BS-7374-1990.pdf

BRITISH STANDARD BS 7374:1990 Methods of test for Water vapour transmission resistance of board materials used in buildings Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 14:30:48 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7374:1990 This British Standard, having been prepared under the direction of the Timber Standards Policy Committee, was published under the authority of the Board of BSI and comes into effect on 30 November 1990 © BSI 08-1999 The following BSI references relate to the work on this standard: Committee reference TIB/26 Draft for comment DD 146:1986 ISBN 0 580 19003 X Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Timber Standards Policy Committee (TIB/-) to Technical Committee TIB/26, upon which the following bodies were represented: American Plywood Association Association of British Plywood and Veneer Manufacturers British Woodworking Federation Council of the Forest Industries of British Columbia Department of the Environment (Building Research Establishment) Fibre Building Board Organization (FIDOR) Finnish Plywood International Furniture Industry Research Association Institution of Structural Engineers National House-building Council National Panel Products Association Timber Research and Development Association Timber Trade Federation United Kingdom and Ireland Particleboard Association Amendments issued since publication Amd. No.DateComments Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 14:30:48 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7374:1990 © BSI 08-1999i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Definitions1 3Determination of water vapour transmission resistance by the wet cup method2 4Determination of water vapour transmission resistance by the dry cup method5 Appendix A Conversion of units8 Figure 1 Test dish assembly wet cup method1 Figure 2 Example of report of wet cup water vapour transmission test4 Figure 3 Test dish assembly dry cup method6 Figure 4 Example of report of dry cup water vapour transmission test7 Table 1 Conversion factors8 Publication(s) referred toInside back cover Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 14:30:48 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7374:1990 ii © BSI 08-1999 Foreword This British Standard has been prepared under the direction of the Timber Standards Policy Committee. It supersedes DD 146, which is withdrawn. Two methods of determining the water vapour transmission resistance of board or sheet materials used in buildings are given. The first, referred to as the wet cup method, is based on a method originally described in DD 146. The second, called the dry cup method, has been adapted from BS 3177 where it was used to measure the water vapour transmission of packaging films. For most building materials, which tend to be hygroscopic in nature, the wet cup method is to be preferred. In this test the test specimen attains a moisture content which is closer to its likely service condition. The results are consequently more meaningful than those obtained from the very low moisture contents prevailing in the dry cup method. However, for a few non-hygroscopic materials the dry cup procedure can be satisfactory. It is important to specify which test procedure has been used as the two methods can give quite different results. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 14:30:48 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7374:1990 © BSI 08-19991 1 Scope This British Standard describes two methods of test for determining the water vapour permeability resistance of board or sheet materials used in buildings. They are known respectively as the wet cup and dry cup methods. The wet cup method is intended for use when testing hygroscopic materials (for example wood-based panels and gypsum plasterboard). The dry cup method is intended for use when testing non-hygroscopic materials such as plastic sheeting. Appendix A gives guidance on the conversion of units. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this British Standard the definitions given in BS 6100 apply together with the following. 2.1 water vapour transmission resistance the water vapour pressure differential across a sheet material, when expressed in meganewtons per square metre (MN/m2), to produce a steady-state water vapour flow of 1 g/(m2·s) through the sheet material concerned, normal to the plane of the sheet NOTEThe water vapour transmission resistance (Rv) should be expressed in meganewton seconds per gram (MN·s/g). 2.2 water vapour transmission resistivity the water vapour transmission resistance adjusted to a notional 1 m thickness of the material concerned NOTEThe water vapour transmission resistivity is expressed in meganewton seconds per gram metre MN·s/(g·m). 2.3 water vapour permeance the reciprocal of water vapour transmission resistance NOTEThe water vapour permeance is usually expressed in grams per meganewton second g/(MN·s) but it can be expressed in micrograms per newton hour Èg/(N·h) (see Appendix A). 2.4 water vapour permeability the water vapour permeance adjusted to a notional 1 m thickness of the material concerned NOTE 1The water vapour permeability is expressed in gram metres per meganewton second g·m/(MN·s). It is thus the reciprocal of water vapour transmission resistivity. All dimensions are in millimetres (dimensions for smaller dish assembly in parentheses). 1) For samples over 25 mm thick see 3.2.1. Figure 1 Test dish assembly wet cup method Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 14:30:48 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7374:1990 2 © BSI 08-1999 NOTE 2The concept of resistivity (or permeability) is meaningful, within the context of this standard, only when applied to homogeneous materials where the transmission characteristics of one thickness may be estimated from tests on another thickness of the same material and when moisture transmission does not occur by capillary action. 3 Determination of water vapour transmission resistance by the wet cup method 3.1 Principle The sheet material to be tested is sealed to the top of a dish containing a saturated solution of potassium nitrate so as to form a lid. The dish assembly is then stored in a conditioning atmosphere. The water vapour transmission is calculated from the rate of decrease in the mass of the dish assembly. 3.2 Apparatus 3.2.1 Test dishes, light and rigid and made from material such as aluminium which is resistant to corrosion and impermeable to water vapour. They shall normally be dimensioned to accord with the larger size shown in Figure 1 and especially for use with thick samples. Where the thickness of the material exceeds 25 mm, dishes with deeper rims shall be used. In no case shall the distance between the rim and the top of the specimen exceed 30 mm. NOTEWhere there are limitations of space, and where the material under test is known to be of uniform vapour resistance, for example to within 20 % of the mean, over the specimen area designated, the dish diameter may be reduced to the smaller size shown in Figure 1 (dimensions in brackets) with a corresponding reduction in the diameter of the area of sample exposed. 3.2.2 Saturated salt solution. A saturated solution of potassium nitrate shall be used within the dish. Surplus solid undissolved potassium nitrate shall be present both before and at the end of the test, but shall not be exposed above the surface of the solution. NOTEIf the solution is heated during preparation it should be allowed to cool to room temperature before incorporation within the test dish assembly. Potassium nitrate in solid form is an oxidizing agent and should be handled accordingly. 3.2.3 Anti-surge grid, to avoid wetting of the sample by the salt solution during handling. The grid shall be resistant to corrosion. The depth of the grid shall be such as to break the surface of the liquid in the dish. The top of the grid shall be at least 6 mm below the test specimen. The grid shall not reduce the exposed liquid surface by more than 10 % and the spacing of grid elements shall be not less than 5 mm. 3.2.4 Sealing compound. The compound used for sealing the test specimen to the top of the dish shall adhere firmly to both the surfaces of the specimen and to the dish and it shall be capable of accommodating moisture and temperature-induced dimensional change of the specimen under test without cracking. It shall be impermeable to water vapour so that when used with an impermeable test specimen the change in mass of the test assembly shall not exceed 0.01 g per 24 h. NOTEA suitable sealing compound is microcrystalline wax to which about 10 % plasticizer has been added. It is melted in accordance with the manufacturers recommendations and applied in liquid state. Application is aided by the use of a sealing template as shown in Figure 1, which is removed when the sealing compound cools sufficiently. The edge only of the template should be lightly greased beforehand with petroleum jelly to facilitate removal. 3.2.5 Humidity enclosure. The cabinet or room in which the test dishes are exposed shall be capable of being maintained at 20 ± 2 °C, 65 ± 5 % r.h. and at 23 ± 2 °C, 50 ± 5 % r.h. It shall have storage racks, as a means of circulating the conditioned air. The air circulation shall be sufficiently rapid to maintain the specified humidity at the surface of the test specimen. The design of the cabinet or room shall be such that the moisture does not condense on the interior surfaces in the vicinity of the specimens during the test. Care shall be taken to ensure that the humidity enclosure is not loaded to such an extent that the rate of moisture interchange causes the humidity to vary beyond the limits specified. 3.2.6 Balance, accurate to 0.01 g and of sufficient capacity to accommodate the test dish assembly. 3.3 Preparation of test specimens The circular test specimens shall be cut to the diameter shown in Figure 1 and shall be representative of the material being examined, taking into account, where appropriate, variations within and between sheets and batches. The faces shall be designated 1 and 2 respectively. Where the two faces of the material can be distinguished face 1 shall denote the face exposed to the higher water vapour pressure in service. Tests shall normally be conducted with both face 1 and face 2 facing the interior of the dish. Five test specimens shall be used for each face. This shall be stated when the results are reported. In the case of a sheet or board which has been laminated with a film or veneer of high vapour resistance it is of paramount importance that there is a tight seal between the impermeable layer and the cup. NOTEThe integrity of this seal can best be checked if the impermeable layer faces outwards from the dish. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 14:30:48 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7374:1990 © BSI 08-19993 3.4 Conditioning Condition each specimen in the humidity enclosure at either 20 ± 2 °C, 65 ± 5 % r.h. or at 23 ± 2 °C, 50 ± 5 % r.h. The conditioning shall be continued until the mean change of mass per 24 h of the specimen is less than 0.1 % of the final specimen mass. 3.5 Procedure Take the required number of dishes and place an anti-surge grid in each dish. Distribute a small amount of solid potassium nitrate evenly over the base of each dish. Fill the dish with a saturated potassium nitrate solution to just below the top of the grid. Centre the test specimen on the support ring of the dish. Lay the sealing template centrally in place, to obtain the designated exposed diameter which determines the effective area. NOTE 1The annular recess of the dish can conveniently be pre-coated with a film of sealing compound to facilitate subsequent sealing. Run molten sealing compound into the annular recess round the test specimen until it is 3 mm to 5 mm above the upper surface. Break any air bubbles in the compound by using, for example, a small gas flame or hot wire, and allow the compound to harden sufficiently before the template is removed. Inspect the dish to ensure the seal is satisfactory and remove any sealing compound on the outside. Take care not to damage the test area nor to allow saturated salt solution to contact the specimen at any stage. Place the dishes in the controlled humidity enclosure at either 20 ± 2 °C, 65 ± 5 % r.h., or 23 ± 2 °C, 50 ± 5 % r.h. NOTE 2As a consequence of the temperature control imposed on the potassium nitrate solution the humidity of the air within the test dish closely approximates to 92 %. The conditions prescribed are a compromise between the need to achieve a realistic average moisture content in the sample during testing, and the need to maintain a sufficient vapour pressure differential to permit the test to be completed in an acceptable time. The conditions specified yield a vapour pressure difference of 11.8 mbar1) (92 % to 50 % r.h.) or 6.3 mbar (92 % to 65 % r.h.) respectively. NOTE 3Whenever wood-based panel products are to be evaluated it is essential that the conditioning atmosphere is 20 °C, 65 % r.h. Throughout the procedure handle the dishes with care to ensure that the liquid within does not contact the specimen. NOTE 4Handling technique can conveniently be checked by the use of a transparent “dummy” specimen. At recorded time intervals during the test make successive weighings of the charged dish assemblies such that the test is completed before the cumulative loss in mass exceeds one-half the initial mass of the saturated salt solution. If samples are removed from the conditioned atmosphere for weighing, take precautions to ensure that dishes are out of the cabinet for as short a time as is practical and for the same period under the same conditions at each successive weighing. NOTE 5Weighing should preferably be carried out no more than once per 24 h, to minimize disturbance in any cabinet used, unless the resistance to the transmission of water vapour is low, in which case weighing at shorter intervals will