Water Vapour Transmission in Wall Structures Due to Diffusion and Convection

Abstract

New equipment for building physics tests has been built by the Laboratory of Structural Engineering at Tampere University of Technology (TUT) which allows studying the moisture behaviour of shell structures under different conditions. The equipment has been developed on the basis of earlier thermal transmittance testing equipment and has taken altogether some 4 years. The new equipment consists of a warm and a cold chamber - the examined structure is placed between them. The warm chamber is used to model indoor air conditions while the cold chamber models outdoor air conditions. The equipment incorporates numerous measurement and control instruments that are computer-controlled. Accurate and fast regulation of conditions requires an effective control program that continually maintains an equilibrium between various factors. In tests, the controllable variables are indoor and outdoor air temperature and relative humidity (RH) as well as the pressure difference across the examined structure. Tests can be conducted either under constant or varying conditions. The building physical test equipment has many features that together make it a novel and versatile apparatus such as: • all indoor and outdoor air conditions can he controlled simultaneously during tests • structures can he tested in indoor and outdoor air conditions that correspond to real-life situations (e.g. RH of outdoor air can be made to correspond to the actual value also under freezing conditions) • all controllable condition variables can he set freely within the control range • all measurements and adjustments are automatic, accurate and quick as they are computer -controlled • in the test, the moisture flow rates into the structure hy diffusion and convection can be measured separately. • structures can he tested under constant conditions or conditions may be changed cyclically • as a result of the air tightness of the equipment and the installation technique of the element, the air flow rate through the tested structure is controlled • the test opening is large (area: 1200 x 1200 mm2, depth: 400 mm) which means that the same phenomena occur in the tested structure as in actual structures ( e.g. convection inside structure) • the control and measurement systems of the equipment can he augmented or changed as needed • the equipment may he rotated as needed so that wall, roofing deck and base floor structures can be tested in the proper position • the control and measurement systems developed in connection with the building of the equipment can also be used in other laboratory tests The new test equipment was used to study the transmission of water vapour in timber framed external wall structures due to diffusion and convection. Conducted tests gave the following results: Effect of diffusion 1. All wall structures perform well from the viewpoint of diffusion, if • the moisture increase of indoor air is small and • water cannot enter the structure as a result of moisture leaks 2. A structure permeable to moisture is clearly more at risk for condensation than one with a vapour barrier. -->The internal wall surface must have sufficient water vapour resistance (5:1 rule). 3. lf the internal wall surface has proper air and vapour barriers, both cellulose and mineral wool insulation can be used. 4. The moisture-retention capacity of wood-based materials delays the onset of condensation but is not always enough to prevent condensate from forming. 5. Surplus moisture retained by materials increases the risk of condensation. In the case of wood-based materials the risk is high since they can retain a lot of moisture. 6. A windshield sufficiently permeable to water vapour must he attached to the external wall surface in order to allow surplus moisture to exit. There must also be a functioning ventilation gap outside the windshield. Effect of convection 1. Ali wall structures perform safely with regard to convection if • underpressure prevails in the building or • the structure has a solid air barrier 2. Intemal underpressure does not reduce moisture contents of a perforated wall structures significantly compared to a situation where there is no pressure difference. However, underpressure prevents the build-up of overpressure in a building. 3. Internal overpressure increases the equilibrium moisture contents of a perforated wall structure and increases its risks for condensation and moulding. 4. Condensation is possible in a moisture-permeable wall structure despite pressure difference (diffusion). 5. The risk of condensation exists for a wall structure with a vapour barrier only if the structure has holes clear through the internal surfaces and there is overpressure. 6. The use of cellulose insulation slows down increases in RH values in overpressure situations at holes, but in the final end the moisture rates of the structure correspond to those of a mineral wool wall. 7. When holes penetrate only the air/vapour barrier, RH values are not affected by pressure difference. This appiies also to the attachment of the inner sheet to the bracing through an air/vapour barrier. 8. Joints of the inner sheet and air/vapour barrier at the bracing have no effect on the RH values of the wall structure in over- or underpressure situations. A 200-mm overlap is sufficient at the joint of an air/vapour barrier. Taping of joints is always recommended but is not necessary at the bracing. Other test results 1. The more permeable the wall structure is, the more calculated diffusion values differ from the real-life situation. -->It is difficult to determine the moisture specifications of a wall structure permeable to moisture. 2. A permeable wall structure is incapable of increasing indoor air humidity in winter. 3. Overpressure raises significantly temperatures on the internal surface of the windshield in structures with holes clear through the internal surface. 4. Exterior cladding increases the temperature in the ventilation gap compared to outdoors whereby the RH values on the internal surface of the windshield drop in the case of airtight and vapourtight structures. 5. Under winter conditions the wall structures do not generally provide temperature or RH conditions conducive to mould growth.