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Foam and durability 08 May 2016 09:55 #1

Let's be up front about this. The main reason I signed up with Mattress Underground is I want the discount. I can't justify the expense of a new mattress until July, but it has not stopped me from researching. This site is the go to place for that.

Because of my budget, I have pretty much at this point narrowed down my choice down to the Ghost Bed and the latex Brooklyn Bed. It is hard to argue against the Brooklyn Bed, but the Ghost Bed consistently gets such great reviews.

But what I want to discuss is foam density and durability. What does pounds per square feet mean?. There has to be depth, otherwise you simply can't equate it to weight. I think it is one foot by one foot by one inch. But is a four pounds per square feet foam twice as durable as two pounds? I doubt it. I think it it is more likely a logarithmic function.. But it turns out there is no simple calculation to answer that..

Density has absolutely nothing to do with quality. The major durability issue with foam is softening over time, often called flex fatigue. But there is no measurement for this. All we have is statistics. The probability of in-use softening is greater with lower density foams than denser foams. But how much? It is just as easy to err when making a high density foam as it is when making a lower density foam. It is only probable that higher density foams will be more durable than low density foams.

The magnitude of the degree of probability is not yet known, and, presently, only a statistical trend exists

This all makes it hard to compare apples to apples. A three pound foam made with high production specification can be more durable than a cheap foam made in China. And once you start adding stuff like gel in the foam .. all comparisons are out the window..

Anyway, yesterday I took my wife to a mattress store and had here try innerspring, viscoelastic, and latex mattresses. It turns out she is not as picky as I thought. I tried them out too, just to make sure that as a side sleeper, I would not have a problem with the firmer latex. I didn't..

I don't know. maybe I could get away with the cheaper alternative of the Tuft and Needle or even Arctic Cloud mattress. Lisa did not hate the viscoelastic beds,. I do sleep hot though.. I know no one can make this decision for me, but any thought would be appreciated. :-).

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Foam and durability 08 May 2016 11:06 #2

Hi Jimmie,

I'm not sure what you've read since you found the site but just in case you haven't read it yet ... the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice ... and perhaps more importantly know how and why to avoid the worst ones.

Two of the most important links in the tutorial that I would especially make sure you've read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best "match" for you in terms of "comfort", firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn't turn out as well as you hoped for and post #13 here which has more about the most important parts of the "value" of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you.

It is hard to argue against the Brooklyn Bed, but the Ghost Bed consistently gets such great reviews.


I would keep in mind that while other people's comments about the knowledge and service of a particular business can certainly be very helpful, you are the only one that can feel what you feel on a mattress and I would be cautious about about using anyone else's suggestions, experiences or reviews on a specific mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you. In many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words ... reviews or other people's experiences in general won't tell you much if anything about the suitability, quality, durability, or "value" of a mattress for any particular person (see post #13 here ).

But what I want to discuss is foam density and durability. What does pounds per square feet mean?. There has to be depth, otherwise you simply can't equate it to weight. I think it is one foot by one foot by one inch.


Density is defined as weight per unit volume and in North America it's usually expressed as pounds per cubic foot (not square feet which is a surface area not a volume). In other countries it is often expressed as kilograms per cubic meter.

But is a four pounds per square feet foam twice as durable as two pounds? I doubt it. I think it it is more likely a logarithmic function.. But it turns out there is no simple calculation to answer that.


You're right there is no simple calculation because mattresses generally use multiple layers of different materials. With polyfoam and memory foam it's similar to a reverse exponential curve with a steeper curve at the lowest densities and with a flattened curve at higher densities. Any type of blend of latex would be a durable material relative to the two other types of foam.

Density has absolutely nothing to do with quality.


That depends entirely on how you are defining quality. On this site I use quality as being most closely related to durability and with memory foam and polyfoam density is the single most important (although not the only) variable involved in durability.

The major durability issue with foam is softening over time, often called flex fatigue. But there is no measurement for this. All we have is statistics. The probability of in-use softening is greater with lower density foams than denser foams. But how much? It is just as easy to err when making a high density foam as it is when making a lower density foam. It is only probable that higher density foams will be more durable than low density foams.


You are correct that loss of firmness is a much bigger durability issue than loss of height and is the biggest reason that a mattress will lose the comfort and/or support that was the reason you purchased the mattress in the first place. Unfortunately the loss of firmness (virtual impressions) isn't covered by mattress warranties which only cover defects in a mattress and defects are generally defined as a loss of height (visible impressions) that is more than the warranty exclusion (generally .75" to 2" depending on the mattress and the manufacturer).

While there is no way to specifically quantify how long any mattress will last for a specific person or predict exactly when they will decide to replace it because it is no longer suitable or comfortable for them (because this is the only real measure of durability or the useful life of a mattress that really matters) and because there are too many unknowns and variables involved that are unique to each person ... if a mattress is well inside a suitable comfort/support range and isn't close to the edge of being too soft when it is new (see post #2 here ) and you have confirmed that it meets the minimum quality/durability specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer and the chances that you would have additional "bonus time" would be higher as well.

There is also more detailed information about the variables that can affect the durability and useful life of a mattress (and the materials inside it) in post #4 here and the posts it links to as well.

The probability of in-use softening is greater with lower density foams than denser foams. But how much? It is just as easy to err when making a high density foam as it is when making a lower density foam. It is only probable that higher density foams will be more durable than low density foams.


Because there are many variables involved ... the goal is to shift the odds in your favor as much as possible. In other words it's an exercise in risk management rather than a specific calculation. A good analogy would be playing poker where you ask someone "will I win" if you have AA in the hole and the answer would be "the odds are highly in your favor" and if you have 2-7 offsuit in the hole the answer would be "the odds are highly against you". In either case the final outcome may not be what the odds "point to" although the odds will eventually catch up to you if you continuously play low probability hands (or buy mattresses with lower quality and less durable materials).

This all makes it hard to compare apples to apples. A three pound foam made with high production specification can be more durable than a cheap foam made in China. And once you start adding stuff like gel in the foam .. all comparisons are out the window..


I would agree that there is more uncertainty and a higher risk with foams that are made in China (see post #6 here ).

There is also more about gel foam materials in post #2 here but I would use density as the most meaningful comparison with gel foams as well.

I don't know. maybe I could get away with the cheaper alternative of the Tuft and Needle or even Arctic Cloud mattress. Lisa did not hate the viscoelastic beds,. I do sleep hot though.. I know no one can make this decision for me, but any thought would be appreciated. :-).


Assuming that the materials and components in a mattress you are considering are durable enough for your body type and meet the quality/durability guidelines here relative to your weight range ... the choice between different types and combinations of materials and components or different types of mattresses are more of a preference and a budget choice than a "better/worse" choice (see this article ). The best way to know which type of materials or which type of mattresses you tend to prefer in general terms will be based on your own careful testing or your own personal experience because different people can have very different preferences.

In its simplest form choosing the "best possible" mattress for any particular person really comes down to FIRST finding a few knowledgeable and transparent retailers and/or manufacturers (either locally or online) that sell the types of mattresses that you are most interested in that are in a budget range you are comfortable with and that you have confirmed will provide you with the all the information you need about the materials and components inside the mattresses they sell so you will be able to make informed choices and meaningful comparisons between mattresses and then ...

1. Careful testing (hopefully using the testing guidelines in the tutorial) to make sure that a mattress is a good match for you in terms of "comfort", firmness, and PPP ... and/or that you are comfortable with the options you have available to return, exchange, or "fine tune" the mattress and any costs involved if you can't test a mattress in person or aren't confident that your mattress is a suitable choice.

2. Checking to make sure that there are no lower quality materials or weak links in a mattress you are considering relative to your weight range that could compromise the durability and useful life of the mattress.

3. Comparing your finalists for "value" based on #1 and #2 and all the other parts of your personal value equation that are most important to you.

Phoenix
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