Old box spring under a new latex mattress?

Hi everyone,

So - what’s been your experience? Is this just the new way to make a couple extra hundred dollars or is the “flex steel” foundation a must for a latex mattress? Has anyone bought a new latex mattress but not the foundation then regretted it? Or gone back and got the matching foundation after the fact?


Hi ZhivagosGirl,

While I am not “everyone” and I would welcome any other people’s experience with this … I can share some of the design theory behind the choices between flex foundations (box springs), semi-flex foundations, and rigid foundations and why each may be necessary (or not) and how they affect the mattress on top of them.

NOTE: For more information about different types of foundations and specific suggestions and links to some good choices see post #1 here.

In general terms … a foundation or box spring is designed to do one or more of three things (besides just adding height to the sleeping surface).

  1. Act as an actual layer of the sleeping system itself and in combination with the mattress is a key part of the performance and feel of the mattress. In these cases … the characteristics of the box spring are designed into the overall sleeping system itself and are an essential part of how the mattress is designed to perform in terms of pressure relief, alignment, and overall feel. In these cases it is just as important as the mattress itself to achieve the design goals of the sleeping system. These would typically be “active” flex support systems with some type of springs which have more give under the mattress and can also be designed in many different ways to change the way the mattress feels and performs.

  2. Act as a “shock absorber” to protect and lengthen the life of the mattress. This would apply mostly to innerspring mattresses because sudden shocks or more sudden compression can weaken or deform the innerspring unless there is something under them to help absorb the shock so the innersprings or border wire in the mattress don’t become weakened or deformed. Either flex or semi-flex systems can work well here. In these cases a flexing box spring can lengthen the life of an innerspring mattress.

  3. Act to provide an even rigid and non flexing surface to support the mattress and make sure that any compression is all in the mattress itself. This would apply mostly to foam core mattresses (without any springs) which can absorb shock all by themselves without the risk of deformation of an innerspring. They are usually “designed” to be self contained and to achieve their design goals they require an evenly supportive non flexing surface.

  4. Act to provide ventilation under the mattress. While a solid surface platform or foundation would be the most supportive type of support system … the lack of ventilation under the mattress in combination with other risk factors can contribute to the risk of mold or mildew or the accumulation of dust mites in a mattress (see post #10 here).

With most innerspring mattresses, (with the exception of many pocket coil mattresses) … either or both #1 or #2 can be important. Which is “best” depends on whether there is a specific foundation that has been designed as part of the sleeping system or if the mattress just needs a “shock absorber” to protect it and lengthen it’s life. Many pocket coil mattresses don’t require a flexing box spring and do best with a firm non flexing or semi-flex foundation.

With a foam core mattress (either polyfoam or latex) … a rigid slatted or grid system where the spacing of the slats or grid is small enough to prevent the foam from sinking through any gaps (and latex being more flexible than polyfoam needs smaller gaps) are the norm. For a mattress with a latex support core the gaps of a slatted foundation should be a maximum of 3" and preferably less. I also have some reservations with the use of wire grid foundations with an all latex mattress (see post #10 here).

This doesn’t mean that a semi flex or active box spring can’t be used with a foam mattress … only that it is usually not necessary or even desirable and can change the properties of the mattress unless it is part of the design or is part of how you tested a mattress that you decided to buy.

In some cases … especially with thinner foam core mattresses … flexing foundations are designed as part of a sleeping system (a 6" latex mattress that would otherwise be too firm that is put on a flexible box spring to add more “give” would be an example). The thicker the mattress … the less effect the innerspring underneath will have. There are several different types of foundations that can be used here such as a tension adjustable slatted foundation (where different areas can be adjusted for firmness), flexible slatted foundations (which have some give) or a zoned innerspring (with the middle being firmer to help with holding up the heavier pelvis region while it can “allow” lighter wider areas like the shoulders to sink down more) but in general … at least in North America … foam core mattresses (including latex mattresses) are designed with a firm rigid and non flexing foundation in mind. The next best choice would be a semi-flex which will have little if any effect on the feel and performance of the mattress. The best choice is normally a rigid non flexing slatted foundation unless there is a specific reason to choose otherwise.

If you are testing a mattress that is designed to use a flexing foundation and you have tested it this way (or vice versa), then using the same type of foundation would be necessary to duplicate the performance and feel you experienced in the store

So the bottom line is that a foam mattress that is designed for it and that you tested with a firm, even, non flexing foundation can use any firm even non flexing foundation that provides even support and is suitable for the weight of the mattress and people on it. For a mattress where there was a flexing box spring under it that is an integral part of the overall feel and performance of the mattress (specifically designed as part of the sleeping system’s performance) … then they should be purchased together. For a mattress that needs some “shock absorption” but not so much the type of flex that adds to how the mattress performs and feels … then any semi-flex foundation that provides suitable strength and shock absorption would be fine.

If you test a mattress that has a flexing foundation and you purchase it with a non flexing foundation (or vice versa) then it can change the feel and performance of the mattress.

If you already have a suitable foundation/box spring of the type that you need and it is not a specific part of the sleeping system design … then using the one you have will be fine as long as it has not weakened and is still up to it’s “original” specs. Some rigid slat foundations for example will last decades and they certainly don’t need to be changed when you buy a new mattress that is designed for a rigid foundation. A box spring which has some areas where some of the springs have weakened or that doesn’t provide even firm support across the entire surface should be replaced both for the sake of performance and longevity. This can be tested with firm compression using your knee all across the box spring surface to make sure there are no areas that are softer or compress more than others. This will help keep the mattress level (and improve alignment) and act as a shock absorber rather than allowing the mattress to “bend” too much into the weakened area of the box spring which could shorten the life of the mattress and alter the way the mattress is designed to perform.

Hope this helps.


PS: I should add that none of this deals with specific warranty “rules” of some manufacturers which may require a specific foundation for the warranty rather than a certain “type” or with fire code issues where a specific foundation has been tested and passed as a prototype in combination with a certain foundation. This only deals with performance issues, the "suitability of dieferent foundations, and how different types of foundations will affect different types of mattresses.