Hi mattressseeker,
The breathability, ventilation, and the ability to control humidity and microclimate of the upper layers and materials in a mattress are a much more important factor in the sleeping temperature of a mattress than the deeper layers (and there are also many other factors that can affect the sleeping temperature of a mattress as much or more than the type of foam used in the comfort layers as well) so the breathability of the polyfoam in the support core wouldn’t be a significant issue in temperature regulation.
Yes … this is more an issue with the type of foundation or support surface you use than the bottom layers in a mattress. Having airflow under a mattress vs having a solid surface platform (regardless of the bedframe you use) is a good idea with any mattress because it can help with moisture retention and humidity in the mattress and reduce the risk of mold, mildew, and dust mites in your mattress (see post #10 here). The breathability of the bottom polyfoam layers in the mattress (or any type of support layers) isn’t a significant issue in terms of temperature regulation or how it affects ventilation in the upper layers of the mattress.
Phoenix