Does it come down the fact that PU foam just doesn't last? Is that the essence of the aversion that people seem to have? Surely there are variations wthin the class that would allow for some to be superior to others. When assessing foam top layers or cores is there a density or an ILD to look for? Like is 2.0 lb density not enough ?

Given $600 wouldn't it be wiser to get a foam bed than a lower end S brand?  (Just for the sake of the argument)

I do wonder...

I would say that is the main complaint. I know it was for me, when a very expensive bed started sagging after only 5 weeks and I ended up sleeping in a hammock-like hole.

I don't know how long this has been going on, since the last bed I bought was a Sealy Posturpedic. That was at least 15 years ago, and that  bed still doesn't sag noticeably.

Expensive bed + very flimsy,cheap foam on top = Outrageous.

Budgy, do you know anything about the durability of the so-called "memory foams" which apparently are made from polyurethane? Are those the "viscoelastic" foams you are talking about? How is viscofoam different from regular PU foam in terms of how it is made, if you know?

Thanks very much!

By general rule of thumb polyurethane foam is mostly made out of TDI (Toluene Diisocyanate), roughly 2/3's of the weight of polyurethane is made from this chemical.  Almost all of the remaining 1/3 is made up of polyol's, alcohols that essentially act as bonding agents.  When we talk about soy based foam or 'plant based foam' they are only referring to the alcohol portion of the foam mix, or a portion of the polyol content being replaced.  It is impossible to make 100% natural polyurethane.  The other part of making polyfoam is the blowing agent used. Gases pumped/blown into the liquid to turn it into a foam, this makes up very little of the content but different blowing agents can give very different properties....on the chemistry side of things this is where memory foams vary the most from regular polyurethane foams.  

Memory foam is also much denser than typical polyfoams which generally leads to them lasting longer.  The density of foam in a pillowtop mattress can range between 0.6~2.2 lbs density (very rare to see above 1.8lbs). Memory foam ranges usually between 3~8lbs density. But it is important to note that regardless of density all PU foams do soften with use. High-quality memory foams can however provide good pressure relief and be somewhat resistant to permanent body indentations. Some people can keep a Tempur-Pedic for a long time because they don't usually permanently impress very much and some people find them to be more comfortable as they soften over time.  Others find them to soften too much and replace the beds quicker. The savvy shopper should choose their memory foam mattress based on this knowledge that they do get softer with use.

The big advantage with latex based foams is that they are more resilient, specifically natural rubber does not change dramatically with use.

Thank you very much, Budgy! Very enlightening.

So TDI is this?

http://www.epa.gov/ttnatw01/hlthef/toluene2.html

Looks like you're on solid ground to suggest to folks complaining about breathing problems and other adverse health reactions to look at what their mattresses are made of.

Personally, I won't consider buying a mattress primarily composed of PU, whatever other "comforts" they may offer 

- www.

Don't believe anything about mattress "warranties". It is difficult to get a warranty replacement on a mattress due to the body impressions because they only cover it if it's an inch or an inch and a half or something like that, something that is very unlikely to happen yet you'll still have back pain from it. Also mattress warranties are null and void if you have the slightest stain on it, even a small ink mark or tiniest of stains or dirt marks.

As Budgy or someone said, it's also the fact that pu foam does not conform to your body the way latex does, either. That said, if I was going to spend only $300-400 on a mattress, I'd probably go with HR (pu) foam and add a layer or 2 of latex on top. I was actually able to buy HR foam at a wholesale supplier for around $80 for a 4" thick piece of 34ILD foam. So then one could get a couple inches of latex for a couple hundred dollars... and yes, have a better mattress than junkly springs with junky pu foam on top.