Insulating yourself from the heat-sucking ground and providing comfort from your bones grinding into the hard ground or tent platform are critical to have a good night’s sleep when backpacking. Sleeping pads come in 3 main types: closed cell foam (CCF) pads, inflatable pads (air mattresses), and self-inflating pads.
Many backpackers choose to sleep on a CCF pad because they have had bad experiences with punctures in the past using inflatable pads, while some simply are able to sleep comfortably enough on a CCF pad and choose it for its low weight and cost. And many backpackers use a combination of both: some put the CCF pad below their inflatable to provide protection from punctures, while others put the CCF on top to get the most insulative benefit (discussion of that is beyond the scope of this article, but trust me that all things being equal you will sleep warmer with the CCF pad placed between you and the inflatable than between the inflatable and the ground).
Comparison of the pros and cons of using CCF vs inflatable pads
|Closed Cell Foam Pads||Inflatable Pads|
What is closed cell foam?
Closed cell foam refers to how the air pockets in the foam are not interconnected, resulting in a foam that is generally denser, waterproof, more insulative, more resistant to damage, and fairly stiff and uncompressible. Open cell foam has air pockets that are interconnected, resulting in a spongier foam that is generally less dense, less insulative, will absorb water or allow it to pass right through, and is usually relatively easy to damage. Open cell foam usually compresses more and is more comfortable to sit on; it is used in mattress and seat padding and inside some inflatable and self-inflating pads. In the past, some “foam” sleeping pads used open cell foam encased in waterproof fabric, but as closed cell foam technology has improved and diversified, different types of CCF have become the clear choice for foam sleeping pads.
Almost all CCF sleeping pads sold today use low density polyethylene (LDPE) because of its flexibility, low weight, durability, and moderate softness. Polyurethane foam (PU) is still very occasionally used; it is heavier (more dense) but does usually provide more insulation per inch of thickness. Many sleeping pads in the past (as well as exercise and yoga mats still today) were made of polyvinyl chloride/nitrile rubber (PVC/NBR or Ensolite); these pads deteriorated under extended UV exposure and are no longer used. Note you will sometimes see a CCF sleeping pad’s material listed as polyolefin foam – this is not very descriptive as polyolefin can refer to polyethylene or any of a wide range of other plastics; in these cases, I would assume the pad is made of LDPE.
Polystyrene (Styrofoam) is another type of closed cell foam we are all familiar with. Commonly used in packaging as well as house insulation, it provides more insulation per weight than polyethylene, but because it is rigid and breaks easily, it is not suitable for backpacking except perhaps as a small sit pad.
Types of Polyethylene Foam
There are two main types of LDPE foam used in pads: extruded and cross-linked, with cross-linked being further subdivided whether it is chemically cross-linked or physically cross-linked. Extruded polyethylene (abbreviated as PE, PEF, or LDPE) is produced by blending molten polyethylene polymer with a gas under high pressure. The hot liquid-gas mixture is then extruded onto a conveyer at atmospheric pressure where the gas quickly expands to form bubbles and the polymer cools into a solid, trapping the gas in interstitial cells. A common additive to LDPE is vinyl-acetate, referred to as EVA foam (ethylene-vinyl-acetate), which makes the foam softer, more rubbery feeling, and more UV resistant.
Chemically cross-linked polyethylene (abbreviated as PEX, XPE, or XLPE) involves adding chemicals that create bonding in the liquid polymer before initiation of the foaming process, resulting is smaller cell size and a smoother surface to the foam. Physically cross-linked polyethylene (IXPE) usually uses some type of irradiation to produce even smaller and more consistent cell sizes, resulting in a very fine-textured foam. The direction of the conveyer where the foam expands can also be orientated vertically instead of the traditional horizontal orientation, which affects how the cells form in the foam. There are endless varieties of proprietary blends and processes for making different types of cross-linked polyethylene. Two of the most popular proprietary foams used in pads are Evazote and Plastazote produced by ZoteFoams. Additionally, some foams are produced by laminating the same or different types of foam together; these will appear as having layers of the same or different colors when viewed in cross-section. Some pads are also laminated with a thin layer of aluminum to reflect body height.
The main types of CCF used for pads:
|Low-density polyethylene foam||PE, PEF, LDPE||Low-moderate durability, large cell size and difficult to roll, standard material used in foam swim tubes|
|Chemically cross-linked polyethylene||XPE, XLPE, PEX||Smaller cell size, higher durability|
|Physically cross-linked polyethylene (horizontal foaming)||IXPE||Small uniform cell size, high durability|
|Physically cross-linked polyethylene (vertical foaming)||IXPE||Similar to horizontal foaming, possibly lower thermal conductivity|
|Laminated cross-linked foam||Layers, improved durability, often more difficult to roll|
|Ethylene-vinyl acetate (PEF)||EVA||Softer rubbery feel, standard material used in floor mats|
|Evazote||Type of IXPE with EVA|
|Plastazote||Type of IXPE, less durable, available in low densities|
|Volara||Type of XPE with small uniform cell size|
|Minicel||Type of XPE with very small uniform cell size, soft smooth surface|
Examples of different types of polyethylene foam, with the density indicated in pounds per cubic foot (pcf):
Comparing types of closed cell foam
As each type of foam can be produced across a variety of densities, it is very difficult to make generalized comparisons between the different foam types. Most foams used for CCF pads have a density of 1.5-2.5 lbs/ft3. As density increases, tear resistance increases but the force needed to compress the foam 25% or 50% increases, so there is a trade-off between durability and comfort. Denser CCF is too hard to sleep on and difficult to roll, while less dense foam is softer but less tear resistant. Very low density CCF (<1.5 lbs/ft3) is sometimes used as sheets in packaging. While these very low density foams could still be used for sleeping pads, this requires using a thicker, bulkier pad to avoid bottoming out or shredding your pad to pieces. However, as we will see later, the lower density foams have the highest warmth to weight ratio.
In addition to weight, the other primary characteristic backpackers care about is insulation. In the US, pads are normally rated by R value, a measure of thermal resistance. R value is related to a material’s thermal conductivity – this is a measure of how fast heat passes through the material and has the units Btu*in/ft2*h*F. R value is calculated by taking the material’s thickness in inches and dividing its thermal conductivity, with the units ft2*h*F/Btu. While R value will increase with thickness, thermal conductivity should be constant for a given material. Materials with low thermal conductivity are good insulators, with things like argon gas or perlite under vacuum having extremely low conductivity (thermal energy passes very slowly through space with few atoms). Sometimes the R value for 1 inch of material is given, which will remain constant for a given material, being simply the inverse of its thermal conductivity. Outside the US, R values may be given in metric units (k/W). In the UK, pads are often rated by tog value: 1 tog is equal to 0.57 R value (US).
For a given type of foam, thermal conductivity generally increases with greater foam density. However, as each type of foam can be produced across a range of densities and ccf pads rarely indicate what density of foam was used, it is very difficult to compare the thermal conductivity of different pads. Density can be estimated based on the pad’s size and weight, but as pad manufacturing can add weight, this is only an approximation. Different technologies can further reduce thermal conductivity for a given density. Here is my best attempt at ranking the types of foam from highest conductivity to lowest conductivity as:
non-cross-linked > chemically cross-linked > physically cross-linked (horizontal) > physically cross-linked (vertical)
Laminating foams together and aluminizing the foam can further reduce conductivity. Some pads are formed in an egg crate shape; this should not affect the conductivity of the foam, but may allow insulation in your sleeping bag, quilt, or jacket to loft underneath you and further reduce conductivity.
Next in Part 2, I will survey the many ccf sleeping pads available in the US market and present compiled data comparing them.