HITEX RESEARCH BULLETIN

HITEX Research Bulletins summarise research projects undertaken in conjunction with the University of Auckland. The aim is to provide an understanding of what is happening in the wall of a building for the purpose of building structurally sound and healthy homes for the future.

 

1. SUMMARY

Following the discovery of large numbers of leaking and rotting homes in North America, research was undertaken to investigate water entry rates into the stud cavity and the drying potential of the wall assemblies under different climate loads. MEWS Task 6,7, and 8 Project reports were three of the major outcomes. The research used a combination of laboratory work and computer mathematical modeling with four types of walls (1) Stucco walls, (2) EIFS walls, (3) Masonry walls, and (4) Siding walls. Key results of the projects were:

	The rate of water entry into the wall through a defect increased as a result of increased pressure and increased spray rate. A significant quantity of water entered when there was no pressure.
	A cavity as a second line of defense is effective in arresting the penetration of a leak to the stud cavity. The thickness and method of construction of the cavity did not appear to matter. Cavity thicknesses tested varied from 3mm to 50mm, and cavity types varied from nylon mat to grooves to a complete cavity. Some cavity designs caused the wall to take up moisture without a leak.
	Walls with water leaks into the stud cavity did not dry out while the water flow was maintained.
	The area of focus surrounding the simulated leak was predicted to maintain a relative humidity in excess of 90%.

2. BACKGROUND TO MEWS RESEARCH

In the 1990’s there were a large number of building failures in North America and particularly in condominiums built in Vancouver. Leaking and rotting buildings were discovered in many areas. A Commission of Inquiry was appointed in 1998 to review the adequacy of protection, and accountability to, consumers for faulty condominium construction, and to determine the reasons for, and the factors contributing to, faulty construction. The Commission concluded that climate, economic pressures, the residential building process and building science issues have led to a disintegration in the quality of construction. A consortium called MEWS (Moisture Management in Exterior Wall Systems) was formed and carried out research at the Institute for Research in Construction (IRC) of National Research Council (NRC), Canada.

The purpose of the MEWS Projects was to investigate water entry rates into the stud cavity and the drying potential of the wall assemblies under different climate loads. The Task 6, Task 7, and Task 8 Project Reports published in November 2002 through February 2003 were three of the major outcomes from the consortium. These reports deal with research done on four different types of wall assemblies in a building envelope. The four types of walls considered were (1) Stucco walls, (2) EIFS walls, (3) Masonry walls, and (4) Siding walls. The wall assemblies included a rain-screen, a barrier acting as the second line of defense, and an air barrier. Parametric analysis was done using IRC's hygrothermal modelling tool hygIRC. hygIRC is a 2-dimensional numerical modelling tool specifically developed for research purposes and it is continuously evolving at the IRC/NRC. The utility and reliability of hygIRC outputs have been established through laboratory measurements and benchmarking exercises.

3.      MEWS 6 TASK PROJECT REPORT

TASK 6 Project involved water penetration trials for a variety of materials of construction, water irrigation rates and applied pressures. The facility used was the Dynamic Wind and Wall test Facility that tests a wall specimen 2.4 x 2.4 metres, static and dynamic pressures of over 2kPa, and spray rates up to 8 l/min-sq.m. The walls were subjected to water entry through purposefully made 6 mm diameter holes located; (1) above a ventilation duct, (2) above an electrical fitting, and (3) corner of a window. The test walls were backed with clear acrylic so that the movement of the water could be seen. Moisture sensors were located in the timber stud cavity that lit up when moisture had effectively penetrated into the stud cavity.

In general the results showed that the rate of water entry into the wall increased as a result of increased pressure and increased spray rate. A number of tests also showed that a significant quantity of water entered when there was no pressure. Any form of cavity was effective in acting as a second line of defence and arresting penetrating to the stud cavity. Cavity thicknesses from 3mm to 50mm were all found to give benefits. Water entry through a vented rain-screen is markedly less than that for a non-vented rain-screen. Key overview statements made by the project team were:

	Cavities: “Those wall assemblies offering the greatest redundancy in performance in respect to management of water intrusion and hence the least vulnerable to water penetration were those incorporating a drainage space.”
	Best EIFS: “EIFS assemblies that incorporated a drainage system, such as vertical grooves present in either the insulation or adhesive coating, had reduced likelihood of water penetration to the sheathing board.”
	Penetrations Detail: “... highlight the importance of adequate detailing about through-wall penetrations”.
	Windows Worst: “Windows … were found to be the most sensitive components in regard to penetration”.

4.      MEWS 7 TASK PROJECT REPORT

TASK 7 Project used mathematical modelling to predict what happened to water leakage to the stud cavity. The parametric analysis was done using IRC’s hygrothermal modelling tool hygIRC on the 4 types of walls. The report introduced a hygrothermal performance indicator RHT index which gives a quantitative value for the amount of time the RH (relative humidity) and the T (temperature) are above prescribed values for 10 day intervals. RHT(95) which is when the RH is above 95%  and T is above 5ºC is reported most widely. The output data is in the form of so-called “drying curves” over a time period of 2 years.

Results of the mathematical modelling done by the MEWS 7 research team in Ottawa, Canada.

Stucco: Results with a leak onto the top of the bottom plate gave humidities on the bottom plate in excess of RH of 80%, and peaking at over 90%. Humidities over 80% were over the entire 2.3 metre height of the wall and through to the vapour barrier inside the gib board. The RHT(95) values peaked at over 2000 in Wilmington but were zero in the dry Phoenix. A cavity behind the stucco helps reduce the RHT(95) index significantly.

EIFS: The EIFS modelled had a 11mm OSB (oriented strand board) sheathing plus 38mm of polystyrene. There was also insulation in the stud space and the vapour barrier was against the internal gib board. No work was done on walls without a full sheathing under the polystyrene. Again results are with a leak onto the top of the bottom plate and gave a RH of 90% plus on top of the bottom plate. The RH of 80% envelope spread 0.5 metres up the wall. The RHT(95) index values peaked at 5,900 in Wilmington. The vapour barrier with the highest value of vapour permeance gave the lowest RHT(95) index values. Simulations done without insulation in the cavity gave lower RHT(95) index values due to the changes in temperature profile and due to the insulation drawing water away from the region of focus. One simulation was done without the vapour barrier against the internal gib board and gave a noticeable drop in the RHT(95) index value.

	Masonry Walls: The walls modelled had cavities of either 25mm or 50mm. The RHT(95) index values for Wilmington reached 3500. The RH again peaked at over 90% and was over 90% for the entire height of the exterior face of the wall with the source of moisture being weather and not the leak.
	Siding Walls: Hardboard and vinyl wall claddings were modelled with no cavities. The RHT(95) index value for Wilmington reached 3,300. The RH again peaked at over 90% and spread 0.5 metres up the wall.
	“Drying Curves”: The report gives a large number of “drying curves” which plot the moisture content of the walls tested against time following the introduction of the leak. An example is given opposite for a simulation in Wilmington with EIFS cladding. The graph shows the amount of moisture held in the wall increasing, and shows that most of the moisture is held in the OSB sheathing, the gib board and the fibreglass insulation batts.

5.  MEWS 8 TASK PROJECT REPORT

Task 8 Project investigated the hygrothermal response of the 4 different wall assemblies when subjected to exterior moisture loads on the cladding, as well as into the stud cavity when a specific deficiency in the wall assembly provided a water leakage path to the stud cavity. Again the hygrothermal performance indicator RHT index was used for the evaluations. The report also uses a Moisture Index (MI) and a Drying Index (DI); the higher the MI the more sever the moisture loading, and the higher the DI the more potential evaporation available. The climate inputs were taken from 7 cities, ranging from the wettest being Wilmington in North Carolina (MI = 1.13) to the driest being Phoenix in Arizona (MI = 0.13). The moisture loads into the wall were calculated using the climate figures, i.e. wind and rain, and thus the moisture load was in the terms of an hourly rate that varied every hour. This is therefore quite different to MEWS Task 7 Project that used a constant rate of moisture entry into the stud cavity. The point for the injection of moisture was in the insulation next to the bottom plate in the stud cavity. The first year modelled starts with the entire wall at T of 20ºC and RH of 50% and the first year is considered a conditioning year. The general approach used was to vary four basic parameters: the climate, the wall cladding system, the materials used in the assembly and the amount of accidental water entry into the wall assembly.

In general the results were as follows:

Walls With No Deficiency: All wall types exhibited significant resistance to water penetration through the field of the wall. The RHT(95) was zero or near zero for all wall types in all areas.

Walls With Small Deficiency: The wall response varied from a RHT(95) of 655 in the hot and dry climate of Phoenix to about 3213 for the warm and wet climate of Wilmington NC. This indicated that the amount of water entering the cavity was more than could be accommodated by the evaporative drying potential of the materials and the makeup of the wall assembly. The walls exposed to climates with severe moisture loads reached a stud cavity RH level above 95% after a few months of climate exposure and this RH remained stable until the end of the 2 year simulation period while the leak was maintained. Even with only a quarter of the moisture load into the stud cavity, there was only a small drop in the RHT(95) index. The figure opposite shows a dark area of RH over 87% around the bottom plate.

Walls with Vented Cavity: The simulations suggested that an unobstructed 19 mm vented cavity behind the stucco cladding appeared to make little difference. The report suggests that for some wall types the cavity contributed to the rate of moisture loading into the stud cavity, and that this was higher than the moisture withdrawal by evaporation contributed to by the introduction of a clear open cavity. Small reductions in the RHT(95) index were predicted when there was uncontrolled airflow through the cavity.

6.      DISCUSSION

A consistent theme through the reports is that a cavity is able to contribute to stopping a water leak in the cladding penetrating through into the stud cavity. The research raises doubts into how a cavity should be designed and it appears that cavities need to be different for different cladding materials. The research said the width of the cavity did not matter and found with masonry that here was no benefit in increasing the width of the cavity beyond 25mm. 

The drying mechanism that the computer mathematical models used appeared to be water transport by permeation through the internal and external wall linings. The results showed that this method of removing water or drying out a wet wall is very slow. It did not stop the RH levels and moisture loadings in the wall remaining high and at levels that one would expect moulds to develop. The MEWS “drying curves” were in reality “wetting curves” as no system tested was able to dry out to the moisture content before the simulated leak began. It is very apparent that to dry out a wet wall, the leak must be stopped first.

North American construction techniques use OSB (oriented strand board) on the immediate outside of the cavity underneath the cladding and a vapour barrier on the immediate inside of the stud cavity underneath the gib board. They also use fiberglass insulation batts in the stud cavity, even with EIFS systems. The OSB is a very moisture absorbent material and the largest percentage of the moisture from any leak ended up in the OSB. Large percentages also went into the fiberglass insulation batts and the gib board.

MEWS projects RHT index evaluations initially started with the minimum RH of 80% and T of 5ºC as this was believed to be the lower limit for the onset of wood degradation. After receiving results of work done in Finland, they agreed to use a higher RH of 95% but acknowledge this needs checking for wood materials used and fungi prevalent in North America (page 1-23). Other than this, there is little reference to moulds and rot and this is surprising seeing as one of the main reasons for the projects was in response to the leaking and rotting buildings found. In New Zealand it is reported that some moulds can sustain growth at a RH of 65%, common rot causing moulds at a RH of 80%, and toxic moulds at a RH of 95% (ref 4). There shows there is a need to research the applicability of this to New Zealand radiata pine species, New Zealand’s climatic conditions, and the preservative treatments of wood, all coupled with the moulds and fungi prevalent in New Zealand.

As an overview, the MEWS projects have not dried out an actual wet wall nor investigated the drying mechanisms that would be involved. They have however given a great deal of understanding to what happens in the event of a leak and what happens to the water. From the results it can be concluded that if there is a leak into the stud cavity of a wall, then the microclimate conditions in the wall will reach and maintain a level such that the growth of the whole range of moulds will become established and the materials may rot. What is not clear is that once the leak is stopped can the wall dry out fast enough to avoid or stop the continuing growth of moulds.

7.  REFERENCES

1.       “Experimental Assessment of Water Penetration and Entry into Wood-Frame Wall Specimens”. Final Report from Task 6 of Mews Project at The Institute For Research In Construction, Canada. Lacasse, M.A. et all. Research Project 133. Published February 2003.

2.       Long-term Performance: Predict the Moisture Management Performance of Wall Systems as a Function of Climate, Material Properties, etc. Through Mathematical Modelling. Final Report from Task 7 of Mews Project at The Institute For Research In Construction, Canada. Phalguni Mukhopadhyaya et all. IRC-RR-132. Published February 2003.

3.      “MEWS Methodology for Developing Moisture Management Strategies – Application to Stucco-Clad Wood-Frame Walls in North America”. Report from Task 8 of Mews Project at The Institute For Research In Construction, Canada. Beaulieu, P. et all. IRC RR-112. Published November 2002.

4.      HITEX Research Bulletin 307.

5.      HITEX Summaries of MEWS Task 6,7 & 8. These are available on the HITEX web site.

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Page last updated Tuesday, 08 March 2005