Mattress Forum

Phoenix

PS: If you can tell a difference between the two dunlops as well, I would put the firmest one in the middle (closest to you). Result may be marginally better.

Lets say the average body length from a spot a few inches above the knees to the top of the shoulders (where almost all the weight is) is about 36" (I'm about 40"  and 6'5" so probably more than most). I would guess that this included about 85% of the body weight. Lets also say that the "average" width of the body in a side profile is about 12" and that an average weight is about 180 (male). I know these numbers are probably not really accurate but they're close enough for this. This means that this 36" bodylength would weigh about 153 lbs (excluding head and legs from just above the knees to the feet)

This would also mean that the number of square inches in this side profile would be 12 x 36 = 432 (assuming this was all flat which of course it isn't). If every point of this area was being supported equally, then the average pressure would be 153 lbs / 432 sq in = .35 psi.

To qualify as a "pressure reducing support surface", a mattress needs to reduce pressure to below 32 mmHg (also called a Torr) on most bony prominences most of the time. To qualify as a "pressure relieving support surface" it has to reduce pressure to below 32 mmHg (Torr) on all bony prominences all of the time. This is also not quite accurate as fixed numbers like this are not considered to be completely accurate or valid anymore but again for the sake of this post it will do. 32 Torr = .62 psi so if the weight was distributed equally throughout the entire side profile ... there would be no detectable pressure anywhere.

Very low levels of pressure relief (levels below most very sensitive people's detection threshold) would be in the low 20's a good part of the time so lets say 23 Torr or .44 psi

So lets say that 60% of this weight (92lbs) was concentrated in the hips/lower body and 40% (61 lbs) in the shoulders/upper body (of course weight distribution varies here).

Lets say that the hips sank into a mattress' upper layers enough to distribute this weight over an area of 12" x 12" sleeping on the side.

This would mean that there was 92 lbs spread over 144 sq in or .64 psi ... right at the level of pressure relief for most people

If the hips only sank into the upper layers enough to distribute pressure over an area of 6" x 6" (too firm or thin upper layers) it would be 92 lbs spread over 36 sq in or 2.56 psi ... which for many people would equal pain.

If there was enough sinking in in the upper layers to distribute this weight over an area of 12" x 18" (say mid thigh to lower waist) it would be 92 lbs spread over 216 sq in or .43 psi ... which is well below detectable for pretty much everyone.

So all you would have to do in this hypothical case would be to have upper layers just soft and thick enough to let someone sink in just enough that their weight was spread over an area just large enough to bring the pressure to a level below their individual threshhold. This is different for different people. For almost everyone though, a larger 12 x 18 area (upper thigh, hips and some waist/lumbar) would give complete pressure relief. This sinking in can be into a thicker comfort layer or it can use a slightly thinner comfort layer and the top part of the next layer. Either way the pressure relief could be identical.

I know these numbers are not accurate because the body is rounded and doesn't respond exactly to a "formula" but it shows how "sinking in" to a "comforming" (point elastic) upper layer can spread weight over a larger area and relieve pressure to levels below detectable.

The same general idea and "rough math" would apply to the shoulder area.

All of this is also for the most difficult challenge of side sleeping. On the back you would have probably closer to 24" (twice as wide) x 36" surface area which is 864 sq in and if even only half of this surface was supporting all the weight you would have 153 / 432 = .35 psi which would not present a pressure problem for anyone.

The same sinking in that relieves pressure should also offer more gentle support (fill in the gaps) in the waist (on the side) or lumbar (on the back) so this is normally looked after as a "side effect" of dealing with pressure distribution issues. Only people who were completely insensitive to pressure and didn't want to sink in at all (liked sleeping on a floor) would likely have lumbar support issues except in cases where a much higher percentage of their weight was "in the middle" or they were too light and/or thin to sink in enough to "fill in the gaps" in which case other options could be used to create firmer lumbar support (reverse zoning for example).

So if you can create enough sinking in to distribute pressure points over a wide enough surface area using comfort layers (and the upper part of middle layers if necessary) and deal with lumbar support, the only real remaining issues are controlling the depth of additional sinking (non comforming) in the middle and lower parts of the mattress to keep the body in alignment. The amount of sinking in allowed by the upper layers controls pressure relief. The amount of sinking in allowed by lower layers has very little effect on pressure relief (it is not comforming) but does control alignment. This is why when people change the firmness of upper layers to solve support issues they will often create new problems and will often not solve their support issues.

This is where choices come in to use mainly upper layers for conforming pressure relief (softer/thicker with firmer under) or using upper layers and the top part of the middle layers for conforming pressure relief (softer/thinner with slightly less firm under). The first method leads to less (non pressure relieving) sinking in of the heavier "bony prominences" in the deeper layers if necessary for alignment. The second method leads to more (non pressure relieving) sinking in of the heavier "bony prominences" in the deeper layers if necessary for alignment. In either case the pressure relief can be the same, what changes is the alignment that gives the pressure relief.

If all of this can be done on the side for side sleepers, then the much simpler issues involved in back sleeping (mostly stopping hammocking and supporting the lumbar as pressure would rarely be an issue in this position) would be relatively easy to solve using the middle and lower layers.

All of this is because I was curious to see if I could apply some "rough math" to some of what we're doing here and see how close it came. It seems that these examples at least are relatively close to what can actually happen.

Now back to real life (laughing).

Phoenix

In one sentence give me the summary, LOL, of what I can learn because I am still struggling with side hip sleeping pain.  If you want to post it to another thread, go ahead, but I had to reply to your math posting, LOL.  I want to learn, but I can't understand

[QUOTE:]

This is where choices come in to use mainly upper layers for conforming pressure relief (softer/thicker with firmer under) or using upper layers and the top part of the middle layers for conforming pressure relief (softer/thinner with slightly less firm under). The first method leads to less (non pressure relieving) sinking in of the heavier "bony prominences" in the deeper layers if necessary for alignment. The second method leads to more (non pressure relieving) sinking in of the heavier "bony prominences" in the deeper layers if necessary for alignment. In either case the pressure relief can be the same, what changes is the alignment that gives the pressure relief.

So it seems like for most people you can have 3-4" of comfort material over 3-4" of support material. So why do most mattresses have 3 or even 4 layers (9-12"). Do the 3rd and 4th layers really do anything? There is obviously less compression the deeper you go, how different is 2 layers (say 22 over 28) on wood vs those same 2 layers on a firm core (say 40 ILD) or over another soft 22 layer (like you have)?

OK OK... but it wont have any reasons why

"You can control pressure issues by creating a deep enough cradle to spread out the weight (lowering psi pressure on different parts of the body) and the different methods of doing this are how you control alignment at the same time".

Does that work?

Phoenix

Much, much better Phoenix; but HOW do you control the pressure issues by creating a deep enough cradle?   My husband sounds just like you, LOL.  I have to tell him (nicely of course) you take too long too explain something, just TELL ME!

The trick is figuring out what density (ILD) to put on the top layer over the underlying layer (bla bla bla)........  Just TELL ME!
 

If you had someone that didn't like sinking in and they were fine in terms of pressure relief with a 1" or 2" layer on top and firmer underneath, they may not sink in quite as far as they need to for alignment. In this case you could put a softer 1 or 2" layer under a firmer layer to create alignment without affecting how far they sank into the upper layers (their cradle). Subjective feel can play a really big part in how someone likes their mattress and in some cases it can even affect their perception of pressure or alignment. If a softer layer is under a firmer one, it will start to compress "first" before the one above it (this is not black and white as the very top of the firmer layer would compress first) and it can change how the bed responds and feels quite dramatically in some cases without directly affecting pressure relief.

All of this is not quite so "black and white" of course since some layers ... especially in the middle ... play multiple roles but in general you need thicker support layers than comfort layers. Thicker layers also have more "flexibility" as they have a bigger "range" of useful responses. For example 4" firm under 2" soft may work well for someone in most of their sleeping positions but may not "have enough left" to deal with say side sleeping or unusual positions or movement. 2-3" in the comfort layer is "typical" and that with one or more support layers totalling around 6" is usually fine to accomplish what most people need. Beyond this you are usually dealing with either "feel" or unusual issues.

Mattresses in general are usually thicker than they need to be. The Europeans have "figured this out" more than North Americans.

Phoenix

PS: I should add that my mattress was 2 sided so it would always add up to being thicker than a 1 sided mattress. If I had gone one sided it would have been much different and thinner.

LOL. If we take this back to the other thread then I can "hopefully" make a few suggestions without "confusing" things here (I get confused REALLY easily with "multiple" threads :)). Part of the difficulty is that it's not just ILD but layer thickness and "interaction" that makes a difference and we have to do the "best we can" with the layers you have. Can you let me know what is happening with your current configuration?

Phoenix

What would you have done if you went 1-sided? Just curious...

I was bothered last nite by a lump in the middle of the mattress and so I opened it up and sure enough the convo foam had bunched up inside the cover. So I decided to take it out (reducing foam compression even more, hoping to make it a little softer) and, upon inspection, I can see that it is compressing where we sleep, and firmer/higher in the middle and on the edges. I wasn't really sure what to do so I zipped the mattress cover with just the sleepez foam in it and put the convo foam on top. Then, since I don't have a cover for it, and to maybe hide the unevenness a bit, I put my thin (1") wool topper over it. So thats how it is right now, but I can still see and feel the compressions in the convo foam. And I still don't know if it feels soft enough (definitely not near that pillow top experience)...