Technical Properties Affecting Foam Mattress Performance

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A foam mattress is usually made from at least a couple of different types of foam, sometimes more. Retailers will often give the properties of the top layers of a latex or memory foam mattress in their literature. The following foam properties apply to all layers: polyeurothane foam, latex foam, or memory foam.

  1. Density - A measurement of the mass per unit volume. It is measured and expressed in pounds per cubic foot (pcf) or kilograms per cubic meter (kg/m3). In other words, a 1 foot square block of 5lb (pound) density foam will weigh 5lbs.

  2. Indentation Force Deflection (IFD) - also known as Initial Load Deflection (ILD) -- A measure of the load bearing capacity of flexible polyurethane foam. IFD is generally measured as the force (in pounds) required to compress a 50 square inch circular indenter foot into a four inch thick sample, typically 15 inches square or larger, to a stated percentage of the sample's initial height. Common ILD values are generated at 25 and 65 percent of initial height. The top layer of a memory foam bed is usually between 12 - 15 ILD. High resiliency base foam used in bedding is from 20 – 40 IFD.

  3. Support Factor - Support Factor = 65% IFD¸25% IFD determined after one minute of rest or recovery. When the support factor is known it can be used in conjunction with a known 25% IFD value to determine the 65% IFD value. Foams with low support factor are more likely to bottom out under load.

  4. Flex Fatigue (Dynamic Fatigue) -- A normal softening or loss of firmness over time and with use. In general, fatigue is measured in the laboratory by repeatedly compressing a foam sample and measuring the change in IFD.

  5. Tear Strength-- A measure of the force required to continue a tear in foam after a split or break has been started and expressed in pounds per inch (lb./in.). This property is important in determining suitability of foam in applications where the material is sewed, stapled, or otherwise anchored to a solid substance.

  6. Resilience -- An indicator of the surface elasticity or "springiness" of foam. It is measured by dropping a steel ball onto the foam cushion and measuring how high the ball rebounds.  Memory foam bed has almost no resilience. High resilience foams are used in the base layer of mattresses.

  7. Hysteresis -- Measurement of ability to maintain original support characteristics after flexing. Normally, the firmness (IFD) is tested at 25% indentation, 65% indentation and again at 25% on the way up. Hysteresis is the percent of 25% IFD loss measure as a compression tester returns to the normal (25% IFD) position after measuring 65% compression. Lower hysteresis values, or less IFD loss are desirable. Current research indicates that hysteresis values may provide a good indication of overall flexible foam durability. Low hysteresis in conventional foam is equal to less IFD loss in durability and fatigue tests.

  8. Air Flow-- Amount of air expressed in cubic feet per minute, that can be drawn through a 2" x 2" x 1" foam sample at .5-inch water pressure differential. Air Flow is measured by a test. High air flow equates to very open foam cell structure.


Foam Mattress Properties and Shopping

What technical information do you need to understand when selecting a new foam mattress? Foam manufacturers don’t necessarily make all of their technical data available. Even if you did want to play Sherlock, and chose a foam bed based on an objective analysis of facts, the information would not be readily available from mattress retailers.
 
Also, the several properties of foam listed above all work together to affect performance. In other words, you can change any one property and have a dramatically different performance. The variables are just too complex to contemplate. It’s best left to the experts to decide the proper mix.
 
But you also want to be aware that some beds are not made as well as others. Mattress fabricators can use less expensive materials to do the job. You need to know enough about the technical properties of the foam to select a quality bed.
So what’s a consumer to do? How can you combine an objective analysis of facts with your intuitive feeling about what bed is right for you? You can do a few things.


Summary of Foam Mattress Properties - What You Need to Know

First, know the foam density. Most retailers have this information. The rule of thumb is that a 5lb density (or thereabout) memory foam for the top layers of the mattress is optimal. However 3lb and 4lb foam is perfectly acceptable.  5lb foam is more durable than foams of lower density; it offers proper support and yet can be made softly comfortable.
 
Second, understand ILD.  ILD is a measurement of firmness. Many retailers will have this information. The memory foam top layers of a bed usually fall within the 12-16 ILD range.  The lower the ILD, the softer the foam will feel, and the higher the ILD, the firmer it will feel. The firmer base layers of high resiliency foam at the bottom of your mattress are firmer, 20 – 40 ILD.
 
Third, understand the concept of support factor. It is the quality of foam that holds your body in a comfortable position over a period of time. A support factor will not be supplied by your retailer, but it is not necessary. A mattress does not have proper support if it “bottoms out.” This means that the top layers of foam fully compresses, allowing your bottom, hips, or shoulders to bump against the firm support layer of foam at the bottom of the mattress. Bottoming out creates pressure points, and to some degree cancels out the benefits of having a memory foam mattress.
 
Fourth, anticipate flex fatigue. Flex fatigue is the normal softening of a foam mattress over time and with use. The bedding industry uses the term “body impression” to describe the softening of foam in areas of higher use. Higher density foam experience flex fatigue at a slower rate. Firmer foams will enjoy a longer usable lifespan. Different foam mattresses will soften within different time frames, but will retain their ability to provide quality support long after the natural softening begins.