THE ROLE OF DEW POINT IN SUSTAINABLE 
ENVIRONMENTAL MANAGEMENT

Understanding Dew Point

Use IPI’s Dew Point Calculator to learn more about how 
dew point, temperature, and RH interrelate at: www.dpcalc.org

(A)

The capacity of air to hold water 
increases as air is warmed, and 
decreases as air is cooled

 » The Dew Point determines the preservation environment that can be achieved because 
it determines what combination of Temperature and Relative Humidity can be achieved

 » Temperature (T), Relative Humidity (RH), and Dew Point are interrelated

 » At a constant Dew Point – as the T rises, the RH falls, and as the T falls, the RH rises

 » Dew Point is typically the limiting factor of a mechanical system’s ability to provide a 
good environment for long-term preservation of collections

General Recommendation 
for Preservation:  

Make it as cool as you can 
while maintaining a moderate 

RH – understanding Dew 
Point is the key.

The Dew Point is the temperature at which air containing a specific amount of water becomes saturated. Dew Point can be used as an 
indicator of the total moisture content in an environment Think of the diagrams below as a series of “buckets” – the bucket size increases or 
decreases with temperature (A), while the amount of water in the bucket remains the same regardless of the temperature change (B).  

(B)

The actual amount of water in the 
air does not change with changes 
in air temperature

55o    60o    65o    70o       75o         80o

Capacity to hold water

Air Temperature

Actual water present

55o    60o    65o    70o       75o         80o

Air Temperature

Dew Point

Concept A:

Concept B:

55o    60o    65o    70o       75o         80o

Capacity to hold water

Air Temperature

Actual water present

55o    60o    65o    70o       75o         80o

Air Temperature

Dew Point

Concept A:

Concept B:

Overview

 

imagepermanenceinstitute.org



(C)

Relative Humidity represents 
how saturated the air is with 
water vapor

Special Thanks to Peter Herzog of Herzog/Wheeler & Associates, St. Paul MN, for helping IPI develop these tools and concepts

Relative Humidity is the percentage of the bucket that is full based on the moisture content of the air (actual water present) and the capacity 
of the air to hold water (based on temperature) (C). 

Concept of Relative Humidity

100%     84%    70%       59%       50%            42%

Relative Humidity

Absolute Humidity
(.0092 lbs/lb)

Actual water present

55o    60o    65o    70o       75o         80o

Capacity to hold water

Air Temperature

Actual water
present

45o   50o   55o    60o    65o    70o       75o         80o

Capacity to
 hold water

Air Temperature

100%     84%    70%       59%       50%            42%

Absolute Humidity
(.0092 lbs/lb)

Capacity to
 hold water

100%    84%        69%     58%       50%       42%            35%

Absolute Humidity
(.0076 lbs/lb)

100%     84%       68%       58%        48%            41%             34%           29%

Absolute Humidity
(.0062 lbs/lb)

Capacity to
 hold water

Actual water
present

Actual water
present

The environment is at its Dew 
Point when the air is completely 
saturated – i.e., 100% RH.  The 
dew point can be reduced by 
cooling the air below the current 
dew point (dehumidifying to 
remove moisture), which reduces 
the capacity of the air to hold 
moisture (shrinking the bucket). 
Excess moisture in the system 
will condense out. The lower 
the dew point, the lower the 
RH can be maintained at cool 
temperatures.

Move up
to

humidify

Move down
to

dehumidify

 

imagepermanenceinstitute.org