What are the Differences Between Memory Foams?

Memory foam refers only to the “viscoelastic” behaviour exhibited by the foam. Memory foams can encompass a wide range of quality, durability, comfort, and other characteristics. Here’s a few things you should look for when considering the purchase of a memory foam product.

Density

This is the most reported memory foam specification, and while it isn’t perfect, it’s the one you
should look at most.

Density refers to the weight of a given volume. For instance, higher quality memory foams weigh
approximately 5 pounds per cubic foot (80 kg/m3) of volume or more. The reason density is a relatively good indicator is for two reasons:


First, memory foam density is typically increased by increasing the amount of additives to the foam mixture. These additives usually give it it’s “memory” effect. The higher the density, the better and more obvious this “memory” effect appears. Low density memory foams will deform quickly, and rebound equally as speedily. This means they will tend to feel more like “regular” foam and less like “memory” foam.

Secondly, increased density generally correlates with improved mattress life. Higher density foams will tend to last longer, and will not “collapse” or form dents where you sleep.

Note that softness was not mentioned! Remember, styrofoam has an extremely low density but would not be comfortable to sleep on. Lower density foams may be sold as "softer" but what they really are is simply a cheaper option for the manufacturer.

Be careful! Some unscrupulous dealers will put large meaningless numbers on their product which you may mistake for the density. Others may report the density as “pounds per TWO cubic feet” which has the nice effect of doubling the final number – but is a shameless attempt to deceive you. It is easy to check this – simply calculate the volume of your mattress, multiply by the density, and it should be quite close to the shipping weight. If it’s off significantly, this is a warning sign!

For instance, a 4″ thick queen mattress is 202 cm by 152 cm by 10 cm (yes, I’m from Canada). This gives us a total volume of 307,040 cubic centimeters, or ~10.8 cubic feet. If we assume it is made of 5lb/ft3 (80 kg/m3) memory foam, this means it should have a shipping weight of approximately 54 lbs (25 kg). A little bit either way shouldn’t be an issue, but significant deviations from this should be a warning sign.

I personally would not reccommend anything less than a density of five pounds per cubic foot. While this does eliminate a large number of products, it is still quite possible to obtain reasonably priced memory foam at this quality. This is a similar density to that used by high end memory foam beds (typically greater than 5lb/ft3 (80 kg/m3)).

Indentation Load Deflection

Specifications like this give engineers a bad name. While a valiant attempt at quantifying how “soft” a mattress is, the extraordinary variation in mattress composition and thickness makes it very difficult for a single number to accurately reflect a general quality like “softness” or “comfort”. Even if these numbers are produced, it then becomes difficult for the average person to say what number they prefer, and accurately compare between mattresses. It’s an extraordinary precise number that manages to be nearly useless from a consumer standpoint due to it’s precision. As such, the ILD is largely a quality control measure, more valuable to the manufacturer than the consumer.

ild

The test itself uses an “indenter”, shown on the right. This typically has a contact area of 50 square inches. The test involves pressing this indenter a specified distance into the foam, then measuring the force required to do so. A typical test is 25% deflection for a 4 inch sample, which means that the sample is compresses from 4 to 3 inches and force measured.

I don’t put much weight on this number for two main reasons. First, comparing ILD values is really only useful for foams with similar properties and manufacturing. Our entire intent is to compare between manufacturers, so this means ILD will give us little information.

This leads into my second point – it’s an extraordinarly limited test, initially designed for classic “elastic” foams. The major characteristic of memory foams is their viscoelasticity, which means that its response to load varies over time. The ILD test does not take time into account, which is fine for conventional “elastic” foams but completely misses the point of viscoelastic memory foams. As such, I do not view ILD as a useful “consumer level” metric. Choose based on comfort reviews and testimonials, not a single number.

Other Numbers

There’s a ton of different ways to measure foam, and as a result there’s a ton of numbers thrown around. They mean very little unless you are a chemical engineer dealing with the manufacture of these foams. The major measurement a consumer should pay attention to is the density, and following that reviews and testimonials on comfort.

Nerd alert: there are two valuable tests (creep and stress relaxation) typically done on viscoelastic materials that I haven’t bumped into for memory foams. If anyone has any quality data in this regard please contact me or post in the comments below.

About the Author

lives and works in Ontario, Canada.

2 Responses to “What are the Differences Between Memory Foams?”

  1. Geoff – Thank you so much. This is the best, well-written, clear & concise information I’ve found on memory foam. I’m likely going to buy a Bobopedic (queen size mattress is $800, w/a 4″, 5.3 lb density top layer) but now I have some clear guidelines & good questions to ask (density of base layer for 1) before I lay down the buckaroos. You have been a huge help. Maybe I will build my own! Thanks again. -Pat

  2. I impact test foams on a regular basis and you are correct about the ILD number. We also use deflection and rebound to determine memory foam properties and these seem to help but when you start layering with different materials all these specs go out the window.

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