Collection of information about the building and surroundings

Summary – applicability of internal insulation

While the first part of the visual assessment focused on visible moisture damage, further assessment is needed to evaluate whether the building envelope is sufficiently robust to meet the changed hygrothermal conditions caused by adding internal insulation. Although the building envelope in its present state might seem OK, or a plan is made to take the needed measures to improve the condition, this is not necessary sufficient to ensure that internal insulation will work with regard to moisture safety. Both the weather, especially the driving rain load and the indoor climate can affect the building’s ability to function after adding internal insulation. Especially if the building contains moiture sensitive or frost sensitive parts or has a defect rain water collection system.

If more details are needed, additional material parameters have to be obtained via measurements either on site or in the laboratory. This will serve as input data for a realistic prediction of the hygrothermal behaviour via simulation models. The more detailed and precise the information about the state of the building before renovation, the safer is the application of internal insulation.

Table 4‑11 summarises when internal insulation is applicable, might be applicable or is not recommended. The indoor climate in the winter season should generally not exceed 50 % relative humidity, if it does a climate control system should be installed etc. See also Table 4‑1.

Table 4‑11: Assessment tool for the applicability of internal insulation, focusing on moisture load, frost damage and indoor climate, adapted from (Steskens, Loncour , Roels, & Vereecken, 2013)

Aspects of weather that causes degradation of building components:

• wind effects on buildings: internal-external pressure difference (convection), highest load at corners of buildings and roofs where flow separation occurs;

• ambient air temperature: chemical and biological degradation usually accelerate at higher temperatures, freezing and thawing are especially harmful for porous materials as brick;

• solar radiation: has a great impact on the material surface temperature but can also change the atomic structure of a building material (destroys the bonds between the atoms);

• moisture: air humidity, condensation, precipitation, groundwater, higher vapour content of the ambient air in summer and lower in the winter, driving rain (horizontal component of rain during windy conditions, part of the rain is absorbed, part may penetrate into cracks and joints), freezing, deterioration by decomposition, corrosion of reinforcement.

In addition to the large-scale classification of the location, it may be useful to investigate the local climate more closely. Risk factors result from local differences in driving rain, the humidity level of the outside air and the drying potential due to solar radiation. Risk factors and associated possible causes are illustrated in Table 4‑12. Other aspects are also relevant, listed in Table 4‑13.

Table 4‑12: Location and local climate. Risk factors and associated possible causes

Table 4‑13: Other possible risks related to the building and the surroundings

Aspects related to driving rain load, moisture sensitive parts, frost sensitive parts, rain water collection systems and indoor climate are described in more detail in section 4.3.2 - 4.3.6.

If easy accessible and relatively new, drawings, inspection reports etc. can help when preparing a more detailed building assessment, looking for causes to moisture damage seen at the exterior (see ‘Typical Damages’), as it often is necessary to open constructions to identify causes. Useful planning documents and data sources are listed in Table 4‑14. If no plans are available, it is recommended to carry out a building survey covering the necessary constraint and connection points.

Table 4‑14: Possible planning documents and data sources

In most cases, energy efficiency renovation of a building goes hand in hand with system technology renewal. The planned improvement of the building envelope will reduce the final energy requirement of the building. In the case of planned further use of installations or at least parts of thereof, existing components must be suited for this. This includes:

• heating systems (boilers, pumps, additional chimney?)

• ventilation system (supply and return air, heat recovery, only return air?)

• distribution and transfer system

• hot water preparation.

The type and location of the heating system (radiators or surface heating, pipe routing, temperatures etc.) as well as the presence of a ventilation system can influence the effect of a planned internal insulation measure and thus limit or expand the selection of potential insulation materials. For example, critical design points can be mitigated by well thought-out positioning of the heating pipes. It should also be borne in mind that special fasteners are required when installing radiators to the internally insulated outer wall.

Building services and connection details to pay attention to:

• building services: cold water pipes (no laying in or outside of the insulation level, otherwise high risk of frost)

• building services: hot water pipes (drinking water, heating pipes) are problematic, especially energetically, but can contribute to local heating (drying).

• avoid leaks, e.g. due to airtight sockets and switch inserts, cable lines free of empty conduits

• wooden beam ceilings : material and constructive thermal bridge in the area of the ceiling supports, with internal insulation the beam head becomes colder than before, possible insulation, elimination of leaks,

• lower/uppermost floor slabs

• jamb (sides)

• non-insulated floor slabs

Particularly in the case of listed buildings, the existing situation is such that it is often difficult to carry out a sensibly designed internal insulation measure due to structural constraints. In addition, there are requirements under monument protection law that, for example, window face widths cannot be changed, that historic valuable facades must not be painted (and thus protected against driving rain) or that no changes are permitted to brick face facades (e.g. sheeting in the area of projections). Sometimes compromises have to be made here, which may also have planning consequences.

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Driving rain load

A good assessment of the driving rain load on the façade is a decisive step when designing an internal insulation system. The driving rain load in combination with the façade materials and façade thickness are the key elements that determine the risk for moisture and frost damage. The rain load on the façade is dependent on the surrounding environment, orientation of the façade, the height of the building and protection of the façade by overhangs and/or exterior finishings. There exist no strict guidelines related to driving rain load, but as general rule, caution should be exercised when the rain load is high.

In general, a façade will have a higher rain load if (see Figure 4–22):

• it is oriented towards to the main driving rain direction in an open region with hardly any surrounding buildings

• it is a relatively high building (e.g. a multistorey appartment building)

• the building is located in a region with high precipitation, e.g. the coastal area or on a hill side

• hardly any protection is provided by cover stones, sills, roof overhangs, etc.

If a high driving rain load is to be expected, care should be taken when designing the internal insulation system. In this case it is advisable to ask the opinion of an expert or to perform a more in depth study to evaluate whether additional protection of the façade is necessary.

Figure 4–22: Driving rain load on a façade will in general be higher for a) a free standing building in an open region compared to terraced houses in the city, b) a high rise building compared to a low rise building and c) facades without physical protections due to overhangs etc.

Wind driven rain is one of the most important moisture sources affecting the hygrothermal performance and hence, durability of building facades. Wind driven rain is rain that is given a horizontal velocity component by the wind, which drives the rain drops against the windward façade of the building. The amount of wind driven rain impinging upon building facades is governed by different parameters such as wind speed and direction, rainfall intensity, environmental topology, the building geometry, and the position on the façade. As a result, data on wind driven rain impinging upon a building façade is typically not available. Standard meteorological data measured by weather stations only provide wind speed, wind direction and horizontal rain fall. To quantify the amount of wind driven rain on a specific façade of a specific building semi-empirical quantification methods can be used, such as the European Standard EN 15927-3 (EN-ISO 15927-3, 2009). In specific cases, more precise, but also more expensive numerical quantification methods such as computational fluid dynamics can be used.

To get a first idea of the driving rain load on a building façade, checking the location of the building, the orientation of the façade and the protection of the façade often provides a first evaluation of the applicability of internal insulation. Weather data of a nearby weather station is important information to get an idea of the driving rain load in the region of the building. Typically this weather data is collected in the free field and local terrain roughness and obstructions (surrounding buildings, trees,…) highly influence the final driving rain load on the façade. Nevertheless precipitation and wind load data from a nearby site will be decisive information to determine the most susceptible wall orientations.

Figure 4–23 shows an example of mean wind speeds for Bremen, Stuttgart and Magdeburg. A slightly higher mean wind speed from the west and south-west sector is recorded for all locations. To evaluate the driving rain load for a building, it is not only the wind speed itself that is decisive, but also the amount of precipitation that strikes the façade together with the wind from a given direction. Figure 4–24 illustrates that if the precipitation is summed up over the different points of the compass, the dominating wind driven rain comes from south- and south-west directions for the three given locations.

Figure 4–23: Mean wind speeds by wind direction for the locations Bremen, Stuttgart, Magdeburg in 2017. Data source: FTP-server of the DWD (ftp://ftp-cdc.dwd.de/pub/CDC)

Figure 4–24: Precipitation totals by wind direction for the locations Bremen, Stuttgart, Magdeburg in 2017. Data source: FTP server of the DWD (ftp://ftp-cdc.dwd.de/pub/CDC)

In addition to the local driving rain load, protection on the façade may strongly influence the impinging and absorbed rain. Constructive elements such as large roof overhangs, correct window sills, etc. strongly reduce driving rain loads (see Figure 4–25). The water absorption of the driving rain load can be further reduced by finishing renders, paints or hydrophobic treatment of the walls.

Figure 4–25: Visualisation of rain trajectories on a façade in case of a roof overhang. For small droplets in a low velocity wind field (left figure) the roof overhang shields the façade completely for impinging rain droplets. For larger droplets in a high velocity wind field (figure at the right) shielding is less, but still larger than the geometrical shielding.

If a first check reveals the façade will suffer from high driving rain loads and has insufficient protection against it and no possibility of strengthening it, hygrothermal simulations are advisable to check whether the use of an internal insulation system is nevertheless possible and whether it can be applied without problems.

Knowledge on the wind driven rain load impinging on the the building facade is essential as a boundary condition for reliable hygrothermal simulations. Determining the wind driven rain load on the building facade is complex, though, due to the large number of parameters that determine the load: wind speed and wind direction, rainfall intensity and raindrop size distribution, building geometry and local topology, orientation fo the facade and position on the facade. To assess the moisture load impinging on the building facade semi-empirical models or catch ratio charts predicted by numerical simulations based on computational fluid dynamics can be used.

Semi-empirical models are based on a combination of theoretical formulae with coefficients that have been obtained from wind driven rain measurements. The best known and widely used illustration is EN-ISO 15927-3 (EN-ISO 15927-3, 2009), but other approaches are available as well. In the standard, hourly measurements of rainfall, wind speed and wind direction are first cumulated in an airfield annual index IA [l/m²]:

with v [m/s] hourly mean wind speed, r [l/m²] hourly total rainfall, D [°] hourly mean wind direction from North, Θ [°] wall orientation, and N [-] is 8760, the number of hours in a year. The summation is only taken over all hours for which the cos(D-Θ) is positive, hence only when wind is blowing onto the wall.

The airfield index is then transformed to a wall index IWA [l/m²], via:

with CR [-] the roughness coefficient, CT [-] the topography coefficient, O [-] the obstruction factor, and W [-] the wall factor. The roughness coefficient accounts for the variability of mean wind velo­city at the site due to 1) the height above the ground, and 2) the roughness of the terrain in the direction from which the wind is coming. The topography coefficient accounts for the increase in mean wind speed over isolated hills and escarpments (not undulating and mountainous regions) and is related to the wind velocity upwind to the hill or escarpment. More information on how to quantify these can be found in the standard.

The exposure of the wall should be assessed by determining the horizontal distance to the nearest obstacle, which is at least as high as the wall, along the line of sight from the wall. When the obstruc­tions are being assessed, account should be taken of possible changes such as the felling of trees in the development of a housing estate.

Table 4‑15 below lists possible values of the obstruction factor.

The amount of rain incident on a wall depends on the type of wall, its height and other factors such as overhangs or the orientation of bricks etc. within the structure. In addition the amount of incident rain varies significantly over the surface of a wall due to the flow of air around corners, over the roof etc. The wall factor W for the appropriate position on the wall is illustrated for a two storey gable, a two storey eaves wall, and a two storey building with flat roof in Figure 4–26.

Table 4‑15: Values of obstruction factor depending on the distance of obstruction from the wall.

It should be noted though that the obstruction can vary significantly at different points along a long wall.

Figure 4–26: Wall factor dependent on the building design.

Semi-empirical models are attractive because they are relatively simple to use. Their accuracy, how­ever, is limited. More detailed information on the driving rain load can be obtained with numerical simulations based on computational fluid dynamics. In these simulations (Blocken, 2004), the wind flow around buildings and the resulting wind-driven rain on buildings is calculated and than translated to a catch ratio description:

rwdr = η.r                             (4.4)

with rwdr [l/m²] the wind-driven rain and η [-] the catch ratio. The catch ratio depends on rainfall, wind speed (and wind direction), and is typically obtained through catch ratio graphs, examples of which are shown in Figure 4–27.

These simulations are however quite time-consuming, and can hence not be performed for each and every configuration. A restricted spectrum of catch ratio graphs for different buildings and building clusters is available in literature.

Asssessment of the degree of exposure to WDR is summarised in Table 4‑16.

Figure 4–27: Catch ratio graph. Catch ratio (η) as a function of wind speed (U10), amount of precipitation (Rh) and position on a wall (Blocken, 2004).

Table 4‑16: Summary of assessment of the degree of exposure to WDR

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Moisture sensitive parts

A fundamental part of the planning for application of internal insulation is the examination of the existing structure. In particular, this includes locating and documenting the condition of wooden beam ends. At the beginning of the last century, square section floor joists were the most widespread construction method for floor slabs. They were used on all floors and consisted of floor joists (wooden beams), with floor boards or similar on the top and grooves and lining boards or similar as ceiling towards the floor below. Pugging and pugging boards or similar were fixed to the wooden beams to ensure fire resistance and sound insulation. The floor joists were supported either directly by the brick masonry or a wall plate (wooden beam) placed within the wall. An example is seen in Figure 4–28.

Installing internal insulation will expose floor joists and wall plates to lower temperatures and higher relative humidity. i.e. increasing the risk of mould growth.

Figure 4–28: Joint between external wall and floor joist supported by a wall plate (marked by red box) (left). Cross-section of floor joists in normal floors (right, A) and above basement (right, B) (Engelmark, 1983).

If available, the location of beam ends in the building can be determined from old drawings. If they are not available, detailed investigation should be carried out by qualified experts.

Damage at wooden beams can have different causes and they have to be eliminated prior to application of an internal insulation. In many cases an increased moisture content can be identified. The cause of damage can be due to poor maintenance or due to a single event.

Damage can be caused by:

• The external wall not sufficiently being dimensioned for the existing driving rain load or having damage, in both cases causing moisture penetration due to driving rain.

• Water leaking or condensation taking place in the area of water pipes on the façade or within the external wall. This can lead to permanent or event-based moisture exposure of the wall (Figure 4–29b).

• The wall being salt-loaded and constantly moist as a result of the hygroscopic effect of the salts.

• The beam end covered with vapour-tight materials, e.g. tar board or bitumen sheeting, not being able to transfer moisture from the inside to the surrounding masonry.

• The beam head being filled with air. If the air gap is connected with the indoor climate, infiltration can cause high humidity loads.

• A diffusion-tight coating of the facade. This can lead to an accumulation of moisture in the masonry coming from the inside which cannot dry to the outside.

• Anchors acting as a thermal bridge. Condensate occurs on the cold iron, the outermost point of which is usually further outside than the beam end (see Figure 4–29a).

• Damage events that have led to a long lasting moisture penetration of the masonry or the wood, e.g. accidents with washing machines, leaking baths, tap water or fire-fighting water, broken or blocked gutters and downpipes, leaking eaves connections and/or leaking roof coverings.

• Consequences of war damage, as some buildings have been exposed to the weather unprotected for years or inadequate, improvised repairs persist.

• Earlier insulation measures.

• High relative humidity in the masonry or the wooden beam itself and inadequate rain protection during the construction period.

• Addition of internal insulation can yield excess moisture at the interface due to condensation, as internal insulation creates a steep temperature gradient, and there is a risk of the temperature reaching dew point temperature at the interface.

Figure 4–29: Possible damages as a result of a) an anchor or b) a damaged water pipe.

In addition to the aforementioned causes of moisture damage to wooden beam ends, the following points must be observed by a suitable expert:

• Assessment of the ceiling structure (e.g. by uncovering or endoscopy), determination of dimensions and all materials

• Assessment of the bearing

• Assessment of existing insulation measures

• Assessment of weak points in the wall construction: hollow masonry, recesses, etc.

• Assessment of the existing wall construction - materials and their properties

• Checking for infestation with wood-destroying organisms (e.g. fungi, insects). Search on any wood preservation measures carried out earlier.

• Localisation of damaged beam ends incl. condition mapping

The bearings can be evaluated with regard to their hazard potential using Table 4‑17. For each type of situation, both the original condition (uninsulated) and the condition after adding internal insulation is listed.

Table 4‑17: Bearing types for wooden beam ends in masonry (WTA 8-14, 2014). Bearing can either be by the external wall itself, by a bracket bearing, or a combined bearing (wall plate and the external wall)

The application of an internal insulation system changes the temperature conditions in the existing construction. The temperature level in the wall decreases and this leads to a reduction of the drying potential. This is illustrated in Figure 4–30. For the beam end itself as well as for the surroundings, the relative humidity eventually increases. For critical constructions, this increases the risk of destruction of the wooden beam end by wood-destroying fungi.

Figure 4–30: Beam end before (a) and after (b) application of internal insulation (WTA 8-14, 2014)

To reduce the risk of destruction, planning and executing the installation of internal insulation must done carefully. At the beginning of the refurbishment, defective beam ends have to be replaced or repaired. All anchors, if present, should be removed due to their thermal bridge effect and replaced by statically suitable measures. The same applies to wall plates that are no longer intact. Before installing new wooden beam ends and the subsequent application of internal insulation, depending on the condition of the wall, drying may be needed (see also ‘Assessment of state of the building envelope’).

Driving rain protection:

Buildings with a low driving rain load usually have a low risk of damage to the wooden beams. The same applies to buildings for which a suitable protection against driving rain is guaranteed due to constructive measures and/or an intact continuous external plaster.

Brick-faced façades have a lower resistance to driving rain. Therefore, the material properties of the bricks and the joint material must be examined prior to the refurbishment. If necessary, a joint refurbishment and in some cases a hydrophobic treatment has to be carried out. Their effect on the construction should be checked by means of hygrothermal simulations. It is advisable to first depict the actual situation to then compare it with the refurbished construction. The evaluation of the results requires an experienced planner. At the construction site, the success of the hydrophobization measure also has to be checked.

Driving rain load is further described here.

Sealing layer:

A sealing layer underneath the bearing ensures hygric decoupling of the wooden beam end and prevents capillary moisture from the masonry from entering the wooden beam end. If a permanently dry masonry can be ensured, no barrier layer is necessary.

In case of a high moisture content it is important that not all sides are sealed, as this will hinder the moisture to be transported away from the beam.

The sealing layer have to be designed to in such a way that protrusions are avoided. Furthermore, the sealing layer should be protected from damage by ensuring a smooth mortar bed.

Airtight connection of the beam:

According to practical experience and the current state of European research, beam ends should be walled in in such a way that there is no contact between the beam flanks, the beam top and the end grained wood and the mortar or surrounding masonry (WTA 8-14, 2014). To avoid moisture damage due to convection of air from the inside, a convection-inhibiting connection must be provided. This can be achieved, for example, with pre-compressed sealing tapes, plasterable adhesive tapes or other suitable adhesive tapes, as shown in Figure 4–31. It is also possible to fill the gaps with frayed insulating material. In addition, larger cracks in the wooden beam have to be closed. In most cases, a convection-tight design is not necessary.

Figure 4–31: Suitable sealing variants a) plasterable adhesive tape, b) pre-compressed sealing tape, c) wood plug in crack (Kautsch, et al.), d) plasterable adhesive tape with a mesh embedded in the plaster (Kautsch, et al.). a) and b) are from a laboratory setup, with the connections made according to specifications of the manufacturer. In c) and d) the beam is seen from below.

If the ceiling is not opened and the intermediate beam area remains uninsulated, a convection-inhibiting connection to the ceiling layers should be made. The hygrothermal conditions at the beam end itself are slightly improved by the thermal bridge effect. In addition, the intermediate beam area acts as a heat and moisture buffer between the interior and bearing.

All types require detailed planning. Their careful implementation has to be checked during the construction process. In the case of very critical construction details, monitoring can ensure safety in subsequent years.

As with hydrophobization, it is advisable to verify the construction details by means of a hygrothermal simulation. At this point, the wood moisture with the limit value of 20 M‑% (DIN 68800-1, 2012) is often mentioned as the evaluation criterion. Below this a degradation of wood by fungi or other organisms can be excluded. However, the temperature dependence cannot be represented with this rigid limit value. For this reason, there were investigations in the past that consider the dependencies of temperature, humidity and time with wood degradation (Viitanen, et al., 2010). Based on this, a boundary line was derived in (Kehl, Ruisinger, R., & Grunewald, 2013), which can also be used for the evaluation of the hygrothermal simulation (WTA 6-8, 2016). Below a relative pore air humidity of 95 % (26 M‑%) at 0 °C and 86 % (20 M‑%) at 30 °C no wood degradation takes place (WTA 6-8, 2016). In individual cases, exceeding can be tolerated if it is ensured that the construction does not tend to humidify in long term. Crossing the boundary line in the first year after refurbishment is to be named here as an example. In the following years, the construction has to have a sufficiently large drying potential so that a long-term moisture condition occurs below the limit line.

Even before the start of the simulation, the wood moisture content of the existing structure can be checked using the measurement methods listed in Table 4‑18. A detailed method description can be found in ‘Measurements’.

Table 4‑18: Moisture measurement. Classification and acuuracy of different procedures.

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Frost sensitive parts

Both porous building materials as well as installations embedded in the external walls, can be highly sensitive to frost. In order to assess frost sensitive parts in the construction such as piping embedded in the wall, it is preferable to locate drawings of the construction at an early stage in the investigation. If no drawings are available, an on-site investigation is highly important. This would also reveal whether the external surface of the building is already subjected to frost damage (see the consequences of frost damage on external surfaces). In this case, internal insulation is not recommended without further assessment. Installation of internal insulation decreases the temperature of the existing wall, and the risk of frost damage in the masonry and embedded installations increases.

When installing internal insulation, it is recommended to replace embedded piping with a frost resistant solution and ensure that the old piping system is drained and plugged. If it is not possible to do changes in the piping system, it is recommended to use heating coils to ensure that the pipes do not crack due to frost.

To reduce the risk of destruction of the porous building materials, it should be considered to improve the driving rain protection, if possible, or to impregnate the wall (see ‘Moisture Damage’). In both cases to reduce the external moisture load.

Frost problems related to a defect water collection system are described in in ‘Rain water collection system’.

If no frost damage is detected in the existing wall and the general state of the wall seems OK, it is assumed that the risk of frost damage after adding internal insulation is limited. However, if frost damage is detected during the visual assessment or if the building façade contains elements deviating from a plane surface, e.g. façade ornamentation, frost damage after installing internal insulation might be expected. Likewise, if embedded piping has shown problems with frost prior to internal insulation, one should assume a worsening of the situation after decreasing the wall temperature.  

Frost damage occurs when the volume expansion of water during water-to-ice phase change, in cases where building materials or piping are sufficiently wet or waterfilled, leads to pressure that exceeds the tensile strength of the material. Essentially this leads to microcracks and spalling/crumbling of materials, or bursting of pipes, which in turn can cause even more moisture damage.

The outer surface layers of historic masonry walls are normally exposed to the highest risk for frost damage, mainly manifested through scaling of the outer surfaces. An example is shown in Figure 4–32. Both the moisture and temperature levels of porous building materials depend on the wall orientation. The prevailing direction for wind-driven rain in Europe is South-West while the lowest facade temperatures occur in North-faced facades. It is subsequently difficult to predict the most exposed orientation with respect to frost damage.

Table 4‑19 presents a summary with regard to determination of frost sensitive parts, and possible mitigation.

Table 4‑19: Summary for determination of frost sensitive parts with regard to internal insulation and possible mitigation.

Frost damage in porous building materials can originate from a variety of physical frost impacts, of which the volume increase of the water-to-ice phase change is the most widely known. Three conditions must be fulfilled for frost damage to occur in porous building materials:

• The material must be sufficiently wet

• Phase change must happen in the material

• The material must be sensitive to frost damage

The two latter also applies to embedded piping, which can burst when sufficiently filled with water, for the expansion to rupture the pipe. The application of an internal insulation system changes the temperature conditions in the existing construction as illustrated in Figure 4–33. The temperature level in the wall decreases, potentially increasing the frequency of freezing point passages and the duration of temperature below 0°C. Depending on the moisture load and the state of the external wall this increases the risk of frost damage at the external surface.

Embedded piping that was previously kept warm by internal heat loss through the wall, may become in risk even if it is located close to the internal side of the existing wall, as the temperature decreases significantly in the entire wall. Different materials have different sensitivities to frost damage, and hence respond differently when exposed to the same climatic exposure. Consequently, to predict and prevent frost damage in real world situations, both the climatic exposure and the mechanical resistance of the material should be considered.

To eliminate the risk of damage to embedded piping or other installations in the walls, these should be replaced. Water pipes should be drained and plugged. If that is not an option, heating coils should be used to ensure safe temperatures around the piping systems.

Figure 4–33: Beam end before (a) and after (b) application of internal insulation (WTA 8-14, 2014)

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Rain water collection system

It is important to do an overall inspection of the rain water collection system shortly after heavy rains. This is to capture all leakages in both vertical and horizontal rain water piping on the façades and above. Horizontal chutes can also be inspected during dry periods by filling them with water, to control that the inclination is sufficient to drain the water and that there are no flaws or blockages.  

Things to observe and details to inspect after a heavy rainfall is for instance (Table 4‑20):

• Wet spots/areas on the façade around water collection systems; see examples of poor water collection systems and water sensitive areas and details in Figure 4–34 to Figure 4–37.

• Areas on the façade where rainwater has been accumulating. It can appear as water stains on the façade, usually in conjunction with water collection systems and water chutes where water can spray both below and above these. Signs are usually obvious if the problems have been ongoing for some time.   

• Icicles from eaves, chutes and piping when the temperature is below 0 °C. Icicles are usually a sign of poorly insulated roofs or congested pipes, which can result in further blockage of the rain water collection system during cold periods.

• Locally damaged joints close to the water collection system, indicating high water load on the facades. Damaged joints increase the risk for further leakages into the construction. 

It is recommended to regularly assess the building façades to avoid future problems with rain water collection systems.

If damaged joints are found (see Figure 4–35), they should be repaired and filled with a mortar suitable for the type of masonry used once the water collection system has been repaired or maintained. 

Check around friezes. Friezes often have horizontal surfaces and joints in which rain water can be collected and stay.

Table 4‑20: Summary for determination of problems and possible mitigation for rain water collection systems

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Indoor climate

Installing internal insulation increases the thermal comfort and reduces the risk of mould growth on the interior finishing as the indoor surface temperature increases in winter. This can increase the use of the area closest to the external wall by those present in the building. Further, the indoor climate will react faster to heat inputs. This means that the indoor temperature will increase faster when the heat is turned on and less energy is needed to increase the temperature in spring and autumn, where heating is not needed all the time. However, there is also a significant increase in risk of overheating in summer, which might result in thermal discomfort.

It is well known that the exterior climate strongly determines the heat and moisture response and hence the durability of the building façade, and this both before and after thermally upgrading the wall. Examples of degradation and impact of the driving rain load are given here. But there is also a strong interaction between the building walls and the indoor climate. So will indoor insulation strongly reduce the thermal capacity of the building elements itself (Hens, 1998) and hence decrease the reaction time of the indoor climate to heat inputs. This occurs due to the fact that external walls no longer serve as heat storage after internal insulation is installed. The reduced thermal inertia of the building has some advantages, e.g. lower heat requirements in intermittent heating period and quicker reaction time when the heating is switched on, but drawbacks as well, e.g. a significant increase of overheating risks in summer (Al-Sanea & Zedan, 2001), which might result in thermal discomfort.

On the other hand, the internal insulation will increase the indoor surface temperature in winter, rising the thermal comfort and in general reducing the risk on mould growth on the internal finishing. Finally, depending on the type of internal insulation, the temperature and relative humidity of the indoor climate to a large extent determines the risk of interstitial condensation. Interstitial condensation is an important issue for internal insulation. Previous insulation systems mainly focused on avoiding interstitial condensation by a vapour tight barrier. Nowadays also vapour tolerant systems are available on the market. The different types of insulation systems will be discussed in ‘Insulation Systems’. But, as the indoor climate is a decisive parameter when designing all systems, this section focuses on the assessment of the indoor climate itself and more specifically on moisture sources in the indoor environment and their impact on the indoor humidity.

Assessment of internal moisture sources and corresponding climate classes

Indoor air always contains moisture in the form of water vapour. The indoor air moisture content is dependent on the air exchange with the outdoor due to ventilation and/or infiltration, moisture exchange via the building envelope, moisture buffering in indoor materials and internal moisture sources. Usually, the moisture generated by inhabitants is the most important moisture source in the building. Humans release moisture by respiration and perspiration. In addition moisture is released to the indoor air due to various activities such as bathing, showering, cooking, dish washing, and cloth washing and drying. The balance between moisture production and removal by ventilation determines the indoor conditions: a high moisture release with a poor ventilation will result in a high indoor humidity and hence a larger risk on interstitial condensation, but also a higher change of possible other problems such as mould growth at thermal bridges. A good balance between moisture release and ventilation is hence crucial.

The indoor heat and moisture load can vary significantly from building to building depending on its use, the buildings properties and the outdoor climate. However, a rough division can be made between residential and commercial buildings. The moisture generated in residential buildings by the inhabitants usually varies over time: more moisture is generated in the mornings and late afternoons due to cooking, cleaning, showering etc. Previous studies (IEA Annex 14, 1991) estimated the average moisture release of a typical family (two adults and two kids) to be more than 13 kg per day.  In office buildings loads are concentrated during office hours and also often coincide with solar gains. Similarly, for commercial buildings, moisture production is high during daytime. In most office and commercial buildings the ventilation system compensates for the moisture production and the excess moisture is removed. However, in naturally ventilated buildings, often the case for historic buildings, the indoor humidity level is a constant balance between indoor moisture production, air exchange with the outdoor environment and vapour stored and released by condensation/evaporation and buffering in materials.

On average, the indoor relative humidity can be calculated from the following equation:

with pi, psat,i en pe respectively the indoor, the indoor saturation and the outdoor vapour pressure, R the gasconstant of air (462 [J/kg/K]), n the number of air changes per hour, V the indoor volume and Gvp the moisture production rate in the indoor. This equation states that if moisture is released in the building, the indoor vapour pressure is on average always higher than the outdoor vapour pressure. The difference between both increase with increasing moisture release and decreasing ventilation rate. Note that, although the vapour pressure is higher indoor than outdoor, the relative humidity often is not as the saturation vapour pressure is highly temperature dependent.

Even though a lot of data is collected on typical indoor moisture sources and their corresponding moisture release (see e.g. IEA ECBCS Annex 41 (IEA Annex 41, 2007)), most of the time no information is available on the exact moisture release inside a specific building. Therefore, simplified empirical models have been developed in the past to assess the humidity levels of the indoor environment. These models are based on large-scale field measurement data of various buildings, mostly collected in North and West Europe on natural ventilated buildings. Typically relationships between indoor-outdoor vapour pressure difference and outdoor temperature are found, subdividing buildings in different indoor climate classes. This concept is also incorporated in the informative annex to the European Standard ISO 13788 on hygrothermal performance (ISO 13788, 2012). Buildings’ indoor humidity environment is classified as very-low, low, medium, high or very high (Figure 4–38). Also the informative Annex of EN 15026 (EN 15026, 2007) presents a simple way to assign indoor conditions to buildings in the absence of more precise measured or simulated data. In this model, indoor temperatue and relative humidity are assigned according to the daily mean outdoor air temperature (Figure 4–39).

Figure 4–38: Simplified approach to determine the internal humidity class of a building as proposed in Annex A of EN ISO 13788 (ISO 13788, 2012): the indoor outdoor vapour pressure difference (Δp [Pa]) and difference between outdoor and indoor moisture content (Δv [kg/m3]) is given as a function of the monthly mean outdoor air temperature (A) for different classes of buildings, ranging from very dry storage buildings (class 1) to very wet special buildings such as breweries, swimming pools etc. (class 5).

Figure 4–39: Simplified approach to determine the internal boundary conditions, proposed in Annex C of EN ISO 15026 (EN 15026, 2007): the daily mean humidity is presented as normal (A) and high (B) occupancy humidity levels depending on daily mean external outdoor temperature (θe).

Most building energy simulation software allow calculating the moisture balance of the indoor space in addition to the thermal balance. At that moment, much more detailed information can be included in the models compared to the commonly used empirical relationships of incorporated in ISO 13788 (ISO 13788, 2012) or EN ISO 15026 (EN 15026, 2007). Note though, that the moisture balance is a continuous equilibrium between moisture release, air exchange, moisture buffering and hence requires not only a more precise description of the moisture production, but also a reliable assessment of the moisture buffering capacity, ventilation system and air tightness of the building.

Some general guidelines concerning indoor climate

Apart from lowering the energy losses through the building fabric, applying an internal insulation system typically also increases the thermal comfort as cold surface temperatures are avoided. Combined with a proper detailing of all junctions, this might also reduce the risk of mould growth on the interior finishing, depending on the specific circumstances. Despite all benefits, there might be some drawbacks with respect to the indoor climate that have to be taken into account. The most important ones are listed below:

• When applying internal insulation in historic buildings one has to be aware that the thermal inertia of the building façade is no longer available from the inside. Depending on the overall heat storage capacity of the building (concrete floors versus wooden floors), this might result in a higher overheating risk in summer. Installing appropriate solar shading devices might be advisable in that case.

• Vapour open or vapour tolerant internal insulation systems are only recommended in buildings with a low indoor air moisture content. For buildings corresponding to high indoor moisture classes, such as swimming pools, an in depth hygrothermal study by experts is anyway advisable.

• When internal insulation is part of an overall renovation, one has to be aware that the overall hygrothermal response of the building may change. A renovation, including e.g. installing windows with a better thermal performance, often results in a more airtight building. At that moment, introducing a ventilation system might be crucial to maintain a healthy and moisture safe indoor climate.

Table 4‑21: Summary table for indoor climate and mitigation of elevated RH.

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