Hi Danibug,
The foam we currently have isn't from FBM and has held up over the past 6 years just fine with light use. If I add the 2" topper to 3" of the poly, won't the latex be alot firmer and hotter in the TIGHT sunbrella cover? I'm toying with the idea of leaving the cushions poly 4" with the sunbrella cover and adding the 2" latex on top, but I need to cover it with something.
If the foam you have is HR rather than conventional HD then this would be a good thing.
There are two separate influences involved when you are layering materials inside a cover. the first is that if you add a layer of foam to another layer on a firm surface ... the two layers together will tend to be softer overall than either one by itself. This is different from the "feel" which will depend on which layer is on top and which is softer than the other.
If you put these same two layers inside a tight woven cover like Sunbrella (as opposed to a stretch knit cover) then the more the cover compresses or "squishes" the foam inside it the firmer the two layers will be compared to the same two layers that are not compressed. All foam gets firmer with deeper compression so how compressed it is is a big part of how firm it will feel. This is why compression modulus (support factor) plays such a big role in how firm a foam feels. If the Sunbrella cover only has room for the polyfoam and you try to "squish" the latex inside this will firm it up considerably. If you put the latex on top of the existing cover ... then it will definitely need a cover of its own because latex will degrade much more quickly with exposure to air and light.
Don't forget that ILD is only a rating at a certain percentage of compression of a foam and different compressions will have different ILD's and an ILD measurement that is taken at 25% compression (which is standard) will change depending on the thickness of the foam that is being tested (25% is different with different thicknesses). The "standard" thickness that is used to test the IFD of polyfoam is 4" and the standard thickness used to test latex is 6" so the two are not directly comparable to each other.
Which of the two "influences" are dominant (the softening that comes from thicker layers or the firming that comes from compression) will depend on how tightly they are compressed inside the cover. Either way though ... the two layers together will decrease the likelihood of "bottoming out" and allowing the feel of the firm surface below ti to come through. Even if it is compressed inside the cover, the starting ... ILD will be firmer and it will get progressively firmer yet but will be much less likely to bottom out (reach maximum compression).
While latex has a more open cell structure than most other types of foam (and Talalay is more open than Dunlop) ... compressing it will reduce breathability yes. The ventilation and ability of a fabric to wick or store moisture away from the body can also play a significant role in temperature regulation. This is why in general natural fibers (like cotton or wool) or semi synthetic viscose fibers (like bamboo or eucalyptus) are cooler than most synthetic fibers which don't absorb moisture although some of them can wick it away fairly effectively.
Do you think the latex will sleep cooler and be truer to it's rated ILD number if it's on top of the tightly cover poly or make no difference?
It will perform more like a "typical" layer of foam of a certain thickness and of a specific 25% ILD would perform if it was on top of the cover rather than compressed inside it.
I worry shoving the 2" into the covers as is...a very tight fit...will make the latex sleep hot, be too firm and defeat the whole purpose of latex (especially the Celsion).
I would probably tend to avoid compressing the foam too much as well. While it will affect ventilation (which is the primary reason that combinations of materials are cooler or warmer) ... it won't affect the phase change gel in the Celsion which is a secondary cooling technology and compression would have little effect on this. The ventilation and ability to wick away and store moisture away from the body of all the layers combined (foams, cover, any protector or mattress pad, sheets, and blankets) will all have a combined effect which together determine the temperature regulation of the mattress. One layer that is not breathable can minimize or negate the ventilation effect of the layers below it although they won't negate the effect of any thermally conductive or phase change materials below them.
What kind of cover do you recommend? I can sew it out of any fabric and will have to since the cushions are odd shaped. I would prefer a Polyester blend mostly to avoid the mold and mildew issues that happen with cotton.
This would probably depend on the overall design goals and the primary use of the system. Stretch knits would affect the softness and feel of the foam less than tighter and less stretchy weaves and in general synthetic will be warmer than natural or semi-synthetic viscose fibers like rayon which may make a good choice.
What ILD would be best you think for 2" of the Celsion Latex on top of the HD Poly about 35 ILD? 15, 21, or 27 ILD?
I've never slept on something like what you are building so I really don't know but since Celsion (now called Talalay GL fast response) only comes in the 3 ILD's you are mentioning ... 15 would be way too soft, 21 would probably be too soft for sitting, so I would probably go with 27 which would have less risk of bottoming out on such a thin mattress.
Phoenix