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International Journal of ArchitecturalHeritage: Conservation, Analysis, andRestorationPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/uarc20
The Reduced Gradient Approach (RGA):An Alternate Method to OptimizingHumidity Conditions in House Museumsin Cold ClimatesRussell Richman a , Kim D. Pressnail b , Lori O'Malley c & NastassjaLiebenow da Ryerson University , Toronto, Ontario, Canadab University of Toronto , Toronto, Ontario, Canadac PCL Constructors Canada, Inc. , Mississauga, Ontario, Canadad Halsall Associates Limited , Toronto, Ontario, CanadaPublished online: 08 Dec 2010.
To cite this article: Russell Richman , Kim D. Pressnail , Lori O'Malley & Nastassja Liebenow (2010)The Reduced Gradient Approach (RGA): An Alternate Method to Optimizing Humidity Conditions inHouse Museums in Cold Climates, International Journal of Architectural Heritage: Conservation,Analysis, and Restoration, 5:1, 48-59, DOI: 10.1080/15583050903131363
To link to this article: http://dx.doi.org/10.1080/15583050903131363
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THE REDUCED GRADIENT APPROACH (RGA): ANALTERNATE METHOD TO OPTIMIZING HUMIDITYCONDITIONS IN HOUSE MUSEUMS IN COLD CLIMATES
Russell Richman,1Kim D. Pressnail,
2Lori O’Malley,
3and
Nastassja Liebenow4
1Ryerson University, Toronto, Ontario, Canada2University of Toronto, Toronto, Ontario, Canada3PCL Constructors Canada, Inc., Mississauga, Ontario, Canada4Halsall Associates Limited, Toronto, Ontario, Canada
The modern approach to artifact preservation comprises an aggressive interior environment
with high relative humidity levels. Maintaining high interior humidity coupled with typical
interior temperatures in house museums has proved to be detrimental to their building
envelopes. Such envelopes are typically un-insulated and often experience excessive, uncon-
trolled air leakage. Thus, the potential is high for interstitial condensation and the deleterious
effects that typically accompany it. For house museums, solutions to mitigate this problem
are limited because the degree of intrusiveness on original structure and associated costs are
primary concerns. This article presents research on a simple, low-cost, non-destructive
preservation control technique applicable to house museums and similar designated historical
structures. The reduced gradient approach involves manipulation of the interior temperature
regime to minimize the water vapor pressure differential across the building envelope
throughout the year while maintaining the desired indoor relative humidity. Such an
approach will minimize the amount of condensation due to air leakage. Two separate studies
present the design and application of the reduced gradient approach when applied to a house
museum in the cold climate of Toronto, Ontario, Canada. The results show that this approach
as a successful preservation strategy for house museums with an interior environment that
does not jeopardize artifacts nor the building structure.
KEY WORDS: house museums, preservation, building envelope, artifacts, relative
humidity, interior environment, condensation
House museums provide opportunity for immersion into a historic way of life byexperiencing first handthe structures and the artifacts within them (Cabral, 2001;Pavoni, 2001). However, the preservation of the building structure and the artifactswithin them has become a problem in cold climates (Said, et al. 1999). The buildingenvelopes in cold climates are generally challenged to separate a warm, moist, interiorclimate from a relatively cold, dry, exterior climate. In doing so, the main deleteriouseffect on building envelopes in cold climates is condensation, either due to bulk airleakage, vapordiffusion or a combination of the two (Hutcheon and Handegord, 1995).Much of the housing acrossCanada rely onwood framed constructionwithwood based
International Journal of Architectural Heritage, 5: 48–59, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1558-3058 print / 1558-3066 online
DOI: 10.1080/15583050903131363
Address correspondence to Russell Richman, Department of Architectural Science, Ryerson
University, 350 Victoria Street, Toronto, Ontario, Canada, M5B 2K3. E-mail: [email protected]
Received 20 February 2009; accepted 20 June 2009.
48
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sheathing/siding and pulp based interior finishes (Hutcheon and Handegord, 1995).Interstitial and surface condensation can be detrimental to the organic elements withinthe wall (Straube and Burnett. 2005).
Without the use of treatment processes or specially designed, environmentallycontrolled glass display cases, preservation of artifacts within a house museum and thestructure itself are highly dependant on the relative humidity (RH) of the interior air.Sustained fluctuations in RH will induce changes in a material’s moisture content, acatalyst for potential deterioration (Michalski, 2000; Mandrioli, Caneva, and Sabbioni,2004; Pavlogeorgatos, 2003; Lstiburek and Carmody, 1993; Camuffo et al., 2001). Theideal environmental conditions to preserve the integrity of a mixed collection of historicartifacts has been determined to be 21�C � 1�C and 50% RH � 5% year round (TexasHistorical Commission, 2008; Erhardt et al., 1997; Michalski, 1998; Cassidy, 1994;O’Malley, 1997; Craddock, 1992). Due to technological advancements in constructionpractices andmaterials science,modernmuseumsare able tomaintain these strict interiorenvironment conditions without compromising the integrity of the building envelope.However, studies have indicated that these conditions are difficult to attain even for newstructures since the high RH leaves little or no room for error in the construction of thebuilding envelope (Timusk, 1987).Historic homes, such as the ones comprisingCanada’sdwindling house museum stock, generally do not have these advanced technologies. Assuch, the application of the ideal interior environment in the current day puts thepreservation needs of the artifacts ahead of the structure (Michalski,1996; Brown andRose, 1996). By heating and humidifying cold winter air to levels that are conducive toartifact preservation, the potential for condensation on cold areas within the buildingenvelopedue to uncontrolled air leakage is significantly increased. This poses an ongoingconflict between the preservation needs of the artifacts and the structure.
Conversely, the original operating conditions of the house did not induce suchmoisture problems. The relatively ‘‘loose’’ construction allowed the interior environ-ment to easily mimic the outsideenvironment, removing much of the vapor pressuredifferential across the building assembly. Due to excessive air leakage, moisture carriedby the air was removed through exfiltration at a rate faster than it could accumulate.When the rate of condensation surpasses the rate of drying,deleteriousproblems such asmold growth or decayof wood can occur (Straubeand Burnett, 2005).
There are many solutions available to mitigate moisture and condensationproblems. Conventional solutions to reduce condensation potential include:(i) redu-cing indoor RH by providing more ventilation, (ii) air-tightening the building envel-ope, and (iii) depressurizing the interior during cold periods (Timusk, 1987). However,for house museums with minimum operating budgets, preservation of the artifactsmust be done simply and inexpensively. This rules out many elaborate mitigationtechniques involving major retrofits or highly sophisticated technology.
As both the historic structure and artifacts are valuable parts of our heritage, itmust not become a question of whether one should be sacrificed for the other. Instead,a reasonable compromise that balances the needs of both elements should be imple-mented. One such method developed at the University of Toronto is the reducedgradient approach (RGA). This method is neither costly nor intrusive—an idealalternative for house museums. By lowering the indoor temperature during the heat-ing season and raising it during the cooling season, the vapor pressure differentialacross the envelope is reduced. This significantly reduces the potential for condensa-tion within the wall assemblies (Hutcheon and Handegord, 1995). The RGA attempts
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to return the interior environment to one more representative of the environmentwhen the structure was occupied as a home. This article presents the development,implementation, and monitoring of the RGA for a house museum in Toronto,Ontario, Canada. Two separate studies were conducted to measure the effectivenessof applying the RGA in a working house museum.
1. THEORY: THE PSYCHROMETRICS BEHIND THE REDUCED GRADIENT
APPROACH (RGA)
The fundamental concept behind the RGA is to minimize the average vaporpressure gradient across the building envelope, thus minimizing water vapor diffusionin addition to the potential for condensation due to air leakage. This minimization isaccomplished bymimicking the exterior temperature trend throughout the year.Indoing this, the interior environment more reflects the original operating conditionsexperienced during historical times.
There are numerous combinations of temperature and RH resulting in a given-vapor pressure. By defining a target vapor pressure and RH, a set of boundarytemperatures can be calculated as outlined below using Equations 1 through 5. Thenomenclature used for the following equations can be referenced in Table 1.
The water vapor pressure of the air at any given temperature is given as(Hutcheonand Handegord, 1995):
Pwv ¼ RHð ÞPSat (1)
Where the saturation water vaporpressure can be defined by Antoine’s Equation(O’Malley, 1997):
Psat ¼ 133 � exp 18:67� 4030:2
235þ T
� �� �(2)
The RGA aims to reduce the vapor pressure gradient across the building envelopesuch that:
PWVout¼ PWVin
(3)
Substituting Equation 3 for interior conditions yields:
PWVout¼ RHinð ÞPSatin (4)
Substitution of Equation (4) followed by isolating T yields:
Tin ¼4030
1867� logPWVout�logRHin
log 1:33
� �24
35� 235 (5)
As expected, the interior temperature is the sole variable that can be manipulated withease in order to minimize the vapor pressure difference across the building envelopeand to lessen the potential for condensation.
Figure 1 shows the largepotential for condensationandmoldgrowthassociatedwithhigh indoor environmental loads typical of museums for artifact preservation in during a
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typicalTorontowinter. Sustained conditions of this nature have resulted in deteriorationofhistoric structures. Brown and Rose (1996) suggest that most artifacts can be adequatelypreserved in environments of varying temperatures and RH levels provided that fluctua-tions are neither extreme nor rapid. Although the optimum value of RH remains at 50%,the risk to artifacts in varying the RH by � 10% is low 15 (O’Malley, 1997); therefore, arange between 40% and 60% RH can be considered acceptable when determining theboundary temperatures associated with the RGA.With this in mind, Figure 2 presents thereduced impact associated with lowered interior temperatures resulting from adopting theRGA. The indoor environment is altered such that it still preserves the artifacts butminimizes the potential for condensation in the building envelope.
Ideally, no point in the exterior wall assembly should drop below the dew pointtemperature of interior air exfiltrating outside, however, this point cannot be guaran-teed. Instead, what is possible is ensuring that no point in the wall is sustained belowthe dew point temperature of the interior air. For decay of wood components to occur,it is not wetting and drying that causes damage, but sustained wetting accompanied byinsufficient drying (Hutcheon and Handegord. 1995; Straube and Burnett, 2005).Therefore, if sustained periods of condensation within the wall assembly can beavoided, preservation of the wall will be achieved.
If outdoor environmental conditions year round were constant, maintaining theindoor conditions would be simple, however in cold climates this is not the case. Inregions such as Canada, temperature conditions can change significantly from day today. Daily fluctuations in temperature cannot be avoided; however as a starting point,sustained periods of condensation can be minimized by attempting to bring the netmovement of moisture through the assembly to zero,which can be achieved by
10%
Dry Bulb Temperature
85% Web Bulb Temperature
Hum
idity R
atio (
Pounds o
f m
ois
ture
per
pound o
f dry
air)
Relative Humidity20%
30%
40%
50%
60%
70%
80%
90%
90
75
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65
60
55
50
3025
Dew
Poi
nt Tem
pera
ture
Potential for Fungal Growth
Condensation Occurs
Interior Conditions21°C, 50% RH
Exterior Conditions–10°C, 90% RH
Va
por D
iffusio
n
Figure 1. Psychrometric chart illustrating the potential for condensation within a building envelope during
the winter. (Figure is provided in color online.)
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matching the indoor vapor pressures to the average outdoor conditions. It is assumedthat by doing so, the overall vapor pressure gradient will be minimized and anycondensation that does occur will not be sustained and can dry to the exterior.
2. DEVELOPMENT: THE REDUCED GRADIENT APPROACH (RGA) FOR
TORONTO, ONTARIO
Tables of climatic normal values (Environment Canada, 2008) provide daily andmonthly average outdoorweather data used as a basis fromwhich the target indoor vaporpressure canbedetermined.Using the daily average temperatures, an average temperatureprofile for each month was developed in order to create the target vapor pressure profile.
Using the city of Toronto as an example, Figure 3 shows the resulting range inindoor air temperature required each month to maintain RH conditions that support theneeds of a housemuseum.Aproposed temperature profilewith a balance between artifactand envelope preservation and thermal comfort is also included in the Figure. Figure 3depicts the accepted range between 40% and 60 % RH for artifact preservation.Theproposed profile optimizes artifact preservation with occupant comfort. In addition, thetransition period between 40% and 60%RH is gradual over several months, minimizingany risks associated with acute fluctuations and artifact deterioration.
3. APPLICATION: A CASE STUDY IN TORONTO, ONTARIO
The RGA was applied to a house museum in southern Ontario. The ParshallTerry House, located at Todmorden Mills within the Don River Valley of Toronto,
Dry Bulb Temperature
Hum
idity R
atio (
Pounds o
f m
ois
ture
per
pound o
f dry
air)
Dew
Poi
nt Tem
pera
ture
10%
85% Web Bulb Temperature
Relative Humidity20%
30%
40%
50%
60%
70%
80%
90%
90
75
80
70
65
60
55
50
3025
Potential forFungalGrowth
CondensationOccurs
Interior Conditions10°C, 50% RH
Exterior Conditions–10°C, 90% RH
Va
por D
iffusio
n
Figure 2. Psychrometric chart illustrating the reducedpotential for condensationafter applicationof the reduced
gradient approach (RGA) within a building envelope during the winter. (Figure is provided in color online.)
52 R. RICHMAN ET AL.
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was established as a house museum by the Ontario Heritage Act in 1975. The house isa single storey, traditional balloon-framed, Georgian/Regency cottage style woodstructure, originally constructed in the 1830s. Retrofits were conducted in 1967comprising the addition of thermal insulation in the attic and electrical resistanceheating elements on the floor of each room. This retrofit program was intended torestore the structure; however, a completely undesired outcome arose.
An investigation (Cassidy, 1994) found that preservation of the house and itsartifacts were threatened by high indoor RH, mold, excessive air leakage, groundwaterpenetration, and condensation. The sustained exposure to high relative humiditiesplaced it in an environment that threatened the artifacts and the structure itself.Although the numerous layers of oil based paint on the interior acted as a vaporretarder, the interior warm, moist air was leaking out of the building, condensing onnumerous interstitial surfaces within the exterior wall assemblies. Remedial repairs wererecommended immediately and carried out within 2 years of the original investigation.Following the repairs, the RGA was implemented to minimize the potential for surfaceand interstitial condensation within the envelope and preserve the structure.Boundarytemperatures utilized were similar to those presented in Figure 3.
Two follow-up monitoring studies were performed in an effort to evaluate thesustained application and impact of the RGA at this house museum. Study A wasperformed during the first year of implementing the RGA. Study B was performed 2years after utilizing the samemethodology and equipment. Both studies utilized type Tthermocouples to measure air and surface temperatures. Relative humidity sensorswere utilized in multiple interior and exterior locations. Dataloggers were used tocapture hourly readings. Readings were verified by weekly measurements of tempera-ture and RH utilizing hygrometers and sling-psychrometers.
The temperature and relative humidity conditions measured during Study A(O’Malley, 1997) are presented in Figure 4. Throughout this year, the Terry houseexperienced two occasions during the winter months when the indoor RH droppedbelow 40%. This decrease was associated with abnormally cold winter exterior tem-peratures as shown in Figure 5. This cold snap effectively rendered the upper bound-ary temperature too high. Due to the limitations of the heating system during this
0
5
10
15
20
25
30
35
Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Jan.
Tem
per
atu
re (
°C)
40 % R.H.
Proposed
60 % R.H.
Figure 3. Proposed temperatureprofile for ahousemuseuminacold climate. (Figure is provided in coloronline.)
REDUCED GRADIENT APPROACH 53
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time, the lowest obtainable temperature was 10�C, preventing the design temperatureprofile from being adhered to until March. Following this period, Figure 4 shows themeasured temperature within the design boundary profile.
During Study B (Richman 1999), similar deviations from the design temperatureprofile were observed as shown in Figures 6 and 7, although for different reasons thanStudyA. Subsequent to Study A, new thermostats were installed allowing the house tosustain temperatures as low as 6�C; however, during the coldest months the interior
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40% RH
60% RH
60% RH
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EL
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Figure 4. Comparison of the measured and target interior temperature and relative humidity for Study A.
Data from O’Malley (1997). (Figure is provided in color online.)
–10.0
–5.0
0.0
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20.0
Sept. Oct. Nov. Dec. Jan. Feb. March
Tem
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1995/96
Normal's Tables
Figure 5. Comparison of average monthly exterior temperatures. (Figure is provided in color online.)
54 R. RICHMAN ET AL.
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temperature remained consistently at approximately 10�C. This error was attributedto employee modifying the thermostat for comfort reasons and an auxiliary heater inan attached ‘‘warm room’’ losing heat to the historic structure.
Regardless of the reasoning, this temperature increase shifted interior RH out-side the target zone. With exception of some warm periods in November, December,and February, the weather experienced during this study followed the average
0.0
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2-Sep 22-Sep 12-Oct 1-Nov 21-Nov 11-Dec 31-Dec 20-Jan 9-Feb 1-Mar 21-Mar
Tem
per
atu
re (
°C)
Actual
Target
40 % R.H.
60 % R.H.
Figure 6. Comparison of the actual interior temperature with that of the target interior temperature for
Study B. (Figure is provided in color online.)
35
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2-Sep 22-Sep 12-Oct 1-Nov 21-Nov 11-Dec 31-Dec 20-Jan 9-Feb 1-Mar 21-Mar
Rela
tive H
um
idity (
%)
Actual
Target
40%
60%
Figure 7. Comparison of the actual interior relative humidity to that of the target profile for Study B.
(Figure is provided in color online.)
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temperature profile provided by the normalstablevalues relatively well. These warmerthan average periods account for the RH spikes above 60% shown in Figure 7.
Despite the deviation from the design temperature profile, the vapor pressuredifferential experienced at the Terry house during both studies was reduced signifi-cantly. Figure 8 shows targeted vapor pressure levels representative of both studies tobe acceptable. Figure 9 shows the net vapor pressure differentials across the buildingenvelope for a representative sample of both studies. Isolated events occurred whenthe vapor pressure differential was monitored to be greater than the target values.However, these events were considered acceptable due to the home’s past perfor-mance. It was assumed that any accumulated moisture would be removed byairleakage exfiltrating through the sufficiently permeable envelope. The reported averagevapor pressure gradients were 140 Pa in Study A and 160 Pa in Study B.
4. RECOMMENDATIONS
Although effective, it is evident that improvements can be made to the RGA.The importance of accurate, site specific, temperature and relative humidity data has
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Study B
Target
0
VA
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UR
PR
ES
SU
RE
(Pas
cals
)
500
1000
1500
2000
2500
Figure 8. Comparison of the actual interior vapor pressure and the desired interior vapor pressure based on
the reduced gradient approach (RGA) for both studies. (Figure is provided in color online.)
02-Sep
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Net
Vap
ou
r P
ress
ure
Dif
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(P
a)
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22-Sep 12-Oct 01-Nov 21-Nov 11-Dec 31-Dec 20-Jan
Study A
Study B
09-Feb 01-Mar 21-Mar
Figure 9. Sample of net vapor pressure differentials for both studies. (Figure is provided in color online.)
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been discussed. As the method depends greatly on knowing these conditions, ratherthan depending on past averages, temperature profiles should be regularly updatedand altered throughout the life of the structure to adequately follow the actual out-door conditions from year to year. This approach might require the development andinstallation of a dynamic system that is capable of capturing instantaneous climatedata and adjusting the interior environment to it accordingly.
Until such a system is incorporated into a house museum, specific regulationsmust be adhered to so that success of RGA as a moisture controlling method can beassured. This includes ensuring no alternate heat sources are present and that thermo-stats are not adjusted away from the pre-determined target temperatures. Recognizingthis need, organizations such as the Ontario Museum Association are hosting work-shops to educate museum staff and volunteers about the importance of meeting thepreservation needs of the structure and the artifacts rather than compromising one forthe other (Ontario Museum Association, 2007). If such conditions prove to be tooextreme for visiting patrons during the colder months, an alternative could includeclosing the house museum to the public in during the winter or sparingly make use ofradiant heaters that do not alter the temperature of ambient air. Regardless of whichapproach is taken, it must be made clear that the adherence to the specified tempera-ture boundaries is paramount.
Based on what has been observed by the studies conducted on the Parshall Terryhouse the RGA has been a success. As with any new approach, the long-term effects ofits application on a structure need to be further observed in detail. The significant gapin literature pertaining to the use of this mitigation strategy on house museumscanonly be filled by continuing research and monitoring.
5. CONCLUSIONS
By manipulating the temperature of the interior environment, the RGA re-exposes a house museum to conditions that better represent those conditions presentduring the original operation, thereby supporting the long-term preservation of thestructure. In combination with these temperature manipulations, restricting relativehumidity to the range of 40% to 60% and controlling its rate of change will ensure theintegrity of the artifacts is kept sound. By implementing these measures the vaporpressure differential and condensation potential due to air leakage through the build-ing envelope is reduced to manageable levels.
Results of two studies completed on the Parshall Terry House Museum showsthe RGA as an effective moisture management strategy to maximize preservation ofboth the structure and artifacts within it. The RGA is an economically viable,effective, long-term solution in managing the moisture problems of house museumsin cold climates. Although in each study indoor temperature and moisture controlwere not ‘‘ideal’’, indication of moisture damage is no longer observed and overallbuilding performance is improving—and the potential for further improvement exists.More stringent adherence to temperature restrictions, as well as the acquisition ofmore site-specific climate data to develop more accurate temperature profiles, willrender significantly better results through the RGA. Although expectations are good,the true success of the long-term performance however can only be estimated untilfurther, and much needed, research and monitoring is conducted.
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By restoring building environment to conditions that better represent the con-ditions of the past, these historic buildings will endure and the immersive value ofportraying a part of our history can be preserved for all people, present now, and in thefuture.
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concerns: A guide for collectors and curators.Washington: Smithsonian Institution Press, 15–22.Environment Canada. 2008. Canadian climate normals or averages 1971–2000. http://www. climate.weatheroffice.ec.gc.ca/climate_normals/index_e.html (accessed July 16, 2008).
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Canada: National Research Council of Canada.Lstiburek, J., and J. Carmody. 1993. Moisture control handbook. New York: Van NostrandReinhold.
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Preservation Technology 27(3):25–29.Michalski, S. 1998. Climate control priorities and solutions for collections in historic buildings.
Historic Preservation Forum 12(4):8–14.Michalski, S. 2000. Guidelines for humidity and temperature in Canadian archives. InCanadianCouncil of Archives Preservation Committee Information Bulletin 15. Ottawa, Canada:
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Table 1. Nomenclature
W Total mass of water vapor transmitted [ng] RH Relative humidity [%]
M Permeance [ng/s�m2�Pa] Pwv Water vapor pressure at temperature, T [Pa]
A Cross-sectional area of flow path [m2] PSat Saturation water vapor pressure [Pa]
� Time [s] T Temperature [K]
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