Insulation that breathes: That may sound like a contradiction, but it is the core principle behind one of the most important developments in modern thermal insulation. Vapor-permeable insulation refers to insulation systems and structures that allow water vapor to diffuse through the building component in a controlled manner, rather than completely blocking it. Anyone who understands this principle realizes that moisture within a building component isn’t necessarily a problem—it only becomes a hazard when it accumulates unchecked and can no longer escape.
- What vapor-permeable insulation means and how it differs from vapor-barrier systems
- The physical principles of vapor diffusion and the sd-value as a key metric
- Which insulation materials are considered diffusion-open and why this is not a universal characteristic
- How diffusion-open insulation is used in roofs, walls, and floors
- The role of the Glaser verification and the DIN 4108 standard in this context
- When vapor-permeable systems are appropriate and when vapor-retardant or vapor-barrier solutions are a better fit
- Common design errors and how to avoid them
- How vapor-permeable insulation fits into the broader context of moisture protection and sustainability
Definition: What “diffusion-open insulation” means
Diffusion-open insulation refers to insulation systems whose design allows water vapor to migrate through the building component without causing a harmful accumulation of moisture within the cross-section. The term encompasses two aspects: on the one hand, the property of individual insulation materials to allow water vapor to pass through easily; on the other hand, the concept of an entire building component structure designed so that moisture that has penetrated can dry out again. Vapor-permeable insulation is therefore not a single product, but rather a design strategy.
The opposite of this are vapor-barrier or vapor-retardant systems, in which a layer of polyethylene film, aluminum laminate, or similar material virtually completely blocks vapor transport. Such systems are appropriate and technically sound in certain situations, but they require that no moisture enter the building component at all, because otherwise it cannot escape. Diffusion-open systems follow a different philosophy: they accept that moisture penetrates building components and ensure that it can also escape again.
To understand this, it is essential to clarify the terminology. Diffusion refers to the physical process by which water vapor moves through materials due to a partial pressure gradient, without requiring a pressure difference or air flow. This process is slow, continuous, and inevitable. Even materials considered to be airtight are permeable to water vapor to a certain extent. The question is not whether diffusion occurs, but whether it leads to condensation within the building component and whether any resulting condensation can dry out again.
The sd value: The key measure of vapor permeability
The most important parameter for assessing the diffusion permeability of a material or layer is the sd value, also known as the water vapor diffusion equivalent air layer thickness. It indicates how thick a stationary layer of air would have to be to offer the same resistance to water vapor diffusion as the material in question at its actual thickness. The unit is meters. A material with an sd value of 0.1 m allows vapor to pass through as easily as a ten-centimeter-thick layer of air, while a material with an sd value of 100 m corresponds to one meter of still air and is therefore practically vapor-tight.
The sd value is calculated as the product of the diffusion resistance coefficient μ (Greek: μ) and the material thickness in meters. The μ value is a dimensionless material parameter that indicates how much less effectively a material conducts water vapor compared to still air. By definition, air itself has μ = 1. Mineral wool typically has a μ value of 1 to 2, wood fiber boards have a μ value of about 3 to 5, expanded polystyrene (EPS) at μ = 20 to 100, and polyurethane foam (PUR) can reach values of μ = 30 to 150. Aluminum and glass fall in the range of several thousand, making them practically impermeable to water vapor.
In practice, the rule of thumb is as follows: Materials with an sd value below 0.5 m are considered diffusion-open; those between 0.5 m and 1.5 m are considered diffusion-inhibiting; and layers with sd values above 1,500 m are considered vapor barriers. These thresholds are not defined by standards but have become established as guidelines within the industry. The decisive factor is always the overall structure of the building component, not the sd value of a single layer. A diffusion-open insulation material installed behind a vapor-tight interior lining cannot function as a diffusion-open material.
Comparison of Insulation Materials and Their Diffusion Properties
Among commonly used insulation materials, mineral wool (glass and rock wool), wood fiber, cellulose, hemp, sheep’s wool, and perlite are considered vapor-permeable. They all have low μ-values and allow vapor to pass through largely unimpeded. Wood fiber insulation materials, which are available both as blow-in insulation and as panels, combine vapor permeability with a pronounced ability to buffer moisture: They can absorb moisture by capillary action, temporarily store it, and release it again when conditions are more favorable. This hygroscopic property goes beyond mere vapor diffusion and makes wood fiber particularly suitable for renovation projects.
In contrast, rigid polystyrene foam (EPS, XPS) and polyurethane foam (PUR, PIR) are diffusion-resistant to nearly vapor-tight. They are excellent thermal insulation materials with high compressive strength, but their closed-cell structure allows virtually no vapor to pass through. This makes them technically suitable for certain applications, such as perimeter insulation in the ground or the insulation of flat roofs with an inverted roof system. In structures that rely on vapor permeability—such as ventilated wooden roof structures or vapor-permeable facade systems—they are, however, unsuitable or require careful planning of the adjacent layers.
Foam glass occupies a special position: It is completely vapor-tight and, at the same time, capillary-inert—meaning it is unable to absorb water. It is therefore specifically used where moisture from the ground or from wet rooms must be completely kept out. Foam glass is the opposite of a vapor-permeable insulation material, but it fulfills an important function within its niche. Choosing the right insulation material is therefore always a matter of context, not of abstract material quality.
Natural Insulation Materials and Their Unique Properties
Natural insulation materials such as hemp, sheep’s wool, seaweed, or straw are increasingly being used in environmentally conscious construction projects. Their vapor permeability is generally very high, and many of them possess pronounced hygroscopic properties. Sheep’s wool, for example, can absorb up to one-third of its own weight in moisture without significantly reducing its insulating effect, and it releases this moisture again as ambient humidity decreases. This ability to regulate moisture is a real advantage in structures exposed to temporary spikes in humidity.
However, natural insulation materials also place special demands on installation and protection against permanent moisture penetration. Organic materials are susceptible to mold and rot if they remain too damp for extended periods. Diffusion openness alone does not protect against damage if the building structure does not allow for sufficient drying or if construction moisture has not completely dried out before the structure is sealed. Proper planning and careful execution are particularly important when using natural insulation materials.
Applications: Roofs, Walls, and Floors
In pitched roofs, diffusion-open insulation is particularly common and well-founded from a building physics perspective. Wooden rafters and wood sheathing are moisture-sensitive materials that are prone to mold and rot if moisture accumulates over time. A roof structure with a diffusion-open underlayment on the exterior and a vapor barrier on the interior allows moisture that has penetrated the structure to be transported outward, while limiting the ingress of indoor air moisture from the inside. The sd-value of the interior vapor barrier should be significantly higher than that of the exterior underlayment so that moisture can dry out toward the exterior. The basic rule among experts is: tighter on the inside than on the outside.
In exterior wall insulation, diffusion-open insulation is primarily used in external thermal insulation composite systems (ETICS) with mineral wool and in rear-ventilated facade systems. Mineral wool boards in ETICS allow vapor to pass through, which can then escape via the diffusion-open exterior plaster. Ventilated facades even provide active drying through the airflow in the ventilation layer. Both systems require that the interior plaster and interior building components do not have excessively high vapor resistance, which would prevent moisture from drying out toward the exterior.
In wood-frame construction, which has become increasingly important in Central Europe, the issue of diffusion openness is particularly complex. Wood-frame constructions with cellulose or wood-fiber insulation between the studs and an exterior wood-fiber insulation board serving as a wind-pressure barrier can be designed to be fully vapor-permeable, provided the interior cladding does not have an excessively high sd-value. Alternatively, moisture-adaptive vapor barriers are used, whose sd-value changes with ambient humidity: In winter, when the indoor air is dry and vapor pushes from the inside to the outside, they increase their resistance; in summer, when moisture can escape inward, their sd-value decreases. These intelligent films combine the principle of diffusion openness with active protection against winter condensation.
In floor applications, diffusion-open insulation is less common because floors are generally not exposed to significant vapor diffusion from the inside to the outside, and because moisture can rise from the ground via capillary action, which requires other protective measures. Here, pressure-resistant, vapor-tight, or capillary-passive insulation materials predominate. Exceptions include wooden-beam ceilings above basements or crawl spaces, where diffusion-open insulation between the beams protects the wooden structure, provided the crawl space is sufficiently ventilated.
Glaser Method and DIN 4108: Normative Foundations
The most important computational tool for assessing condensation formation in building components is the Glaser method, named after the German building physicist Herbert Glaser, who developed it in the 1950s. For a defined building component cross-section and standardized climatic boundary conditions, the method calculates whether and where within the component the temperature falls below the dew point of the vapor passing through—that is, where condensation occurs. It takes into account the thermal resistance and vapor diffusion resistance of all layers. The result indicates whether condensation forms within the building component and whether it can dry out again in the summer.
The Glaser method is enshrined in DIN 4108-3, the section of the German thermal insulation standard that deals with moisture protection. The standard specifies limit values for permissible amounts of condensation and requires verification that the amount of moisture that dries out in the summer exceeds the amount of condensation that forms in the winter. For diffusion-open systems, this can generally be demonstrated without difficulty because the thin vapor barrier facing the exterior allows for rapid drying. In the case of vapor-tight systems, however, it must be demonstrated that no condensation occurs because drying is not possible.
The Glaser method has limitations that are well known among experts. It does not account for capillary moisture transport, hygroscopic storage, or air flows within the building component. For building components containing hygroscopic materials, such as wood fiber or cellulose, it therefore yields overly conservative results: It overestimates the risk of condensation because it ignores the buffering effect of the materials. For such structures, hygrothermal simulation programs such as WUFI (Heat and Moisture Transient) are available, which simulate the actual moisture balance of a building component over a period of several years and provide results that are significantly closer to reality. In design practice, WUFI is increasingly being used as a supplement to or replacement for the Glaser method, particularly for innovative or eco-friendly insulation systems.
Common Design Errors and How to Avoid Them
The most common error in diffusion-open structures is confusing a vapor retarder with a vapor barrier, as well as the incorrect placement of these layers. A vapor retarder with an sd value of two to four meters on the interior side of a rafter roof is sensible and intentional; a vapor barrier with sd values exceeding 1,500 m in the same location would destroy the diffusion-open character of the structure and prevent moisture from drying out toward the interior. It is equally problematic to replace a diffusion-open underlayment with a vapor-tight membrane simply because the latter is cheaper or more readily available. The structure thereby loses its functionality without this being apparent at first glance.
Another common mistake concerns airtightness. Diffusion-open insulation and airtightness are two distinct concepts that are often confused. A diffusion-open building component can and should be airtight: Air currents through joints and cracks transport moisture many times faster than diffusion and can generate significant amounts of condensation within the component in a short time. The airtight layer—usually a vapor barrier or airtight sheathing—must be carefully installed and seamlessly sealed at connections. Being vapor-permeable does not mean that air is allowed to flow through the building component.
Finally, the significance of construction moisture is often underestimated. Newly installed wood structures, damp concrete slabs, or plaster that has not fully dried contain significant amounts of moisture that must dry out after the building element is sealed. If a diffusion-open structure is sealed too early, before the construction moisture has dried out, the moisture can in principle escape, but the drying time is significantly prolonged and the risk of mold growth increases. Proper construction using diffusion-open systems therefore also requires careful moisture management on the construction site.
Vapor-Permeable Insulation in the Context of Sustainability and Building Culture
Diffusion-open insulation is not merely a building physics strategy; it represents a specific approach to the building as a physical system. Those who embrace diffusion-openness accept that moisture is an inevitable part of a building component’s life cycle and plan for its management rather than trying to prevent it. This philosophy aligns with the behavior of traditional building structures made of wood, clay, and solid masonry, which have functioned for centuries without vapor barriers because their materials could absorb, buffer, and release moisture.
In the context of ecological construction, diffusion-open insulation is of particular importance. Natural insulation materials such as wood fiber, cellulose, or hemp are not only diffusion-open but also renewable, CO₂-storing, and, at the end of their useful life, compostable or recyclable. They fit into a circular economy model that is difficult to reconcile with vapor-tight plastic foams. For building owners and architects who view sustainability not only in terms of energy efficiency but also in terms of materials, vapor-permeable systems are therefore a logical choice.
At the same time, it would be wrong to reject vapor-barrier systems across the board. In wet rooms, in industrial buildings with high moisture production, on flat roofs with certain construction types, or when insulating reinforced concrete structures, vapor-tight solutions may be the only sensible choice. The key lies in choosing the right approach for each situation and planning it out thoroughly. Diffusion-open insulation is a powerful tool, but not a universal panacea.
For architects, designers, and building physicists, this means that the decision for or against vapor-permeable insulation must be made at the beginning of the planning process, not at the end. It influences the choice of insulation materials, membranes, cladding, and connection details. A subsequent change in the design concept on the construction site almost inevitably leads to compromises that jeopardize the structure’s functionality. Those who understand the physics of vapor diffusion and are familiar with the properties of available materials can design buildings that remain permanently dry, healthy, and energy-efficient without having to sacrifice the durability and expressive power of natural materials.











