A building component that allows water vapor to pass through without trapping it sounds like a simple property. In fact, it involves a complex interplay of building physics, materials science, and careful design. What exactly does “diffusion-open” mean, why is this property so important for the durability of buildings, and when does it become an indispensable design criterion? The answer goes to the heart of modern moisture protection.

• What “diffusion-open” means in physical terms and how the term is correctly defined
• The difference between vapor diffusion, vapor diffusion resistance, and the sd-value
• Which building materials are considered vapor-permeable and why this is not an absolute category
• How vapor-permeable constructions prevent moisture damage and when they fail
• The relationship between airtightness and diffusion-openness as frequently confused concepts
• The role of the Glaser verification and the DIN 4108 standard in the evaluation
• Typical applications: wood construction, exterior walls, roofs, and interior insulation
• Common misunderstandings and design errors when dealing with vapor permeability

What does “vapor-permeable” mean: Definition and physical principles

“Diffusion-open” refers to the property of a building material or component structure to allow water vapor to pass through its cross-section without significantly impeding this vapor transport. The opposite is diffusion-tight or vapor-barrier: a material that almost completely prevents water vapor from passing through. Between these two extremes lies a broad spectrum, which is quantified by the so-called water vapor diffusion resistance factor (Greek: Mü, symbol µ). This dimensionless factor indicates how much more strongly a material inhibits vapor transport than a stationary layer of air of the same thickness. By definition, air itself has a µ-value of 1; a material with µ = 10 inhibits diffusion ten times more effectively than air.

In practice, the so-called sd value—the water vapor diffusion-equivalent air layer thickness, expressed in meters—is calculated from the µ value and the layer thickness. The sd value describes how thick a stationary layer of air would have to be to produce the same diffusion resistance as the component in question. A material with µ = 5 and a layer thickness of 0.1 meters has an sd value of 0.5 m. The classification into diffusion-open and diffusion-tight is based on these sd values: layers with an sd value below 0.5 m are considered diffusion-open, while layers with an sd value above 1,500 m are considered vapor barrier-equivalent. In between are diffusion-retarding layers, which slow down vapor transport but do not prevent it.

Water vapor diffusion is the process by which water vapor molecules migrate through solid materials due to a partial pressure gradient. The driving force is not air movement, but rather the difference in water vapor partial pressure between the two sides of a building component. In winter, the vapor pressure on the warm interior side of an exterior building component is generally higher than on the cold exterior side, so vapor diffuses from the inside to the outside. In summer, this gradient can reverse. A diffusion-open construction allows this transport to occur without moisture accumulating inside the building component and causing damage.

Diffusion Openness and Water Vapor Permeability: The Spectrum of Building Materials

No building material is absolutely diffusion-open or absolutely diffusion-tight; it is always a continuum. Mineral building materials such as sand-lime brick, aerated concrete, or ordinary concrete have µ-values between approximately 5 and 150, depending on their bulk density and pore structure. Wood has µ-values between 20 and 200, depending on the grain direction; wood fiber boards range from 3 to 20, depending on their density, and are therefore considered extremely vapor-permeable. Mineral wool has a µ-value close to 1—meaning it offers hardly any more resistance than air—and is thus the most diffusion-open of all common insulation materials. Expanded polystyrene (EPS), on the other hand, has µ-values ranging from 30 to 70; extruded polystyrene (XPS) even reaches values from 80 to 250; and rigid polyurethane foam can have values ranging from 30 to over 100. Polyethylene films, such as those used as vapor barriers, have µ-values in the range of several tens of thousands.

The same principle applies to plasters and paints. A mineral lime plaster, with µ values around 15 to 35, is significantly more vapor-permeable than a synthetic resin plaster, which has values ranging from 50 to over 200. Silicate-based exterior paints are considered vapor-permeable, whereas emulsion paints with a high plastic content can significantly increase the sd value of a wall surface. This fact is often underestimated in practice: Applying a diffusion-inhibiting paint coating at a later stage can impair the function of a carefully designed, diffusion-open wall structure.

It is important to distinguish between the µ-value of a single material and the sd-value of an entire layered structure. The sd-values of a wall cross-section consisting of multiple layers are added together. The decisive factor for assessment is not a single layer, but the ratio of the sd-values from the inside to the outside: To ensure that moisture diffusing into the building component can dry out again, the outer side must be more vapor-permeable than the inner side. This principle, known in building physics as “more airtight on the inside than on the outside,” forms the basic rule for any wall construction designed to prevent damage.

Diffusion-open versus airtight: A fundamental misunderstanding

Hardly any other pair of terms is more frequently confused in practice than “diffusion-open” and “air-permeable.” Diffusion-openness describes exclusively the molecular transport of water vapor through the structure of a solid material, driven by differences in partial pressure. Air tightness, on the other hand, refers to the convective transport of air—that is, the physical mass flow of air through joints, cracks, connections, and leaks—driven by pressure differences. These two transport mechanisms are fundamentally different in physical terms and require different design measures.

A material can be both vapor-permeable and airtight. Low-density wood fiber boards are a good example: They allow water vapor to diffuse through their structure but, when carefully processed and the joints are properly sealed, form an effective airtight barrier. Conversely, a construction can be air-permeable and simultaneously diffusion-resistant if, for example, a membrane with a high µ-value is poorly bonded: The membrane inhibits diffusion but allows convective moisture flows to pass through leaks, which can account for many times the amount of moisture transported by diffusion.

This difference has significant practical consequences. Convective moisture ingress through air leaks is generally orders of magnitude more effective than vapor diffusion. A small leak in the airtightness layer can introduce more moisture into a roof structure than the total diffusion through a well-designed construction. This is why building physicists emphasize that airtightness is even more important for moisture protection than diffusion openness. Diffusion openness is the second line of defense: it allows moisture that has entered the building component despite good airtightness to dry out. Both properties complement each other but are not interchangeable.

The Glaser Method and Classification According to DIN 4108

In Germany, the computational evaluation of the moisture behavior of exterior building components is performed using the Glaser method, which is enshrined in the DIN 4108-3 standard. The method, named after the German building physicist Helmut Glaser, who developed it in the 1950s, checks whether condensation forms in a building component cross-section under defined steady-state boundary conditions and whether this condensation accumulates during the condensation period or can be dissipated again during the evaporation period. The analysis uses monthly average values for temperature and relative humidity for both indoor and outdoor climates.

The Glaser method has known limitations: It does not account for the hygroscopic storage effects of building materials, capillary transport processes, or sorption. Materials such as wood, clay, or cellulose insulation can temporarily store moisture and release it again with a time delay, which the Glaser method does not account for. For such structures, hygrothermal simulation methods such as the WUFI (Transient Heat and Moisture) program are more suitable, as they simulate transient moisture and heat transport processes over several years. Nevertheless, the Glaser analysis remains the standard method required by standards for verifying minimum thermal insulation and is, in practice, the most common starting point for evaluating wall assemblies.

DIN 4108-3 also defines the limits for permissible condensation volumes and requires that the evaporation rate during the dry season exceed the condensation volume. For diffusion-open constructions, this verification is generally unproblematic because the moisture can dry out quickly. Problems arise with constructions where a vapor-barrier layer is located on the exterior, thereby preventing drying, or where the dew point lies within a moisture-sensitive layer, such as in a wood structure or thermal insulation made of organic material.

Diffusion-open constructions in timber framing, roofing, and interior insulation

Wood-frame construction and solid wood construction

In timber construction, diffusion openness is a central design principle. Wood reacts to changes in moisture by swelling and shrinking, which—under prolonged exposure to moisture—leads to cracking, warping, and, in the worst case, fungal infestation. A typical diffusion-open wood-frame structure consists, from the inside out, of a vapor barrier with a variable sd-value, cavity insulation made of mineral wool or wood fiber, a wood-fiber insulation board serving as a plaster base or wind barrier, and exterior plaster or a rear-ventilated facade. The vapor barrier on the interior side has a significantly higher sd value than the exterior layers, thus fulfilling the basic rule of “more impermeable on the inside than on the outside” without making the exterior side vapor-impermeable.

Moisture-adaptive vapor barriers, also known as “intelligent” vapor barriers, adjust their sd value depending on the relative ambient humidity. In dry winter conditions, they increase their diffusion resistance, thereby protecting the structure from incoming vapor. In summer, when the structure needs to dry out, their sd value decreases, allowing moisture to be transported inward. This property is particularly valuable in structures that can become damp from the outside in summer, such as roofs with dark roofing materials that heat up significantly and create an inward-directed vapor pressure gradient.

Pitched Roofs and Flat Roofs

For pitched roofs with between-rafter insulation, the vapor permeability of the underlayment is critical. A diffusion-open underlayment with sd values below 0.3 m allows moisture that enters the insulation layer to diffuse outward into the rear ventilation layer. Diffusion-tight underlayment membranes would prevent this drying process and cause moisture to accumulate in the rafter space. Distinguishing between diffusion-open underlayment and diffusion-tight vapor barriers is fundamental in roof design; both products look similar but serve opposite functions.

With flat roofs, the situation is more complex. Inverted roofs with insulation above the waterproofing intentionally use a diffusion-tight waterproofing layer because the insulation is on the outside and the waterproofing protects it from temperature fluctuations. Warm roofs with insulation below the waterproofing, on the other hand, require careful calculation of condensation conditions, since the waterproofing forms a diffusion-tight seal on the exterior. Here, the vapor barrier on the interior is particularly important to minimize vapor ingress into the insulation layer.

Interior Insulation as a Special Case

From a building physics perspective, interior insulation of exterior walls is the most challenging application scenario regarding diffusion openness. Because the insulation is on the interior side, the exterior wall is cut off from the room’s heat and continues to cool down. The dew point of the indoor air can now be reached within the wall assembly, at the interface between the interior insulation and the existing wall. Diffusion-open interior insulation systems made of calcium silicate or wood fiber can absorb and release moisture by capillary action, which distributes condensation and reduces the potential for damage. Diffusion-tight interior insulation systems made of foam glass or XPS, on the other hand, shift the dew point level entirely outward and prevent any moisture absorption within the insulation layer; however, they require absolutely airtight connections to rule out convective moisture ingress.

Common Misconceptions and Design Errors

A widespread misconception is that “diffusion-open” automatically means “good” and “diffusion-tight” means “bad.” This oversimplification is incorrect and leads to design errors. Diffusion-tight layers are not only permissible but necessary in certain locations within the building component structure: A vapor barrier on the interior side of a well-insulated roof protects the structure by minimizing vapor ingress from the outset. The problem arises not from the vapor barrier itself, but from its incorrect positioning or from leaks that negate its protective effect.

Another common mistake is to evaluate individual layers of a structure in isolation, without considering the overall structure. A vapor-permeable exterior plaster is of little use if there is a vapor-impermeable XPS composite thermal insulation panel underneath it, which largely prevents vapor transport to the outside. Conversely, a diffusion-inhibiting interior coating applied retroactively can disrupt the balance of a carefully planned, diffusion-open structure. The assessment must always cover the entire layered structure from the inside out.

Finally, vapor permeability is occasionally misunderstood as a substitute for airtightness. Some designers and contractors conclude from a material’s diffusion-openness that a carefully designed airtight layer is unnecessary because “the structure can breathe.” This conclusion is untenable from a building physics perspective. Convective moisture ingress through air leaks far exceeds diffusive transport and can lead to damage even in diffusion-open structures if airtightness is inadequate. Diffusion openness and airtightness are not alternatives but complementary requirements.

Diffusion Openness as a Long-Term Design Principle

What does “diffusion-open” ultimately mean for architecture and building design? It means that building envelopes must not be conceived as static barriers, but rather as dynamic systems that absorb, transport, and release moisture. This understanding has become established in modern building physics and influences the choice of materials, the sequence of layers, and the detailed design of exterior walls, roofs, and floor slabs alike.

Energy efficiency requirements have increased rather than diminished the importance of diffusion openness. Thicker insulation layers, airtight building envelopes, and lower heat inputs mean that building components dry out more slowly and moisture ingress is offset less quickly. Designs that used to function despite suboptimal diffusion conditions—because heat and natural infiltration ensured sufficient drying—may fail under modern efficiency standards. Careful planning of diffusion conditions is therefore no longer optional but mandatory.

Architects, civil engineers, and specialist planners who use the term “diffusion-open” precisely and handle the associated parameters confidently make better decisions regarding material selection, the detailed planning of connections, and the assessment of renovation projects. Buildings that are properly designed with moisture physics in mind require less maintenance, last longer, and provide a healthier indoor climate. This is not an abstract promise, but the measurable result of consistent building physics planning, which begins with an understanding of a single, seemingly simple question: What does “diffusion-open” mean?