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Technical Note (National Research Council of Canada. Division of Building
Research), 1964-04-01
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DBR Studies Relating to Rain Penetration (April 1962 to April 1964)
Ritchie, T.
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DIVISION OF BUILDING RESEARCH
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PREPARED BY T. Ritchie CHECKED BY P. J. S. APPROVED BY N. B. H.
PREPARED FOR C. 1. B. Working Commission on Rain Penetration
April 1964.
SUBJECT DBR STUDIES RELATING TO RAIN PENETRATION
(APRIL 1962 TO APRIL 1964)
Since the last meeting of the C. I. B. Working Commission on Rain Penetration, held in Copenhagen in 1962, studies of the rain penetration problem have been continued by the Division of Building Research. There have been four main aspects to recent DBR work on rain penetration: (1) the promotion of improved designs of exterior walls, including the use of the "open rain screen" principle; (2) construction of an apparatus for the testing of windows and of joints in walls for rain resistance; (3) prep-aration of a map of "driving rain index" for Canada, and a detailed
investigation of the basis for calculation of the index; (4) measurement of the degree of wetting of masonry materials by rain.
DESIGN OF WALLS TO OVERCOME MOISTURE PROBLEMS
Several articles have been issued by the Division (Canadian Building Digests No. 30, 40, 42, 48 and 50) dealing with moisture prob-lems in buildings and steps that should be taken to overcome them or minimize their effects. One of the articles (CBD 40) describes the "open rain screen" method of constructing panel walls and windows, following the methods developed by the Building Research Institute of Norway.
As a further step in promoting the better design of walls in regard to moisture and other problems a seminar on the design of, exterior walls was held by the Division in February 1964, which was attended by more than 220 architects, engineers, and representatives of contracting firms and manufacturers and suppliers of building materials. The causes of moisture problems in buildings and the principles to be followed in reducing or eliminating them were the main subject of the
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-seminar. Because of the excellent response to this seminar (many who
wanted to attend were turned away because of limited accommodation), it is planned to repeat it later in the year.
The practice of introducing humidification into heated, occupied buildings other than single dwellings is growing rapidly in Canada.
Increased winter humidities have been the rule in owner -occupied single dwellings of wood-frame construction which are readily arranged to avoid
difficulties. Other buildings of commercial, industrial, and institutional
types have not been constructed to the same thermal standards and now are experiencing very serious problems resulting from increased
humidities in cold weather. Notable advances have been made in recognizing
the important role of air leakage in producing condensation within walls, and of the ice lensing mechanism, formerly recognized only in soils but now known to occur in certain mortars, in displacing masonry units and otherwise disrupting masonry constructions under certain critical combina-tions of condicombina-tions.
The open rain screen concept fits well into the pattern of wall design for humidified buildings for cold climates and indeed becomes an essential part of an optimum rational solution to the combined problems of control of heat, water vapour, air and rain (see, for example, Canadian Building Digests 42 and 50).
A study has been undertaken of the air pressure difference
across exterior walls of buildings. Two tall buildings, one in Ottawa and
the other in Montreal, have been fitted with the necessary equipment for
this work. Pressure differences occurring during periods of high winds
are naturally of interest for the structural loads which they represent. The generally smaller, sustained pressure differences arising from buoyancy effects due to differences between indoor and outdoor temper-atures are becoming of increasing interest, particularly in tall buildings humidified in winter, because of the role of air leakage under these
conditions in promoting wetting and freezing effects in walls. These
pressure differences are discussed in more detail in Canadian Building Digest 23.
APPARATUS FOR TESTING WINDOWS AND CURTAIN WALL JOINTS FOR RAIN PE NE TRAT ION
An apparatus based on the Norwegian method of test has been
constructed in the Division of Building Research. It is designed to test
a sample 8 ft (2. 4 m) wide by 8 ft high. Instead of a single row of
nozzles, as used in the Norwegian apparatus, four banks of nozzles are
used, which travel vertically a distance of 2 ft (0.6 m). The apparatus
has the capacity of maintaining an air pressure difference of 2.5 in. (6 cm) of water across the test wall.
The characteristics of the apparatus are being determined at
present; a program of testing has not yet been drawn up. It is possible
that initially the apparatus will be used to compare the results of the Norwegian test method with those of a test method proposed by the
American Architectural Aluminum Manufacturers Association. The latter
test makes use of stationary water nozzles and finer droplets of water.
CANADIAN MAP OF DRIVING RAIN INDE X
As requested by the Working Commission at its last meeting a map of the driving rain index for Canada has been prepared (DBR
Tech-nical Note 398). The method of calculating the index was similar to that
used by the British Building Research Station.
The driving rain index for a locality is calculated by this method as the product of the annual rainfall and the mean wind speed. If the wind speed during the periods of rainfall at a location differ s from the average wind speed (which appear s to be a distinct possibility) there would be a corresponding difference in the calculated driving rain index. A study has therefore been undertaken in the Division to determine the difference between the mean wind speed and the windspeed during
rain-fall. Five Canadian stations have been selected and their 10 -year
weather records are being studied in order to compare the driving rain index based on the two wind speeds.
The directional effect of driving rain is also being studied for
the five stations. The wind speed during periods of rainfall is being
obtained for each of the 8 directions and this wind speed, combined with a suitable proportion of the total annual rainfall. will be used to calculate a directional driving rain index.
WETTING OF MASONRY MATERIALS FROM RAIN
Experiments were started in the Division to determine the amount of moisture taken up by various masonry materials exposed to
rain. Samples of brick, stone, and mortar were set out on racks facing
the four directions. at two locations, Ottawa and Halifax. The driving
rain index for these two places is quite different (Ottawa := 2.3,
Halifax = 5.8 sq m per sec).
The samples consisted of five kinds of bricks (four clay and
one concrete), three mortars, and one stone. The mortar and stone
samples were formed into the shape of a brick. Four sides of the
samples were sealed with a polysulphide sealant, with the front and back
surfaces unsealed. The samples were exposed on a rack with unsealed
surfaces vertical. Only one of the unsealed surfaces could be wetted
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-the bricks from -the rack and weighing -them. The rack was made in such a way that the samples can be taken off and replaced easily.
For each type of material similar samples were exposed at Halifax and Ottawa, and at each place samples were exposed to the four
directions. The results of the tests, from the start in October 1963 to 1 April 1964, have provided the following interesting information:
(1) The same material exposed at Ottawa and Halifax did not have greatly different moisture content, even though there is a wide difference in driving rain index for the two places (2.3 vs. 5.8).
(2) The moisture content varied widely from one type of material to another.
(3) The moisture content of samples easily wetted by rain usually fluctuated greatly due to rapid drying out after being wetted by rain.
(4) There was a pronounced directional wetting effect (east is the direction of greatest wetting in both Ottawa and Halifax).
In addition to the bricks being used for moisture content measurements, other bricks have been fitted with a thermocouple at the mid-point and a continuous record of the temperature is being obtained. Such bricks have been exposed on the racks at Ottawa and Halifax to the four directions. The temperature records have enabled a determination to be made of the number of cycles of freezing which the samples have undergone.
The following information has been obtained from the temperature records:
(l) At both stations, but particularly at Ottawa, there is a considerable directional effect on the temperature of the samples (because of the sun falling on samples facing east, west, and
particularly south). As a result, the sample facing south in Ottawa had twice as many freezing cycles in the cold months as the sample facing north. A similar directional effect occurred in Halifax but it was not as pronounced.
(2) Samples had more freezing cycles in Halifax than in Ottawa, especially the samples facing east which at both places corre-spond to the condition of highest moistur e content.
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-CONCLUSIONS
The problem of rain penetration of walls continues to receive attention in the Division of Building Research. The latest findings of research on moisture problems of buildings were presented to architects and builders at a building science seminar on the design of exterior walls. More equipment has been obtained for laboratory and field studies of the rain penetration problem. Studies of the driving rain index are being continued, and interesting information is being obtained of the effect of rainfall on the moisture content of various masonry materials.
