There've been a number of posts recently about sleeping bags not performing as rated and corresponding suggestions that it's not the bag but, instead, the sleeping pad that doesn’t have a sufficient R-value to prevent convection cooling, etc.
Some have suggested adding a thin CCF (closed cell foam) pad on top of whatever primary sleeping pad is being used to eliminate or lessen the cooling effect of otherwise low R-value pads.
Although relatively cheap, “thin” CCFs (e.g., from Gossamer Gear, Wal-Mart, etc.) are much more bulky in comparison to an effective alternative I created in an effort to stay as ultra light as possible without unnecessarily sacrificing functionality or incurring inordinate cost.
What I did was to start with a Thermo-lite Bivy 2.0 (pictured below and available for $26.50 from Amazon - http://www.amazon.com/Adventure-Medical-Kits-Thermal-Bivvy/dp/B000WY2962 - and maybe cheaper elsewhere).
As you can see, the Bivy 2.0 is a semi-mummy shaped emergency “bag” with a foot box that vents.
The DIY process
1. I first put my mummy shaped sleeping pad on top of the flattened Bivy 2.0 and traced an outline of the Bivy around my sleeping pad. (I let the air out of my pad; I found that it doesn’t really matter much if the pad is “airless” or not).
2. Using an exacto knife, I next cut the Bivy 2.0 along the pattern I had traced. This left a 2-ply (i.e., folded in half) "mummy shaped" remainder (see pictures below).
PLEASE NOTE: I laid out the pattern in such a way that I cut off the vented foot box from the piece left.
3. After this, I cut the 2-ply remainder of the Bivy 2.0 in half (again using the exacto knife and cutting a very narrow strip where the two "sides" are folded to make one piece).
PLEASE NOTE: if you want to use a “sandwich” approach – i.e., with the pad in the middle and both modified Bivy 2.0 sides around it - then don’t make the cut I did.
4. After doing the above, what’s left are two separate pieces, both identically shaped and each having an "inside" (dull) and an "outside" (shiny) (see pictures below of each side and its appearance).
5. I next put one piece of this modified Bivy 2.0 on top of my mummy sleeping pad and the sleeping bag on top of it. (NB- In an effort to prevent the Bivy 2.0 material from moving around on the pad, I first "painted" the pad on top with Silnet "dots", which effectively "fastened" the cut Bivy 2.0 material to the pad. I also "dotted" the bottom of my sleeping pad and tent to prevent any similar "slippage" from movement/pitching on non-level ground. I could have "dotted" the Bivy 2.0 material but found it unnecessary to prevent "sliding".
End result: the "half" Bivy 2.0 mummy piece I cut/use weighs 56 grams (i.e, 2 oz.) and is comparatively MUCH lighter and VERY much more compact (about fist size) than any CCF I’m aware of. It also reflects back roughly 80-90% of the radiant heat that would otherwise be lost using a low R-value sleeping pad (such as my custom model) and prevents virtually all heat loss through convection cooling.
(For those wanting an even lighter approach, you can use a “space blanket” - i.e., aluminized Mylar. However, the Bivy 2.0 material is MUCH quieter.)
And no, I don’t make these/sell them/have a pattern for them.