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The Problem of achieving weathertight joints
Garden, G. K.
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i ! :
SYMPOSIUM
ON WEATHERTIGHT
JOINTS
FOR WATLS
- OSLO . NORWAY. SEPTEMBER
1967
Ser
THl
N21t2
no. 25, I
c . 2
i
BI,DG
The problem
of qchieving weothertight joints
BY G. K. GARDEN,
NATIONAL RESEARCH
COUNCIL
DIVISION
OF BUILDING
RESEARCH,
CANADA
:-'.gs3s
A N A L Y T [ }
BUILDING RESEATT
. LIBRARY
oEt 16 19EB
N A T I O N A L . R E S E A R C H C O U TUnn/NnC
Technical Paper No. 265
OTTAWA' JuIy 1968
NRC e874
Price 10 cents
The problem of achieving weathertight joints
BY G. K. GARDEN,
NATIONAL
RE.SEARCH COUNCIL
DIVISION OF BUILDING
RESEARCII,
CANADA
The protection of a space from the variations and un-pleasant conditions of the weather is one of the main reasons for constructing a building, yet, throughout history, man has been plagued by the problems of leak-age. Rain and air leakage, the two basic problems con-sidered under weathertightness, detract from the success of a building, but are of far greater significance with respect to the durability of its parts. Although we have learned to control leakage to a tolerable degree in some building systems, the problem of achieving weathertight 'joints
is still urgently in need of attention.
It ,seems reasondble to assume that the attention paid to preventing air and rain leakage would have been related to the severity of the problems believed to result from each. Air leakage, although it contributes to ther-mal discomfort, has normally been treated lightly and so could not have been eonsidered to detract appreciably from the service life of a building. Rain penetration, however, must have been recognized as the cause of more serious problems for there have been many innova-tions in wall and joint design in the attempt to prevent, or at least minimize, its occurrence. These attempts to prevent rain penetration have even had a pronounced influence on architectural styles.
A study of past and present building practices shows that there have bebn two basic approaches to the design of walls and Joints with respect to the control of rain penetration. One is to shed the water by overlapping each piece of material of the wall surface; the other is the attempt to eliminate all openings that might let water enter the construction.
The approach of shedding water by the use of over-Iapping units such as shingles, siding, and thatch has been very successful in preventing rain penetration. A system of overlapping units, however, did not prevent air leakage and another layer of matenial was necessary to control this problem. Because this approa.ch was not being followed in the design of all joints in a wall, particularly at windows and doors, walls covered with overlapping units were not completely free of rain penetration problems. Organic matenials, in particular wood, were normally used in these systems because of the difficulty of hanging other materials on a wall. Because of the problems of rot and fire associated with organic materials, masonry construction normally held more appeal for permanent buildings and those in urban areas. Systems of overlapping units, however, have been the most common method employed to prevent rain penetration throu.gh roofs of all types of buildings. On roofs, more duradle materials could be held in place and clay, slate, and metal tiles have been used.
It must have been generally reeognized that rain penetration could be controlled best by using overlapping units. At many wall joints and with some materials and methods of construction, however, this approach could not be followed. In these cases, there appears to have been no choice but to resist rain penetration by attempt-ing to eliminate all openattempt-ings that would permit the entry of water. This approach has been followed in the design and construction of solid masonry walls.
Traditionally, masonry walls, constructed of small units
cemented together with mortar, were quite thick and more or less uniform throughout. They were reasonably stable but in response to ageing, loading, and tempera-ture and moisture changes, differential movements between units occurred. The movements were small in small units and with the weak mortar normally used, extremely small cracks developed in almost every joint. Despite the many microcracks, thick masonry walls were successful in preventing through-wall penetration of rain except during prolonged or wind-driven rain storms. Their success derived from the length and small size of the leakage paths and the large moisture .storage capacity of all the materials of the wall. These
walls also provided reasonably good air leakage resist-ance,
Partial rain penetration of masonry walls does not show at the buildirtg interior and may have been less frequently recognized as a problem. ft is partial penetra-tion, however, that has played the main part in masonry wall deterioration since it causes frequent wetting of wall materials, In mild climates, masonry walls served for many centuries despite periodic leakage. In regions where freezing occurred, the deterioration of damp masonry was often quite dramatic. Masonry units were displaced and mortar joints had to be repointed fre-quently. Unless well maintained and periodically repaired, masonry walls can be completely destroyed in a relatively short period of time. Freezing of dry masonry has rela-tively little ill effect. Wetting of masonry materiaJs, even through only partial penetration of rain, therefore, is a key factor in masonry wall deterioration.
Throughout history, the inability of masonry vralls to prevent rain penetration must have been Senerally recognized. This is shown by the many practices followed in the attempt to improve their performance. Cornices and belt courses have been used in an attempt to reduce the amount of water that contacted the wall surface. Stucco and other renderings haVe been used in the attempt to produce a crack-free, monolithic exterior surface. It was recognized that in the United Kingdom a masonry wall would inevitably leak, thus cavity wall construction was adopted. With a properly drained clear space between two walls, the outer wall leaked as antici-pated but the inner wall remained dry. fn Canada, it has been common to use furring to separate the interior finish from the exterior masonry wall to protect it from moisture problems.
More extreme measures had to be employed in regions of more severe climate. Buildings along the Atlantic Coast of America, which were built of English half timber with masonry infill, were quite unsatisfactory. Even the application of stucco did not stop the rain leakage and the finishing of exterior wall surfaces with overlapping units was necessary. Scottish settlers in Eastern Canada built with stone masonry and, again, because of the unsatisfactory performance of these wa.lls under the severe weather conditions, it became a com-mon practice to cover them with shingles or weather boards, at least on the weather wall. These examples also show that importation of building systems and stylee by immigrants or through architectural fancy, without
regard for differences of climate, has resulted in the construction of many unsatisfactory buildings. Even today this practice is often blindly followed, much to the dismaf of owners, builders, and architects. The apparent success of a building system in one climate cannot be considered to indicate that the system, if followed in another cli.mate, will produce a good building.
'We
frequently read that masonry problems have be-come more common and more severe in the last fifty tb seventy-five years. Failure to recognize and allow for the different, conditions that result from changes in the materials and methods of construction and th1 new interior environmental conditions could be the main factor in producing this situation.
Changes in rnaterials and methods of construction have altered the performance of masonry walls and not always for the better. The materials of a structural building frame and the enclosing masonry walls have different thermal and moisture responses and are subjec-ted to different environmental conditions. If these diffe-rences are not reeognized and if rio allowances are rnade for the differential movements that tend to occur, severe cracking of the masonry walls frequenily results. Deflection of structural spanning members, especially when creep in concrete is involveil, causes iurther cracking of supported masonry walls. The increase rn size of masonry units increases the differential move-ments and the width of the cracks at mortar joints. Improved ,bond and increased tensile strength, gained with new mortar materials, cause areas of masonry to act as large integral units. Instead of microcracks deve-loping in eaoh mortar joint, major cracks at less frequent intervals occur and are frequently eontinuous through the masonry units. Different,ial movements are of great significance and have contributed to the increased inci-dence of problems in walls and joints.
The introduction of the structural building frame relieved the masonry of vertical loads, and walls could be made thinner. The reduction in mass of the wall reduced its moisture storage capacity and its rain penetration resistance. Its resistance to air and heat flow was also reduced. The use of materials that are more dense and less absorptive further reduced the moisture storage capacity and more water leaked through the waJl more quickly.
The improve ability to maintain more desirable interior environments through central heating, ventilating, and air conditioning systems has introduced other factors that influence wall and joint performance. The desire to maintain a higher interior temperature and yet conserve heat energy, coupled with the reduced heat flow resistance of the thinner walls prornoted the use of insulation mate-rials. Because these materials would deteriorate if exposed to the weather, Lhey were applied to the inside of the wall Although insulation in this position performed the function for which it was intended, the ,rnasonry wall was subjected to a much greater range of tempera-tures. The increased thermal expansion and contraction resulted in greater differential movement and cracking of the walls. The waJl, in reaching much lower tempera-tures in cold weather, was also subjected. to more fre-quent freezing.
Maintenance of a higher interior temperature during cold weather and the trend to taller buildings increase the inherent chimney effect in a building which, in turn, increases the air leakage through the walls. Mechanical ventilation systems, in causing the interior air pressure to be different from that outside, further incr:ease air leakage. The leakage of air through joints has seldom been consideretl a serious problem, although it contributes to the entry of dirt and odours and causes cold drafts
and some heat loss. If the dewpoint temperature of the interior air is higher than the exterior temperature, severe condensation within the wall can result from air exfiltration. Under these atmospheric conditions, the simple process of convective interchange of interior air with the air in cold wall and window spaces can also cause serious condensation. Humidification of heated buildings in winter provides more water for conden-sation at even higher temperatures and causes rnore moisture to accumulate at air leakage paths further increasing wall and joint problems. Air leakage has become too important a factor in joint performance to be treated as lightly as it has been in the past.
Changes in the environmental conditions being main-tained inside buildings have imposed more rigorous functional requirements upon joints. A designer must now consider the control and the interdependence of air, heat, and moisture flow both inward and outward through a wall and its joints. Failure to do so will increase the frequency and severity of wall problems. Modern technological advances have caused a greater awareness of what should be achieved in our buildings. foday, we are not prepared to tolerate even infrequent minor failures of building walls and standards of accep-table performance have risen. In published correspon-dence and papers, written in t782, Mr. Thomas Jefferson, the Architect'President of the United States, discussed vapour condensation and rain leakage problems of masonry walls. With respect to rain leakage, he said, <<In a house, the walls of which are of well-burnt brick and good mortar, I have seen the rain penetrate through only twice in twelve or fifteen years.>> This statement clearly shows that this wall, which he considered to be exceptionally good, actually leaked. Furthermore, this performance must have been better than normal for hirrr' to use it as an example of accomplishment, indicating that leakage from titne to time must have been tolerated under their standards of acceptable performance, The belief that there are now more problems in masonry wallS can perhaps be partly dttributed to today's higher performance criteria and our increased awareness of what should be possible of attainment through modern technology.
Joints, other than those in masonry walls, which were dependent upon the eli,mination of holes through which water could pass, were also prone to eventual failure, Because a sheet of glass had to be held in the wall by retaining its edges, ths overlapping unit approach could not be followed and a perfect seal of the glazing joint was essential if rain penetration was to be prevented. Oleoresinous materials were used for this seal but, in time, they dried, cracked, and leakage occurred. Painting of the glazing material resealed the cracks but repainting and even regla?ing were necessary from time to time. Joints between the window sash and frame also depended for weathertightness upon their ability to be sealed but the overlapping unit approach was followed where pos-sible. The drip mouldings at the window head an on the sill rail of a sash were actually overlapping units and these joints performed exceptionally well while leakage occurred at the jambs,
With the introduction of new wall systems, such as metal and glass and large precast concrete panel curtain walls, there has been very little improvement, if any, in the performance of $oints. Despite new terminology, there are still only the same two approaches to the design of joints for the control of rain penetration. Because of their familiarity with past masonry and glazing practices and their inability to understand the real reasons for the successful performance of joints in systems of overlapping units, designers have followed
the approach of attempting to gain a perfect seal at the
exterior surface. Because of the failures that resulted,
many sealant materiaJs have been developed that have fantastic capabilities. That joints are still failing despite
the use of these materials indicates several things:
sealant materials are heing improperly used or their
properties are being overestimatedi the basic approach
followed in most joint designs must be wrong or truly
successful joint performance simply cannot be achieved.
Weathertight joints have been achieved, even without
the aid of marvellous new materials and exceptional
workmanship. The problem of achieving weathertight
joints must, then, be one of design. Materials and
work-manship are important factors in joint performance
but a faulty design, no matter how well the work is
performed, is prone to failure. Overestimating the ability
of a material to perform a critical function or to retain its required properties over a reasonable period of time should be considered a fault in design even if due to false
information. Faulty design results from inadequate
knowledge or lack of due consideration of the functional
requirements imposed on each component of the joint,
of the many physical processes and phenomena
influen-cing joint performance, of the interdependence of one
factor on another, and of the true nature and charac-teristics of the materials being used.
Experiences of the past must neither be ignored nor
blindly followed. They must be re-examined in the light
of new knowledge and understanding in order to
deter-mine the reasons for success or failure. The influence
of new materials and methods of construction and the
new, more rigorous functional requirements imposed on
walls and joints must be considered when comparing past and prese8t experience. For designers to appraise thelr past experiencg as well as to design new systems,
they must be provided with detailed, scientifically sound
principles upon which to base their analyses and design.
It is the responsibility of building research to develop
and delineate these principles. It is hoped that the
exchange of information and experience at this
Sympo-sium will bring us much closer to the realization of this
fundamentally important goal and the solving ot the
problem of achieving weathertight joints.
B6surn6
L'infiltration de la pluie dans le bdtiment est un
pro-bldme qui date de la plus haute antiquit6. Une r6tude
des m6thodes actuelles et pass6es de la construction
d6montre que, fondamentqlement, iI n'y a eu que deux
concepts dans la planification des murs et des joints.
Dans un cas, on a tent6 d'6liminer tous les passages
accessibles ir I'eau tant dans les murs de magonnerie
que dans la f6nestration. Dans I'autre, on a 6tabli la
d6perdition d'eau au rnoyen du chevauchement
d'616-neents tels que le bardeau, le parement i recouvrement et la chaume. Ces concepts sont encore employ6s de nos jours.
Les divers essais de scellement des surfaces ext6rieures
des bAtiments n'ont Jamais 6t6 tout ir fait satisfaisants.
On peut facilement le constater ir I'usage des corniches,
'chaines, enduits cle mortier ou stucs, finis int6rieurs
sur profil6s et I'usage de cavit6s comme moyens
d'am6lio-rer Ie comportement des murs congus selon ce concept.
,Par ailleurs, la planification selon le concept de
chevau-chement des 6l6ments s'est av6r6e avantageuse pour
pr6venir I'infiltration de la pluie. Dans certains cas oir
les pliieg chass6es sont particulidrement s6veres, on a
m€me employ6 des 6l6ments de chevauchement comme
parement d des murs de magonnerie, A cause des r6sul-tats heureux obtenus selon le concept de chevauchernent,
cette m6thode est commun6ment employ6e pour limiter
I'infiltration de la pluie par les toitures de divers genres
d'6difices.
L'exp6rience du pass6 ne doit ni 6tre rejet6e
totale-ment ni suivie aveugl6totale-ment. Elle doit, cependant, 6tre
r66valu6e d la lumiEre de nos nouvelles connaissances
et de notre meilleure compr6hension du problBme. En
comparant l'exp6rience actuelle avec celle du pass6, il
faut aussi prendre en consid6ration I'influence des
nouve-aux mat6rinouve-aux. des nouvelles m6thodes de construction et
des exigences fonctionnelles plus rigoureuses que I'on
attend des murs et des joints.lDans le but d'obtenir des
joints 6tanches aux intemp6ries, il est essentiel de bien comprendre toutes les exigences fonctionnelles, les divers
ph6nomEnes physiques rattach6s au comportement des
joints ainsi que I'intime interd6pendance de tous ces
