Thermal Performance

Thermal Conductivity

Thermal conductivity is defined as the rate of thermal movement through a material per unit area, per unit thickness, per unit temperature (delta T).

“U” Value (Factor) / (BTU) (British Thermal Unit)

  • BTU is used to describe the heat value (energy content) offuels and the power of heating and cooling systems.
  • BTU per hour, per square foot, per degree Fahrenheit (h/ft2/ ̊F)
  • Watts per hour, per square meter, per degree Centigrade (h/m2/ ̊C)
  • (For metric U-value, multiply the imperial U-value x 5.678)
  • BTU is a measurement of the transfer of heat loss or gain, due to the difference between indoor and outdoor air temperature.
  • One BTU is the amount of heat required to raise the temperature of 1 lb (0.454 Kg) of liquid water 1 ̊F (0.556 ̊C).
  • One BTU is approximately the heat produced by burning a single wooden match.
  • A window with a U-value of 0.30 will lose half the energy that a window with a U-value of 0.60.

“R” Value (Factor)

  • R-value is the resistance to thermal conduction per unit thickness of a material.
  • R-value is the inverse of U-value. The higher the R-value, the more effective or resistant to heat loss the building’s insulation will be.

 To convert:

“U” = 1/R, “R” = 1/U

The chart provides the R-values and U-values for window materials based on a test sample measuring 1 cubic inch. (1″ x 1″ x 1″)

Thermal Efficiency Comparison

Fiberglass Cladding Systems

Fiberglass is the most energy efficient cladding system material offering resistance to heat loss, reduced heating and cooling costs, and improved occupant comfort. A simple touch test demonstrates fiberglass’s thermal efficiency; in cold climates fiberglass is warm to the touch, in hot climates it’s cool to the touch. What is being felt in a fiberglass cladding is the lack of heat transfer. Fiberglass cladding systems do not need a thermal barrier within the frame, as required by aluminum and steel cladding systems, as the whole fiberglass frame is a thermal barrier.


Relative humidity (RH) is the ratio of the amount of water vapor molecules actually present in the air, as a percentage of the maximum moisture the air can hold at a given temperature. The warmer the air temperature the lower the percent RH. When warm moist air meets a cold surface, the air temperature drops, raising the RH, until the RH is 100%, where it meets its dew point. Any moisture in the air over 100% is squeezed out and deposited on the closest coldest surface.

The ideal humidity comfort level is a broad range, from 30-50% RH. Below 30%, anyone with asthma or bronchial problems may have difficulty breathing. Building occupants may think they have a perpetual cold, but are suffering from the effects of lowhumidity. Low humidity dries out nasal passages and aggravates mucus glands simulating a cold. Cuts and wounds heal slower in this environment.

If there is condensation on windows, there is condensation in the walls. If not dealt with, condensation will lead to:

  • conditions suitable for mold growth and subsequent health issues for occupants
  • the damage of household fabrics and discoloration of paint and wallpaper, and
  • the reduction in the effectiveness of insulation, deterioration of drywall, and structural deterioration.

Lineal Thermal Expansion

The following materials have known coefficient of linear expansion rates:

Expansion and contraction for each degree in temperature (C or F):

  • Vinyl is seven times greater than fiberglass.
  • Aluminum is three times greater than fiberglass.
  • Wood does not expand and contract due to temperature change, but is subject to swelling and shrinkage due to moisture absorption.

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