Low-Tech Solutions to Boost Low RH in Winter Climates

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Through the manipulation of room temperature, many exhibit collections can be protected from physical damage resulting from the dry interior conditions produced by winter heating.

Why is it difficult to maintain relative humidity (RH) in winter climates?

Northern climates that experience low winter temperatures require a considerable supply of heat in order to maintain temperatures adequate for human comfort (e.g. 68° F). Central heating causes interior humidity levels to plummet to very low levels in museums without humidification systems. The conventional approach of humidifying the building's atmosphere is often not possible or practical as this technique has financial and structural drawbacks.

• The expense of installing humidification/dehumidification equipment for building-wide control can be prohibitive.
• Older buildings which are insufficiently insulated and lack vapor barriers cannot maintain desired RH levels even when moisture is added.
• Humidification of buildings in wintertime frequently leads to structural deterioration: the building envelope (e.g., walls and windows) can be damaged in an attempt to maintain even moderate RH levels (e.g., 40 - 60 % RH). Moisture diffuses through the walls, condensing and/or freezing as it meets exterior temperatures, encouraging rotting of wood products, corrosion of metal components and freeze/thaw cycling that weakens masonry.

Which are the climatic/geographic regions of particular concern?

Central and northern areas of the United States and Canada (see map) commonly experience interior relative humidities of 10 - 30% RH during the winter heating season. These are well below the minimal acceptable RH levels. Because conventional winter humidification is a problem in these geographic regions, many conservation scientists currently recommend the acceptance of a less restrictive and more achievable preservation standard for moisture-sensitive materials, that is, a lower limit of 30 - 35 % RH with limited fluctuation.

Are there low-tech solutions to increase humidity in winter?

Two low-tech methods are available for raising RH in winter; both help protect collections from needless damage, decrease the danger of damage to the buildings housing the exhibit, and lower overall energy costs. The techniques are based on the manipulation of interior temperature by:

• manual lowering of interior temperature;
  
• electronic control of interior temperature using a humidistat.
  
  

1. Manual Lowering of Interior Temperature

The temperature range in an exhibit room can be incrementally lowered in winter to prevent a dangerous drop in relative humidity. Temperature is controlled by a thermostat that is manually adjusted downward in periods of very low RH (commonly the months of December through March). The RH will still vary relative to changing exterior conditions; however, lowering the temperature range will help moderate the more drastic daily changes in RH. The environment must be carefully monitored using a hygrothermograph and adjustments made as necessary.

The following projection of the potential RH gain from manually lowering a thermostat indicates the usefulness of this technique.

When winter exhibit room conditions are.…....68°F and interior RH is 20%: Simply lowering the temperature to 60° F….…increases the RH to 25%. Lowering the temperature further to 50° F……increases the RH to 38%.

2. Humidistatically-Controlled Heating Technique

Temperature, in this method, is manipulated electronically by a humidistat to maintain an appropriate level of relative humidity. As a result, interior temperatures are constantly adjusted to maintain a stable relative humidity within a predetermined range (set points). The humidistat senses the relative humidity and sends the information to a control panel. If the RH level is too high, the control panel switches on the heating system until the RH returns to the desired level. If the RH is too low, the heating system remains off until the desired RH is obtained or a minimal temperature limit is reached.

In order to avoid excessively low or high temperatures, a low and high temperature limit is set on the thermostat connected to the control system. When the low or high temperature limit is reached the temperature remains at that limit and the RH must remain uncontrolled.

When the lower temperature limit is set at 40°F or less, humidifiers should be disconnected and water pipes in the building should be properly drained or kept above freezing with heating cables.

Are there any classes of materials which should not be exposed to lower temperatures?

Problematic materials which could be damaged by low temperature exposure include paintings on stretched canvases, wooden objects with cross-grain structures and inlay, and wood with delicate finishes. If very low temperatures are involved (below 32°.), archival or photographic materials should be removed from the exhibit area and moved to a more controlled environment. Consult a conservator for a more detailed assessment of your collection and to identify any particularly sensitive objects.

What are the special requirements/recommendations for using the humidistatic method?

Any institution considering the humidistatic control system must have an engineer work with them from the beginning to plan and set up the system. The engineer determines whether the system is viable for your situation and what additional measures need to be taken, if any, for its implementation. Experience has shown that the following recommendations are key to the successful use of this method.

Recommendations for humidistatically controlled heating technique.

1. Use qualified specialists.

Use an engineer who is familiar with humidistat control principles. Include a good service contract with the firm that installs the system.

2. Use a good quality humidistat.

Less expensive residential humidistats lose their accuracy as soon as the RH falls below 35% and/or temperature dips below 50° F.

3. Put one staff member in charge.

The staff member responsible for overseeing the humidistatic controlled system requires training. This person must be in charge of the system at all times (i.e., heat cannot be turned up indiscriminately for human comfort, or doors left open).

4. Ensure adequate air circulation.

Good air circulation is critical to preventing condensation problems. Condensation and mold outbreaks are possible if cold air meets with warmer air. The engineer should determine if fans should run continually.

5. Raise and lower the temperature slowly.

When an object's temperature is much cooler than the air, the dew point can be reached and condensation can occur. Temperature should not change more than 15° F per day. Massive objects such as furniture retain or gain heat at a much slower rate (thermal inertia) than the surrounding air. Covering furniture with muslin can provide a buffering effect against condensation.

6. Raise large objects from the floor.

Large organic objects, such as furniture, can trap moisture at the floor level. A simple solution is to raise these items up on blocks.

7. Remove objects from exterior walls.

All objects hanging on or positioned against exterior walls have a higher risk of moisture condensation. Low exterior temperature transfers first through the walls of a building; upon meeting the interior surface, condensation can occur in poorly insulated structures.

8. Use caution when moving objects.

Some materials (for example - furniture with hide glue joins) become more brittle at lower tempera- tures and require very careful handling until the objects become acclimated to a warmer atmosphere.

9. Acclimatize objects gradually.

Objects that are moved from a humidistatically controlled building/room to a normally heated space are subject to stress and condensation unless acclimated slowly. To alleviate this danger, objects can be placed in a sealed, clear polyethylene bag with a thermometer before relocation. Allow the temperature inside the bag to equilibrate to a higher temperature before opening the bag. This may take between 1 and 24 hours depending on the size of the object.

Suggested Further Reading

Gates, Michael, 1990. "Northern Innovations: Humidity Control for Collection Storage in Dawson City, Yukon" Muse, Spring. pp. 65-69.

Kerschner, Richard, 1992. "A Practical Approach to Environmental Requirements for Collections in Historic Buildings, Journal for the American Institute for Conservation, Volume 31. pp. 65-76.

Lafontaine, Raymond H., 1967. "Humidistatically Controlled Heating: A New Approach to Humidity Control in Museums Closed for the Winter Season," Journal of the International Institute for Conservation-Canadian Group (IIC-CG) Journal Vol. 7, pp. 35-41.