Weight: 2.2 oz
Bathtub Wall Height: ~6 inches
Material: Polycryo Window Insulation (Shrink wrap)
Time Required: 15 min
1. Dollarama 40" x 64" Window Insulation Kit - $1.25 each (x2)
-Using window insulation material for a cheap and light backpacking groundsheet is nothing new, but usually the kits still cost at least $10 - this Dollarama kit is a steal. The only problem is that the dimensions will be too small for most people to use just one sheet, hence the second kit. I haven't been able to find the product online, so if you don't have a Dollarama where you live you'll probably have to spend a few dollars more on one of the standard kits. On the upside, you won't have to bother with the size issue and can skip the first step below.
2. Small Length of Cord/String/Rope
-This is just for creating the tie-outs on the corners, so any scraps will work (twine, shoelaces, anything).
3. Measuring Tape
How To Make It
1. First we need to fix the dimensions of the Dollarama kit. The width is fine at 40" since we'll be using some of the material for the bathtub walls later. To make it longer we'll attach the two kits together using the double sided tape from the kits. This is faster said than done, since the tape will stick to everything once you've attached it to the edge of the first sheet and removed the backing. Go slow and don't be afraid to overlap a little extra. Groundsheets don't need to be pretty. If you really make a mess of things, well, they only cost $1.25 each.
2. Now the sheet is far too long, so it needs to be cut down to size. Cutting one of the sheets in half will give us a finished length of around 7' (after making the bathtub walls), which should be a good size for most. It also means we can make two groundsheets out of three kits, bringing the average cost down to a nice $1.88 each. The sheet should now have dimensions of 40" by ~96", which is the approximate size you'll want to cut your sheet down to if you started with a different window insulation kit.
3. Now it's time for the walls. There are two main advantages to bathtub walls when using a groundsheet under a tarp - they prevent any pooling or running water from spilling onto your groundsheet and also block spray and splashing rain from getting you and your gear wet. Making them is easy. Just attach a 6" strip of the double sided tape 6" from the corner of the sheet, then fold and press the sheet from the corner while lining up the edges, bringing the 'walls' together and resulting in a two-layer right-angled triangle of extra material. Repeat on the other corners.
Change the length of tape and the distance from the corner for higher or lower walls.
4. The last step is attaching tie-outs to the corners. It's not required, but the walls won't really keep their shape without some tension on the corners (although placing heavy objects like water bottles in the corners gets you close). Take some of the extra material on each corner, scrunch it up thin so you can tie it, and attach your cord with a sheet bend knot. Other knots will also work since there won't be very much tension on the line. I strongly recommend tying rather than punching any holes in the sheet since it has very low tear strength. If you've got a pyramid style shelter or another shaped tarp where the pitch is fairly consistent, then shock cord and clips will work well to attach the corners to the shelter. If you use a flat tarp you may have to use longer tie-out lines or other methods.
And we're done! If you haven't used polcyryo as a groundsheet before, you might be comforted to know that I used one sheet for an entire thru hike of the CDT, and then used the same sheet a year later for the entire PNT. It's certainly not bulletproof, and I did repair a couple small punctures along the way with tape, but if you're careful with your campsite selection it can last quite a while. If you're still unsure, carry a spare. It's still lighter than many alternatives.