COLD WEATHER USE OF CLOSED CELL FOAM PADS IN HAMMOCKS

There are two classes of closed cell foam pads and they perform differently in hammocks.

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 on both the top and bottom of the pad. 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 its open surface cells, and this phenomenon is made worse in windy conditions. This condition can be modified and pretty much remedied by careful selection of another pad or sunscreen that acts as a vapor barrier (or air tight surface) against the open surface cells. The advantages of closed cell foam pads with open surface cells are (1) more cushioning with less weight and (2) more R-value with less weight when used as the designer intended. For hammock users, it pretty much means more bulk for the same R-value and this is an important issue when trying to use a hammock in temperatures below about 50°F.

How to make use of pad thickness and R-value.

In my research, 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 (it’s a SWAG). These two assumptions allow me to construct the Table 1, showing the relationship between Effective Pad Thickness, R-value, Insulative Value 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 (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 typically does 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. (I think it would be possible to design open surface pads that were stackable in such a way that their R-values would be additive, but I haven't seen any.)

The Insulative Value 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 Insulative Value 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 Insulative Value 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 Insulative Value 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 be able to 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, just in case you misfigured something. 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, that is a life threatening scenario!

Comfortable Temperature Limit (CTL) 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.

Table 1. Relationship Between Effective Pad Thickness, R-Value and Insulative Value.

			CTL	CTL	CTL	CTL
Effective			70*F Baseline	60*F Baseline	50*F Baseline	40*F Baseline
Pad Thickness	R-value	Insulative	Temperature	Temperature	Temperature	Temperature
(in.)	(by thickness)	Value (*F)	(deg. F)	(deg. F)	(deg. F)	(deg. F)
0	0.00	0	70	60	50	40
1/8	0.54	7	63	53	43	33
1/4	1.09	13	57	47	37	27
3/8	1.63	20	50	40	30	20
1/2	2.17	27	43	33	23
5/8	2.71	33	37	27	17
3/4	3.26	40	30	20
7/8	3.80	47	23
1	4.34	53	17

Table 2. Various Pad Dimensions

	Pad length	Pad Width	Pad Thickness	Rolled Dia.	Volume	Weight
	(in.)	(in.)	(in.)	(in.)	(cu. in.)	(oz.)
Sun Screen	66	27	3/32	6.5 x 3/4	132	5
Blue Foam	77	25	3/8	6.1	722	10
Blue Foam 3/4	56	25	3/8	5.2	525	7.3
Coleman convoluted	72	24	5/8	7.6	1080	15
Coleman Ultralight	72	24.5	5/8	7.6	1103	22
Ridge Rest	72	20	5/8	7.6	900	14
Ridge Rest 3/4	48	20	5/8	6.2	600	9
Ridge Rest 25	77	25	5/8	7.8	1203	19
Ridge Rest Deluxe	72	20	3/4	8.3	1080	18
Link Rest	70	24	7/16	6.2	735	13
Link Rest 3/4	46	24	7/16	5.1	483	8
Z-Rest	72	20	3/4	5 x 5.5	550	15
Z-Rest 3/4	51	20	3/4	5 x 3.8	380	11

Table 3. How R-Value Is Affected By Open Surface Cells.

	Pad Thickness	Min. Thickness	R-Value	R-Value
	(in.)	(in.)	(Published)	(by min. thick.)
Sun Screen	3/32	3/32		0.4
Blue Foam	3/8	3/8		1.6
Blue Foam 3/4	3/8	3/8		1.6
Coleman convoluted	5/8	1/4		1.1
Coleman Ultralight	5/8	5/8		2.7
Ridge Rest	5/8	7/32	2.6	1.0
Ridge Rest 3/4	5/8	7/32	2.6	1.0
Ridge Rest 25	5/8	7/32	2.6	1.0
Ridge Rest Deluxe	3/4	3/8	3.1	1.6
Link Rest	7/16	7/16	1.9	1.9
Link Rest 3/4	7/16	7/16	1.9	1.9
Z-Rest	3/4	3/32	2.2	0.4
Z-Rest 3/4	3/4	3/32	2.2	0.4

Dave Womble

aka Youngblood

Jan. 2003

edit Sept. 2003

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