In these posts I will be talking about materials, ergonomics, as well as construction methods for bedding, for the purpose of both selecting, as well as DYI application. Please understand that this is not intended to be a substitute for the highly important aspect of subjective testing of a mattresses. So contributions stating this fact will be superfluous to the intended conversation. I’m also aware that some of the information provided will be redundant to some people existing knowledge.
I hope to here of others thoughts on the topic!
The following is quoted from the book, (Polyurethane Handbook, by Oertel):
Density and hardness are used for adults. The density should not be low. A grade that is to light leads to premature softening in use. This results in a larger deflection, which decreases air and humidity transport. The registered trade association for the mattress industry in Germany stipulates that 2lb/ft3, as the minimum density.
PU foam is used as a minimum for especially high requirement applications. A minimum density of 2.3lb/ft3, a maximum decrease in compression hardness of 25%, and a maximum decrease thickness of 5% are specified.
What is PU foam? What does “compression hardness and decrease thickness” mean?
The combination of foam slabs of higher density and hardness as, for example, bonded material, with soft foam types of higher resiliency on both sides gives a product an almost ideal force-deflection curve, which combines high comfort with good durability.
Another type is the reversible mattress. In this construction one grade of foam is used on one face of the mattress, while a foam of different hardness is used on the other. An added improvement in the force-deflection curve is achieved if cavities is provided in the interior of the mattress.
The foundation foam should be as firm as possible.
In North America … compression hardness is measured by a material’s Indentation Force Deflection which is a measure of the firmness of the material. With latex it’s usually called ILD (Indentation Load Deflection) and is measured slightly differently from polyfoam. There is more about ILD/IFD in post #6 here.
All foam materials will soften and break down over time and this results in both the loss of height (decrease thickness) and loss of firmness (ILD/IFD). The foam will soften and the ILD/IFD will become lower under the heavier parts of the body particularly where weight is more concentrated. Mattress warranties cover loss of height (visible impressions in a mattress) that are greater than the warranty exclusion (usually from .75" to 2" or sometimes more depending on the mattress design and the quality or the materials) but they don’t cover the loss of firmness in the materials or “virtual impressions” which can only be felt but not seen. These virtual impressions from foam softening but without significant loss of height leads to the gradual or with lower quality materials the more rapid loss of suitable comfort and/or support in a mattress. This loss of comfort and support is the main reason that most people will need to replace their mattress. There is more about the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here and the other posts it links to.
I am also looking for information about compression variations of using different weight loads on the same foam.
Edit: I suppose “compression hardness is measured by a material’s Indentation Force Deflection which is a measure of the firmness of the material.” is what I’m looking for here. But does this include compression thickness?
IFD or ILD can be measured at any percentage of compression or with any thickness of material but with polyfoam the “standard” in North America is 25% compression.
Compression modulus numbers are also usually available to mattress manufacturers which is the ratio of the force needed to compress a material by 65% divided by the force needed to compress the same material by 25% of its thickness and compression modulus (AKA support factor, sag factor, or comfort factor) is also one of the criteria that is used to differentiate HR polyfoam from conventional polyfoam (see here).
Some areas of the world use “standard” IFD testing at 40% instead of 25% which would of course produce completely different numbers and some materials (such as latex) are typically tested on a full core which is generally 6" thick rather than a 4" thick piece of material which would also produce different results because the thickness of the material being tested will have a significant effect on its ILD/IFD ratings.