Firmness is the second most important criteria for people shopping for a new memory foam mattress (density being the most important). Unfortunately, firmness is often misunderstood by consumers and misused by retailers. Avoid falling into the ILD trap by reading these revealing findings first:
Most of the time it's impossible to compare mattress firmness based on memory foam ILD ratings alone because:
ILD ratings tell you how firm the viscoelastic foam layer is. It doesn't tell you, however, how firm the other foam layers in the mattress are. Since firmness depends largely on the mattress core, it would not be accurate to compare ONLY the firmness of the memory foam layer (which is why foam samples are nice, but not helpful).
You can directly compare the firmness of one memory foam comfort layer to another as long as all other properties are identical: size, density, cell structure, resin quality, temperature-sensitiveness, environmental conditions. This is impossible to do.
ILD is more or less abstract. It's just a number that may be useful as a rough guideline when shopping for a visco foam mattress. This property cannot and should not substitute personal comfort as a criteria. In other words, don't buy a memory foam mattress just because it has adequate firmness if it's not comfortable.
We don't believe density alone is enough to base a $1,000+ buying decision on. Firmness, on the other hand, is not that relevant either. However, knowing how ILD works and common ILD ratings can have significant benefits:
Bottom line, we strongly suggest that you ask about density AND ILD ratings before buying a memory foam mattress, if only to show the salesperson that you are an educated shopper that they cannot risk giving false information to (or they risk losing the sale). If we were to buy a memory foam mattress today, here's the ideal memory comfort layer combination we would look for:
If you want to learn more about Indentation Load Deflection of memory foam, read this special report.
Table of Contents
A prime myth of foam mattress firmness is that density is what makes a visco mattress soft or firm. This is not true. You can have very soft or very firm high density viscoelastic foam. This is emphasized by the Polyurethane Foam Association (PFA) themselves on their website in the Density Standards and Guidelines section.
Firmness is, in fact, determined by the ILD rating (Indentation Load Deflection), a property all foams have. Industry experts actually use IFD (Indentation Force Deflection) to measure firmness, which is about the same thing. Here's how it works.
IFD/ ILD measures the amount of force, in pounds, required to indent a 50" inch disc into a 15"x15"x4" viscoelastic foam sample and make a 1" indentation. For example, 14 ILD means it would take 14 pounds to make a 1" indentation in the foam. That doesn't seem like a lot, does it?
After all, the human body weighs significantly more than that. Does that mean you will sink 2-3 inches deep in a memory foam mattress with 14-15 ILD? No, and here's why.
In order to be considered supporting and firm, ILD numbers for conventional foam mattresses range between 24-31 (medium firm). So how come ILD ratings for most viscoelastic foam mattresses on the market range between 10-16? Wouldn't that be too soft?
This significant difference between ILD ratings for memory foam versus other types of foams made us curious and eager to learn more. After some reading, here's what we can tell you. Visco memory foam ILD readings are lower, we believe, because the cell structure in memory foam is different than that of other types of foam:
Viscoelastic foam ranges between super soft 10 and semi rigid 120. The higher the rating, the firmer the feel, the lower the rating, the softer the feel. Thanks to improved weight distribution and lack of springiness, memory foam mattresses can have ILD values very close to the bottom threshold without being too soft. In fact, softer viscoelastic foam contours and relieves pressure much better than the firmer type (17 and over).
Having said that, it's important to remember that the above applies to the memory foam comfort layer laid on top of the mattress core. Viscoelastic memory foam is too soft (doesn't have enough support) to make a mattress entirely of it which is why it's mainly used as a comfort layer. The bulk of the support comes from the firm mattress core but the comfort layer does add to it to some extent. While visco foam ILD doesn't reflect mattress firmness, here's why it's important.
If the visco foam is too soft (ILD/IFD ratings of 10 and below), the mattress may no longer provide adequate support and comfort. Coupled with lack of proper density and thickness, the mattress's viscoelastic foam layer may bottom out. In other words, a poor quality comfort layer may ruin a perfectly good mattress.
We asked doctor Michael Breus, Ph.D, renowned expert in sleep disorders, featured expert on WebMD and AOL and author of Good Night: The Sleep Doctor's Guide to Better Sleep and Better Health? what he thinks of memory foam firmness. Here's what he said:
I agree that most memory foam mattresses have an ILD of 12 to 15. However, none of this helps determine the functionality / quality of visco foam. It's important to remember that there is really no such thing as a supportive visco foam. Visco-elastic foam is a cushioning foam not a support foam.
We've come across several misleading statements in our research and we're eager to explain them to our readers.
All polyurethane foams, including viscoelastic foams have this physical property. To say that a visco foam mattress has no ILD would be like saying that water is not wet. We've only come across this statement twice during our research but it's important that consumers are aware of it.
Some websites say that ILD reflects the ability of viscoelastic foam to push back against the force pressing down on it. This is not true. Unlike other types of foams, visco foam does not push back. In fact, its open-cell structure absorbs shock and relieves pressure. What it does measure is how much weight it takes to make an indentation.
We've been one of the first to embrace the idea of heat-responsive memory foam that changes its ILD at body temperature. We're now able to explain this statement more clearly. Viscoelastic foam, by nature, responds to temperature. It is viscous in colder conditions and elastic when warmer.
The temperature range memory foam responds to can be closer to what your body can generate or much higher (150 F and over, for example). Visco foams that respond to the latter give the impression that they are NOT heat-responsive which is true, in a way (as consumers, we're only interested in visco mattresses that respond to what our body can generate).
We've had different retailers and manufacturers tell us that ILD ratings do not change with temperature. We have found proof to the contrary in the Tempurature and Humidity Effects on IFD page from the Polyurethane Foam Association website.
In other words, if you measure the firmness of a memory foam mattress while you're sleeping on it, you will get a lower value. IFD readings do, in fact, change with body temperature.
Viscoelastic memory foam is too soft and unsupportive to make a mattress entirely of it which is why it's mainly used as a comfort layer. For a memory mattress made of 2 or more layers, most of the support comes from the mattress core (the thicker and firmer of the two layers).
Memory foam does, of course, add to it, which is why its physical properties are important since you don't want to sink right through that layer. Memory foam, however, is useless without a firm, supportive core of 30 ILD and over.
Firmness cannot be ignored when shopping for a memory foam mattress. Consider this scenario. You buy a luxurious, 6" thick, 5.5 lb memory foam mattress with ILD of 10. This mattress will be so soft that you will sink right through all those 6 inches, despite its high density. Take this other example.
You buy a cheaper, 4", 3lb memory foam mattress with ILD of 15. This mattress will be firm but also, less supportive due to the lower density. It may appear to be supportive in the beginning, but over time, as the open cell structure begins to crumble, the remaining firmness will not be enough to keep you up.
Firmness, however, is not, on its own, a clear indicator of mattress quality. It is just one of the several quality indicators along with density, foam core properties, thickness, temperature-sensitivity, type of cell structure and resin quality. Memory foam ILD ratings are truly relevant only when considered along with all these factors and not judged independently.
Most manufacturers and retailers fail to mention ILD ratings on their websites for 2 reasons:
We've had many readers ask us about this specific property of memory foam so we've decided to investigate and clear things up once and for all.
We've contacted 31 manufacturers and retailers telling them about our special report on memory foam mattress firmness and asked them for ILD ratings. We've also had a friend posing as a potential customer contact the same manufacturers and retailers for the same information for comparison purposes. Here is the list of companies with respective ILD ratings (sorted alphabetically by company name).
|Company Name||Foam Mattress ILD||Manufacturer/Retailer||Notes|
|A1 Mattress||11-13*||Retailer||*Sells Sleep Aid Factory mattresses 11-13 ILD firmness.|
|Absolute Comfort On Sale||15||Retailer|
|Ahh Memory Foam||10-13*||Retailer||Failed to reply
*Sells Isotonic mattresses commonly 10-13 ILD firmness.
|Back Be Nimble||13||Retailer and Mattress Manufacturer|
|Bergad||15-17||Mattress and Foam Manufacturer (Isoform memory Foam)|
|Carpenter||10-13||Foam Manufacturer (Isotonic memory foam)|
|Dormia (Abed)||14||Mattress Manufacturer|
|Flo Beds||Mattress Manufacturer||Told us "latex is the only foam measured in ILD" which is inaccurate|
|Foamex||13||Foam Manufacturer (Sensus memory foam)|
|Foam Online||12-15||Foam and Mattress Manufacturer||This range was given to our customer friend. We were given a 10-18 ILD firmness range.|
|Foam Order||13||Foam and Mattress Manufacturer|
|Foam Source||15||Retailer and Mattress Manufacturer||Uses Sensus foam which is usually found in 13 ILD firmness.|
|Go Mattress||Mattress Manufacturer||Unaware of their product's ILD|
|Healco||11-13*||Retailer||*Sells Sleep Aid Factory mattresses commonly 11-13 ILD firmness.|
|Knox Foam||9-14||Foam and Mattress Manufacturer|
|Memory Foam Canada||13 *||Mattress Manufacturer||Failed to reply
*Uses Sensus foam commonly 13 ILD firmness.
|Memory Foam Factory||10-13*||Mattress Manufacturer||Failed to reply
*Sells Isotonic mattresses commonly 10-13 ILD firmness.
|My Luxury Mattress||12-15||Retailer|
|Relax The Back||15*||Retailer||Failed to reply.
*Sells Tempur-Pedic mattresses
|Rocky Mountain Mattress||13*||Mattress Manufacturer||Failed to reply
*Uses Sensus foam which is commonly 13 ILD.
|Tempur-Pedic||15||Foam and Mattress Manufacturer (Tempur foam)|
|Sealy||Mattress Manufacturer||Failed to reply.|
|Serta||Mattress Manufacturer||Expressly declined to participate.|
|Simmons||Mattress Manufacturer||Failed to reply.|
|Sleep Aid Factory||11-13||Foam and Mattress Manufacturer|
|Sleep Innovations||12-16||Mattress and Foam Manufacturer (Novaform memory foam)|
Did we leave you out? Do you want to correct some of the information in this table?
Email us the ILD ratings of your mattress's memory foam layer and we'll publish them on this page.
As you can see from the table above, ILD for most visco foam mattresses indeed fall between 12 and 16. Tempurpedic ILD is "nearer 15" we were told. The one thing that surprised us most is how much of a difference a point on the ILD scale makes. For example, Tempur-Pedic mattresses with 14-15 ILD are sensibly firmer than viscoelastic mattresses with 13 ILD. Is this relevant?
Unfortunately, this comparison may be totally irrelevant as we'll see by the end of this report.
|We felt that, overall, manufacturers and retailers have been open and honest about the firmness of their mattresses.||However, manufacturers and retailers are not readily open towards customers. In other words, if customers don't ask, they won't provide answers to this simple, but important question out of their own free will.|
|We felt that, overall, manufacturers and retailers who did answer us gave the same information as they did to our customer friend.||One can only wonder why manufacturers and retailers are afraid or unable to share such a basic feature of their products, one which should be as clearly displayed on their website as the density of the products.|
Firmness measurements of visco elastic foam mattresses and beds can be affected by temperature, humidity and recovery rate (how quickly the foam retakes original shape).
Unfortunately, it's impossible to tell whether:
We can only assume and hope that ILD ratings for memory foams on the market today have been measured in similar conditions but there are no guarantees that that is the case.
It is also possible that retailers willingly or unwillingly provide customers with false information regarding ILD ratings (out of ignorance or out of the desire to make their product comparable with leaders like Tempur-Pedic).
Mattress firmness can also be affected by a wide range of factors including: foam sample thickness, firmness of mattress core (usually placed directly underneath the memory foam layer), any fabric that insulates it from the body, resin quality, etc.
You can see, then, that ILD rating comparison is often irrelevant.
As you can see, many companies provided an ILD range while some provided a single ILD number. Indentation Load Deflection is not an absolute property of visco elastic foam and a range (12-14 ILD for example) can indicate either of the following:
An unlikely, but still possible reason for giving a ILD range, can be the following:
Given the large number of factors that affect mattress firmness, quality and comfort, we must tell you that the ultimate test is your own personal preference. One low-risk way of finding out if this type of bedding suits you is a visco memory mattress trial.
In Touch (on Viscoelastic Memory Foam) [PDF] - One of the most informative and authoritative papers we could find on viscoelastic memory foam.
Polyurethane Foam Association - The PFA gives customers the chance to get in touch and learn more on viscoelastic and other polyurethane foams.
A Critical Eye On The Latest Mattress Technologies