Joint Industry Foam Standards and Guidelines


Published: 7/94

Temperature and Humidity Effects on IFD

"Relative humidity" is defined as the amount of moisture vapor in the air "relative" to the amount of moisture vapor the air will hold at a specific temperature. For example, a relative humidity reported as 80% at 75 degrees F means that the air contains, at that specific temperature, 80% of the moisture that the air could contain. Another way to look at relative humidity in the above example is that 20% more moisture could be added to the air at that temperature. If 20% more moisture were added to the air at 75 degrees F, the air would then be completely saturated, and the relative humidity would be 100%. Relative humidity is always reported as a percentage value.

"Absolute humidity" is the quantitative amount of moisture in the air regardless of the temperature. Absolute humidity is usually reported in grains of moisture per cubic foot (or cubic meter) of air. Typical absolute humidity values in the wintertime are 10 to 30 grains of moisture vapor per cubic foot, while typical values for the summertime are 120 to 140 grains per cubic foot.

The other effect of temperature and humidity is related to the temperature and humidity during the actual measurement of the IFD. The effects of temperature and humidity during the measurement of IFD are generally reversible. For example, the following scenarios are actual measurements on actual pieces of foam:


  • 25% IFD of a piece of foam in a laboratory conditioned to 70 degrees F and 50% relative humidity..................................... 32 lbs

  • 25% IFD of the same piece of foam when placed in the plant a week later at 90 degrees F and 88% relative humidity........................ 26 lbs

  • 25% IFD of the same piece of foam when placed back into the conditioned laboratory a week later.........................................32 lbs


  • 25% IFD of a piece of foam in a laboratory conditioned to 70 degrees F and 50% relative humidity......................................32 lbs

  • 25% IFD of the same piece of foam in the plant warehouse a week later at 32 degrees F and 40% relative humidity.............................39 lbs

  • 25% IFD of the same piece of foam one week later placed back in the conditioned laboratory @ 70 degrees F and 50% relative humidity......32 lbs

It is easily seen that the IFD changes due to temperature/humidity are reversible and are directly related to the temperature and humidity. Furthermore, with the technology available today, the magnitude of the actual change in IFD is not quantitatively predictable.

There is a very good reason that the quantitative relationship between IFD and temperature/humidity is not yet predictable. It is fairly certain that the chemical structure of the foam has much to do with the temperature/humidity related IFD changes. Because the temperature/humidity IFD relationship varies with different foam types, development of a universal temperature/humidity IFD relationship statement is impractical.

As the IFD increases, the effects of both relationships of temperature and humidity also increases. For example, the effects of both summertime regression and changes only due to measurement temperature/humidity are much more significant on a 36 IFD foam than on a 15 IFD foam. The reason for this is probably due to the magnitude of the IFD number. For example, if the IFD change is 20% on both foam types, 20% of a 36 pound IFD is 7.2 pounds while the same 20% change of a 15 IFD foam is only 3 pounds.

Thus, temperature and humidity must be considered carefully and thoroughly in the day to day management of IFD specifications by both the foam manufacturer and the furniture manufacturer.

The question arises, with the variations caused by temperature and humidity, how can one devise an acceptance testing procedure and not have to maintain huge inventories awaiting test results from controlled temperature and humidity laboratories?

Day-to-day acceptance of foams, tested in non-controlled temperature and humidity conditions, can be accomplished by working closely with one's vendors. Some general knowledge about IFD variation with temperature and humidity is necessary.

It is generally recognized that the key point to remember is that the IFD/temperature/humidity relationship is an inverse relationship; that is, as the temperature/humidity increases, the IFD decreases.