Hygrothermal Performance of Timber-Framed External Walls in Finnish Climatic Conditions: A Method for Determining the Sufficient Water Vapour Resistance of the Interior Lining of a Wall Assembly
|Kustantaja||Tampere University of Technology|
|Tila||Julkaistu - 2007|
|Nimi||Tampereen teknillinen yliopisto. Julkaisu|
|Kustantaja||Tampere University of Technology|
This study looked into the moisture performance of timber-framed external wall assemblies in Finnish climatic conditions by examining the effects of the diffusion of water vapour in wall assemblies.
The study has been preceded by decades of discussion about the need of a vapour barrier in the interior wall lining of external wall assemblies. Much research has been conducted related to this subject worldwide in the last 70 years, and studies have shown that in Nordic climatic conditions the interior wall lining always requires sufficient water vapour resistance against the diffusion of the water vapour emitting from indoor air. On the other hand, it has also been found that external wall assemblies can be implemented without incorporating a tight vapour barrier into the interior wall lining. Furthermore, in more southern climatic conditions the interior wall lining may have to be permeable to water vapour when the direction of the diffusion flow is from the outside in. Yet, despite numerous studies, agreement has not been reached on the minimum water vapour resistance that should be required of the interior wall lining of a timber-framed wall assembly under different conditions. Consequently, different guidelines and regulations for water vapour resistance values of interior wall linings exist also in countries located in highly similar climatic conditions.
Comprehensive assessment of the moisture performance of wall assemblies requires establishing the performance criteria and limit values that an assembly must meet, the outdoor and indoor conditions to be used in designing moisture performance, and the assembly solution and used materials as well as their building physical properties. The acceptability of the moisture performance of an external wall assembly depends essentially on all these factors during the service life of a building.
The aim of this study has been firstly to create a method which allows examining the moisture performance of timber-framed external wall assemblies in different situations. The first phase involved setting the performance criteria, limit values and reference boundary conditions for analysing wall assemblies under Finnish climatic conditions. Then, the effect of the different properties of the assembly layers on the moisture performance of the wall was examined followed by the setting of minimum values for the water vapour resistance ratio between the interior and exterior linings of different wall types based on selected criteria and study conditions.
An attempt has been made to ensure the reliability of the developed analysis method by conducting different laboratory and field tests and calculational analyses in connection with the study. External wall assemblies were examined in the laboratory with building physical research equipment developed during this study, the key building physical properties of the materials used in assembly tests were determined for calculational analyses, external wall assemblies were also analysed in field conditions in a one-family house and in test houses at the test field of Tampere University of Technology, indoor air excess moisture was analysed in field tests of timber-framed one-family and row houses, and the performance of the used heat, air and moisture transfer simulation model (HAM model) was verified by tests conducted in various situations.
Moisture condensation and mould growth within were chosen as the performance criteria of the external wall assembly. Limit values were selected according to the following principle: The temperature and relative humidity conditions of the exterior wall lining must not be more critical than those of the exterior wall lining of the most critical but still acceptable wall assembly (reference wall) where those conditions result only from the effect of outdoor air conditions. Also, the temperature and relative humidity conditions of the interior wall lining must not be more critical than those of indoor air. The limit value in the moisture performance analysis of the assembly was the maximum continuous condensation time of the moisture condensation analysis, and in the mould growth analysis the maximum mould index.
In Finnish climatic conditions it is not possible to set design requirements for timber-framed external wall assemblies that allow no condensation or mould growth at all in the exterior wall lining. The maximum continuous condensation time of this study was 34 days in the exterior wall lining and 0 days in the interior wall lining. The respective maximum mould index was 1.96 in the exterior wall lining and <1 in the interior wall lining. When these performance criteria are applied, moisture condensation is generally the design criterion for diffusion.
Moisture reference years (MRYs) representing a 30-year period of Finnish climatic conditions were selected for both performance criteria. The climatic conditions of four localities were selected to represent Finnish climate. Moisture reference years were selected so that only 10% of the years are more critical with respect to the examined criterion than the selected years.
Indoor air conditions were selected so that standard temperature was 21°C and excess moisture values varied between 0 and 8 g/m3 in winter. On the basis of the field study, the recommendable design value of excess moisture in Finnish one-family and row houses is 4 to 5 g/m3 in winter conditions. In connection with the field study a set of design curves for excess moisture as a function of outdoor air temperature was drawn.
The studied walls are typical assemblies used in Finland and other Nordic countries. All walls had the following structural layers: cladding, ventilation gap, sheathing, thermal insulation, air/vapour barrier and interior board.
On the basis of conducted field tests, diffusion analyses of timber-framed external wall assemblies need not be complemented by separate analyses of the impact of driving rain, wind, solar radiation and surface undercooling since these factors can be factored into the thermal surface resistance value of the sheathing in a calculational analysis of the northern wall. This facilitates significantly the study of the moisture performance of a wall assembly.
The best ways to increase the reliability of the performance of an exterior wall assembly against the harmful effects of diffusion from indoor air are to increase the water vapour resistance of the interior wall lining and thermal resistance of sheathing. Low water vapour resistance of the sheathing is an important factor for good moisture performance of the assembly.
The research results indicate that a plastic vapour barrier behind the interior board is safe in all typical indoor and outdoor air conditions occurring in Finnish climatic conditions. With this assembly solution relative humidity as well as the risk for moisture condensation and mould growth are at their lowest on the interior surface of the sheathing since indoor air moisture does not affect the moisture performance of the assembly.
In Finnish climatic conditions an assembly may be implemented also without a plastic vapour barrier in the interior wall lining. Then, the required water vapour resistance ratio between the interior and exterior wall linings depends on the sheathing and thermal insulation used. The key properties of the layers which improve the moisture performance of a moisture-permeable assembly are the high thermal resistance and low water vapour resistance of the sheathing.
Hygroscopic thermal insulation retards the wetting of the assembly in autumn and its drying in spring. Therefore, high moisture capacity of thermal insulation can also improve the performance of the assembly if the water vapour resistance of the sheathing is low.
When the excess moisture of indoor air is 4 to 5 g/m3 in winter conditions, the minimum water vapour resistance values between the interior and exterior wall linings are typically in the 0 to 80:1 range. If sheathings highly permeable to water vapour are used, the required resistance ratio typically varies within the 0 to 40:1 range.
The present specification of the Finnish Building Code of 5:1 for the water vapour resistance ratio between interior and exterior wall linings is not sufficient in most cases. That water vapour resistance ratio is acceptable only if the thermal resistance of the sheathing is high.
The water vapour resistance of the paper-based air barrier membranes presently on the market is normally so low that many assemblies implemented with them do not even meet the existing specification for the resistance ratio between interior and exterior wall linings (min. 5:1) if the interior finishing is excluded from the analysis.
The method for analysing external wall assemblies created in connection with this research may also be used, where applicable, to analyse the moisture performance of layered wall assemblies in other climatic conditions. Neverthless, the limit values for moisture condensation and mould growth must always be redefined for the climatic conditions of other countries.