The amount of weight or pressure required to compress the mattress to that extent is the ILD. Both measure the force it takes to compress a foam latex mattress by a measurable amount. With latex, "there's a straight-line relationship between ILD and density", according to Lynn Holt, latex expert. In other words, denser mattresses are firmer and thus have a higher ILD, while less dense mattresses tend to be less firm and thus have a lower ILD.
In the foam mattress industry, the standard "density" measurement is translated as the weight of a 12 inch x 12 inch x 12 inch block of a particular foam. A higher density measure corresponds to a higher ILD. If all foam mattresses are measured using ILD or density, does that mean all foam mattresses are the same?
Some companies would have you believe so, but natural botanical latex possesses properties that synthetic materials, even those with a similar ILD, simply do not have. Botanical latex mattresses have a much quicker recovery time than memory foam.
Memory foam very slowly regains its shape, while latex springs back immediately. Latex foam actually bounces — the same properties that make a rubber ball bouncy apply to the foam as well.
The same doesn't hold true for memory foam. Here's what he said:. 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. 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 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. 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. 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 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. 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. Did we leave you out? To learn more about memory foam, checking out our Memory Foam Buying Guide.
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