24.06.2026

Vapor-Permeable Insulation: An Overview of the Basics and Requirements

Building design

Gray residential building in a contemporary architectural style – Photo: gett_urban / Unsplash

Insulation that “breathes” may sound like a contradiction at first: after all, thermal insulation is supposed to retain energy, not allow air and vapor to pass through. But diffusion-open insulation elegantly resolves this apparent contradiction by combining thermal protection with moisture regulation. This concept lies at the heart of modern building physics and is just as important for the durability of building structures as it is for a healthy indoor climate. Anyone who understands what “diffusion-open insulation” means understands one of the most important principles of contemporary construction.

What “diffusion-open insulation” means and how it differs from diffusion-tight systems
The physical principles behind vapor diffusion and water vapor permeability
Which insulation materials are considered diffusion-open and why
How the sd-value functions as a parameter and how to interpret it
When vapor-permeable insulation is appropriate and when vapor-impermeable systems are a better fit
The role played by wall construction, the sequence of layers, and the Glaser method
What typical mistakes occur during planning and installation
How vapor-permeable insulation is used in new construction and renovation

Definition: What does “diffusion-open insulation” mean?

“Diffusion-open insulation” refers to insulation materials and systems that allow water vapor to diffuse through their structure to a significant degree. The term refers to a building material’s property of not blocking water vapor but rather allowing it to pass through the material. In contrast, there are diffusion-tight or vapor-barrier materials that largely prevent the transport of water vapor. Diffusion openness is not an absolute property, but rather a gradual one: Every building material allows water vapor to pass through to varying degrees, and the classification as diffusion-open or diffusion-tight depends on the reference value.

The parameter used to describe diffusion openness in building physics is the sd value (water vapor diffusion-equivalent air layer thickness), measured in meters. 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. An sd-value below 0.5 meters is considered diffusion-open, values between 0.5 and 1,500 meters are considered diffusion-inhibiting, and values above that are considered vapor-barrier or vapor-tight. This value is calculated as the product of the material thickness and the dimensionless water vapor diffusion resistance factor µ (My), which is a material constant and indicates how much less effectively the material conducts water vapor compared to still air.

In practice, this means that diffusion-open insulation allows moisture originating from the interior or generated within the building component to migrate through the structure and be transported to the outside. This is particularly valuable when moisture from various sources enters the wall assembly and is not intended to be permanently trapped. Diffusion-open insulation systems rely on the principle of moisture regulation through equalization, not through blocking.

Physical Principles: Vapor Diffusion, Dew Point, and Condensation in Building Components

Water vapor moves through building components by diffusion—that is, due to the concentration difference between areas of higher and lower vapor concentration. As a rule, the indoor air has a higher water vapor concentration than the outdoor air, especially in winter. The vapor pressure indoors is then higher than outdoors, and water vapor migrates through the wall assembly from the inside to the outside. This process is physically inevitable as long as a pressure difference exists, and it occurs throughout all building components, albeit with varying intensity depending on the material.

The problem arises when the migrating water vapor encounters a layer whose temperature is below the dew point. At this point, the vapor condenses into liquid water within the cross-section of the building component—a process that building physicists refer to as interstitial condensation. Liquid water inside a wall or roof damages insulation materials, wooden structures, and plaster, promotes mold and rot, and reduces thermal insulation performance. Vapor-permeable insulation systems address this risk not by preventing vapor transport, but by ensuring that moisture that has penetrated the structure can dry out again.

The classic calculation method for assessing condensation formation in building components is the Glaser method, named after the German building physicist Helmut Glaser. It calculates the distribution of temperature and vapor pressure across the cross-section of the building component and indicates whether and where the temperature falls below the dew point within the component. The method is simplified and does not account for either capillary transport or hygroscopic storage, but it provides a normatively recognized basis for evaluating wall assemblies. In Germany, it is enshrined in DIN 4108-3, which sets requirements for moisture protection of exterior building components. For more complex structures, particularly in the case of diffusion-open systems with hygroscopic materials, experts recommend supplementary hygrothermal simulations, such as those performed using the WUFI program.

A key principle in diffusion-open wall assemblies is the so-called vapor barrier rule: the diffusion resistance of a structure should decrease from the inside to the outside. The inner layer may slow the vapor slightly, but the outer layer must allow it to pass through much more easily. Only in this way can moisture that has penetrated the building component dry out toward the exterior. If this rule is reversed, moisture will accumulate inside the wall.

Comparing Insulation Materials: Which Materials Are Considered Diffusion-Open?

There is a considerable range of insulation materials in terms of their permeability to water vapor. Mineral fiber insulation materials such as mineral wool (glass wool and rock wool) are among the most permeable materials available. Their µ-value is around 1, which means they allow water vapor to pass through almost as easily as still air. They are therefore the benchmark for vapor-permeable insulation and are used in exterior walls, roof structures, and facade systems where moisture tolerance is required.

Natural insulation materials such as wood fiber insulation boards, cellulose insulation, hemp, flax, and sheep’s wool also have low µ-values, typically between 1 and 5, and are therefore considered vapor-permeable. They have the added advantage of a pronounced hygroscopic buffering effect: they can temporarily store moisture and release it again when humidity drops, without immediate condensation occurring. Wood-fiber insulation boards are particularly commonly used as exterior insulation on wood-frame structures because they preserve the vapor permeability of the entire structure while also providing good thermal protection in the summer.

Expanded polystyrene (EPS, Styrofoam) and extruded polystyrene (XPS) have µ-values ranging from 20 to 150, which are significantly higher, and are considered to be diffusion-inhibiting to diffusion-tight, depending on their density and thickness. Rigid polyurethane foam (PUR/PIR) is even denser and is used in systems where a vapor barrier is specifically required. Phenolic resin foam behaves similarly. These materials are not inherently inferior, but they require a different design approach: If the insulation material is vapor-impermeable, the entire wall assembly must be designed to account for this barrier effect, and moisture ingress must be controlled through other measures.

Calcium silicate boards, which are primarily used as interior insulation and for mold prevention, are highly capillary-active and vapor-permeable. They can absorb moisture, transport it via capillary action, and release it again without visible condensation. Their µ-value ranges from about 3 to 6. Perlite and expanded clay, used as loose-fill insulation materials, are also vapor-permeable and are used in cavities and ceiling structures.

Overview of typical µ-values for selected insulation materials

Mineral wool (glass wool and rock wool): µ ≈ 1
Wood-fiber insulation boards: µ ≈ 1 to 5
Cellulose insulation (blown-in): µ ≈ 1 to 2
Hemp, flax, sheep’s wool: µ ≈ 1 to 3
Calcium silicate boards: µ ≈ 3 to 6
EPS (expanded polystyrene): µ ≈ 20 to 100
XPS (extruded polystyrene): µ ≈ 80 to 150
PUR/PIR rigid foam: µ ≈ 30 to 100

Wall Construction and Layer Sequence: How to Properly Plan Diffusion-Open Insulation

The effectiveness of vapor-permeable insulation does not depend solely on the insulation material itself, but on the entire wall construction and the sequence of its layers. The basic principle is that the vapor resistance must decrease from the inside to the outside. Specifically, this means that the innermost layer may have the highest sd value, while the outermost layer must be as diffusion-open as possible. Only then can moisture that has penetrated the wall assembly dry out toward the exterior without accumulating inside.

A classic example is wood-frame or solid wood construction. Here, the inner lining often consists of a vapor barrier with an sd value of about 2 to 10 meters, followed by a layer of mineral wool or wood fiber insulation, and finished on the outside with a diffusion-open wood fiber insulation board or a rear-ventilated facade. The outer layer has an sd value of less than 0.5 meters; the inner vapor barrier slows the influx of moisture but allows enough to pass through so that the structure can dry out from the inside in the summer. This system is proven and widely used in timber construction.

In solid construction using masonry or concrete, the situation is different. Solid brick masonry has µ-values between 5 and 10 and is therefore only moderately vapor-permeable. An exterior external thermal insulation composite system (ETICS) made of mineral wool with mineral plaster maintains the structure’s diffusion openness because both the insulation material and the plaster have low sd values. An ETICS with EPS insulation and silicone resin plaster, on the other hand, is significantly less vapor-permeable, which may not be a problem with well-dried masonry but can lead to issues with moisture-laden masonry because the moisture cannot escape to the outside.

Retrofit interior insulation of exterior walls requires special care; this is a common measure in the renovation of older buildings where exterior insulation is not possible for design or technical reasons. With interior insulation, the dew point shifts toward the now-colder exterior wall. If a vapor-impermeable interior insulation system is installed, the exterior wall can no longer dry out toward the interior, and moisture accumulates inside the wall. Vapor-permeable interior insulation systems made of calcium silicate or wood fiber, combined with capillary-active plasters, solve this problem by absorbing moisture, distributing it via capillary action, and slowly releasing it again. However, these systems require careful planning and a precise understanding of the wall’s initial moisture content.

Common Mistakes and Misconceptions Regarding Vapor-Permeable Insulation

A common misconception is to equate vapor-permeable insulation with air permeability. Diffusion and convection are two fundamentally different transport mechanisms. Diffusion is the molecular transport of water vapor through a material, driven by differences in concentration or pressure. Convection is the transport of air—and thus also moisture—through flow, i.e., through cracks, joints, or unsealed cavities. Diffusion-open insulation can and should be installed in an airtight manner. Airtightness and diffusion openness are not mutually exclusive; they are independent properties.

Another misconception is the assumption that diffusion-open insulation is automatically better or safer than diffusion-tight systems. This is not universally true. Diffusion-tight systems function perfectly when they are consistently planned and executed, and when no uncontrolled moisture ingress from the outside—such as from driving rain or rising damp—is expected. Diffusion-open systems offer greater tolerance for design and installation errors because they can balance out moisture, but they are not a panacea. Both system philosophies have their merits, and the choice depends on the building component, the location, the intended use, and the existing condition of the structure.

The importance of connection details is also often underestimated. A vapor-permeable insulation layer that is not carefully connected to window reveals, baseboards, or roof edges loses much of its effectiveness. Convective moisture ingress at leaky points cannot be compensated for by the insulation material’s diffusion-openness. Proper execution of the connections is therefore just as important as the choice of material itself.

Finally, the sd-value of plasters and coatings is often overlooked. A vapor-permeable insulation material behind a vapor-impermeable interior plaster or a vapor-barrier wall paint loses its function because the vapor is already blocked at the surface. Silicate paints, mineral plasters, and clay plasters have very low sd values and are compatible with the logic of a diffusion-open system. In contrast, emulsion paints and synthetic resin plasters can have sd values of several meters and should be avoided in diffusion-open constructions.

Applications: Diffusion-Open Insulation in New Construction and Renovation

In new construction, vapor-permeable insulation is used particularly consistently in wood construction. Wood-frame construction, solid wood construction, and cross-laminated timber (CLT) structures benefit from vapor-permeable insulation systems because wood, as an organic material, is sensitive to persistently elevated moisture levels. Wood-fiber insulation boards as exterior insulation, cellulose insulation as blow-in insulation in cavities, and mineral wool as between-rafter insulation are typical combinations that combine thermal insulation with moisture tolerance. These constructions are supplemented with an interior vapor barrier that limits vapor ingress without completely blocking it.

When performing energy-efficient retrofits of existing buildings, choosing the right insulation system is particularly challenging because the moisture content of the existing building structure, its usage history, and the structural details of the building must all be taken into account. Vapor-permeable exterior insulation systems made of mineral wool with mineral plaster are widely used in renovation projects because they allow damp existing walls to dry out toward the exterior. The moisture content of the masonry should be measured before insulation is installed, as insulating on a damp substrate slows down the drying process and, in the worst case, can lead to frost damage.

Separate rules apply to flat roofs and inverted roofs. The classic warm roof with a vapor barrier beneath the insulation layer uses a vapor-tight system. The inverted roof, on the other hand, places the insulation above the waterproofing, which requires extruded polystyrene (XPS) as a water-resistant but diffusion-tight insulation material. Diffusion-open insulation materials are not suitable for inverted roofs because they cannot withstand prolonged moisture penetration. In pitched roofs, however, diffusion-open between-rafter insulation made of mineral wool or wood fiber, combined with a diffusion-open underlayment, is standard.

Vapor-Permeable Insulation in the Context of Holistic Moisture Protection

Diffusion-open insulation is not an isolated product feature, but rather part of a comprehensive design philosophy that views thermal insulation and moisture protection as inseparable. A wall assembly is always a system in which each layer interacts with every other layer. The decision to use a vapor-permeable insulation material has implications for plasters, paints, vapor barriers, and connection details. Anyone who selects only the insulation material without carefully considering the entire layered structure risks errors that will only become apparent years later.

The increased requirements for the energy efficiency of buildings have heightened—not diminished—the importance of moisture planning. Thicker insulation layers shift the dew point further outward and alter the drying dynamics of building components. At the same time, airtight building envelopes make controlled moisture transport via diffusion more important, because uncontrolled convection is no longer available to balance moisture levels. In this context, diffusion-open systems offer greater tolerance for errors but require thorough planning.

Architects, building physicists, and contractors who understand the interplay of heat, moisture, and diffusion can develop structures that function reliably over the long term without requiring extensive maintenance or premature renovation. Knowledge of diffusion-open insulation is not a niche topic for specialists, but rather a fundamental skill that affects every design decision in building construction. Buildings that are properly insulated and properly designed protect their occupants, the building structure, and the climate alike—and do so for decades to come.

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.