Window insulation film
Encyclopedia
Window insulation film is a plastic film which can be applied to glass windows to reduce heat transfer. There are two types in common use designed to reduce heat flow via radiation
and convection
respectively.
Typical absorption for a silvered film is 65% for visible and infra red with 99% for UV. This type of film sticks directly onto the glass.
This film is attached to the window frame using double sided pressure sensitive tape
. The effect is to create a double glazed system with a still air layer about 0.5 inches thick between the film and the glass. This restricts the convective air flow which efficiently transfers heat onto the inside glass surface. The reduced heat flow also can prevent condensation
which is triggered when the temperature of the inside surface falls below the dew point
. Assuming an outside temperature of 0C with wind velocity 15mph and inside temperature 20C condensation occurs at only 30%RH relative humidity
with a single glazed system compared with 60%RH for a double glazed system. Condensation also transfers an additional 2200 J/ml which is significant.
The film is a heat-shrink plastic which allows a hair dryer to be used to remove creases and improve optical clarity. Since the thermal conductivity
of air (0.024 W/mK) is much lower than glass (0.96 W/mK) the heat flow via conduction could be theoretically reduced by 97.5% though this is limited in practice by slow air movement in the convective cell formed between the film and the glass.
However, a typical window insulation film kit will not prevent convection unless the frame is less than 1 inch from the glazing. The top sash
of a typical double hung window is 1-2 inches behind the lower sash which is 2-3 inches behind the window frame. This is too far to eliminate convection. In these situations, the kits work by reducing infiltration not convection, and are similar to interior 'storm windows.' The ideal distance for 2 sheets of glazing is about .5-.75 inches (refer to insulated glazing
).
Convection control film is often used in winter when the outgoing heat flow is more important, especially for single-glazed windows where the temperature difference is often sufficient to cause condensation.
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
and convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
respectively.
Solar control film
This works by reflecting the infra-red component of solar energy (often 700W/sq M) and absorbing the UV component. Some films are also silvered or tinted to reduce visible light.Typical absorption for a silvered film is 65% for visible and infra red with 99% for UV. This type of film sticks directly onto the glass.
Convection control film
Pressure sensitive tape
Pressure-sensitive tape, known also in various countries as PSA tape, adhesive tape, self-stick tape, sticky tape, or just tape, is an adhesive tape that will stick with application pressure, without the need for solvent, heat, or water for activation...
. The effect is to create a double glazed system with a still air layer about 0.5 inches thick between the film and the glass. This restricts the convective air flow which efficiently transfers heat onto the inside glass surface. The reduced heat flow also can prevent condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
which is triggered when the temperature of the inside surface falls below the dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
. Assuming an outside temperature of 0C with wind velocity 15mph and inside temperature 20C condensation occurs at only 30%RH relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
with a single glazed system compared with 60%RH for a double glazed system. Condensation also transfers an additional 2200 J/ml which is significant.
The film is a heat-shrink plastic which allows a hair dryer to be used to remove creases and improve optical clarity. Since the thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
of air (0.024 W/mK) is much lower than glass (0.96 W/mK) the heat flow via conduction could be theoretically reduced by 97.5% though this is limited in practice by slow air movement in the convective cell formed between the film and the glass.
However, a typical window insulation film kit will not prevent convection unless the frame is less than 1 inch from the glazing. The top sash
Sash
A sash is a cloth belt used to hold a robe together, and is usually tied about the waist. The Japanese equivalent of a sash, obi, serves to hold a kimono or yukata together. Decorative sashes may pass from the shoulder to the hip rather than around the waist...
of a typical double hung window is 1-2 inches behind the lower sash which is 2-3 inches behind the window frame. This is too far to eliminate convection. In these situations, the kits work by reducing infiltration not convection, and are similar to interior 'storm windows.' The ideal distance for 2 sheets of glazing is about .5-.75 inches (refer to insulated glazing
Insulated glazing
Insulated glazing also known as double glazing are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope....
).
Applications
Solar control film is an effective way to control excessive sunshine during the summer months.Convection control film is often used in winter when the outgoing heat flow is more important, especially for single-glazed windows where the temperature difference is often sufficient to cause condensation.