PEM2 datalogger and HOBO Pro Series logger

A Review of Recent Ideas and Annotated Bibliography

Ellen Carrlee, Alaska State Museum Conservator, October 2010


For a long time, the climate ideal for museums was 50%RH and 70F with very little fluctuation.  This has been problematic because it costs money and fossil fuels to achieve that target, and many buildings simply cannot achieve it.  In addition, there have not been enough scientific studies on real materials to prove that this is the correct target.  The new way forward in thinking about museum climate involves an understanding that deterioration happens by different mechanisms (biological, chemical, mechanical) and that you have to balance the risks and benefits for each on.  Very low RH slows down biological and chemical risks, but increases risk of mechanical damage, for example.  The beginnings of a shift in our conception of appropriate collections preservation climate began in the mid 1990’s, but it has taken longer to reach the dawning of a new era.  A review of the recent literature suggests that a summary of the subject can be split into three areas: causes of damage, methods of analysis, and recommendations by material.

In thinking of the needs of a future integrated facility for the Alaska State Library, Archive, and Museum, I am struck by how the library/archive needs are really quite different from the museum needs.  To oversimplify, I think it is because the threats to paper are mainly biological and chemical, and therefore they are less concerned with RH and more concerned with temperature.  In general, they need RH to be lower than we like it at the museum, and fluctuations are not as threatening.  Our museum, with lots of paintings and organic ethnographic materials, is keen to have a bit more RH.  There are competing needs that have to be weighed.  Light fading risk goes down at low RH, but fracture risk goes up.  Higher temps reduce RH and therefore help prevent mold but low temps reduce chemical self – destruction.  Just as we’ve looked at light fading risk and recommended light levels for different categories of materials, we’ve started to consider temp and RH needs based on materials.  But that’s another posting (and a lot more reading for me!)


  1. Biological
  2. Chemical
  3. Mechanical/ Physical

Some people will say that RH is more important than temperature, and others will say just the opposite.  Seems like the answer depends on what kind of damage you want to prevent.  If it is biological, you need to keep your RH low.  There is an easy-to-remember rule of thumb that mold tends to flourish if you have stagnant air in a dark place over 70% RH and 70F for at least two days.  Also, insects and pests need moisture to live.  So for the biological risk, looks like temperature has the edge.  Chemical usually refers to rate of deterioration, like things becoming more acidic or corrosion happening.  It is hard to say if temp or RH is more important here.  Some sources will say that for approximately every 5C DROP in temperature, you double the life of your artifact.  Anyone remember the Arrhenius Equation from chemistry class?  That one says for every 10degrees C the rate of a chemical reaction doubles. Think about how fast sugar dissolves in hot tea and how slowly it dissolves in iced tea.  So temp is definitely important for those chemical reactions that cause degradation, but a lot of those equations involve water.  So RH is important, too…metal corrosion, dye fading, lots of things.  For chemical changes, short fluctuations in temp/RH don’t matter much, it is the situation over the long haul that is most important.  In general, room temperature is too warm for ideal long-term preservation of most organic materials as well as paper, leather, dyes, and film.  Mechanical damage seems to really connect strongly to RH and damage from movement caused by shrinking and swelling as moisture content goes up and down.  Now some of that risk is influenced by how brittle something is, and things get more brittle as they get colder, so temperature has a role, too, but not quite as much as RH.  Some sources separate out “physical” as another category to show how something might get torn apart because there are various materials all restrained in association with each other (composite or mixed-media), like drums or paintings, but this seems to fit OK in the “Mechanical” category.  James Reilly has a good description: To avoid mechanical damage, stay in the 20-70% RH range and keep outside excursions as short and infrequent as possible.  Seasonal extremes are worse than short-term events.  Short term excursions don’t matter as much because materials take quite a while to equilibrate, weeks and not hours or days.  The Smithsonian folks are more conservative with RH and in general suggest 30-60% as being OK for most collections.


How do they know what is really causing damage?  How do we know that damage didn’t just happen before it even came into the collection, or as a result of inherent vice in the material itself?  Apparently, those original targets of 50% and 70 degrees F were based on trying to replicate conditions of a slate quarry in the UK where valuable collections of the British Museum were kept during WWII, and the allowable fluctuations were based on the limitations of the HVAC equipment at the time.  When I was in school (late 1990’s) I was taught that what we really wanted was to keep that line on the hygrothermograph as flat as possible, and that big, rapid fluctuations were really bad for the artifact.

Proofed RH fluctuations Michalski the idea in a nutshell is that you look at the highest and lowest extremes of RH the object has been exposed to in the past, and then you avoid ever getting to that boundary again.  Whatever damage happened at that limit has happened, and as long as you stay away from that extreme, you’ll probably be OK.  Let’s say there was a crack in a wooden item that happened at low RH.  If you keep away from that extreme, the crack will probably just open and close with RH fluctations without getting any worse.  If you get closer to the extreme, the number of times that crack can open and close without consequence is limited.  Remember, this is just for mechanical damage.  That’s not the case for biological or chemical damage at all.  Also, conservation treatments may erase that proofed RH because they introduce new materials  and stress distribution in the object.  I have especially seen this in the treatment of large organic materials like kayaks…they show no major change for years before conservation treatment, and then in the year or two following treatment you see changes.  If the treatment was done right, those will result in the repairs opening up or flexing and not in new damage to the artifact.  I’ve come to think of these artifacts as experiencing a “period of convalescence” after treatment.

Isoperms Don Sebera’s work in the early 1990’s, looking into relationships between temp, RH and preservation as a matter of activation energies for decay reactions.  Quantitative graphical measures of relative permanence, such as if paper will last 100 years at the accepted standard, it will last only 3 years at 95F and 80%RH, but 1200 years at 50F and 40%RH.

IPI decay metrics  the Image Permanence Institute (a non-profit research lab at the Rochester Institute of technology) has done amazing work (grant funded by the NEH, the Mellon Foundation and IMLS.) to quantify things.  Great quote from James Reilly: “One cannot manage what cannot be measured”  An important aspect of this new way forward is to realize it is based on algorithms, modeling, predictions and other scientific ways of guessing how materials are supposed to behave.  Since deterioration mechanisms of each material are complex and dynamic, it is difficult to get real info about damage to real artifacts.  The whole IPI scheme is a great hypothesis, and maybe the best one we’ve got, but there is precious little in hard data and proof.  For each of the three forms of RH/ temp deterioration, they’ve picked something to be the canary in the coal mine and then projected possible damage to vulnerable organic materials using math.  Biological decay is predicted by mold growth, chemical decay is predicted by a model of deterioration for organic materials based on the hydrolysis of cellulose acetate (common plastic used for photographic film), and the physical decay is predicted by the equilibrium moisture content for an imagined block of wood from an “average species.”  Tim Padfield has a good article describing the science behind the preservation index and the TWPI http://www.conservationphysics.org/twpi/twpi_01.php

The Marvelous Marion Mecklenberg  OK, I have to be uber-caffeinated to read anything he writes, but the man has done great work, co-authored with many really stellar people and in general, when I think of cold hard data I think of Dr Mecklenberg and his colleagues,  in particular Tumosa, Erhardt, and McCormick-Goodhart.  They did research starting in the early 1980’s to measure dimensional changes to different materials with mechanical testing to predict stresses and strains, as well as modeling to show how much climate change was required to cause irreversible changes.  In 1994, they announced new guidelines for museum climate, suggesting that 50 +15% moderate fluctuations were OK, and that for general collections, 30-60% RH was mechanically safe.  Temperature in the human comfort range was fine, but avoid going below 13C (55F) due to some materials undergoing phase change and getting brittle, like acrylics.  Of course, some materials need more stable environments, like degraded materials, drums, veneers etc.  The current Smithsonian guidelines as of publication were 45 +/-8% RH and 70 +/-4F. 


My understanding of the current trend is based on the articles below.  The links were good as of October 2010, but if they fail for you, just try googling them.

____________“Rethinking the Museum Climate”  (2010) Meeting, April 12-13, 2010 MFA Boston http://blog.conservation-us.org/blogpost.cfm?threadid=2227&catid=175

____________ “The Plus/Minus Dilemma: A Way Forward in Environmental Guideleines”  (2010)  3rd IIC Roundtable, 13 May 2010, Milwaukee Wisconsin at the AIC annual meeting  http://www.iiconservation.org/dialogues/Plus_Minus_trans.pdf

In addition to the balancing ideal preservation environment with the comfort needs of visitors and employees in the same spaces, there is growing influence of economic and environmental concerns (carbon footprint) since trying to maintain a rigid target is resource-intensive.  Loan requirements are more strict than many institutions can achieve, and institutions often ask that the borrower keep more rigid standards than the lending institution can manage.  We need to be more honest about what are actually able to pull off.  Michalski thinks a 10% fluctuation is OK  He thinks the biggest issue is mixed media or composite artifacts.  National Gallery of Canada is now going with 44% RH +/- 3% in winter and 50% +/- 3% in summer, with a year-round goal of 71F +/- 2%

American Society of Heating, Refrigeration and Air-Conditioning Engineers. (2007) Chapter 21: Museums, Galleries, Archives and Libraries. ASHRAE Handbook, Atlanta, GA

The 1999 edition included a section on Museums Libraries and Archives for the first time.  Revised edition in 2003.  Michalski was lead author for the T and RH section.  Next one due out in 2011 will probably include more about “proofed fluctuations.”  The introduction here describes threats in decreasing order of seriousness: light, RH, temperature, air pollution, pest infestation, shock/vibration, natural disaster, theft/vandalism and misplacing objects.  IPI preservation index is specifically mentioned.  Lots of detail and references cited in the sections regarding biological, mechanical and chemical damage.  Conservative RH for no mold to occur at any temp on collections is under 60%.  Mechanical damage section seems to point toward +/- 10% as being a maximum acceptable RH fluctuation, with damage being reported when the fluctuation was closer to 20%, especially on cracked cabinetry and paintings.  Chemical damage section focuses on acid hydrolysis of paper, photos and magnetic media and recommends cold and dry but without a specific range given.  Response times of artifacts suggest that anything under an hour is probably not an issue, such as a 15 minute HVAC cycle, as long as dampness is not promoted.  In the design parameters section, Class A is considered optimum for most museums, with AA being high in energy costs without a lot of benefit.  10% seasonal swing in RH and 5% short-term fluctuation is OK.  Temperature set between 59 and 77F. Constant air volume is recommended.  Variable air volume tends to have poor humidity control, inadequate airflow, maintenance disruption, leaks, and inflexibility and is not recommended for collections housing.  The next version of this handbook is expected to come out in 2011.  Most engineers and architects are familiar with temp and RH issues only though this handbook, and conservators are wise to be familiar with its contents.

Anderson, Maxwell. (2010) Revising the Gold Standard  http://www.theartnewspaper.com/articles/Revising-the-gold-standard-of-environmental-control%20/20549

Brief article for general audiences, written by the director and chief executive of the Indianapolis Museum of Art.  There are three considerations of RH: set point, allowable fluctuation, and seasonal adjustment.  Some discussion of where the old target standards came from.  IMA 50% +/-8% (6% per 24 hours) and 70F +/- 4 (2 per 24 hours) 

ANSI/NISO Z39.79-2001 Environmental Conditions for Exhibition of Library and Archive Materials.  American National Standard developed by the National Information Standards Organization. http://www.kb.dk/export/sites/kb_dk/da/kb/nb/bev/Z39-79-2001_Udstillingsstandard.pdf

Set point for temperature should be below 72F with a variation of up to 5degrees on either side of your set point.  RH set point should be between 30 and 50% with a variation of up to 5% on either side of the set point.  Seasonal drift is not to exceed 5% per month. 

Bizot Group draft Guiding Principles (Int’l Group of Organizers of Large-Scale Exhibitions)

This organization is made up mostly of directors of large museums, and these are the people who will ultimately decide what temp and RH limits are in loan contracts between museums, and therefore the key to the big game of chicken: who will change their standards first?  Will other institutions still lend to that daring pioneer?  The IMA has already taken a step toward less restrictive temp and RH limits.

Chicora Foundation (1994) “Managing the Museum Environment.” http://cool.conservation-us.org/byorg/chicora/chicenv.html

For each 14 F rise in temp, double the rate of deterioration for paper.  Humidity is more important than temp and should be controlled first.  In contrast to some other sources (Reilly 2008) this article advocates making the humidity the controlled factor and allow the temperature to fluctuate.  Aim for 40%-55% with a daily fluctuation of +/- 3% and 65-75F with daily fluctuation of +/- 5%.  These are not necessarily the ranges we are still talking about in 2010, but specifying how much fluctuation is allowed per day is helpful.  A timeframe needs to be included when you are talking about fluctuation.  Discussion of isoperms.  Really interesting section on the major HVAC components.  A Constant Air Volume System properly filters air and keeps flow to prevent stagnant pockets of air, as opposed to a Variable Air Volume system, which delivers heated or cooled air to each zone and is cheaper but creates humidity instability.  Hmmm, how much humidity instability I wonder?  Electronic controls are more responsive than pneumatic, and the controls ought to be in the occupied spaces and not within the ductwork itself.  I need to compare this to what ASHRAE 2007 says….

Conrad, Ernest A. (2007) “Climate Control Systems Design and Climate Change.”  Contribution to the Experts Roundtable on Sustainable Climate Management Strategies, held in April 2007, in Tenerife Spain.  http://www.getty.edu/conservation/science/climate/paper_conrad.pdf

Author has a master’s in environmental engineering and has been designing systems for major museums and libraries for 25+ years.  Apparently, mechanical engineers look at statistical weather data, mostly published by ASHRAE in 30-year averages since 1950.  Humidistatic heating often used in historic buildings where they don’t want to add in moisture…instead, they just drop the temperature to put RH in the appropriate realm.  Mentions the New Orleans Charter for Joint Preservation of Historic Structures and Artifacts (1990-91) which was apparently written to establish a philosophy regarding balance between protecting collections, historic building structures and comfort of occupants.  Thrust of the article was to discuss impact of a 1 degree C global increase in temperature.  Mostly it seems that the risk of high RH could increase from an expansion of the tropical zone, and more dehumidification would be needed, thus causing more pollution. Greater risks are associated with increased incidence of extreme weather.  While the author seems very expert in his own field, the topic of the article seems to be more about speculative effects of climate change, which is not his realm of professional expertise, and he doesn’t cite references.  

Conrad, E. A. (1996) “Environmental Monitoring as a Diagnostic Tool.” In Preservation of Collections: Assessment, Evaluation, and Mitigation Strategies. Papers presented at the workshop, Norfolk, Virginia, June 10-11, 1996. Washington, D.C.: the American Institute for Conservation of Historic and Artistic Works. pp. 15-20.

Conrad, Ernest A. (1995) Balancing Environmental Needs of the Building, the Collection, and the User. East Norwalk, CT: Landmark Facilities Group.

Erhardt, D., C. S. Tumosa and M. F. Mecklenburg, (2007) “Applying Science to the Question of Museum Climate.” In Museum Microclimates, Contributions to the Copenhagen Conference, 19 – 23 November 2007, ed. T. Padfield and K. Borchersen. National Museum of Denmark, 2007: 11-18.   http://www.natmus.dk/graphics/konferencer_mm/microclimates/pdf/erhardt.pdf

If you need to cut to the chase, read this article and then the ASHRAE chapter.  Lays out the history of the traditional standards, which had little scientific evidence and were best guesses and outside factors like limits of HVAC systems, buildings, or local climates.  The Boston MFA installed heating, air washing and humidification in 1908 and determined that 55-60% was the best RH for paintings.  Air conditioning technology improved, and by 1941 it was seen in the National Archives and Library of Congress, among other institutions. The WWII slate quarries in Wales so often mentioned used heating to achieve an RH of 58%, the average RH in the National Gallery of London from measuring the equilibrium moisture content of blocks of wood in the gallery. Some places like Canada used lower RH values (around 25%) because higher values were hard to maintain in winter. The idea that objects were “use to it” was used as justification without scientific proof.  There is also a prevalent notion that more constant is better, although the +/- 2% fluctuation sometimes dictated is hard to even measure accurately with the methods we commonly use.  “Specifying climate control requirements and telling the engineers to implement them is easier by orders of magnitude than the research required to justify the specifications.”  Mecklenberg and his colleagues, in particular Tumosa, Erhardt, and McCormick-Goodhart) did research starting in the early 1980’s to measure dimensional changes to different materials with mechanical testing to predict stresses and strains, as well as modeling to show how much climate change was required to cause irreversible changes.  In 1994, they announced new guidelines for museum climate, suggesting that 50 +15% moderate fluctuations were OK, and that for general collections, 30-60% RH was mechanically safe.  Temperature  in the human comfort range was fine, but avoid going below 13C (55F) due to some materials undergoing phase change and getting brittle, like acrylics.  Of course, some materials need more stable environments, like degraded materials, drums, veneers etc.  The current Smithsonian guidelines as of publication were 45 +/-8% RH and 70 +/-4F. 

Grattan, D. and S. Michalski, (2010) “Environmental Guidelines for Museums – Temperature and Relative Humidity (RH).” Canadian Conservation Institute. October 2010. www.cci-icc.gc.ca/crc/articles/enviro/index-eng.aspx

Current trend away from a target and toward embracing a range.  Linking RH fluctuation to measurable damage.  Cooling makes RH go up, warming makes RH go down.  Damage is three kinds: biological, chemical, and mechanical.  Fluctuation is the main threat, and your “class of control” defines the allowable degree of fluctuation.  Libraries and archives ought to keep things cool as long as it is not too damp.  Around 30% RH or less.

Hughes, Susan.  UK National Archives, Conservation DistList posting July 15, 2010

The British Standard Institution (BSI) is working on new standards packaged as Publicly Available Specification (PAS 198) for environmental conditions in museums libraries and archives, which will influence storage, display, and loans in the UK.  Expected out in May 2011.

Image Permanence Institute (2005) Step-by-Step Workbook: Achieving a Preservation Environment for Collections.  Image Permanence Institute, Rochester Institute of Technology.  Rochester, NY August 2005

As opposed to the “target” set point recommended for many years, the new way forward in temp/RH involves an understanding that deterioration happens by different mechanisms (biological, chemical, mechanical) and that you have to balance the risks and benefits for each one…very low RH slows down biological and chemical risks, but increases risk of mechanical damage, for example.

They have a holistic approach to the preservation environment, which includes a Preservation Management Process: 1. Understand 2. Evaluate 3. Take Action.  As part of understanding, it includes recommendations for how to understand your own HVAC system.  The ”schedules” section of mechanical plans are the part you want.

Great explanation of the importance of dew point.  The temp at which water will condense out of the air, so it is a measure of the absolute amount of water in the air.  If you have that amount of water outside when it is cold, the RH will be pretty high, but then bring it in and warm up the air and the RH goes down.  An example I looked up for rainy Juneau Alaska: if it is 40F and 90%RH outside, that’s a dewpoint (absolute amount of water) of 37.  Take that inside and heat it up to 70F, you get an RH of only 30%.  Also of interest: materials change temperature very quickly, in less than a day.  But it could take weeks or months to equilibrate to changes in RH (faster at higher temps, though.)

An important aspect of this new way forward is to realize it is based on algorithms, modeling, predictions and other scientific ways of guessing how materials are supposed to behave.  Since deterioration mechanisms of each material are complex and dynamic, it is difficult to get real info about damage to real artifacts.  This implies that the whole IPI scheme is a great hypothesis, and maybe the best one we’ve got, but there is precious little in hard data and proof.  For each of the three forms of RH/ temp deterioration, they’ve picked something to be the canary in the coal mine and then projected possible damage using math.

Biological: Measured by the Mold Risk Factor. Number 1-5, with anything above 1 indicating mold growth. Mold growth as a problem above 65%RH.  Spores won’t germinate below that, or if the temp is below 36F.  Germination happens in two days under favorable conditions (86F is ideal) but three years or more in marginal conditions.  Estimates came from published mold studies on stored food grains, and then mathematical modeling by Doug Nishimura of IPI and models from Stefan Michalski of CCI.  Higher temps also mean insects eat more and breed faster.

Chemical: Measured by the TWPI.  Higher numbers are better.  Temperature and humidity are the “speed controls” on natural aging, and they also combine with other agents of deterioration to accelerate decay.  “Deterioration is cumulative and progressive, and it doesn’t reverse itself when conditions improve.”  Example: milk doesn’t un-spoil when you put it back in the fridge after leaving it out for a few days.

Physical:  Measured by “dimensional change metrics” (shows up as EMC on software) with 2.5% or higher being bad.  They are estimating this aspect of deterioration from the behavior of an imagined block of wood from an “average species.”  Unclear what that means, really. 


PEM Preservation Environment Monitor.  Name of the IPI’s dataloggers.  Cost about $350 each in 2010.

Climate Notebook software sold that reads and interprets data from various loggers.

Preservation Index  (PI) Suggests, in years, how long it will take for a vulnerable organic material to become noticeably deteriorated under the conditions given.  Higher number (over 50) is good, with very good environments up around 200.

Time-Weighted Preservation Index (TWPI) A quantitative model of organic decay, tries to give an idea what is going on over a longer period with fluctuating conditions.  Higher number (over 50) is good, with very good environments up around 200.

Preservation Calculator: You enter temp and RH, it tells you the rate of natural aging from chemical change, and risk of mold for organic materials in general.  It gives you a PI number, too.

Dew Point Calculator: You can calculate temp, RH or dew point if you know two of the three.  Then it tells you the PI, mold risk and mechanical damage risk.  Using this helps know if your mechanical systems are humidifying or dehumidifying.  Also, IPI says keeping summertime dew points low is one of the most important factor of all in preservation.  If dew point is high outside, just cooling the air isn’t enough.  If you just cool it without taking water out, your indoor RH is way too high.

Equilibrium Moisture Content: Amount of moisture in a material at certain RH.  Above 2.5% in the Climate notebook interpretation indicates dangerous dimensional change.

Mecklenberg, M.F.  (2007) “Determining the Acceptable Ranges of Relative Humidity and Temperature in Museums and Galleries.”  Smithsonian Museum Conservation Institute 2007. 

2004 Smithsonian standards were around 45% RH +/- 8% and 70F +/- 4degrees.  Part one of the article is 57 pages on structural response to RH and part two is 29 pages on structural response to temperature.  In a nutshell, it seems that most materials are OK in the RH range of 30-60%, with 30-37% and 53-60% being cautionary regions.  For temperature, above 74F gets into increased chemical activity and should be avoided.  Down to around 55F is OK also, but below that we get into the glass transition temperature of many materials (acrylic paint has a Tg around 50F) and therefore risk brittleness and cracking.  A lot of this research involves paintings and wood.

Michalski, Stefan  (2007) “The Ideal Climate, Risk Management, the ASHRAE Chapter, Proofed Fluctuations and Toward a Full Risk Analysis Model”  Contribution to the Experts’ Roundtable on Sustainable Climate Management Strategies, April 2007, Tenerife Spainhttp://www.getty.edu/conservation/science/climate/paper_michalski.pdf

“Proofed RH or T” is the largest fluctuation to which an object has been exposed in the past.  Risk of further mechanical damage from fluctuations that are smaller than the proofed value is low.  Ie, once the crack happens, it just opens and closes and doesn’t tend to get worse, especially if you stay away from those past extremes.  This is for mechanical damage.  Mold, chemical aging different.  When can repeated stress be handled indefinitely?  For tough materials, at about ¼ the stress that causes the damage can be repeated over and over OK.  For brittle stuff, about 1/10th.  So the safe zone is smaller than the proofed zone.  But even a little below the proofed value means there are many cycles needed.  Go straight to risk assessment based on past climate records.  Furniture, paintings other mixed collections susceptible to fracture from fluctuations: the worse their past the better their future.  “previous fluctuation history”  Response times of the objects?  Extreme fluctuations are sometimes shorter than the response time of the material.  Conservation treatments “erase the safety margin achieved by the fractures from historical conditions.”  Light fading risk goes down at low RH, but fracture risk goes up.  Higher temps reduce RH and help prevent mold, low temps reduce chemical self destruction.  Many collections survive well in non-ideal conditions, and many museums who claimed to reach the ideal didn’t.  People want a specification to deal with reality rather than the ideal.  He’s working with ICCROM on web-based manual for risk assessment.  This idea of proofed fluctuations is also going to appear in the 2011 ASHRAE chapter?  The part that is problematic in my mind regarding proofed fluctuations is new incoming artwork and artifacts whose proofed fluctuation may be smaller than that of the collecting institution overall?

Michalski, Stefan (1994) “Relative Humidity and Temperature Guidelines: What’s Happening?” Canadian Conservation Institute Newsletter. No 14. September 1994 pp. 6-10

Some basic concepts: low temps can cause some materials to get brittle, while high temps can promote chemical deterioration of many materials, especially acidic paper, nitrate/acetate films, celluloid and rubber.  Each drop of 5 degrees C doubles the lifetime of an artifact.  High RH promotes mold, corrosion, deliquescence, and promotes degradation through chemical self-destruction.  In most Canadian museums, the “proofed RH” seems to be +/- 25%.  Keeping within 20% +/- is suggested as reasonable, (although other articles suggest that 20% is too great, and maybe 10% is more appropriate?)  Useful table “Effect of Incorrect RH and Incorrect Temperature on Museum Materials”  The Abbey Newsletter, Vol 18 No 8 Dec 1994 has a review of this article by Ellen McCrady, a book and paper specialist.  She said the article was accurate for paintings and objects, but weak in the realm of paper and photos. 

Ogden, Sherelyn. (2007) NEDCC Preservation Leaflet 2.1 Temperature, Relative Humidity, Light and Air Quality.  Basic Guidelines for Preservation.  North East Document Conservation Center. http://www.nedcc.org/resources/leaflets/2The_Environment/01BasicGuidelines.php

Deterioration doubles for every 18 F temperature rise.  Libraries and archives prefer RH 30-50%, the lower end of that the better.  Excessive fluctuations in RH can lead to dimensional changes, cockling, flaking inks, warped covers, and cracked emulsion on photographs.  She does not specify what fluctuation over what time period would be excessive.

Pacifico, Michele F and Thomas Wilsted.  (2009) Archival and Special Collection Facilities: Guidelines for Archivists, Librarians, Architects and Engineers.  Chicago.  Society of American Archivists.  

Padfield, Tim (2009) “Lecture on Archive Environmental Standards.” 


BS5454:2000 is the British Archive Standard, is difficult to enforce and seems rather arbitrary.  16-19C for handled stuff, 13-16C for stored stuff, and an RH 45-60% with a 5% fluctuation.  Science still cannot measure very slow deterioration.  Interesting historical and archaeological examples:

St Catherine’s Monastery in Sinai Egypt dates back to the 6th century.  RH 10-30%, temp 8-30C

Desert of Eastern Turkistan, 3rd century papers buried in sand in an abandoned building.  RH 30-50%, temp -10 to 25C

Padfield, Tim (2005) “How to Keep for a While What You Want to Keep Forever”

Respected scientist and consultant in the field of museum climates.  “The standard we have in conservation are particularly unconvincing and the evidence that supports them is shaky and controversial”

Padfield, Tim (2004) “The Preservation index and the Time Weighted Preservation Index”  http://conservationphysics.org/twpi/twpi_01.php

Article about the Image Permanence Institute’s standards for preservation storage environments.  “Preservation index” links temp/RH to chemical deterioration (as opposed to biological or mechanical) based on a study of the deterioration of a common photographic film, cellulose acetate, through hydrolysis.  “Time Weighted Preservation Index” is a way to indicate the situation taking the variation of the room over time.  Padfield explains the chemistry (law of mass action and Arrhenius law) for the degradation of the cellulose acetate reaction and how both temperature and RH impact it.  He has a slightly different interpretation of what is going on chemically than IPI does, but doesn’t seem to quibble with their product.

Rawlins, (1942) “The Control of Temperature and Humidity in Relation to Works of Art.”  The Museums Journal (Museums Association) 41: 51-90

Very early and influential text setting the standard at .  “Target “60F 60% 

Reilly, (2008) “Specifying Storage Environments in Libraries and Archives.”  From Gray Areas to Green Areas: Developing Sustainable Practices in Preservation Environments, 2007, Symposium Proceedings by the Kilgarlin Center for Preservation of the Cultural Record, School of Information, The University of Texas at Austin.  Published online: September 2008, http://www.ischool.utexas.edu/kilgarlin/gaga/proceedings.html

“One cannot manage what cannot be measured”  In contrast to some other articles (chicora 1994)  he advocates that temperature matters more than RH.  Like Michalski’s articles, Reilly explains the risks in terms of biological, chemical and mechanical damage.  This model is really helpful.  Biological damage (especially mold) happens with high RH, usually above 65 for a few days or more.  Chemical damage is accelerated with heat and moisture, and examples include corrosion, yellowing, embrittlement, and fading.  For chemical change, short term fluctuations don’t matter much.  Mechanical damage mostly relates to hygroscopic materials, and the damage would be cracks, deformations, delamination etc.  High RH causes hygroscopic materials to swell and soften, while low RH causes them to shrink and become brittle.  For avoiding mechanical damage, you need to stay between 20-70% RH and keep excursions outside that range as short and infrequent as possible.  Seasonal extremes are worse than short-term events because materials take considerable time to equilibrate…weeks or months, not hours or days.  Room temperature is too warm for ideal preservation of many organics, paper, leather, dyes, film, etc.

Ryhl-Svendsen, Morten, Lars Aasbjerg Jensen, Poul Klenz Larsen and Tim Padfield.  (2010)  “Does a Standard Temperature Need to be Constant?” Presented at the Going Green conference, British Museum, April 2009  http://www.conservationphysics.org/standards/standardtemperature.php

Article argues for allowing temperature fluctuation greater than traditional standards allow while keeping RH controlled.  Uses three institutions in Denmark as an example.  Royal Library in Copenhagen is about 65F and 50%, permanently air conditioned.  Arnamagnaean archive in Copenhagen University has buffered conservation heating, which allows the temperature to fluctuate as the outdoor temp fluctuates but keeps a steady RH.  Museum store in Ribe, Denmark has dehumidification only, with temp of 50-60F and a steady RH as long as the equipment is functioning properly.  When it does not work, RH reaches dangerous levels.  Ventilation is an issue, and the authors recommend recirculation of the inside air through a filter.

Sebrera, Donald K.  (1994) “Isoperms: An Environmental Management Tool”  Washington DC: The Commission on Preservation and Access  http://cool.conservation-us.org/byauth/sebera/isoperm/

Credited with being the first person to explore preservation, temp and RH relationships using a “kinetics model based soley on theory and assumptions about the likely activation energies of decay reactions.” (quote from the IPI Step-by-Step workbook)

Thompson, Garry. (1978) The Museum Environment  Butterworth Heinemann

Until recently, the contemporary standard for museum environment.  50% RH +/- 5% and 70F +/- 3 degrees.  Allowable fluctuation apparently based on the switching differentials of the best available HVAC systems of the 1970’s.  Part of the reason this book is so influential is that the author worked for the Scientific Department of the National Gallery, London and examined the available scientific evidence.  He also was aware that there was little proof for the limitations, and qualified his statements carefully.  He chose 55% largely as a midpoint between 70% risk of mold growth and the then-prevalent notion that below 40% organic materials become brittle 

Weintraub, Steven.  (2006) “The Museum Environment: Transforming the Solution into a Problem.”  Collections: A Journal for Museum and Archives Professionals Vol 2. No 3 (February 2006) pp 195-21



  1. Mechanical Engineering Projects…

    […]TEMP AND RH REVIEW FOR MUSEUMS 2010 « Ellen Carrlee Conservation[…]…

  2. Hi, I’m a HVAC Engineer with over 30 years experience.
    This article was really useful for me.

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