First of all, thermal insulation is indispensable today for both new and old buildings. Not only are energy-saving measures required (and encouraged) by law and policy, but most homeowners are genuinely concerned about using less energy and becoming less dependent on fossil fuels. The first step in saving energy is to ensure that the building enclosure is as well insulated and sealed as possible. But what exactly do we need to consider?

Exterior insulation: building science versus design

Typically, a building physicist’s first choice is exterior insulation. This has two advantages:

• All structural components are protected and evenly covered, keeping moisture and potential frost damage away from the component.
• The thickness of the insulation is not at the expense of the interior, which should be available as valuable floor space.

However, as is often the case, every advantage has its disadvantages: the cost of exterior insulation is relatively high, as scaffolding must be erected and the entire facade must be worked on. In particular, the transitions between walls and a roof overhang or recessed windows, pose great challenges in terms of planning and workmanship. If too little attention is paid to these details, structural damage is inevitable. At the same time, the application of exterior insulation on historically valuable buildings usually means that the design will suffer, especially if, for example, building-defining elements such as timber framing, windowsills, natural stone walls and/or stucco elements are covered. The seemingly “clean line” of a plastered exterior insulation and finish system (EIFS) in such cases leads to the deterioration of our building culture and cityscapes. As an alternative, there are facades with rainscreen siding systems using wood, fiber cement, or slate.

The material makes the music

But there are also design-related advantages and disadvantages when it comes to materials. The classic material choices of plastered EIFS are still mineral wool and expanded polystyrene (EPS) insulation, protected on the surface by multiple, but very thin layers of mesh-reinforced plaster. These are often only 3 to 5 mm (almost 1/18 in to just over 3/16 in) thick and consist of synthetically hardened special plasters. These plasters are hardly able to buffer or absorb moisture from precipitation, as a directly plastered masonry wall has done effortlessly for centuries (using up to 30 mm / 1.2 in thick lime-cement plasters). The result is that the moisture remains on the surface and, in combination with the organic (plastic) substrates, leads to rapid algae and fungal growth. In order to counteract this, biocidal additives are usually mixed into the coatings of synthetic EIFS systems, which, after being washed off the facade, enter the ecological water cycle unfiltered and can thus develop their biocidal effect on humans and animals (aquatic animals, fish, and so on).

What is not ideal for designers and contractors is that EIFS systems are usually predefined “system solutions” in which unauthorized deviations (e.g. avoiding biocidal paints) can also lead to the loss of warranty protection. This means that contractors are not free to make their own changes in the design of details and the choice of materials.

Using purely mineral-based plasters and finishes

So if you would like to avoid biocidal additives in the finish plaster and paint, you need to think and build differently in the layers below: EIFS systems based on wood fiber, cork, reed, calcium silicate, mineral foam, or perlite insulation may be used. These insulation materials are also covered with multiple layers of mesh-reinforced plaster, but this can be made much thicker due to the higher strength of the insulation material. The moisture balance of the facade is achieved by combining purely mineral-based finish plasters with a sorptive insulation material, which does not reduce the insulation capacity of the insulation material. In addition, it is possible to use pure mineral paints (e.g. silicate paints) for the finish coat, which do not provide nutrients for mold or algae growth. A small advantage: Because of the much higher strength of the insulation material and finish coat in such systems, failures in the base area around the building are much less common than in mineral wool insulation systems.

1 Polystyrene (EPS) is commonly used as plastered facade insulation in energy-efficient retrofits.
2 Significant algae growth forms above the window due to residual moisture on synthetic exterior insulation (EPS).
3 Exterior insulation and finish systems (EIFS) made of wood fiber insulation can be plastered and finished with purely mineral-based materials.

Interior insulation: maintaining the outer appearance of the building

If it is important to maintain the outer appearance of a building – either because the building is a heritage building or because the owner and architect would like to keep the existing facade – interior insulation is often the only option. Building physicists usually consider this to be “second best,” not because it is inferior or less effective, but mainly because the proper design and execution of interior insulation requires a greater deal of attention to detail and extra care. There are several factors that need to be considered in the design of interior insulation:

• Exterior walls are usually interrupted on the inside by horizontal ceiling joists or interior walls. The areas where ceilings and interior walls connect with exterior walls represent thermal bridges, which are particularly prone to condensation problems.
• Since the insulation material is on the warm side of the wall assembly, the heat storage capacity of the wall assembly is reduced.
• In order to avoid cooling the actual facade / exterior wall assembly too much and risking frost damage, the thickness of the interior insulation assembly is limited. Calculations for the moisture behavior of the wall assemblies are therefore essential in the design phase.
• Interior insulation reduces the available floor space and often conflicts with the desire to preserve visible historic wall finishes (wood paneling, and so on). Such historic finishes would need to be carefully removed and reinstalled.

The maximum thickness of interior insulation is a hotly debated topic among experts. The goal is to achieve the best possible thermal insulation of the building enclosure. However, this usually means 15 – 20 cm (6 – 8 in) thick interior insulation, where the frost point migrates far into the existing construction and can cause frost damage. Therefore, interior insulation thicknesses of approximately 6 – 8 cm (2.5 – 3 in) are usually considered the upper limit, depending on the type of insulation material and the water vapor diffusion behavior of the wall assembly.

What at first glance appears to be an unacceptably low insulation thickness in comparison to the legal requirements (in new buildings, an average of 16 – 30 cm / 6 – 12 in of insulation is currently used) is put into perspective when we consider that the first 5 cm (2 in) of the insulation cut the transmission heat losses through the building enclosure more than in half. However, an extra 5 cm (2 in) do not keep all the heat inside, but rather the following principle applies: “Half the benefits for twice the effort” or “The thicker the insulation layers, the less effective further increases in insulation thickness are.”

The key to CO₂ reduction lies in existing buildings

Because many owners of older buildings are reluctant to incur the expense of bringing their building up to new construction standards with 20 – 30 cm (8 – 12 in) of insulation, many upgrades are not made at all.

It would therefore be much more consistent to measure energy improvements in existing buildings compared to the previous situation rather than new building standards. This is especially important since the small annual increase in new construction is not able to achieve significant energy savings overall.

4 Installation of wood fiber insulation EIFS on the IBN building
5 Historic buildings lose their unique character when exterior insulation is installed
6 Interior insulation is a reasonable retrofit strategy to preserve the existing character of the building.
7 Wood fiber insulation can be installed from the inside and then finished with a plaster. In this example, a clay plaster was applied.
8 A vapor retarder (shown in blue) on the warm side of the room prevents unwanted moisture from entering the wall assembly.
9 Interior insulation made of unfired clay bricks with plant- or mineral-based aggregates not only provides thermal insulation, but also an active moisture buffer on the inside.

Heat transfer = moisture transfer

In any case, designers and installers of building insulation need to be aware that it is not only the ability of the insulation material to retain heat that is essential for a successful installation, but also the moisture behavior of the wall assembly and the selection of building materials.

In a building enclosure assembly, any heat transfer is also associated with a moisture transfer. To protect the building enclosure from condensation, vapor retarders or barriers are usually installed on the interior side of soft insulation materials. In particular, insulation materials with poor sorptive properties, such as mineral wool or fiberglass, must be properly protected against moisture penetration. If this is not achieved, the insulation material not only loses its insulation effect, but also carries the risk of mold growth. Natural insulation materials – such as wood fiber, hemp, flax, sheep’s wool, cellulose, reed, and so on – can handle moisture much better because they have better sorptive properties. However, they should generally also be protected with at least a vapor retarder. In this context, not only the value of the water vapor diffusion equivalent air layer thickness (s_d value) or permeance is important, but also the sealing of the vapor retarder membranes, including all connections to building components. In practice, this is often done incorrectly, using inappropriate tapes or overlooking holes and tears in the vapor retarder membrane. For interior insulation that is highly sorptive – such as unfired clay bricks with plant- or mineral-based aggregates – such membranes can be omitted because any incident moisture can be easily buffered in the material and then returned to the indoor air. This means that this type of insulation can still have a positive effect on the indoor climate.

Minimizing risks before optimizing savings

Despite all the euphoria when choosing the best possible energy-saving strategies, the focus should always be on what is “feasible.” It is therefore necessary for those involved in the project to make a realistic assessment of how much risk a structure or retrofit concept can carry. Can the airtightness of a vapor retarder be maintained across all structural details? What happens to the insulation material if, contrary to expectations, it becomes damp? Can the moisture dry out or is there a risk of hidden structural damage? The risks associated with the structural design should also be carefully considered in building biology-based and sustainable retrofit projects. Building biology-based and sustainable building focuses on reducing risks through careful design and error-tolerant construction. In ecologically built houses, neither the occupants with their biological needs and human shortcomings should become a “disruptive factor” nor should the tradespeople be overburdened with overly complicated and error-prone construction details whose energy-saving potentials have been (mathematically) optimized.

IBN comment

In order to reliably prevent condensation with interior insulation and the subsequent formation of mold and structural damage, we recommend consulting a specialist. You are welcome to contact our Building Biology Consultants IBN.