I need a queen mattress for a murphy bed.  The weight limit is 80 pounds.  I had been hoping to get a tempurpedic type mattress but of the stats I can find they are all too heavy.  Are there any queen size memory foam mattresses that weigh 80 pounds or less? If not, other recommendations?

Usually a good memory foam mattress has only about 3" give or take (you will see more in some constructions) of the heavier memory foam and then the rest is a HR polyfoam which weighs less so it would be worth checking out some of the memory foam mattresses in queen which may just squeak by. You would probably be just OK with something up to about 8 or 9 inches depending on the type. An all latex mattress would be too heavy in a queen but a mattress with good quality poly with latex on top may also just make it.

Some of the thinner innersprings would also be in the range but they would usually have poly in them since latex and memory foam or larger amounts of cotton and other natural stuffings that are compressed would probably put it over .... depending on how much steel was in the innerspring. I would look at options here too.

Phoenix

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This 10" memory foam is on sale at Costco.   It says it weighs 79 lbs.  Some like it, so say not enough back support, some say too firm, so hard to say.  If back support is the issue, I would not go with a memory foam mattress.  Great return policy at Costco.

You might want to look at Ikea as well.  They have some thinner lighter ones.  This one has coils and latex for 82lbs.   They have a lighter foam ones (this one has 1" latex over foam - 73 lbs.).  Click on package measurement/weight to get the specs.

Overstock, walmart.com, etc. should have some as well.  As Phoenix said, you won't be able to get all latex at that weight unless it is very thin.  I think latex is around 12-15 lb.s per inch (queen size).