How to Reduce Condensation with Reflective Aluminum Foil Insulation
This entry was posted on December 15, 2011.
As well as reflecting and blocking radiant heat, radiant barriers and reflective foil insulation are also great at reducing condensation. Today's "Staple Tab" Bubble Foil Insulation is what is used in between in walls, ceilings, and crawlspaces. Years ago reflective insulation was also used but in a different design. The product was multiple accordion aluminum insulation which basically comprised of layers of reflective foil with trapped airspaces. Here is an informative description on how accordion insulation was used to help reduce and control moisture.
"Prevent Damage to Wood, Plaster, Paint, etc.
As air becomes colder, it can hold less vapor in suspension. The degree of saturation increases until a dew-point is reached and condensation occurs.
Heat flows from warm to cold by conduction. A material which is in contact with air colder than itself on one side and air warmer than itself on the other side, will continuously extract heat from the warmer air by conduction and lose it to the colder air. As the contacting warmer air becomes cooler by this action, the amount of vapor it can hold in suspension without condensing becomes smaller.
The denser and bulkier the material, the more heat it can extract before attaining room temperature, if it ever does. The scientific construction of multiple layers of aluminum, fiber and reflective air spaces minimizes condensation-formation on or within this type of insulation, when installed in building spaces.
THE REASON CONDENSATION IS MINIMIZED
Since the sheet of aluminum adjacent to the warm, inner air of a building weighs only about 1/4 oz. per sq. ft., it does not need to extract much heat from that air to attain and remain at room temperature. Since the emissivity of the aluminum surface is only 3%, little heat is lost by radiation on the cold side. This further enables the metal to remain at about room temperature and not extract much heat from that warmer air.
Since the other sheets of aluminum and fiber retard heat flow by inner as well as outer convection, and since conduction is slight through the preponderant low density air spaces, each sheet's other surface faces a space which is only a little colder than the aluminum itself. But because warmth flows to cold in conduction, the aluminum will give off a slight amount of heat to the colder space, thereby slightly increasing the vapor retaining capacity of that space. The successive reflective spaces and sheets of aluminum and fiber behave similarly. Since each aluminum surface is slightly warmer than the air it faces on its cold side, there is no extraction of heat. The reverse is true.
CONTINUOUS, ALMOST IMPERVIOUS VAPOR BARRIER
When multiple accordion aluminum is used, fortuitous vapor and water ( like rain) which intrude into building spaces will, as vapor pressure develops therein, gradually flow out as vapor through exterior walls and roofs because vapor flows from areas of greater to areas of less vapor density. Since the vapor cannot back up through the continuous, almost impervious metal, it will flow out because exterior walls and roofs have substantial vapor permeability compared with aluminum, far greater than the required 5 to 1 ratio.
Of great importance is the use of a continuous metallic vapor barrier, of almost zero water vapor permeability, without "breaks" every few feet, to retard the outward flow of vapor which originates inside the house. Infiltration under the flat stapled flanges of multiple aluminum is slight.
To obtain MAXIMUM, uniform-depth protection against heat loss and condensation formation, it is necessary to use the new edge-to-edge multiple aluminum*, each sheet of which stretches from joist to joist.
The U. S. Bureau of Standards has prepared a helpful and informative booklet, "Moisture Condensation in Building Walls," which discusses causes and cures. Use the coupon to get a free copy from us."