I think the biggest hidden info so far has been what ILD and at what thickness to use when building a mattress, especially as coil foam mattress.
For comfort layers and for complete foam mattress I have found some guidance from Flobeds and Foamite. Flobeds share exact ILD’s in their designs and foamite gives a “Ergo number” which is the weighted average of the ILD of foam layers (all foam mattress).
These are really good reference points. But I am trying to build an innerspring mattress and I wonder how the reference ILD’s used in Flobeds comfort layer can be translated to an innerspring martress?
I guess the answer to above depends on how innersprings (say the 8" combizone by L&P) generally compare to a latex core? Do innersprings have ILD’s or similar standard characteristics? something really confusing to me is L&P’s website : they say their pocket coil springs have an ILD of 609 !!! If this is the same ILD describing foam then it seems impossibly high! e.g extra firm foam is 50, so springs in way seems 10x firmer!!! Am I missing something?
Also, are there any typical designs so for an innerspring mattress or knowledge that can be shared to help us start choosing correct thickness and ILD of comfort layers?
ILD (Indentation Load Deflection) for latex is measured slightly differently from polyfoam (IFD) and different yet for innersprings. In latex world, ILD is usually measured with a 6" thick layer of foam and it’s basically the weight that it takes to compress a 50 sq ich 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 or springs ILD. There are other variables involved as well but to keep it short … latex manufacturers produce cores that are different thicknesses 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. There is more about ILD/IFD in post #6 here.
Leggett & Platt (L&P) tests for firmness and ILD using Spring Rate Analysis and an Indentation Machine (developed by L&P) that evaluates the firmness of innersprings using tensile/compression testing and also the weight that is needed to compress a spring one inch. The ILD is measured in specific areas such as the shoulder and calf zones and geometric center of the spring units. I’d regard this as an internal scale used to compare units to each other in terms of. You can read more about testing methods used by Legget & Platt here.
The equivalent to latex’s “compression modulus” for an innerspring would be “spring rate”. Because innersprings are “linear” … a single spring that has the shape of a cylinder and has even turns will have an equivalent spring rate of 2.6 (because it would be compressing 2.6 times as far). This means that the compression modulus of Talalay and the spring rate of a cylindrical spring would be similar. Dunlop has a higher compression modulus so it would get firmer faster than this same type of spring … but this is where the similarities end. Most springs have a softer section and a firmer section (which can be achieved with different shapes such as thinner and thicker diameters in parts of the spring or different gauge springs on top of each other and in other ways as well). This means that this type of spring would have multiple spring rates. You can see a graph of the response of variable rate springs here. You can also see a typical compression curve of Talalay latex here.
Unfortunately, there really is no one-size-fits-all DIY. As a general piece of advice, I would avoid trying to design a mattress based on “specs matching” and focus more on the quality, durability, and type of materials you prefer and on how they feel and perform in your testing. Learning how all the mattress specs and designs work together can take years of experience and a great deal of knowledge and a steep learning curve to be successful.
The guidelines in post #1 here are the steps I would follow and I would avoid trying to design your mattress based on specs. The specs that are important are the ones that identify the quality and durability of a material so you have a good idea of how long your mattress may last … the goal is to focus on PPP (Posture and alignment, Pressure relief, and Personal preferences) based on your personal testing and to make sure that the materials inside the mattress are good quality so they won’t soften or break down too quickly and will maintain their original properties over the longer term.
With regards to adequate thickness, this is also best left to you, it’s also important to have an understanding of the specific needs associated with your sleeping positions. We have a resource for that here. Also, you’re most likely aware of the Mattress Durability Guidelines when building your DIY.
Let me know if any questions come up in your reading!
