What Do Density and ILD/IFD Ratings For Memory Foam Mean?
To distinguish between foam mattresses and all of the ingredients that are used in them, and to have a kind of rosetta stone “code” to be able to understand whether you are buying a really good mattress or a promotional grade bed made with the same foam used to use as filler in cheap sofas, more and more consumers want to know about the esoteric and confusing acronyms which differentiate great ingredients from crap.
And while we are happy to explain this technical jargon, I want to be clear that these technical specs alone do not give you a real indication of the comfort of a particular memory foam mattress or topper. A lot of it has to do with the outer fabric, quilting, and other elements, but the durability and level of comfort and support can often be revealed by understanding ILD (Indentation Load Deflection) and IFD (essentially the same thing).
Because high density foams are typically a complex material that has a tremendous range of firm vs. soft, buoyant vs. cradling, and more, and these ingredients also have other characteristics such as their responsiveness to heat and humidity differently due to their different formulations, understanding all of this is pretty complex. They also have differences in their cell structure (the various degree of “openness” of their cell structure) which can also affect a foam feel and pressure relief characteristics. We’ll try to keep it simple and relevant to give you the basics.
Some foam layers may have the same specs in terms of density (which is just the weight of the foam, specifically the weight of a cubic foot of the foam (a cube of foam that is a foot in each dimension) but still have very different sensations. ILD, or Indentation Load Deflection, basically is a number which represents how many pounds of compression on one cubic foot of foam it requires to compress the block one inch in a controlled environment — the higher the number, the more pressure that was needed, so the stiffer the foam. A typical layer of memory foam, as an example, which would be considered fairly soft might have and ILD of 13. Foam used for car seats may have ILD of over 50, on the other hand.
The Polyurethane Foam Association (PFA) actually produces guidelines, rarely disclosed to the public, but rather to manufacturers and fabricators, that offer a range of comparison of various kinds of foam used in bedding production, and in other industries.
The PFA has purposely avoided using the word "comfort" directly associated with ILD properties. Suffice to say, ILD is a part of the comfort equation, but ILD is not always related directly to comfort.
ILD Ratings The Mattress Industry Uses As General Comfort Guidelines- The Down And Dirty Info To Ask Anyone Who Is Trying To Sell You A Mattress
6--12--------------Bed pillows, thick back pillows
12-18--------------back pillows, upholstery padding, wraps
18-24--------------thin back pillows, tufting matrix (the quilt on top), very thick seat cushions
24-30--------------average seat cushions, upholstery padding, tight seats, certain mattress types, quilting (foam used in middle or top layers of mattresses typically fall in this range, described as softer or cushier, more nest like in nature)
30-36--------------firmer seat cushions, slightly firmer, less yielding mattresses
36-45--------------thin seat cushioning and very firm mattresses- excellent for people over 200 lbs., and typically used on base layer (support or foundation layer) for a lot of bed in a box beds as the bottom layer only.
45 and up----------shock absorbing foams, packaging foams, carpet pads, and other uses requiring ultra-firm foams. RARELY used or necessary in mattresses.
The above table should only be used as a beginning guideline. The actual ILD perceived is again going to include the top quilted outer covering and other factors, such as the design type, whether coil systems are integrated into the design, and other parameters.
Bottom line, the sweet spot, in our opinion, for a great bed, that will generally work for the typical human with no health issues, back problems, and with average time in bed at around 6-9 hours, would probably have a build out of 32-36 ILD on the bottom layer, usually 5-6 inches thick. probably a middle layer about 2-3” that has an ILD of 24-28 ILD, and would be topped off with a 2” layer of 20-16 ILD material. Look for a softly quilted cover, preferably a natural fabric, and you’ll be whittling down your options. If you can get the ingredients described above in a queen size for around $700-800, or a king for $800-1000, you’re getting a fairly good deal on a decent mattress.