I have a few questions about HR polyfoam. I am thinking of purchasing a 2 1/2 or 3" 21 ild, 2.5 density polyfoam to use on top of a newer mattress that does not have a thick enough or soft enough comfort layer for me. I am using a 1" 17 ild, 1.5 density right now with a nice mattress pad. It helps somewhat, but I am sinking pretty much right through it. I understand that in order to benefit from a topper, you don’t want to sink too far into it. Is this correct?
I would like to know if a thicker, more dense, polyfoam that is a 21 ild would support my size and weight without too much sinking. I am 5’5" and 120 lbs. I know it is all guess work but I’m afraid to pull the trigger on this ild. The other option I see is a 31 ild, but I think this may be too firm. I can also add a 1/2" of 11 ild HR polyfoam on top of the topper I purchase if needed. I do not like latex and memory foam smells awful to me.
It’s actually not quite correct. All the layers of a sleeping system will all compress simultaneously but they will compress to different percentages of their thickness depending on their relative softness, thickness, and their position in the mattress. The layers closer to the top will generally compress more than the deeper layers and softer layers will compress more than firmer layers. What you want is for the combination of layers (mostly in the upper layers) to compress “enough” so that the pressure relieving cradle on the top of your mattress is deep enough to redistribute your weight and relieve your pressure points. Too little and you may still feel too much pressure and too much and you may have alignment issues.
I would keep in mind that a topper is for pressure relief … not support (which comes from the deeper layers). It’s job is to “allow” the heavier parts of your body where you are experiencing pressure (generally hips and shoulders) to sink in more deeply so that the pressure can be relieved.
How deeply you sink in and how effectively your topper will relieve your pressure points will depend on the topper, on the specifics of the mattress underneath it, and on your weight, your weight distribution, your body shape/surface area, and your sleeping positions.
There are too many variables to be able to predict with any certainty whether any mattress/topper combination will be a suitable match for someone in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) without trying it in person but post #2 here and the topper guidelines it links to can help you use your experience on your mattress to choose the thickness and firmness of a topper that has the best chance of success.
I would also keep in mind that ILD alone isn’t the only factor that determines the softness of a layer (see post #4 here) and that ILD isn’t comparable between different types of materials such as latex and polyfoam (see post #6 here) or between different grades of polyfoam that have a different compression modulus so I would always talk to a supplier about the firmness/softness of any polyfoam topper you are considering because going by ILD alone can be misleading.
The goal is that a topper is “just barely enough” in terms of thickness and firmness that it relieves your pressure points in your most pressure prone position (generally side sleeping) but isn’t so thick and soft that it causes you alignment issues in any other positions you sleep in (back and stomach sleeping).
Thanks Phoenix. I’m unclear about “compression modulus”. I am trying to compare a 1.5 density 1", 17 ild polyfoam that I already have to a 2.5 density HR polyfoam 1 , 21 ild. It is only 4 ild’s difference but how will the difference in densities change the feel?
My mattress is a double sided innerspring with just 1" of 31 ild, 2.0 density foam on each side, with a fairly thin quilted encasement. I thought that less would be better, but it compressed quickly in the areas where we lay and is too firm for my side sleeping. When I fold an old 22ild polyfoam (not sure of the density) in half and even triple it up, it feels pretty good, so that’s why I am looking at a 2 1/2" 21 ild polyfoam topper. I’m just not sure about how higher density changes the feel in a similar ild foam.
The IFD of polyfoam is tested at 25% compression with a 50 sq in round compressor foot on with a 4" thick piece of polyfoam so a layer that is 17 ILD would take 17 lbs of pressure for the compressor head to compress the 4" thick layer down to 3". With a 21 ILD layer it would take 21 lbs to compress the material down to 3".
Compression modulus is the rate that a material gets firmer with deeper compression so if a material has a higher compression modulus it would be firmer if the material is compressed more than 25% of its thickness and softer if it was compressed less than 25% of its thickness. It has a different compression curve in other words. Most people would compress a topper more than 25% of its thickness at least under some parts of their body. This is why higher grades of foam are often used in chair cushions because they can feel soft but they also “firm up” faster so there is less chance of the foam bottoming out.
All the factors that can contribute to how soft a topper feels can be quite complex because thickness and to some degree point elasticity also plays a role (see post #4 here) not to mention the specifics of the mattress underneath the topper (a topper will feel different on different mattresses) so if you can’t test a topper in person in combination with your mattress then I would suggest a phone conversation with the supplier who will be in the best position to help you compare the topper or the type of polyfoam you are considering with other densities and ILD’s of foam.