View Full Version : Cold Weather Use of Closed Cell Foam Pads in Hammocks

2003-01-30, 21:30
There is a major difference in how a closed cell foam pad with open surface cells (i.e. Ridgecrest®, Z-rest®, convoluted foam, etc.) performs in the environment of a suspended hammock as compared to a tent floor. This difference is because these open cells are intended to be against an airtight surface. The R-value’s (insulation) specified by the manufacturer depend on this and pretty much indicate that the R-value is determined by the maximum thickness of the pad. When this condition is not meet, the R-value is then determined by something approaching the minimum thickness of the pad. This condition does not occur for closed cell foam pads with solid surfaces (i.e. Link Rest®, standard blue foam from REI, etc.). When a closed cell foam pad with open surface cells is placed against the breathable surface of a hammock it cannot trap and hold air stationary very effectively with it's open surface cells. This condition can be modified and pretty much remedied by careful selection of another pad that acts as an air tight surface against the open surface cells.

I have taken some liberties with the information that I found so that I can try to make some useful comparisons. My hope is that these 'simplifications' do not significantly distort what is really happening. One of the few closed cell foam pads with solid surfaces that I could find manufactures specifications on was the Link Rest® and I am assuming that other closed cell foam pads that have a density between 1.5 and 2.0 pounds per cubic foot (PCF) have an R-value proportional to the R-value of the Link Rest®, which is 1.9 with a thickness of 7/16 inch. The second assumption that I am using is that the standard 3/8 inch blue foam found at REI extends the comfortable temperature range by 20°F. I am basing this assumption on personal experience. These two assumptions allow me to construct Table 1, showing the relationship between Effective Pad Thickness, R-value, Temperature Extension and Comfortable Temperature Limit for closed cell foam pads.

Effective Pad Thickness is the pad thickness for closed cell foam pads with solid surfaces. For closed cell foam pads with open surface cells the Effective Pad Thickness is somewhere between the manufactures stated thickness (pretty much its maximum thickness) and its minimum thickness, depending on how it is utilized.

The R-value is a measure of its insulating capability, with higher numbers indicating higher insulation. Usually, when you stack pads the R-values are additive. I say usually because this may not occur when closed cell foam pads with open surface cells are involved, it does occur when solid surface pads are involved and it might occur if an open surface cell pad is used with a solid surface pad

The Temperature Extension is how many °F of extra comfort obtained when using a particular R-value for the bottom insulation in a hammock. If you don’t wear any clothing when you sleep and are comfortable without an insulating pad as long as the temperature is above 80°F, you will need 60°F of Temperature Extension to be comfortable when the temperature dips to 20°F. Likewise, if you sleep in fleece jacket and pants and are comfortable as long as the temperature is above 50°F without an insulating pad, you will need 30°F of Temperature Extension when the temperature dips to 20°F. Understand how this works? Your metabolism, choice of clothing and anything else underneath you have a direct effect on how much Temperature Extension you need from your sleeping pad. Also, don't overlook the fact that down sleeping bags compress much more than most synthetic sleeping bags...this means that the inexpensive sleeping bags that don't compress much will provide more insulation underneath you than a similarly rated premium quality down bag.

Folks, you got to watch where you are stepping here. You need to take all these factors into consideration when someone tells you they slept comfortable at 20°F with a half-inch thick closed cell foam pad. They probably didn't even need that pad until the temperature got below 45°F… and you might not can sleep comfortable with that same half-inch thick closed cell foam at 45°F. Be careful, make sure you are comparing what you think you are and please try out your cold weather sleeping system somewhere where you have a bailout plan. You don't want to find out that your system only keeps you warm to 45°F when it is 20°F and you have no other way to stay warm.

Comfortable Temperature Limit is exactly what it sounds like, the lowest temperature that you will be comfortable at. I have included several columns in the table to emphasis that the insulation underneath you is not always 'all up to the pad'. The Baseline Temperature is the lowest temperature that you are comfortable at when not using an insulating pad. I have not chosen to extrapolate the Comfortable Temperature Limits much below 20°F because this is unfamiliar territory to me and I fear that any extrapolation into this region might not be applicable, or worse yet, it might be hazardous.





SGT Rock
2003-02-02, 00:17
Good info. So if I understand your point:

1. Use a pad with closed cells on the surface.

2. And/or put a barrier under the pad that is vapor proof.

2003-02-02, 09:08
Yeah Sarge, that is what I was finding when I used my assortment of pads. After corresponding with Cascade Designs and Coleman I got to thinking more about what was going on. I did some crude compression tests on my workbench and that didn't seem to answer what I was experiencing. Then I remembered that Cascade Designs told me that they weighted their pads evenly accross the entire surface area when they did R-value testing...that's when it hit me that the large open suface cells had to 'trap some air' if you wanted to get the rated R-value from them. Also, when I tried to push Cascade Designs for definitive answers as to what pad they recommended at what temperatures, all they would say was that "when it got cold to use a self inflating pad with a closed cell foam pad". I think that configuration works so well because it allows the large open surface cells to 'trap some air', at least when you are sleeping on the flat surface of a tent with an air-tight/water-proof floor. I have had some good results when hammock camping when the open suface cells where not directly in contact with the breathable hammock material. I also think that this means that closed cell foam pads without open surface cells makes more sense when insulation versus packing volume becomes an issue.


SGT Rock
2003-02-02, 11:23
That is very interesting. Shane S reccomends wraping the pad in a neat Sheet which is supposed to be waterproof and comfortable to lay on.

2003-02-02, 14:11
What's a Neat Sheet? Also, tell me what you think about this idea. What if you put an open cell foam pad, like a Z-Rest, inside a VBL inside a hammock? Seems like this would give you warmth without the extra weight of a whole extra pad. Just a thought.


Just thought of something else. I like the idea of the pad with wings, but instead of sewing the two pads together, I thought about sewing velcro patches onto the pads. I know there are problems with adhesives sticking to the pads, but since you are sewing them anyway, why not just sew the velcro on so it's easier to separate the pads in warmer weather?