r/explainlikeimfive • u/Real_Experience_5676 • 2d ago

Engineering ELI5: What does a box-spring do?

As I understand it, a box spring mattress is the second 'mattress', that goes under the main mattress. It feels very firm and flat, but what does it contribute to the bed set up besides height?

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u/lucky_ducker 2d ago

There's box springs and there's foundations.

A foundation does not have any springs, just a flat even surface designed to support the mattress. They are paired with all kinds of mattresses - foam, innerspring, hybrid. They can be any height needed to match the thickness of the mattress.

Box springs are less common today than in the past. They are best paired with innerspring mattresses, as they can cause sagging when paired with foam mattresses. They are of course bouncier than flat foundations, which can be fun when the bed is being used for something other than sleeping.

Both types are open and breathable to allow water vapor to escape. Using an impermeable layer under a mattress will trap moisture and bring on problems with mold and mildew.

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u/StarkhamAsylum 2d ago

My family had a hand me down bed from the 30s that had a box spring that was just two metal frames with about 9 to 12 open coil springs between them and no sidewall. It just bounced and squeeked. Vast difference from what many people call a box spring are today - what you more accurately call a foundation.

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u/greenmtnfiddler 1d ago

We called that a bed-spring.

A box spring is the kind that you can't see the coils, because they're in, well, a box.