Energy-Saving Options to Consider

We define an optimal preservation environment as one that achieves the best possible preservation of collections at the least possible consumption of energy, and is sustainable over time. Your institution has achieved "optimal" when your unique climate control system consistently produces its own best possible storage environment at the least possible consumption of energy. Simply put, do the best that you can with what you have. The complexity of this task will depend on the size of your institution and the type of building and mechanical equipment you are dealing with.

Post and beam, wood-framed or masonry buildings—from barns to standard historic house structures:

Use shutters, windows, porches, curtains, awnings and shade trees to reduce heating and cooling loads. Pull down the shades on sunny summer days to reduce the heat load. Consider adding storm windows. Improve energy efficiency by using appropriate weather stripping and caulking around doors and windows. Consider installing insulation in attics and basements. Add insulation and vapor barriers to exterior walls only when it can be done without damage to historic building structures.

Lower the temperature where collections are housed. Dial down the radiators or close the heat vents in winter—but measure the relative humidity to be sure it stays below 55%, in which case you will need to raise the temperature or dehumidify. Check on your outdoor dew point averages and use IPI's Dew Point Calculator ( to determine what the best balance you can achieve is. Keep the RH between 25% and 55% to avoid the majority of moisture-related problems.

Deal with any sources of moisture that could cause problems for your collections.  Consider high humidity, rain, local bodies of water, wet ground, leaking pipes and broken gutters, moisture in walls, human respiration and perspiration, wet mopping, flooding, and cycles of condensation and evaporation.

Tightly-constructed new metal or concrete-framed structures—from typical modern cultural or industrial buildings:

Many years of experience in monitoring both the behavior of storage area HVAC systems and climate conditions in associated storage areas has demonstrated that these systems can consume more energy than necessary to achieve their desired climates, and that excess energy consumption can easily go on undetected.  You can benefit by visiting each significant energy-consuming element of the system and asking, “Is this element using more energy than necessary to achieve its intended function?”                                                                   

New Stacks Energy0001

Energy Consuming Elements of a Typical Forced Air HVAC System

A. Fans    B. Cooling/Dehumidification    C. Heating/Reheating     D. Humidification    E. Lights


The following examples of potentially excessive energy use are organized by the system components identified in the illustration above:

A.  AHU Supply and Return Fans - Many fan motors are equipped with variable speed drives, making it possible to reduce the fan speed (and thereby the rate of energy consumption) at times when the climate can be maintained with a reduced air flow.  However, this variable capacity is often underutilized or not used at all. In situations where the fan speed can be controlled by a schedule (e.g., slowed down at night) the schedule is either not used at all or used too conservatively (e.g. , slow down 10% for three hours per night when conditions could have been maintained by a slowdown of 50% for eight hours per night). To get the maximum energy savings from variable speed capability facilities staff and collection managers should perform a series of experimental slowdowns to determine when (what time of day, what time of year) and how far the fan speed can be reduced without compromising the quality of storage climates.

Facilities knows where the ducts goB.  Cooling/Dehumidifying - The function of the cooling system is to remove heat that enters the system.  At the storage spaces, heat is added to the air stream by the lights, and the operation of the lights should therefore be managed to be concurrent with occupancy (see E. Lighting).  Likewise, doors to adjacent warmer spaces should be open only when necessary.

For most buildings of this type, with a reasonably constructed envelope, the primary driver of cooling energy consumption is the amount of outside air brought into the system.  The introduction of more outside air than is necessary, whether by applying occupied building code to unoccupied spaces, through malfunction, or other cause, results in greater energy use to perform work on that air. Whenever the outside air temperature is above the desired supply air temperature (usually 60°F or lower) heat must be removed. Additional heat must also be removed whenever the dew point temperature of the outside air is above the desired storage area dew point temperature (typically 45° to 50°F). In warm humid climates, the outside air needs cooling continuously, and even cool northern climates must cool outside air up to six months per year. 

Most systems remove the excess moisture from outside air through a process of sub-cooling and reheating. This sub-cooling can represent a major portion of the total annual consumption of cooling energy.  Excess energy consumption occurs when unnecessary sub-cooling is done and/or when more air is sub-cooled than necessary (see C. Heating for an elaboration).

C.  Heating - The temperature set point in many storage areas is determined by the temperature that must be maintained to prevent high humidity during summer months. Maintaining this temperature in winter may require heating that could be reduced or avoided if the temperature was allowed to drift lower in winter. A large percentage of storage area heating energy is consumed when reheating air that has been sub-cooled for dehumidification. While this process may be necessary when the dew point temperature of the outside air is above the desired dew point temperature of storage spaces, sub-cooling and reheating is not necessary when the outside air dew point temperature falls below the storage space desired dew point. Many systems sub-cool and reheat continuously all year, even in climates where it is unnecessary for several months per year.

Some systems are designed to allow a certain adjustable fraction of the air stream to bypass the sub-cool and reheat coils, thereby reducing energy use in situations and/or seasons when conditions can be maintained without processing all of the air.  Many of these systems could increase the amount of air bypassed without compromising storage climates.  Careful experimentation would likely show that this quantity could be increased during portions of the year, if not all of the time.

In many northern climates, heating needs are linked to the amount of outside air brought into the system.  If the quantity of outside air exceeds the amount required, excess heating energy will be consumed.

D. Humidifiers - Humidifying typically represents a small fraction of storage area annual energy use and therefore presents few energy-saving opportunities. However, in northern climates where humidification energy use is more significant, consideration should be given to the space humidity set point.  Some facilities try to maintain 50% RH in winter, when a gradual seasonal drift from 50% in summer down to 35% in winter may not be detrimental to collections and will reduce humidification.

The amount of humidification required is directly related to the quantity of outside air introduced into the storage area climate control system.  Serious consideration should be given to how much outside air is brought in and when.  Many systems bring in more outside air than is necessary all of the time, and most bring in more outside air than is necessary during periods when the collection storage areas are unoccupied.

E. Storage Area Lighting - The operation of any storage area lighting fixture when there is no one nearby requiring illumination can be considered a waste of electrical energy – energy consumed at both the light fixture and at the cooling system that must remove the heat generated by the fixture. The occupancy of most areas in most storage spaces is very intermittent and an organized and sustained effort is required to prevent the unnecessary operation of storage area lighting fixtures. The quantity of lighting in some storage areas is excessive, in which case consideration should be given to removing some fixtures.


The paragraphs above describe how each energy-consuming component of a storage area HVAC system can use more energy than necessary to maintain the desired climate conditions.  It's clear that the causes of excess energy use do not announce themselves. A reduction in energy use will probably result if each energy-consuming component is carefully examined to see if the way it actually operates coincides with the way it was designed to operate to maintain the desired storage climate.

The following graphic shows the potential energy cost reduction that could result from such a careful examination.  The “typical operation” annual cost is based upon the actual measured performance of a storage area climate control system.  The “optimal operation” annual cost shows the savings that would result from eliminating unnecessary energy use.  The preservation quality of the storage climate would be unaffected by the change from typical to optimal, while the annual energy cost would be reduced by 28%.

Annual Energy Costs