a vapor barrier - when using diffusion-open insulation materials for Aufsparrendämmung but still necessary.

In general, an optimal roof structure is more dense on the inside than on the outside and as permeable to air as it is while maintaining air-tightness.

Rafter insulations for a flat roof

On pitched roofs, rafter insulation is a classic method for efficient thermal insulation, but they can also be used for flat roof insulation. They also provide optimal insulation performance and also offer greater safety, since the supporting structure of the roof is indoors and therefore well protected from condensation and weather. The cost of rafter insulation for a flat roof is between 40 and 100 euros per m2. There are three different methods for the insulation of a flat roof:

Cold roof insulation

The name of this variant of the rafter insulation is explained by the cold air layer, it is located between the insulation layer and the roofing of the roof. Particularly suitable cold roof insulation for flat roofs with low inclination, in which between the ceiling and the roof construction about 30 to 40 cm space is. Above the ceiling, a vapor barrier film is applied, on which there is a gapless insulation layer. Suitable insulating materials are, for example, mineral wool, polystyrene insulation (EPS, XPS) or the synthetic and particularly strong synthetic high-performance insulation material PUR / PIR. Above the rafter insulation, an air gap of at least 10 cm must remain.

Warm roof insulation

The warm roof insulation in the form of rafter insulation is particularly suitable for an almost horizontal flat roof. In this variant, there is no cold air layer between the ceiling and the outside of the roof - the insulation material must be enclosed airtight between the ceiling construction and the roof covering. The tightness of the roof construction is particularly important in this insulation variant, since penetrating moisture only very poorly escape and can lead to mold growth. As an insulating material for this form of Aufsparrendämmung a flat roof XPS is usually used.

Inverted roof insulation

The simplest method for insulation on the flat roof is a so-called reverse roof insulation. The insulation layer is applied to the existing roof insulation. On the insulation is another seal layer. The insulating materials for inverted roofs are mainly XPS or foam glass panels. The roof seal may for example consist of bituminous membranes.

Advantages of rafter insulation:

  • Continuous and complete thermal insulation
  • Exclusion of thermal bridges
  • Protection of the roof truss: The roof truss is located in the "warm" area of ​​the roof.
  • No loss of useful and living space.
  • Interior design: In the case of pure rafter insulation, the wood of the rafters remains visible on the inside of the roof and can therefore be incorporated into the interior design. In addition, damage to the rafters - for example due to pest infestation - is already visible at very early stages.

Disadvantages of a rafter insulation

The disadvantages of Aufsparrendämmung exist essentially in construction costs and the relatively high price, in addition a re-roofing of the roof is necessary.

State promotion of a rafter insulation

Efficient thermal insulation is an essential prerequisite for the energy transition and is therefore also in the public interest. For the attachment of Aufsparrendämmung state subsidies are possible. Prerequisite for the receipt of the funding is a report by a professional energy consultant who, among other things, makes an estimate of how much energy can be saved by this form of insulation. The energy expert usually also handles the drafting and submission of the application. The application for funding must be made before the start of construction works; it is best done directly at KfW.

KfW grant or KfW loan

Funding is possible as a KfW grant or as a KfW loan. The subsidy covers 10% of the investment costs, the upper limit of the funding is 5,000 euros. The loan is currently (2015) with an interest rate of 0.75% and 7.5% repayment subsidy. The maximum loan amount is 50,000 euros.

Tips & Tricks

The installation of a rafter insulation can be supported by the KfW. Both a grant and a KfW loan are available on particularly favorable terms. Prerequisites for this are the application to KfW and an opinion by an energy consultant.

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