Guideline for selecting an internal insulation system

The main goal of this guideline is to provide information to the consultant on how to select an internal insulation system.

Insulation systems – characteristics and preconditions for installation

Internal insulation systems are generally composed of several materials (layers) ensuring the necessary functions, such as insulation, structural stability and finishing. In many cases, the manufacturer delivers all the needed components (insulation material, fixing layers etc.).  

The installation procedure usually includes the following phases:

Concerning phase iv), in general, there are two main installation methods, i.e.:

• i) decide the type of insulation system, thickness and installation method;

• ii) remove everything fixed to the walls to be insulated (e.g. plug sockets, light switches, curtain rails, radiator, etc.);

• iii) clean and prepare the wall surface (i.e. knocking off old plaster if damaged);

• iv) install the insulation system according to the instructions of the manufacturer;

• v) eventual reinstallation of previously removed items.

Concerning phase iv), in general, there are two main installation methods, i.e.:

• directly fixing the thermal insulation to the wall. This method is generally adopted in case of sufficiently rigid insulation boards;

• creating a new internal stud. This method is usually adopted in case of not sufficiently rigid insulation materials.

Since installing an internal insulation system requires removal and re-fixing of several items, such as switches, radiators, etc., it is important to consider the best insulation solution/material for each specific case, also taking into account the possible occurrence of moisture-related problems, such as frost damage, interstitial condensation, mould growth and other damage patterns that may occur when an internal insulation system is applied. In this case, it is advisable to ask the opinion of an expert or to perform a more in-depth study to evaluate the possible occurrence of moisture-related issues and, then, to select the insulation system suitable for the specific case.

As a rule, the surface of the existing wall has to be clean, sustainable and dry (phase iii). Unsustainable plastering, barrier layers, paint and wallpaper have to be removed. Gypsum components in the existing wall construction have to be removed depending on the planned internal insulation system. If board-shaped internal insulation materials are applied, an even surface is required. This means that depending on the condition of the existing surface, the application of a base or levelling plaster may be necessary.

Preparation for installation

As part of deciding which type of insulation system to install (phase i), it might be relevant to clarify e.g. the bearing capacity of the existing wall (cf. some systems are glued directly to the existing wall), and possible material incompatibilities between materials in the existing wall and layers in the insulation system. Provided that the existing masonry is intact and the moisture load can be assumed normal, measurements of moisture properties in the specific case are not required, as most properties in historic brick material don’t differ that much. If this is not the case, knowledge about the drying potential and moisture resistance of the existing wall, as well as capillary and diffusive transport and storage properties is needed. Potential increase of internal moisture loads should also be considered, especially if the use of the building is changed after the renovation. Finally, also fire and noise protection requirements should be considered, cf. national building requirements.

Phase iii) includes the following items:

• Wall texture: The substrate of the existing wall must be clean, dry, dust-free and free from other contaminants.

• Bearing capacity of existing plaster surfaces. Non load-bearing concealed plasters, barrier layers, paints and wallpaper must be removed in order not to hinder moisture transport after the internal insulation has been applied.

• Flatness of the wall: Gypsum components in the existing wall construction must be removed. If panel-shaped internal insulation materials are to be applied, a flat surface is required. This means that, depending on the condition of the existing plaster, it may be necessary to arrange a base plaster or level out.

• Prepare a new base plaster to be applied: It should have the following properties: (1) Compressive strength: 3-5 N/mm² , (2) high porosity, (3) vapour diffusion resistance μ < 15 , (4)  sufficient capillary activity (Aw > 1 kg/(m²h0,5)), e.g. NHL plaster, lime plaster with a low content of cement or special base plaster.

• The drying of the base plaster (if required) must be observed until internal insulation can be applied.

If a sufficiently rigid insulation material is adopted (e.g. EPS, XPS, PUR, cork, etc.), the insulation layer can be directly fixed to the wall by using a specific adhesive or, when necessary, mechanical fixing (screws). If the retrofit intervention concerns an uneven wall, fixing battens can be used to provide a flatter surface for the insulation installation. The installation is then finished at the interior by using a specific rendering system or through gypsum plasterboards directly glued to the insulation material.

In the case of more flexible insulation material, a new stud wall is generally built, leaving a ventilated cavity between the existing wall and the stud wall, that is finished with gypsum plasterboard.

Non-rigid internal insulation systems

They can be deformed during paving and can therefore be applied to uneven surfaces without further preparation. If insulating plasters are to be applied, make sure that the surface is moisture-resistant. Highly absorbent surfaces have to be prepared accordingly with a suitable primer.

Board-shaped internal insulation systems

Even with board-shaped systems, the absorption behaviour of the surface has to be adjusted according to the manufacturer's specifications. Unevenness has to be evened out with plasters suitable for the system. The size of the permissible unevenness depends on the manufacturer's specifications. Internal insulation systems with vacuum insulation panels tolerate only very small deviations up to 5 mm.

Pre-walled shells

They do not require any levelling plaster. The resulting cavity has to be completely filled with a suitable backfilling mortar to ensure capillary coupling of the facing layer.

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Characteristics of internal insulation systems

Various internal insulation systems and products are available on the market. They mainly differ in terms of material properties, installation method, environmental impact, durability and costs. Based on the hygrothermal properties of the insulation material, the internal insulation systems can be subdivided into two main groups: vapour-tight and vapour-open systems. Table 5‑1 includes a selection of the main insulation systems subdivided according to this classification.

A vapour-tight internal insulation system prevents the warm, moist indoor air from penetrating the insulation. It can be obtained by using a vapour-tight insulation material (e.g. PUR, XPS, cellular glass, etc.) or, alternatively, by coupling a vapour barrier (i.e. a foil that does not permit the vapour to pass through) to a vapour-open material (e.g. mineral wool). Particular caution is always required when a vapour barrier is adopted, as any disconnection or perforation of the barrier determines a possible vapour penetration, thus a potential significant decrease in the system performance. The installation of a vapour barrier requires proper workmanship ensuring an air tight layer and careful use by the residents avoiding perforation of the vapour barrier. In some countries, e.g. Switzerland, internal insulation of mineral wool is also used together with a smart vapour retarder, able to modify its vapour diffusion resistance depending on the relative humidity of the indoor environment. This kind of solution has not been studied as part of RIBuild.

A vapour-open internal insulation system is generally obtained by using a vapour-open insulation material that also enables capillary suction (capillary-active insulation material). The capillary activity of the material, in fact, allows the moisture transport through the insulation material towards the indoor air if the inner surface of the existing wall gets wet, for instance due to interstitial condensation.

Table 5‑1. Examples of internal insulation systems classified based on water vapour diffusion resistance of the composing materials.

To select the proper insulation solution, it is important to know the hygrothermal properties of the materials in the existing wall and the insulation systems. Among them, the thermal conductivity λ [W/(m K)], the capillary activity and the water vapour diffusion resistance factor μ [-] are the most important (RIBuild Deliverable D1.2, 2016).

The thermal conductivity λ [W/(m K)] expresses the rate at which heat is transferred by conduction through a unit cross-section area of the material when a temperature difference exists. All λ-values of the materials composing the insulated wall should be known to calculate the targeted air-to-air U-value (thermal transmittance, [W/(m2 K)]) of the wall, usually specified in building regulations. It is calculated as follows:

where Rsi and Rse are respectively the internal and external surface thermal resistances [m2K/W], while si and λi are the thickness (m) and the thermal conductivity [W/(m K)] respectively, of the i-th layer of the retrofitted wall.

The capillary activity is the ability of some materials to absorb liquid water due to capillary suction and to redistribute it through the material. The pore size in capillary active materials permit liquid water to be absorbed and redistributed towards the room in both vapour and liquid phase, following the inwards capillary pressure gradient (Vereecken & S. Roels, 2016). Generally, capillary active materials, such as calcium silicate (CaSi) and wood fiberboards (WFB), are expensive compared with traditional insulation materials and often characterized by a higher thermal conductivity (typically 50 % higher).

The water vapour diffusion resistance factor μ (-) expresses the relative reluctance of a material to let water vapour pass through. It is calculated as the water vapour permeability of air divided by that of the material concerned. This property is generally used to classify insulation materials in vapour-open insulation systems (with low μ values) and vapour-tight insulation systems (with high μ values). Typical vapour-open insulation materials are mineral wool, cellulose, calcium silicate and wood fiberboard. Typical vapour-tight insulation materials are extruded polystyrene, polyurethane, expanded polystyrene and cellular glass.

Vapour-tight insulation systems are obtained by applying a vapour-tight insulation material or, alternatively, by coupling a vapour-open insulation material with a vapour barrier. In Figure 5–1, a typical Glaser diagram for a vapour-tight internal insulation system based on the use of vapour-tight insulation material is reported. If the μ or the Z-value (water vapour diffusion resistance in absolute values [(m2 s Pa)/kg] of the insulation material is sufficiently high, interstitial condensation can be avoided, in Figure 5–1 seen by the pv curve staying below the pv,sat curve.

In Figure 5–2, a typical Glaser diagram for vapour-tight insulation systems obtained by adopting a vapour-open insulation material coupled with a vapour barrier is shown. If the vapour barrier is characterized by a sufficiently high μ, interstitial condensation can be avoided in the heating season (a). However, during the cooling season, a sufficiently low μ is needed to avoid summer condensation (b).

In vapour-open insulation systems, a vapour-open capillary active insulation material is adopted, generally coupled with specific glue and finishing mortar. In Figure 5–3, the working principle of such an internal insulation system is shown. During the heating season, the temperature and vapour gradients generate an outward vapour transfer. If the temperature between the glue mortar and the insulation is lower than the dew point, interstitial condensation can occur. Good contact between the existing wall and the insulation system is fundamental for the proper functionality of the system. In fact, in case of a discontinuity between the masonry wall and the capillary active material, interstitial condensation can occur at the warm side of the wall (Figure 5–4).

Internal insulation can significantly modify the hygrothermal performance of the existing wall, inducing frost damage, interstitial condensation, mould growth and other damage patterns. When internal insulation is applied to historic buildings, it is then important to assess its hygrothermal performance, to evaluate the possible impact on an existing structure. In Figure 5–5a and b typical ranges of dry λ and μ values for commonly used insulation materials are reported.

In the case of vapour-tight insulation systems with vapour-tight insulation material, μ of the insulation material must be sufficiently high to prevent interstitial condensation. Conversely, if a vapour-open insulation material is coupled with a vapour barrier, the vapour resistance of the vapour barrier should be sufficiently high to avoid interstitial condensation during the heating season and, at the same time, sufficiently low to allow the drying out of the masonry wall during the cooling season.

In the case of the vapour-open capillary-active insulation system, it is important that the hygrothermal properties of the capillary active insulation material and those of the glue mortar are correctly selected to ensure the proper functioning of the system. The glue mortar should have a higher vapour resistance and a lower liquid conductivity than that of the insulation material (Scheffler & Grunewald, 2003). In this way, in fact, the over-hygroscopic moisture is generated between the glue mortar and the insulation material, allowing the insulation material to transport the moisture by capillarity towards the indoor environment.

Finally, it should be noted that the thermal conductivity of capillary-active insulation materials is higher than for traditional mineral wool. The moisture dependency shown in Figure 5–6 and Figure 5–7 is based on DELPHIN using values for a material in dry condition and the thermal conductivity of water. This is why the slope of the two curves in Figure 5–6 are the same. Although the assumption of having the same moisture dependency in both cases can be questioned, notice, only at very high RH values the thermal conductivity increases considerably compared to the values at normal indoor climate conditions. Further, as the capillary-active insulation material (ID 571) absorb more moisture from the air at RH between 80 % and 97 % (Figure 5–7 and Figure 5–8) the difference in thermal conductivity between calcium silicate and mineral wool is higher at a high RH.

Recently, also insulation boards consisting of multiple materials have been developed, such as wood fiberboard with embedded mineral functional layer, PUR in combination with calcium silicate, calcium silicate in combination with PUR, pyrogenic silicas, etc. In Figure 5–9 the working principle of a wood fiber insulation board with an embedded functional layer is shown. During the heating season, water vapour is transported inside the insulation system. The functional layer restricts the vapour transport and a condensation plane arises at the functional layer. Thanks to the capillary active forces of the wood fiberboard, the redistribution of the potential interstitial condensation towards the room side is thus allowed.

A similar working mechanism occurs even in the case of a two-layered capillary active internal insulation system consisting of a retarder layer at the side of the masonry wall and an insulation layer at the room side. The aim of the two-layered system is to force the occurrence of the interstitial condensation between the retarder layer and the insulation layer instead of at the masonry wall, ensuring in this way the functionality of the capillary internal insulation system even in case of an air gap between the original wall and the insulation system. In this case, to locate the interstitial condensation plane between the retarder and the insulation layer, appropriate thermal, vapour and capillary conductivity for both layers of the system should be selected. In particular, the insulation material should have a low thermal conductivity, a high liquid conductivity and a low vapour diffusion resistance. The retarder layer, instead, should have higher thermal conductivity, a lower capillary conductivity and a higher vapour diffusion resistance (Künzel, 1999).

More information about insulation materials and systems is available in (RIBuild Deliverable D1.2, 2016).

(RIBuild Deliverable D3.2, 2019) gives an overview of the hygrothermal performance in practice for a number of internally insulated case study buildings, either RIBuild-cases (14) or published (31) monitoring projects. They cover a wide range of weather conditions (locations), constructions (wall materials, wall thickness, type of constructive details) and typologies (office, residential and others). Different aspects have been analysed, e.g. performance risk factors addressing the construction (See ‘Decision making process’ and ‘Reduction of environmental impact’) or weather conditions (See ‘Reduction of energy and other costs’).

Among 31 case studies published since 2003 and 14 RIBuild case studies, only two damage cases occurred. However, critical moisture contents were measured in several projects for the first monitoring years; in 14 published case studies and five RIBuild case studies. In nine of these cases, critical moisture content could be attributed to a high built-in moisture of the system and the corresponding drying phase in the first year after installation of internal insulation. The length of the drying phase depends on the amount of built-in moisture, drying potential due to the vapour diffusion resistance and capillary activity of the system.

Most of the projects with critical moisture contents showed a strong initial drying process with an acceptable moisture level in the wall after the first or the second year. Further reasons for high moisture levels in the construction were poor thermal resistance of the existing wall, a missing sealing of joist ends towards indoor air, a poor drying potential due to a high vapour resistance of the insulation system in combination with an insufficient wind driven rain protection or an inactive heating system. Other factors like the driving rain load or the air tightness of the construction are not documented in a comprehensive way for all cases.

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Calculation methods

An important step for choosing an internal insulation system is calculation of impact of heat and moisture processes. A calculation method or simulation tool must be chosen. The simplest of combined heat and mass transfer calculation methods is the Glaser model; it is a 1-D model with stationary climate on both sides of the building component only considering thermal conductivity for heat transport and diffusion for moisture transport. Material properties are simplified to be independent of hygrothermal conditions and therefore constant. The method is not considered to be a simulation mainly because the climate is constant. More advanced methods like DELPHIN, WUFI and MATCH operate with transient conditions and hygrothermal dependent material properties and are therefore considered to be simulation tools.

The main purpose of modelling combined heat and mass transfer in building components is to evaluate how the building component will perform in reality without doing physical experiments. The aim of each model is therefore to mimic what happens in reality.

A perfect model would predict the hygrothermal condition in a building component if all boundary conditions and material parameters were known. Unfortunately, there will always be unknown factors as reality is too complex to be described in a model; models are simplifications of reality, e.g. 3-D modelling has only recently become possible, and materials are not perfect. Therefore, in reality material properties will vary within the material. Uncertainties in measurements will make boundary conditions uncertain; time steps in measurements are often different from time steps in calculations, etc. Perfect agreement between calculated and measured values are unrealistic. To increase the reliability of simulation models, (RIBuild Deliverable D3.1, 2018) provides insights in closed technology loop of laboratory experiments and simulation models in the field of internal insulation testing.

Glaser method

This method was developed by German scientist Helmut Glaser. It is also called moisture profile method, or dew point method. The Glaser method was developed to determine interstitial condensation risk and condensation speed in building components. The method can also be used as a fast but not very precise method to assess corrosion and mould growth risks. Glaser restricted condensation to interfaces of layers since interlayer condensation caused difficulties with the mass conservation law. The Glaser method only handles stationary conditions. Further, it does not include capillary suction, which takes place in both brick masonry and capillary-active insulation materials.

Further description of Glaser method is provided in  (RIBuild Deliverable D2.1, 2018), Section 2.3.1.

COND

The COND software was developed at The Institute for Building Climatology at Dresden University of Technology, Germany. The calculations are based on an algorithm developed by Prof. Dr.-Ing. habil P. Häupl in the 1980s. COND was developed to improve the Glaser method especially in relation to certain constructions. In the analysis, many simplifications and idealisations have been introduced that further narrow the scope of the method.

COND is a software for hygrothermal evaluation of one-dimensional building envelope systems, as it calculates the stationary heat and moisture transport. Based on the Glaser scheme standard method according to the German standard DIN 4108-3 (2014) and described in ‘Calculatoin methods’ Glaser that only takes vapour fluxes due to differences in vapour pressure into account, COND additionally includes liquid fluxes, i.e. the redistribution of occurring internal condensate. Thus, the results are more realistic and it is therefore especially useful for multilayer constructions with capillary active internal insulation materials that are used for a thermal upgrade and refurbishment of older buildings.

COND is a simple and fast practice tool for the evaluation of possible moisture damage of the building envelope taking simplified climatic conditions into account. It needs limited input data and climatic conditions and material properties can be used-defined. The outcome is a short report stating whether or not the requirements of the chosen standard are fulfilled can be customized. The report includes a sketch of the construction, material and climate data, temperature, moisture and vapour pressure profiles and individual remarks. This report can be used for documentation purposes. Since COND provides the needed verification according to the German DIN 4108-3 (2014), it is mainly used in Germany, but may also be useful elsewhere if the German standard is regarded as acceptable.

Further description of COND is provided in (RIBuild Deliverable D2.1, 2018), Section 2.3.2.

Eco-Sai tool based on Glaser method

Eco-Sai is a stand-alone tool developed by the University of Applied Sciences of Western Switzerland that combines the calculations of U-value, thermal inertia and life cycle assessment of a construction (homogeneous and inhomogeneous) (www.eco-sai.ch). At present it is the single integrated tool for these three calculations. Eco-Sai can evaluate the characteristics of a construction during the preliminary stage or the project phase, for new buildings or renovation projects. Planners using the CAO Autodesk® Revit® software can also conduct these calculations within Revit®.

Further description of Eco-Sai is provided in (RIBuild Deliverable D2.1, 2018), Section 2.3.3.

DELPHIN

The hygrothermal transport model DELPHIN was developed at Dresden Technical University by John Grunewald. It was extended by air flow, pollutant transport, and salt transport. It has used it as platform for material and transport model development (moisture transport) and for non-linear thermal storage and transport. Simulation program for calculation of coupled heat, moisture, air, pollutant, and salt transport. The program is commercially available in 1- and 2-D. A new 3-D version is being tested. Balance equations are used to carry out numerical analysis of the following transport processes:

• Heat transport in building components and construction details, incl. wall constructions, thermal bridges

• Moisture transport of both liquid and vapour transport, and moisture storage in constructions

• Air transport.

DELPHIN is used as simulation tool in the RIBuild project.

Further description of DELPHIN is provided in (RIBuild Deliverable D2.1, 2018), Section 2.2.1.

In RIBuild, DELPHIN hygrothermal simulation program has been significantly extended, concerning both effectiveness and efficiency. The main focus was on efficiency improvements of both the deterministic simulator and the probabilistic methodology since prior to the initiation of the RIBuild project, both a deterministic simulator and a probabilistic methodology were already available but their joint application to internal insulation solutions required further developments. More detailed description can be found in (RIBuild Deliverable D4.1, 2017) and (RIBuild Deliverable D4.2, 2019).

WUFI

WUFI is designed to calculate simultaneous heat and moisture transport in one- or two-dimensional multi-layered building components in the building envelope based on laboratory and outdoor tests. WUFI is the acronym for "Wärme- und Feuchtetransport instationär" ("Transient Heat and Moisture Transport"). The original basis for the program is given in a thesis by H. M. Künzel and has been developed into the WUFI-family (WUFI-Plus, WUFI-2D, WUFI-Pro and WUFI-ORNL/IBP) which are commercial programmes developed in Germany by the IBP-Fraunhofer Institute for Building Physics. WUFI simulations can be done according to several regulations; EN 13788 (Glaser method), ASHRAE Standard 160 and EN 15026. WUFI-Bio is a post-processor to simulate the risk of mould growth. The results are given as the Mould Growth Index according to the Viitanen model  and in mm mould growth per year according to Sedlbauer’s biohygrothermal model. Many possibilities exist to adjust material properties, outside and inside boundary conditions.

Further description of WUFI is provided in (RIBuild Deliverable D2.1, 2018), Section 2.2.2.

MATCH

MATCH (Moisture and Temperature Calculations for Constructions of Hygroscopic Materials) is a commercial computer simulation program for the calculation of combined transient moisture and heat transport through composite building materials. The one-dimensional model was developed within a research project at the Thermal Insulation Laboratory of the Technical University of Denmark around 1990. It was developed as an alternative to the steady state numerical Glaser scheme that is not feasible - i.e. accurate enough - for hygroscopic materials. It was originally developed for roofs, but useful to most kinds of building constructions. The MATCH model (limited to the vapour region) has been partly incorporated in another simulation program, BSim for whole building simulation, since the early 2000’s.

Further description of MATCH is provided in (RIBuild Deliverable D2.1, 2018), Section 2.2.3.

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