Depends on what memory foam you use those, Tempur are the market leader with the best material and I've not had any heat issues with this matress I just got. Although I understand what you are referring to, foam can cause a heat issue.
PS. Just found Sealy do have some memory foam mattresses.
I honestly don't know which foam I slept on, but it was gruesome. However, it was in a hot climate and that most probably did not help.
I'm not surprised Sealy have foam mattresses - they are one of the best mattress makers in the world (something to stick in the back of your memory the next time you want a new mattress). A Sealy mattress will last a lifetime (maybe I should be writing their advertising blurb
). All the mattresses I have are Sealy and they all have a box spring underneath the mattress - so you get two sets of springs. The one I sleep on is a California King (7 feet long by about 6'6" wide) and because of its size the box spring comes in two separate pieces (the same as two single beds) which are bolted together and then the mattress sits on the top. You never EVER get any sag in a Sealy mattress (more blurb