First off, describe your foam mattress – how thick, expensive, and old is it, and from what retailer? It’s not my regular mattress, but it’s what I sleep on when I’m at my parents’ home, so on average a few nights per month, up to a week over Christmas and a week over Thanksgiving. It is 3 or 4" thick, a memory foam topper. It was several hundred bucks. It was from overstock.com. I have slept on my parents’ before. It is from a local wholesaler, it is a full memory foam, and it feels the same as the topper. You can’t feel below the first few inches of a bed, FYI. If you’re looking for a trial without commitment, I would strongly suggest a topper first.
ETA: I think I have what SeaDragon has, the 4" Sealy.
Do you consider it all-weather, or is it too (I imagine) warm to be comfortable during the summer, unless you really crank up the AC? Can sleeping on a memory-foam bed thermally substitute for sleeping on a down-filled featherbed? It is hot as f-u-c-k to me. Which is good in the winter, torturous in the summer. I have no idea what sleeping on a featherbed is like.
Have you had any problems, from back pain to allergies, that either got better or were exacerbated by switching to a foam mattress? I think it takes getting used to, but I do not think that my nominal back pain was alleviated. My mom, however, feels hers was. This could be due the fact her current memory foam is much softer and to her liking than her previous one. My parents stupidly alternate beds - dad gets hard one for several years, she gets a soft one, and on it goes. Again, this is a big YMMV thing. If you’re inclined to soft enscapulating mattresses, you’ll like it.
How well do these hold up? What would you estimate your mattress’ duration of usefulness? I think this depends on how heavy you are and how crazy your sex is. I wouldn’t imagine they last any less time than a regular mattress. Also, since the technology has been widely available for some time, I wouldn’t recommend getting a brand name Tempur-Pedic. There’s no point. You can’t tell the difference between it and a knock-off. Everything is on the same playing field at this point. I remember sex in the tempur pedic was good but had none of the bounce-back you see with virtually all other mattresses. Basically your partner can go a lot deeper because the mattress isn’t giving as much as a regular one would. Very different, I don’t care for it, but again, YMMV.
Do they have any noticeable or harmful odor or chemical fumes issues (as with VOCs, or volatile organic compounds – the kind of offgassing associated with polyvinylchloride, some paints, and most car interiors)? YES. It was noxious. My parents slept in the guest room for several nights when they first got theirs. My brother has memory foam pillows and he aired them out for at least two weeks (he has pretty bad allergies). Harmful, you have to look that up. But stank? Hell yes.
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And has your mattress improved your sleep?** I prefer a pillow top. My mom swears it’s improved hers, my dad disagrees. I think this is a huge case of YMMV.
Overall, would you buy it again (or replace it when the time comes)? I would keep up with pillowtops because of the heat issue. I don’t particularly like the feeling of “sinking in” either. If you often wake your partner when getting up to pee in the night, they’re a GREAT option because of little motion transfer.