Sum, I tend to agree, but its not too humid during the fall/winter months. July/Aug/Sept are much much different... if my fan stops running for a week I have small black mold spots on the walls...
I'm in the cheap camp with powersailor. adding to the top of old foam.
Doe, Sum, what thickness did you guys get? 4"?
All of the foam we have used both the high density and the dry-fast has been 3 inches thick. It is very firm, but we like it a lot. They have some that is not so firm if that is what one wants.
The lux high quality ....
http://www.foamdistributing.com/products/lux-hq.html
....has a 14 year life expectancy and a 2.8 lb/cu. ft density and a 50 lb. compression rating. It is $122.00 no tax, with free delivery for a 3 inch X 82" X 76".
The lux regular quality...
http://www.foamdistributing.com/products/lux-r.html
...has a 7 year life expectancy and a 1.8 lb/cu. ft. density but still a 50 lb. compression ratting, so might feel the same and it is only $83.25 for a 3 inch X 82" X 76" sheet.
You might be able to get these as cheap as the 'cheap' local stuff that won't be near as good? I've bought foam before in some of the larger cities for the teardrop trailer and none of it was as good as the above foam and I think I even paid more years back. I think they are a good deal.
They also have cheaper, less dense, less firm sheets also...
http://www.foamdistributing.com/products/products.html
This is what we used on the cockpit cushions in 3 inch thickness...
http://www.foamdistributing.com/products/dryfast.html
... and really like it. It is about the same firmness as the high density above, doesn't seem that wet even when it gets wet and dries very quickly. Of course we have it covered in a material that really allows air to pass through, not vinyl.
You live there, so I'd go by your recommendation on the humidity, but I liked the interlocking blocks you posted over putting closed cell on the bottom where the cushion would still be flat on the surface with no air under it,
Sum
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