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Putting the layers together - differential 29 Nov 1999 16:00 #1

An Introduction to Differential Construction

Pressure relief and differential construction.

The common factor in all differential constructions is that they use a comfort layer that is THICKER than the recessed areas or "gaps" in a sleeping profile. If for example you need a pressure relieving cradle that is 3" deep (average side sleeper), then a differential construction could have a comfort layer that was at least 3" thick and usually a little more. In this type of construction, the comfort layer is designed to do all the work of forming a pressure relieving cradle and supporting the recessed lumbar area and the support layers below it are only used to keep the spine in alignment by preventing any further sinking down by the heavier parts of the body. This means that in this construction, a comfort layer needs to have all the qualities that are necessary by itself as it does not “borrow” qualities from the layer below it. This means that for pressure relief, it needs enough softness and point elasticity, and for lumbar support it needs a higher sag factor and resiliency. In other words, if you needed a 3” cradle for good pressure relief then you would use a comfort layer that was about 3” - 3.5” thick. If you needed a 2” cradle for pressure relief, then you would likely choose a comfort layer that was 2” – 2.5” thick. The comfort layer would always be at least as thick as the cradle that you need and usually a little thicker.

The advantages and disadvantages to mattresses with differential construction:

 

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Putting the layers together - differential 25 May 2018 09:42 #2

Hello all,

I would appreciate someone's input on a DIY mattress I am in the process of making for my office. This will be more of a differential build and is a twin size which I used 6 inches HR foam for the base, 34 ILD-2.6 pound, from foamorder.com. The foam seems to be of excellent quality and has a much different feel than the HD foams I have tested( more rubbery feel and bouncier). It's a very supportive foam for me( 5'10", 145 pound side sleeper), too much so is the issue I have been having, even with top comfort layer . On top of this 6 inches I put 2 inches of 19 ILD blended talalay, the mattress is STILL firm but more tolerable. I am thinking I need either an additional 2 or 3 inch topper on top of this yet, to feel fully comfortable as a side sleeper. I know I don't want this mattress to have memory foam in it. As I see it my acceptable options are another 2-3 inch 19 ILD blended talalay latex, or some kind of HD-HR foam.

This is what I have found so far:
Tuft and needle has a 2.9 pound foam topper with cover in 2 inch for $120 in twin, but they can't disclose the ILD to me, just the pound rating. They can only tell me it feels a medium soft and couldn't tell me if it was HR foam or not, just that it's their proprietary foam. They do have a return policy for 100% refund but it would be a bit of a gamble as far as what it would feel like.

Foamorder.com has a 3.2 pound HR foam called Everflex V24, 24 ILD, and they claim it has durability in the 15 year range. That is 100 dollars in twin. I am not sure if 24 ILD HR foam is an acceptable ILD for a comfort layer, since HR is more supoprtive. I assume it would be less pressure relieving than HD or talalay?

Then there is latex in the 19-24 ILD range, which could work in 2 or 3 inch. I have tried 28 and 14 in the past, for comfort layers, but prefer 19 to 24. I wonder if stacking 3 inches of 19 ILD blended talalay would be too soft over the already existing 2 inch layer. But this is a very firm bed due to the HR foam base being so supportive, maybe I could get away with it.

Those are my 3 options as I see it, but I am just wondering if someone could give me their input on these 3 foams or know of another foam which I don't know about. Would 5 inches of plush over 6 inches firm support be in the realm of normal? I realize I am making up for comfort in the top layers because this HR foam is more firm than I bargained for.
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Putting the layers together - differential 25 May 2018 13:07 #3

Have you already adhered the latex permanently to the HR core? If not, a transition layer of about 2" would seem to be what you need. You are on the right track with the ~19 ILD for that. I would look into getting a piece of Energex foam. They make it in 2.5" 20ILD. It is a really good transition layer.
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Putting the layers together - differential 25 May 2018 13:17 #4

Thank you very much for the reply. Nothing is glued together, just free floating foam layers in a mattress cover. Energex was something I thought sounded intriguing, but I have no idea where to get it. Where could a person get 2.5 inches of 20 ILD?
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Putting the layers together - differential 25 May 2018 13:38 #5

I have not looked where to buy it. We use it in our factory. If you search for Energex Topper I would imagine something would come up. If not, let me know and I may be able to help.
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Putting the layers together - differential 25 May 2018 13:48 #6

I did do a pretty extensive search and all I found was a lot of companies using the Energex name, even some memory foams. To be clear, I do not want memory foam in any way. I would want to order from a reputable company which carries the real energex.
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Putting the layers together - differential 25 May 2018 13:57 #7

Understood. Energex is marketed as having the pressure relieving qualities of memory foam with the durability of Latex. It is really just a high quality polyfoam that is best as a transition layer (in my opinion). If you want me to send you a piece, I would be happy to. Just message me.
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Putting the layers together - differential 25 May 2018 14:25 #8

I sent you a message. Thanks for the opportunity.
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Putting the layers together - differential 08 Aug 2018 00:22 #9

I am not entirely sure this is the correct thread to ask this question, but i will post my query anyway.

I am somewhat a voracious reader of this site over an extended period of time.

I want to know if i am correct in my observation that 2 layers of identical polyfoam combined give better cushioning and support than a single piece(of the same material) whose thickness is equal to the combined thickness of two pieces.

I found mattress made of two pieces of 2.5 lb density polyfoam of 2 inch thickness each has completely different experience compared to a single slab of 4 inch thick foam.

Am i wrong in my observation/reasoning that the first two inch layer behaves somewhat independently from the bottom 2 inch layer ?.
Thanks for giving me the opportunity to post the query.
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Putting the layers together - differential 09 Aug 2018 00:24 #10

Hi Rtved.

I am a bit of a bookworm myself and I am glad to have company! ...Welcome to our Mattress Forum! :)

I can certainly see that you've been doing a bit of avid reading on our site as you are asking good questions.

I want to know if i am correct in my observation that 2 layers of identical polyfoam combined give better cushioning and support than a single piece(of the same material) whose thickness is equal to the combined thickness of two pieces. …Am i wrong in my observation/reasoning that the first two inch layer behaves somewhat independently from the bottom 2 inch layer ?.


You are correct that the two 2” layer would respond a bit more independently from each other but using two 2" slabs instead a single 4" solid piece of the same foam type (that is exactly the same ILD and density) would make little if any practical difference. In theory the 2 separate sabs would respond independently and they could feel a little softer than a one-piece slab of the same thickness but most people can’t feel any difference at all provided of course that both foams are the same type (density and ILD) and that there aren’t any cutouts or surface modifications such as the layers being convoluted. You can read a little more about some of the considerations of using multiple layering vs one slab in post #2 here.

All the layers of a mattress compress simultaneously, not sequentially, and they will each compress to different percentages of their thickness depending on their position within the mattress, the firmness of each layer, the compression modulus of the material, the thickness of each layer, and the compression force that they are exposed to. Post #4 here has a bit more about the different specs that can affect how soft or firm a foam feels. The two most important of these would be the ILD and the compression modulus although there are also other foam properties that will affect the "feel" of a foam material as well outside of just its softness/firmness that are more subjective.

I hope this gives you a bit more food for thought and satisfies your reading appetite.
Let us know if you have more "queries" and I or any of the Expert members will be glad to assist you.

Phoenix
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