I bought a memory foam mattress with your help (thank you!), and now I need a new rigid foundation. I’m considering getting a less expensive wooden slat foundation off of Amazon (i.e. https://www.amazon.com/Classic-Brands-Instant-Foundation-Mattress/dp/B003XVKHMY/ with 4" gaps between slats or this on https://www.amazon.com/Easy-Fit-Box-Spring-Foundation/dp/B00471CX3U/ with 3-3.5" gaps) and getting some plywood to lay over top to mitigate the gaps. The plywood would have only about a 3-4" gap to span, so it seems it could support the weight easily and prevent the mattress from sagging into the holes. Is this a bad idea? Thank you!
Hi emurphy,
Both of the foundations you mentioned or any of the options in the foundation post here would work fine for a memory foam mattress without using any plywood on top of the slats. The 3" guideline is for a mattress that has latex in the bottom layer because latex is much more elastic than other foams but for mattresses that use polyfoam for the support core (such as most memory foam mattresses or a latex/polyfoam hybrid) then larger gaps are fine. I would avoid the use of plywood when possible so that there is better ventilation between the mattress and the foundation (see post #10 here).
And congratulations on your new mattress 
Phoenix
Oh, wonderful! So if having a 4" gap wouldn’t be problematic for memory foam with a foam core, then it seems I can use a foundation with larger gaps. Is there any disadvantage to doing that?
Thank you!
In other words, is there an advantage or disadvantage to getting a foundation with 3" gaps over 4" gaps for a polyfoam core mattress? Will the mattress last longer spanning shorter gaps? Are you more likely to have circulation/mold problems with smaller gaps?
Thank you again!
Hi emurphy,
In theoretical terms smaller gaps would be more evenly supportive (provide a larger percentage of surface area under the mattress) even for a polyfoam core and in that way would be “better” but in practical terms it probably wouldn’t make any meaningful difference because of the greater stiffness of the polyfoam although it would also depend to some degree on the firmness of the polyfoam core. As long as your foundation meets the warranty requirements of your mattress manufacturer then it would be fine in practical terms and wouldn’t likely make much if any difference in terms of durability and it would make no meaningful difference in airflow (smaller gaps would still provide plenty of ventilation).
Phoenix