Hi psychmedic,
Yes, you’re totally correct, and that was my error. I was reading too fast and saw Nest and Hybrid and went to the Alexander :S . The Nest Bedding Hybrid Latex does offer an interchangeable layer of 100% Natural Dunlop latex in either a Medium or Firm layer. There is also 2" of wool quilted to the top. It would differ from the Brentwood Home Cedar in the total amount of latex, in that it is not tufted and does not have the flaxseed pads and coconut husk layer, and the difference I described earlier in the spring units (which are more similar than unalike).
It uses 2" of 4 lb of 100% Natural Dunlop on top of the 2" of the 4.7 lb 100% Natural Dunlop that is grooved into the three zones. But there is also the flaxseed layer on top of the spring unit.
ILD is usually the method used to describe latex, specifically Talalay, as there is an almost direct relationship between density and ILD in latex (which is not the case with polyfoam). It’s important to note that ILDs are ranges and not exact numbers.
ILD in latex is usually measured with a 6" thick layer of foam and it’s basically the weight that it takes to compress a 50 sq in round metal foot into the foam by 25% (which would be 1.5"). This is different from polyfoam which is measured with a layer of foam that is only 4" thick so it would only be compressed by 1" (and the ILD number would be lower than latex). So first of all latex ILD is not directly comparable to polyfoam ILD. Some latex manufacturers also produce cores that are a different thickness which would also produce a different ILD rating compared to testing ILD on a 6" core. There are also different testing protocols for ILD so in some cases the ILD would be tested at 40% compression instead of 25% compression which would also produce a different result.
Different materials including latex have a variance across the surface of the 6" core so in one place it may be say 24 ILD and in another part of the core it may be 22 or 26. the ILD rating is usually an average of the range across the surface. Blended Talalay has less variance (=/-2 or so) than Dunlop which may vary by more (+/-4 or so). 100% natural Talalay can have a larger variance than blended Talalay. You can see some of the ILD’s for different densities of Latex Green’s 100% natural Dunlop in post #2 here.
The latex particles in Dunlop settle more in manufacturing so a 3" Dunlop layer that is cut from the bottom half of a 6" Dunlop core can be firmer than a 3" layer cut from the top half and the top would be softer than the bottom of the layer while with Talalay it’s more consistent from top to bottom so it doesn’t matter as much which part of the core a thinner layer was cut from. All the layers cut from a single core will be rated the same even though they may not have exactly the same ILD.
Most people don’t sink exactly 25% into a layer and different types of foam … including latex … get firmer at different rates (this is called their compression modulus also known as sag factor or support factor) as you sink into them more. For example it takes about 3x as much weight to sink into a Talalay layer to 65% than it does to sink into it to 25% (a 24 ILD core would take about 72 lbs of force on the compressor foot to sink in 65%) but with Dunlop it takes about 4x as much weight to sink into the core 65% (which would mean that with 24 ILD Dunlop it would take about 96 lbs on the compressor foot to sink in 65%). This means that Dunlop feels firmer than Talalay for anyone that sinks into a layer more than 25%. Polyfoam would generally have a lower compression modulus than either although HR polyfoam has a compression modulus that approaches Talalay latex.
Talalay is usually accurately measured or at least it’s within a fairly narrow range of the listed ILD but I have seen many Dunlop layers clearly mislisted in terms of the rated ILD (usually rated as being softer than they really are). Some of this is because they use a different system in some places. For example see the Latex Green site for example where they show KGF (kilograms of force) which some people seem to list as ILD when it is much different and lower than the actual ILD. Some Dunlop manufacturers only sell their Dunlop by density and “word ratings” rather than ILD. With Dunlop it’s probably more accurate to compare by its density than by it’s ILD unless someone has specifically measured the ILD somewhere along the supply chain. Of course to make a rough Dunlop to Talalay comparison you would need the accurate ILD of both and then you could assume that the Dunlop will feel firmer if they are roughly the same ILD.
So the bottom line is that Talalay is fairly consistently rated (especially blended Talalay) and would be roughly comparable between manufacturers and suppliers. Blended Talalay is the most consistent and 100% natural Talalay would have a wider ILD range. Talalay is often measured in 9 - 15 different places on the core and then averaged and rated by the “official” rating that the average is closest to.
Dunlop ILD on the other hand would likely be more correctly rated in a range by better manufacturers or suppliers, especially those that sell both Talalay and Dunlop, but may often be mis-rated by others. For example if you see one layer of Dunlop that is 85 kg/m3 density and rated at low 20’s ILD range and another one at the same density that is listed at mid 30’s ILD range you know one of them is probably wrong (and probably the lower one). The accuracy of the rating depends on the source and their knowledge of latex foam and on whether the ILD is a real measurement or just a guess or “mistranslation” from other methods. Dunlop latex that is accurately rated as being in a certain range (it’s never a single number in reality) will also feel firmer to most people than Talalay of the same ILD range unless you only sink into the layer exactly 25%.
In addition to all of this … the “word” ratings can vary widely between different sources and different people have different ideas of what firm or soft is so I would take these with a grain of salt. Either ILD in Talalay or ILD (if it’s accurate) or density for Dunlop would probably be the best way to compare relative firmness between different Dunlop layers and sources if they are the same type and blend of latex. If two layers are a different type or blend of latex then ILD ratings may not be comparable between different layers.
So the best way to get a more accurate idea of the relative ILD’s is to deal with better manufacturers or sources who will usually list their latex more accurately.
In the Brentwood Home Cedar, as listed you would have what more people would rate as a quite soft latex layer upon a medium latex layer. But remember, all of the layers of a mattress work together to provide overall comfort.
Phoenix