12.06.2026

Vapor-Permeable Insulation: Meaning and Applications Explained Simply

Building design

Gray residential building in a functional architectural perspective – Photo: gett_urban / Unsplash

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 almost 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 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 the meter. 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 efficiently a material conducts vapor compared to still air. By definition, air itself has μ = 1. Mineral wool typically has μ = 1 to 2, wood fiber boards have μ = 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 wool 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, store it temporarily, 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 hardly any 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-inactive—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, seagrass, or straw are increasingly being used in environmentally conscious construction projects. Their permeability to water vapor 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 when ambient humidity drops. 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. Professional 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 cavity. Both systems require that the interior plaster and interior building components do not have an excessively high vapor resistance that 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 diffuse 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 transiting vapor load—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 part 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 concept 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 through consistently. 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 diffusion-open 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 building’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.

22.04.2025

Architecture

44 residential units in Saint-Denis from DREAM

Building design

The new building with 44 residential units by DREAM. Photo: Cyrille Weiner

Two decades after the devastating fire in a dilapidated residential building on Rue Fraizier in Saint-Denis, a new construction project marks a turning point in the urban development of the north of Paris. The Parisian agency DREAM (Dimitri Roussel) has realized a residential ensemble with 44 units there – half for rent, half as subsidized ownership according to the “Bail Réel Solidaire” (BRS) model. It is the first project of its kind in Saint-Denis. However, the ambitious gesture is less about architectural showmanship and more about functional, mass-produced housing that strives for social integration.

The new building stands on a site that has been derelict since the fire in 2001. The fire at the time drastically exposed the dilapidated conditions in the old building, which was being used by shark tenants. The ensuing vacancy was perceived not only as a physical defect, but also as a social one. DREAM now sees the project as a contribution to “repairing” the neighborhood – and to re-establishing trust in the urban space.

The 44 residential units are spread across several buildings and follow a clear principle: as much individuality as possible within the standardized production. Almost all of the apartments are open-plan, with many facing in several directions. The majority have generous outdoor spaces – balconies or gardens at ground level. Interior qualities have also been considered: separate entrance areas with storage space, daylight kitchens that can be closed off if required and large window openings with panoramic views are all part of the repertoire.

The floor plan design is based on the charter of Plaine Commune, the inter-municipal association responsible for the area. The urban positioning of the buildings responds to morphological and climatic analyses of the site. A typical planning response is, for example, the staggering and orientation of the volumes to optimize daylight and natural ventilation.

In terms of design, DREAM dispenses with design experiments. Instead, the architectural expression arises from the materiality and rhythm of the façade. Wooden slats, metal panels and open balcony structures made from a combination of wood and metal structure the outer shell. Great importance was attached to prefabrication: The timber frame construction walls, including cladding, windows and shading elements, were manufactured entirely in the factory. The self-supporting balconies also arrived on site pre-assembled.

This strategy has several advantages: Firstly, it increases the quality of execution, and secondly, it reduces the construction time – a factor that plays a particular role in the densely built-up and socially sensitive Saint-Denis. All in all, the result is a residential building that relies on CO₂-reduced construction methods without playing this off visually.

What is striking about the project is the effort to establish communal zones alongside the private living space – a concept that is often referred to elsewhere as “third places”. In Saint-Denis, the elements are simple but effective: a large, inviting entrance area, green inner courtyards with passageways and roof gardens that serve as places to retreat and meet. The lobbies act as semi-public buffer zones between the street and the apartments. Visual references to the courtyard are intended to provide not only light but also social control.

The whole project was designed in collaboration with the public housing association Plaine Commune Habitat. The aim is to appeal to a heterogeneous group of residents – both people on low incomes and young families who want to build up property through the BRS model.

With a construction cost of around seven million euros and a living space of 2,775 square meters (SHAB), the project is within the scope of what is feasible in a subsidized context. The “NF Habitat” certification and compliance with the French thermal insulation regulation RT 2012 with a 20 percent reduction underline the ecological focus.

Those involved in the project include Bollinger+Grohmann (structural engineering), ENEOR (building services), Le Sommer (certification) and Topager for the landscape architecture. Cap-Exe was responsible for coordinating the various trades.

What can be deduced from the project in Saint-Denis for the current housing debate? Certainly not a new type. Rather, it shows how a combination of solid planning, serial production and municipal control can make a contribution to sustainable urban development – beyond creative exaggeration, but also without falling into banal functionality.

The architecture remains restrained but deliberate. It unfolds its effect through everyday use – as a place to live, to meet and to reappropriate a long-neglected urban space.

31.10.2024

Ukraine war: Мы за мир

Building design

As a result of the war in Ukraine, the European architecture scene has quickly taken a public stand against the Russian war of aggression. G+L also stands in solidarity with the Ukrainian people and government.

BIG, David Chipperfield Architects, Foster + Partners, gmp, Herzog und de Meuron, MVRDV, OMA, Snøhetta, Zaha Hadid Architects – as a result of the war in Ukraine, which violates international law, the who’s who of the European architecture scene publicly opposed the Russian war of aggression in a very short space of time at the end of February/beginning of March 2022. Within just a few days, numerous offices expressed their solidarity with the people in Ukraine and with all those who stand for peaceful coexistence – above all via social media. In the case of Chipperfield, HdM, OMA and Zaha Hadid, the public statements were followed by an immediate halt to all construction projects in Russia. BIG also announced in a statement that the office would not be carrying out any projects in Russia or for the Russian government. However, it is not clear from this whether a construction freeze has been imposed or whether there are simply no Russian projects currently in progress.

First the governments, then the private sector. Today, our globalized world also makes it possible for corporations, companies or even planning offices to impose sanctions. So while Apple, Siemens, Starbucks, McDonalds, Coca-Cola, Pepsi and the management consultancies KPMG, PWC, EY and Deloitte are suspending their business in Russia as a result of the war of aggression, or Elon Musk is actively supporting Ukraine with the help of his satellite internet service Starlink, including reception systems, the world of architecture is also drawing its own conclusions. This is worth a special look, as it was or is precisely non-democratic regimes such as Russia or China that have provided the big star offices with unique construction projects in recent years. The M+ Hong Kong designed by HdM only opened at the end of 2021. While at the turn of the year in Moscow, the Renzo Piano Building Workshop RPBW converted the GES-2 power station into a center for visual and performing arts for the V-A-C Art Foundation.

Jacques Herzog on democratic architecture

For us in the editorial team, this immediately (and once again) triggers the question of how political planning can be, but also how political planning must be. What is exciting in this context is that Jacques Herzog in particular has repeatedly publicly addressed the question of democratic architecture. You can think what you like of him and the HdM projects, but he takes a stand. As he did in an interview in 2020 with Lukas Gruntz from architekturbasel.ch. Referring to the historic urban development of St. Petersburg, Venice, Rome and Paris, he said here: “Perhaps more beauty is created in a non-democratic context because the context is more extreme, more radical.” But he also continued: “From our point of view, an enlightened and democratic society, architecture must be anchored in the population and ideally emerge from the needs of the population.” Sentences that should make us think. Now more than ever.

Ukraine war: Coop Himmelb(l)au under pressure over Crimea project

Lighthouse projects in non-democratic regimes must be better considered in future. I wonder what is going through Wolf D. Prix’s head at Coop Himmelb(l)au right now? His office was criticized even before the war of aggression. Since 2020, the Viennese have been planning two of the four cultural buildings that Vladimir Putin wants to be built by 2023. The particularly tricky case is the planned opera house on the Crimean peninsula, which was annexed by Russian occupiers in 2014 in violation of international law(more on this in an SZ-Plus article). With reference to the lighthouse project, Ukrainian President Volodymyr Zelensky imposed economic sanctions against the Viennese architecture firm and six of its representatives on January 21, 2022.

Wolf D. Prix: Coop Himmelb(l)au is building an opera house, not barracks

According to an SZ.de article by Gerhard Matzig, who interviewed Prix on the subject, this was preceded a year and a half ago by threats from the Ukrainian embassy to Coop Himmelb(l)au. Prix would not be allowed to build the opera house in Sevastopol or the architectural firm would soon be ruined. And according to Gerhard Matzig in his article, Prix has now also been advised to distance himself from the project and Putin. When asked by Matzig whether he would do so, Wolf D. Prix sighed on the phone. Prix is of the opinion that he is not building a barracks, but an opera house. As a cultural project, this is not subject to the embargo regulations. Unsurprisingly, as of mid-March 2022, Coop Himmelb(l)au still has no statement on the Ukraine war.

Ukraine war: Russian planners make their mark

But now back to those who openly oppose the war. Because it’s not just the European star offices that are flying the flag. According to SZ.de, a total of 6,500 Russian architects, designers and urban planners also signed an open letter on the website of the Russian architecture magazine “Project Russia” between February 26 and March 4, 2022, calling for an immediate end to the war. The tragedy is that this appeal also fell victim to the “fake news” law against critical reporting on the Russian army signed by Vladimir Putin on March 4, 2022. Only a short version of the campaign with a picture of a dove of peace can now be seen on the site. It says here in Russian: “Unfortunately, we were forced to remove the text of the letter under threat of criminal liability under the law that came into force today. We are for peace!”

One profession, one passion

Meanwhile, however, the Union of Architects of Ukraine also called on the International Union of Architects to expel the Union of Architects of Russia from the organization. “Those who do not condemn Russia’s actions support them,” the Süddeutsche Zeitung quotes the President of the National Union of Architects of Ukraine, Oleksandr Chyzhevsky, as saying in a letter to the UIA. If you let this statement sink in, you have to ask yourself – even if you condemn Russia’s actions in the strongest possible terms – whether we really want to live in a world in which people from one industry, one profession, one passion, go against each other simply because of their nationality. For this very reason, the G+L editorial team would like to join our Russian colleagues: Мы за мир. We are for peace. And we condemn the Russian government’s attack on Ukraine, which violates international law, and stand in solidarity with the Ukrainian people and government.

Ukraine war: bdla and BAK also active

While German landscape architecture firms are still quite reluctant to express their solidarity, the bdla published an official solidarity statement #StandWithUkraine on March 2, 2022. The bdla declared its “deepest regret about the war in Ukraine, the loss of human lives.” It condemns this attack, which violates international law. The bdla’s thoughts are particularly with its colleagues from its partner association, the Guild of Landscape Architects of Ukraine. In the same letter, the bdla refers to the initiative of the Federal Chamber of Architects. This has set itself the goal of becoming active beyond expressions of solidarity. For this reason, the BAK is making its network available to the Ukrainian Association of Architects. The goal: sleeping places for refugees. Find out more here.