HENSOTHERM® and HENSOMASTIK® Fire Penetration Seals for Wooden Construction Elements

The HENSOTHERM® and HENSOMASTIK® solutions, tested and certified according to DIN EN 1366-3, provide effective fire protection and fire sealing for wooden components and construction elements. The versatile, easy-to-apply, and cost-effective seal construction variants restore the fire resistance classification of wall and floor structures made of solid wood, cross-laminated timber (CLT), or DiagonalDübelholz® (DD) elements when these are fitted with openings for the installation of single, multiple, or mixed service penetrations, such as pipe or cable ducts.

The annular gap around single penetrations is sealed using the ready-to-use fire protection filler HENSOMASTIK® Acrylic.

In combination with HENSOTHERM® RM pipe collars and HENSOTHERM® EM endless modular collars, fire seals for combustible pipes or pipes with combustible insulation are created.

Retrofittable fire seals for single cables, cable bundles, and electrical installation conduits can be quickly and easily created using the HENSOTHERM® Service Transit, a ready-to-use intumescent fire protection solution for single penetrations. A quick friction-fitted insertion without additional annular gap sealing is possible.

For larger openings with multiple or mixed services, the HENSOMASTIK® Kombischott 2 x 50 mm is ideal. It consists of two mineral fibre boards, each at least 50 mm thick and with a density of ≥ 150 kg/m³, coated on the outer sides with HENSOMASTIK® 5 KS Farbe or HENSOMASTIK® 5 KS viskos. For fast installation, the required mineral fibre boards are available pre-coated in dimensions of 600 x 1000 mm. As a reserve for quick and easy retrofitted installations, an empty seal can also be temporarily or permanently installed. Seal construction variants with and without a reveal frame are available.

As an alternative, in floors, a fire penetration seal for single, multiple, or mixed services, also in larger openings, can be created using HENSOTHERM® GM 2000 fire protection mortar. This gypsum-based mortar is cast seamlessly around the penetrating services into the opening – a reveal frame is not needed.

Product Features

• Firestop seal constructions tested according to DIN EN 1366-3
• Fire resistance duration up to 90 minutes (fire-resistant)
• Fire protection solutions for single, multiple and mixed penetrations
• Clear construction guidelines and easy installation

Environmental Aspects
Wood construction is particularly sustainable, as wood, being a renewable resource, stores CO₂ and is climate-friendly. Its production requires less energy compared to concrete or steel, reducing the overall CO₂ footprint. Thanks to its high stability, wood enables resource-efficient, lightweight structures that facilitate transport and foundation work. Additionally, it is recyclable and biodegradable, allowing for environmentally friendly disposal at the end of its life cycle. Wood also promotes a healthy indoor climate through natural thermal insulation and humidity regulation. Overall, wood construction is a forward-thinking building method with high ecological efficiency.

• Building products are free from solvents, silicones, halogens, and plasticizers
• Green Products, VOC emission class A+
• Meets the requirements of eco-bau 1 and Minergie-eco
• Free from per- and polyfluorinated chemicals (PFAS-free)
• Environmental Product Declaration (EPD) for all construction products

Construction products with EPD promote circular economy, resource conservation, and waste management. In many countries, environmental requirements for construction projects are increasingly being mandated by law. EPDs provide legal certainty through a standardized data basis and prepare projects for potential future regulatory requirements. Property developers can more easily achieve sustainable building certifications such as DGNB or LEED, increasing the long-term value and attractiveness of their projects.

Cross-Laminated Timber (CLT)

Cross-laminated timber (CLT) is a type of engineered wood product that is becoming increasingly popular in the construction industry as a sustainable and versatile building material. It is made by stacking multiple layers, typically at least three, of wood panels in alternating directions and bonding them together with adhesives. This results in a multi-layered solid wood panel, typically made of 3, 5, 7, 9, or more layers of softwood.

Each layer or lamella is generally made of solid sawn timber or structural composite wood. The cross-laminated arrangement of the layers gives CLT greater strength, stability, and dimensional accuracy. The result is a large, solid panel with consistent structural properties in both directions. CLT panels are known for their high strength-to-weight ratio, making them suitable for various structural applications in buildings, including floors, walls, and roofs. Despite their strength, CLT panels are relatively lightweight, making them easier to transport and handle on construction sites. However, the choice of wood species can affect the structural and aesthetic properties of CLT.

Commercially available CLT typically consists of three to seven bonded layers of cross-laminated timber. The thickness of the panels can vary and is determined by the number and thickness of the individual layers. Common panel thicknesses range from 75 to 240 mm.

The HENSOTHERM® and HENSOMASTIK® Fire Penetration Seals for Wooden Construction Elements have been successfully tested with CLT elements from Stora Enso, Finland (Building Product ETA No. 14/0349). More information can be found at www.storaenso.com.

Although wood, and therefore CLT, is inherently combustible, CLT can offer a certain level of fire resistance due to the charring of its outer layers. The outer layers char slowly, typically at a rate of 0.6 to 0.7 mm/min depending on the wood species and construction, protecting the inner layers and maintaining the structural integrity of the material. The charred layer can act as a protective barrier, slowing the spread of fire and preserving the material’s structural integrity. However, the charring of the inner layers can occur more quickly, at rates of up to 1 mm/min or more. As a general guideline, unprotected three- or five-layer CLT wall panels must be at least 100 mm thick to achieve a fire resistance class of 60 minutes, and at least 120 mm thick to achieve 90 minutes.

DiagonalDübelholz® (DD)

The HENSOTHERM® and HENSOMASTIK® Fire Penetration Seals for Wooden Construction Elements have also successfully been tested with DiagonalDübelholz® (DD), a proprietary development by Sohm HolzBautechnik, Austria, for large-format solid wood elements (Building Product ETA No. 16/0480). The wedge-dovetailed SohmVollholz lamellas are, unlike conventional board-stack or dowel wood, connected to form the DD element using hardwood dowels pressed diagonally and wave profiling, providing structural stability. This connection technique results in a flat and smooth surface.

The production of DiagonalDübelholz® does not require metal fasteners, and with a glue content of only 0.05%, it contains less than one-tenth of the adhesive content of cross-laminated timber (CLT). More information can be found at www.sohm-holzbau.at.

Glued Laminated Timber (GLT)

Glued laminated timber, also known as BS wood or laminated wood, consists of at least three layers of wood glued together in the fibre direction. This multi-layered construction reduces natural weak points, giving the material significantly higher load-bearing capacity and dimensional stability compared to conventional solid wood. First, the softwood used, such as spruce, fir, pine, or larch, is technically dried, planed, and strength-sorted. The prepared lamellae are then glued together using high-quality adhesives in the fibre direction. Finally, finishing is performed to achieve uniform dimensions and a smooth surface.

Glued laminated timber is produced in accordance with EN 14080 in various strength classes. While solid structural timber is made from a single piece of wood, glued laminated timber is made from several carefully sorted and glued lamellae. This compensates for material defects, resulting in higher strength values and more precise dimensions. Thanks to strength sorting and the multi-layered construction, GLT offers up to 80% higher load-bearing capacity than conventional construction timber, excellent stiffness, dimensional stability, and a smooth, four-sided planed surface that is also suitable for visible elements.

GLT is widely used in timber engineering – from load-bearing components in multi-story residential buildings, bridges, and halls, to decorative, visible elements in interior design. Special components with curved or bent shapes can also be realized. Glued laminated timber is made from renewable resources and, thanks to its higher load-bearing capacity, enables the use of slimmer components. The energy-efficient production process and the durability of the material contribute to a reduced CO₂ footprint and sustainable construction.

Solid Structural Timber (Konstruktionsvollholz, KVH)

Solid structural timber refers to solid, single-piece sawn or planed timber that is used as a load-bearing element in construction. It is characterized by its natural structure and homogeneity, without the need for multi-layer gluing processes like those used in glued laminated timber. Solid structural timber primarily uses solid wood, usually from domestic softwood species such as spruce, fir, or pine. After harvesting, the wood undergoes careful drying, often in technical drying facilities, to reduce the moisture content to a suitable level for construction (residual moisture 15%, with a variance of ±3%, as required by DIN EN 15497 for finger-jointed solid timber). It is then machine planed, cut, and, if necessary, strength sorted – processes that ensure consistent quality and precise dimensions.

Solid structural timber offers natural strength. As solid wood, it has high load-bearing capacity that remains constant when properly dried and processed. The machine planning and cutting processes achieve precise dimensions and smooth surfaces, ensuring good workability. Because it is made directly from solid wood without additional gluing processes, it is often more cost-effective and economically favourable compared to more complex engineered wood products. These properties make solid structural timber a reliable material for many classic timber construction applications.

Solid structural timber comes from sustainably managed forests and is a renewable resource. Its production requires fewer energy-intensive processing steps than multi-layer wood products, which contributes to a lower CO₂ footprint.

Typical applications of solid structural timber include load-bearing components in timber frame construction, such as studs, beams, and rafters in residential and commercial buildings. It is also used in roof structures as a load-bearing element in attics or truss constructions, as well as for traditional timber construction methods where the aesthetic of solid wood plays a role in visible elements.

Unlike glued laminated timber, which consists of several bonded layers of wood, solid structural timber is processed as a single, continuous piece. This simplifies the production process, eliminating the complex gluing procedure, which can result in lower production costs. Additionally, the natural wood structure is preserved, which may be desirable for certain applications, such as visible wood elements. However, natural defects and grain direction can cause slight variations in strength. Both materials have their place in timber construction, with the choice depending on the specific application and structural requirements.

Fire Penetration Seals in Wooden Construction Elements– Test Setup vs. Practical Application

When integrating into practical construction, the combination of CLT or DD with drywall and insulation materials provides a holistic solution to meet the diverse demands of modern building construction. The cross-lamination of wood in CLT not only increases structural strength but also contributes to fire resistance. Although wood is inherently flammable, the thickness and mass of CLT panels provide some level of protection. A cladding with drywall further enhances the fire performance of the structure.

Beyond structural considerations, this combination also extends to thermal insulation and energy efficiency. Insulation materials strategically placed between the CLT panels improve the building’s ability to regulate temperature and reduce energy consumption. Drywall in combination with CLT and insulation contributes to a well-insulated building envelope, promoting sustainability and comfort. Furthermore, the natural acoustic properties of wood, especially when combined with additional layers of insulation, aid in sound absorption and create a pleasant indoor climate. Drywall complements this by improving acoustic performance and reducing sound transmission between different rooms.

To cover the various construction variants and layer assemblies of timber components in practical applications, Hensel has therefore conducted all fire tests using only the base layer of CLT or DiagonalDübelholz® (DD), so that the tested compartmentalization constructions can be used in a variety of common layer combinations and configurations.

According to the criteria of DIN EN 1366-3, Section 13.3.4, test results for wall or ceiling constructions made of timber also apply to elements of the same construction type with the same or greater thickness, provided the following conditions are all met:

• The wall/floor is of the same construction as the one tested, e.g., cross-laminated timber (CLT).
• The wall/floor has the same or a higher fire resistance class than the tested one, e.g., REI 60.
• The construction is classified in accordance with EN 13501-2 for the required fire resistance duration.
• The construction is made of the same solid wood slabs as the tested one, e.g., spruce or equivalent softwood.
• The solid wood slabs (as well as the gypsum boards, if relevant) have the same reaction to fire class or higher than the tested one, e.g., Euroclass D-s2,d0.
• The strength class of the wood elements according to EN 338 is the same or higher than the tested one, e.g., C16 / T11 (see below).
• The charring rate according to EN 1995-1-2 of the solid wood slab is the same or better than the tested one, e.g., 0.63 mm/min (see below).
• The thickness of the solid wood slab is the same or greater than the tested one, e.g., a 100 mm thick CLT wall element.
• The number of layers of gypsum boards (if relevant) is the same as tested, e.g., one layer on both sides or none.
• The thickness of gypsum boards (if relevant) is equal to or greater than that tested, e.g., 12.5 mm on both sides.

The correct implementation of HENSOTHERM® and HENSOMASTIK® Fire Penetration Seals for Wooden Construction Elements is the sole responsibility of the executing company. Any required adjustments to the construction variants for local conditions must be coordinated with the fire protection expert! These adjustments include, but are not limited to, the position and fasteners for fire protection sleeves, the execution of any reveal cladding, and the position and filling depth of annular gap fillings.

Load-Bearing Capacity of Timber Components – The Strength Classes of Wood According to EN 338

The EN 338 is a European standard that defines the strength classes of structural timber and wood products. It serves as the basis for the dimensioning and use of wood in construction by classifying the mechanical properties of wood into various classes. These strength classes apply to both softwoods and hardwoods and are essential for the load-bearing capacity and safety of timber components. The standard differentiates between C-classes for softwoods (Coniferous, C14, C16, C18, C22, C24, C27, C30, C35, C40) and D-classes for hardwoods (Deciduous, D30, D35, D40, D60, D70). The number of the class correlates with the characteristic value for bending strength in Newton per square millimetre (N/mm²). Higher values correspond to higher load-bearing capacities.

The strength class according to DIN EN 338 is a key factor in the dimensioning of timber components. It ensures that timber elements have the required load-bearing capacity and can be used in compliance with building codes. Choosing the right strength class is crucial for the cost-effectiveness, safety, and longevity of a timber structure.

Each strength class is characterized by specific mechanical properties: bending strength, compressive strength, tensile strength, shear strength, the modulus of elasticity (stiffness of the wood), and its density.

Timber is sorted either visually or mechanically into quality classes. In visual sorting, trained personnel assess the wood according to uniform criteria, such as those defined in DIN 4074. However, the exact determination of density is not possible with this method. Since density significantly influences strength, the yield in the highest sorting class (S13, equivalent to C30 according to DIN EN 338) is low. Higher strength classes can only be determined through machine grading. Modern sorting machines use X-rays or image processing to capture not only the density but also defects like knots and the modulus of elasticity, allowing sorting up to the strength class C40.

For Cross-Laminated Timber (CLT), C24 or C30 laminates are often used. C24 is the most commonly used strength class for construction timber, as it offers a good balance between load-bearing capacity and cost-effectiveness. Higher classes (e.g., C30, D40) are used in areas with increased load requirements, such as in long-span roof structures or engineered timber constructions. Lower classes (e.g., C14, C18) are primarily used for interior construction or non-load-bearing components.

How Fast Does Wood Burn? Charring Rate and Fire Behaviour of Timber Components According to EN 1995-1-2

In the event of a fire, wood forms a protective layer of charred wood, which insulates the inner core of the component and preserves its load-bearing capacity over a defined period. The outer layer of timber components carbonizes under heat, creating an insulating layer of charcoal (wood carbonization). This layer reduces the heat transfer into the core of the component and protects the remaining structural integrity. The load-bearing capacity remains intact as long as the unburned core of the wood component is sufficiently large.

The European standard EN 1995-1-2 (Eurocode 5: Design and Construction of Timber Structures, Part 1-2: General Rules – Structural Design for Fire) provides methods for determining the load-bearing capacity of timber components in the event of fire and offers specific values for charring rates and burning speed relevant to different timber products. The design is based on the effective cross-section method, where the charred area is subtracted, and the remaining wood mass serves as the load-bearing core. The calculation also takes into account additional safety factors for different components.

The average charring rate (β) is given in mm/min and depends on the wood type, density, and component thickness. There is a distinction between the one-dimensional burning rate (β₀) and the increased, idealized burning rate (βₙ). For components with one-sided fire exposure, such as solid wood walls or ceilings, the standard burning rate β₀ is applied. Columns and beams, on the other hand, are often exposed to fire on multiple sides, causing more pronounced charring in the corners. Therefore, the increased burning rate βₙ is applied in these cases to account for the higher material loss realistically. For softwood solid wood and glued laminated timber (glulam), the one-dimensional burning rate β₀ can be taken as 0.65 mm/min, which corresponds to a material loss of about 4 cm per hour under standard fire exposure.

Cross-Laminated Timber (CLT) consists of cross-laminated layers of boards that stabilize each other. The charring initially spreads in the outer layers, while the crosswise layers slow the spread. Since the individual board layers can be made from different types of wood, they may have different charring rates. The charring rate of commercially available CLT construction elements made from spruce typically has β₀ = 0.65 mm/min for the first layer. Delamination (the separation of layers) can accelerate the charring process, which needs to be considered in fire safety design.

Glued Laminated Timber (GLT) consists of several laminated wood layers with a parallel fibre orientation. Due to its homogeneous structure, it exhibits uniform charring. The charring rate of GLT made from softwoods is typically β₀ = 0.65 to 0.7 mm/min under normal conditions according to EN 1995-1-2.

Solid Structural Timber (Konstruktionsvollholz, KVH) is solid, technically dried timber with defined moisture content and high dimensional stability. The charring occurs uniformly from the outside inward. KVH made from domestic softwoods and beech also follows the usual one-dimensional burning rate of β₀ = 0.65 to 0.7 mm/min.

The charring rate of solid wood can vary significantly depending on the wood species. The burning speed of softwood is generally assumed to be β₀ = 0.6 to 0.7 mm/min, while hardwoods or tropical hardwoods with higher density, such as teak, can have a slower charring rate, up to β₀ = 0.4 mm/min.

Thus, wood burns in a predictable manner, which is crucial for the planning of timber construction projects, especially for multi-story buildings where fire protection plays a central role. Through tested and certified material properties, timber construction can be safely incorporated into fire protection measures and structurally designed accordingly.