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Technical Note (National Research Council of Canada. Division of Building Research), 1963-01-01
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Joint CMHC/DBR Meeting to Discuss Factory-Sealed Double- Glazing
Units
Dickens, H. B.
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...ｾ _ I f
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
TEClHIN leAL
NOTlE
No.
387
FOR INTERNAL USE BY CMHC ONLY
PREPARED BY H. B. Dickens CHECKED BY A.G.W. R.F.L.
DATE January 1963 PREPARED FOR Central Mortgage and Housing Corporation
SUBJECT JOINT CMHC/DBR MEETING TO DISCUSS FACTORY-SEALED DOUBLE-GLAZING UNITS
A meeting was held on 28 November 1962 with representatives of CMHC and DBR to consider the current situation with regard to testing and evaluating factory-sealed double-glazing units. This meeting was held as part of a periodic review with the Corporation of this Division's continuing program which was designed, in the first instance, to develop acceptable laboratory test methods and criteria for evaluation of the degree of sealing provided and main-tained during service life by hermetically sealed units, and to assist the Cor-poration in establishing a sound basis for acceptance of such units. Those
present were Donald F. Marsland and R. F. McDonald of CMHC, and A. G. Wilson, K. R. Solvason and H. B. Di ckens of DBR.
Mr. Wilson briefly reviewed the laboratory weathering and outdoor exposure studies, pointing out that the Corpor.ation's acceptance criteria were based primarily on the former, utilizing measurements of dewpoint temperature and vacuum chamber deflection as outlined in a Technical Note of this Division, No. 363. Mr. Wilson then discussed several limitations of the present accep-tance methods. Many of these limitations are stated or implied in NRC 7042, copies of which were given to the CMHC representatives.
There has been some difficulty in interpreting the results of the initial vacuum chamber test. Some units show leaks when first tested but show no leaks on subsequent vacuum chamber testing. This appears to be related to the use of soft, non- setting, mastics of the butyl variety, which can flow under pressure, rather than the polysulphide-type sealants which were us ed widely in earlier designs. For non-setting mastics the vacuum chamber test appears to be of doubtful value.
2
-Another weakness in the present evaluation methods relates to the 300
F dewpoint criterion. It has been evident from observations of units exposed in the laboratory weathering apparatus that
a
measured dewpoint of 30of
can result in excessive amounts of ｣ｯｮ､･ｮｾ｡ｴｩｯｮ between panes under cycling temperature conditions. This relates in part to errors in the measure-ment of dewpoint. The actual dewpoint between panes is always somewhat higher than the value determined by the present method of measurement. These errors increase as the dewpoint temperature decreases. A laboratory study of the errors has been started. The dewpoint at which condensation will occur under cyclic conditions is also related to the time required for the air space to come into equilibrium with the desiccant. It would appear that a dewpoint of 00F or IaN' er, as measured at present, may be required to o1:;>viateexcessive condensation between panes under service conditions.
The use of a 30 of dewpoint temperature in relation to failure criteria in the laboratory weathering apparatus is questionable for another reason. The dewpoint temperature depends on the amount of water in the desiccant, and increases as the desiccant moisture content increases. The increase in dewpoint with increasing moisture content is rrtuch less at high temperatures than at low temperatures. Thus relatively large increases in desiccant moisture content are required to change the dewpoint from, say, O°F to 30°F, as compared with that required to change the dewpoint from -30°F to O°F. Thus units with slow leaks or which have an excessive water vapour diffusion rate may not be classed as having failed the laboratory weathering _ test when .the 30 F dewpoint is taken as the criterion of failure.
The laboratory weathering apparatus is not effective in identifying units which have high diffusion rates for another reason: the mean vapour
pressure provided during the cycle is not very high and the total time of exposure is relatively short in terms of service life. This is indicated by the fact that some units show more leakage by diffusion when stored in the laboratory at 50 per cent relative hUlnidity than on exposure to the laboratory weathering cycle. If failure occurs in the laboratory weathering apparatus, it does so chiefly by the pumping action of air containing moisture, after the seal is broken by repeated stressing due to temperature cycling.
CMHC indicated some concern over fai lures of units, which had been accepted by these methods, that were occurring in the field. Concern was also expressed by DBR over the length of time taken under the present methods to evaluate a manufacturer's product.
The existing laboratory weathering apparatus accommodates only 24 specimens'. In view of the large numqer of manufacturers, it was possible to allocate only one space per manufacturer. Even on this basis it had been necessary to stop tests on some units after testa on only 6 of the 12 specimens
·
ｾ3
-had been completed, in order to make space available to manufacturers still seeking an acceptance. In the one space available, two specimens were tested during alternate two-week periods for a total of 16 weeks. Thus 4 to 5 months were required as a minimum to determine if a manufacturer might gain
provisional acceptance .. If one of the two specimens failed, an additional 4 months of testing was required. In some instances after provisional acceptance was granted, failures occurred on specimens subsequently tested, perhaps a year or more later. Thus not only was space on the apparatus and the time of the Division wasted, in a sense, but units of doubtful performance were on the znarket for an extended period.
Because of the demands for acceptance testing it was not possible to provide assistance to manufacturers attempting to develop improved unit design. Furthermore, it was not possible to use the laboratory weathering apparatus for further research studies intended to improve evaluation pro-cedures. In order to provide a research facility the Division has under con-struction an additional laboratory weathering apparatus similar to that already in use, but somewhat improved. This apparatus, however, will be of no assistance in overcoming the major problems in relation to the present program of acceptance testing.
For these reasons Mr. Wilson requested that consideration be given to revised testing proc'edures designed to improve the evaluation methods and, at the same time, to reduce the testing time. The main change suggested was the introduction of a new temperature-cycling - high-humidity chamber which would subject the units to a relative humidity of 100 per cent with a temperature
cycle of 80° to 135°F every three hours. It was pointed out that a number of units previously tested in the laboratory we'athering apparatus had also been tested on a trial basis in a prototype of this new high humidity chamber, and
the failures that occurred showed a good correlation with the failures that had taken place on the existing apparatus. Additional failures also occurred, pre-sumably through the diffusion mechanism due to the higher vapour pressure within the chamber. The new chamber could be constructed to handle 96 specimens at one time and would greatly speed up the time for testing since the majority of units that had failed did so within 30 days, and a number of failures occurred within the first ten days. On this basis a maximum test period of eight weeks in the chamber seemed adequate with a dewpoint of O°F set as the criterion for failure. It was thought that this high humidity chamber could be used most effectively in combination with the laboratory weathering apparatus and the following evaluation procedures and criteria were outlined for conside ration in relation to future acceptance testing:
(1) Initial dewpoint and vacuum chamber deflection determinations on each of the 18 specimens as received. Maximum allowable dewpoint would be -40 ° F •
·
..
4
-The vacuum chamber test is considered of doubtful value as an acceptance criterion but it might still be used to select potentially weak units for inclusion in subsequent tests in the high humidity chamber or window weathe ring apparatus.
(2) Exposure of six units in the high humidity chamber for a maximum of eight weeks. Maximum allowable dewpoint would be O°F.
(3) Exposure of six units in laboratory weathering apparatus for a maximum of ten weeks involving a total of 420 cycles. Maximum allowable dewpoint would be O· F. Exposure would be continuous rather than alternating between laboratory and weathering appara-tus every two weeks.
This represents an increase of 100 cycles over the previously accepted criteria for this test but it is considered necessary, in view of the combined laboratory and field experience now available.
It was suggested that provisional acceptance should be granted by the Corporation only after all initial dewpoint and high humidity chamber tests have been successfully completed and at least two specimens have satisfactorily passed the laboratory weathering tests. The laboratory weathering tests on the remaining four units should continue until completed. The window would be considered to have failed to qualify whenever more than one unit fails these tests at which time presumably the Corporation would withdraw acceptance.
, It was explained that the program described above could only be put into effect when the new high humidity chamber and additional laboratory weathering apparatus were ready for service. This would be approximately
1 June 1963. To put all units on a comparable basis, it was suggested that at that time, in addition to new units, all existing factory-sealed double-glazing units with CMHC acceptance could be evaluated in this way. This would eliminate any weak units that might not have been previously detected in the former pro-gram and, perhaps more important, would constitute a requalification of existing units, some of which may have changed their materials or methods of manufacture since original marketing. As a further refinement, it was suggested that the glass in all future 14- by 20-in. test units be standardized at 32 oz glass with i-in. spacer and that a full description of the units, including a cross-section showing complete details of the edge arrangement spacer, etc., should be pro-vided, as well as details of the sealant.
A rough scheduling indicates that since the proposed high humidity chamber could handle 96 specimens, and six units must be tested for each
manufacturer, then tests for 16 manufacturers could be completed in eight weeks. In addition, with two laboratory weathering apparatus in operation accommodating a total of 48 specimens, two units f:rom each of 24 manufacturers could be tested
...'
..
-'"5
-in 10 weeks. On this basis it should be possible to complete all testing necessaryior provisional acceptance of specimens from the more than 30
manufacturers now active, in about six months.
The matter of outdoor weathering was also discussed during which it was pointed out that the main advantage of such testing was in relation to the assessment of the effect of solar radiation on the sealant. The results of out-door weathering tests have not previously been incorporated in the Corporation's acceptance criteria, but should they be so incorporated, it would be necessary to consider mounting each test specimen in a standardized way so that the exposure to solar radiation would be realistic. An exposure period of at least two years was considered: necessary with dewpoint measurements being taken twice yearly. Again a maximum dewpoint of 0OF would appear to be a reason-able cr iter ion.
Finally, it was pointed out that even with this revised basis of acceptance, there was still a number of factors which were not being fully considered. These include the effect of wind pressures on seal failure, the performance of larger-size units, and the durability of sealants in rela.tion to the retention of sealing properties.
The meeting was concluded with Mr. Marsland agreeing to review this whole situation in the light of these remarks and to indicate to the Division at the earliest opportunity the Corporation's decisions regarding the new high humidity chamber and the suggested evaluation procedures and criteria for all future acceptance testing.
