What You Need To Know About Polyurethane Foam and Memory Foam

As with the previous tutorial, “Are Innersprings Good or Bad,” you can either watch the video below, which I designed to be the quickest and easiest way to absorb and digest this topic, or if you prefer to read I’ve included a slightly shortened text version of the video below.  But, I strongly recommend just watching the video and if you can make it to the very end…you’ll win a very special prize.  😛

So in this tutorial, I’ll be talking about everyone’s favorite topic – foam.  As confusing as buying a mattress can be, when you start to differentiate between all of the different types of foams out there, it can make your head spin.  Mattress stores certainly don’t want to make it easy for you, their preference is to just lightly skim over it or not talk about it at all, but I don’t think that leads people to make good choices.  I hope to give you enough knowledge so that you can make sense out of this and have a better understanding of what exactly you’re spending your money on.

So, I’ll first talk about what foam actually is and what it’s designed to do for you.  Then I’ll talk about how you can tell what makes one type of foam better than another, and finally I’ll show you the differences between regular foam and memory foam.

So when shopping for a mattress, you’ll find that there are two main types of foams used to build beds.  The first kind is regular foam, which is known as flexible polyurethane foam.  The second type is memory foam, also known as viscoelastic memory foam.

Mattresses began using foam in the 1950’s as a less expensive synthetic replacement for natural fibers like cotton batting, horsehair and wool.1

Flexible Polyurethane Foam

Flexible polyurethane foam is the most commonly used type of foam, and it’s typically used as a base layer in spring-free memory foam mattresses, or in the top layers of innerspring beds.  But what the heck is foam anyway?  What exactly is it made from?  We only ever see the finished product, not what it was made out of.  You should know what foam is actually made from because it will help you make sense out of some of the newer foams being developed.

Foam is made from two key chemicals:  A polyol and an isocyanate with water, typically this is Toluene Diisocyanate, known as TDI or more recently Methylene Diphenyl Diisocynate, known as MDI.You should be aware that polyurethane foam is derived from petroleum.  In an unreacted state, these chemicals are considered hazardous, but when completely cured and fully reacted they are considered to be inert and non-toxic, though this is still debated by some.(Note: TDI is fully reacted in the foam manufacturing process and can not be found in cured foam.)

This has lead to third party organizations to appear on the scene such as CertiPUR who provide voluntary testing for foam producers and test for volatile organic compound emissions, formaldehyde, ozone depleters, prohibited phthalates, PBDE’s, and heavy metals.

Foam producers will also incorporate other elements such as auxiliary blowing agents (or alternative blowing agents, chemical additives, or through using “forced cooling”) which can make foam softer or lighter, catalysts to change certain properties, surfactants which can help with the formation of foam cells, flame retardant additives as well as various fillers.1

By adjusting the chemical mix, producers can make hundreds of types of foams.  These components are poured and mixed together at a very high intensity and the foam reacts a lot like a loaf of bread.  The final product resulting in a tiny network of interlocking support cells.  Each cell consists of struts and windows.  This elastic cell structure is what allows foam to be compressed and recover over and over again.4

Foam can be made to feel super firm or extremely soft depending on the method used.  The final goal being to meet three general requirements, to be supportive, comfortable and durable.5

Foam Density

So how can you tell whether a certain type of foam is “good” or not?  Well, there’s a few physical properties of foam that can be measured and provide a reliable indicator of quality.  The first and main one is foam density.  Density is actually the most important measurement because it affects durability and support.  The higher the density, the longer the foam will last and withstand compression.  This is because the denser the foam, the more total cellular material there is to cushion to provide support for weight.6

Higher density foams have more total strut material to absorb weight over the long term.  So, this means that there is a direct relationship between density and durability.

Density is a measurement in pounds per cubic foot (pcf), but it’s not weight.  It’s a measurement of mass per unit volume.6

So, it’s a way to measure all the chemistry and combination of materials and “stuff” used to produce the foam.  This measurement also happens to be the one mattress manufacturers are typically least eager to openly discuss.  Technically, no brand has to reveal the foam densities they use as it can be considered proprietary, so it really says a lot about a brand that is willing to share the information when they do.  But generally, a typical range for a flexible polyurethane foam core for a mattress would be between 1.5lbs to 3.0lbs and mattress top layers would range between 1.2lbs to 1.8lbs (pcf).

Indentation Force Deflection or Indentation Load Deflection

The second measurement is the IFD, which stands for Indentation Force Deflection (also known as ILD or Indentation Load Deflection) and is a measurement of foam firmness.  This one is a bit confusing because firmness is independent of foam density.  In other words, you can make foam with a very high density feel either very firm or very soft.

A lot of people associate a high foam density with being firmer, but that’s not the case.  The IFD measurement relates to comfort because it’s a measurement of the surface feel of the foam.  Basically it doesn’t have much to do with durability, it’s strictly a measurement of the softness or hardness of a layer of foam.  In simple terms, it’s measured by indenting the foam 25% of its original height.  You won’t see this measurement used very often because it’s so difficult to describe, but you might often see the IFD or ILD number associated with certain memory foams as well as latex layers.In my opinion, trying to suss out ILD’s of polyurethane foams won’t really help you make your decision, it just doesn’t give you any real usable information.  Density is way more important when it comes to foam.

So I mentioned earlier that foam producers want the foams to be supportive, comfortable and durable.  I mentioned the direct relationship between density and durability, but what exactly does comfort and support even mean?  So, here’s a way to make sense out of those seemingly vague terms.

Just like density has a direct relationship to durability, comfort has a direct relationship to support.  So, comfort can be defined as the ability of the foam to “deflect” at the surface and conform to the body while preventing excess pressure along the body.  And support can be defined as the ability of the foam to ‘prop’ the body into a relaxed, natural alignment while preventing ‘hammocking’ into the sleep surface.

Viscoelastic Foam

All foam in mattresses are ‘open cell.’

This brings me to viscoelastic foam.  You’ll sometimes see viscoelastic foam referred to as “open-cell” foam, but it’s important to realize that both flexible polyurethane foam and viscoelastic foam are open cell.  All foam used in mattresses are open cell.  But the behavior of the cells in viscoelastic foam is very unique because they act almost like liquid, and are able to shift and reorganize when compressed.

Interestingly enough, memory foam wasn’t originally invented to be slept on.  You may have heard the story before, that NASA originally developed the material in the 70’s as a way to help absorb the G-Force astronauts have to deal with during take off and landing and so that people could better survive crashes or impacts.  However, it was never actually used in space shuttles because the first generation versions of the foam had a lot of problems.  It wasn’t until the the early 90’s that the material finally became commercially available after a swedish foam producer finally nailed the formulation and it turned out to be pretty useful material for sleep surfaces.7

In simple terms, viscoelastic foam is “slow recovery” foam, so that when weight is applied to it, the foam slowly conforms to the shape of the object, and when the weight is removed the foam will slowly go back to its normal shape.  This property is what allows it to distribute pressure over the whole surface which results in reduced pressure along the body, which is the #1 main benefit of memory foam.

So how can you tell what kind of viscoelastic foam is good or not? 

Just as higher density equates to better durability for flexible polyurethane foam, the same applies to visco foam.  Density does affect both durability and ability to conform, and generally range between 2lbs all the way up to over 7 pounds per cubic foot.  And usually the higher the density the greater the slow moving, conforming effect.

The mattress industry likes to market memory foam as if it’s the greatest invention since sliced bread, but it’s definitely not for everyone and it’s not perfect.  For example, it’s often touted as offering superior support but actually memory foam is not really that great of a support layer.  It’s much more of a comfort layer.  This is why you’ll never see a memory foam mattress made from memory foam all the way through from top to bottom, they always consist of a layer of memory foam placed over a much more supportive layer of flexible polyurethane foam or an innerspring system.

The overall firmness, support and “recovery rate” (the speed at which the foam returns to it’s uncompressed state) are also affected by temperature and humidity.  This is because viscoelastic foam is temperature sensitive material and there is a direct correlation between temperature and recovery rate.  When it’s really cold, memory foam can become very firm.  Part of what makes the foam conform and adapt to the body is that its responding to the heat from your body, and as a result the foam can collect quite a bit of heat.

Questions or Comments? Post them below!


1.  In Touch, Volume 5, Number 1, May 1995 (http://www.pfa.org/intouch/index.html)
2.  http://www.essentialchemicalindustry.org/polymers/polyurethane.html
3.  Krone, C. A. and Klingner, T. D. (2005), Isocyanates, polyurethane and childhood asthma. Pediatric Allergy and Immunology, 16: 368–379. doi: 10.1111/j. 1399-3038.2005.00295.x
4.  In Touch, Volume 4, Number 3, July 1994 (http://www.pfa.org/intouch/index.html)
5.  In Touch Volume 1, Number 1, February 1991 (http://www.pfa.org/intouch/index.html)
6.  In Touch, Volume 1, Number 2, May 1991 (http://www.pfa.org/intouch/index.html)
7.  http://en.wikipedia.org/wiki/Fagerdala_World_Foams
  • Leanne Stenerson

    I just bought a very soft polyurethane foam mattress to reduce shoulder and hip pain. It has helped my shoulders for going numb but has increased my stiff sore hips. Why is this happening. I have slept on it for 5 weeks. I want to exchange it but don’t know what I should try next. Can you help?