I know, it’s crazy

Just throwing this out there, but would a Tyvek bivy suit your needs? I've seen a few people on here discussing their merits. Probably worth exploration, as Tyvek is cheap online or can be had dumpster diving near construction sites.

I wouldn't count on any longterm results from this setup, but I'd be willing to bet it'll last for a single trip. Another less unsightly method: sew some cheap 3mil painters drop plastic from the home labyrinth superstore into the bathtub bottom. The Equinox bivy runs like $65, but I'd rather make my own.

Cam – I thought Tyvek wasn't 100% waterproof?

Lucas – that's an interesting thought… I'll keep that in mind.

Kier, while Tyvek isn't waterproof like a plastic bag is, it is very water resistant, breathable, cheap, and can be had locally. In fact, after looking at how much trouble you've had in obtaining anything locally, I would wager that there is some sort of construction within 5 miles of your home or work that you could beg some Tyvek from.

And I'm guessing that if you're going from a 4 man tent to sleeping under someone's tarp, that you aren't really equipped or accustomed to sleeping in such a spartan manner, so you won't be using the bivy for your sole form of protection. That's why you're sleeping under someone's tarp, right? And if that's the case, then you're just worried about getting dew or a little oversplash if it rains on your bag? Tyvek will do that well, and it's easy to work with.

You might also consider the Heatsheets bivvy. MSRP of just $16. If you don’t really need full coverage you can use just a blanket or get 2 blankets and tape together for a cheaper bivvy. I’ll be making a tent of these soon. Another poster here made a tent that he used at Philmont.

If the tarp is large enough (find out the size) you may not need a bivy but will need a groundcloth (think 2 mil painters dropcloth). If you need a quick bivy you probably wouldn't even need to sew. Some quick bivy thoughts,

1. Neatsheet- sold in walmarts maybe target
2. Plastic or tyvek bottom/driducks poncho top- if you can get that
3. AMK thermolite bivy- not entirely closed so you have some ventilation
4. Heatsheet bivy
5. Plastic or tyvek bottom/Heatsheet blanket top (tucked in, not sewn)
6. Plastic or tyvek oversized (~6'x7')- simply use it as a groundsheet and pull remainder loosely over bag if weather turns bad; might need something to keep it closed on open end

Tyvek's not 100% waterproof but from what I've read, it takes a fair amount of hydrostatic pressure to get liquid water through it. If you're under a tarp and don't expect to lie in a puddle, it should be good. If you're not sure about the puddle, paint the bottom with your silicone mixture.

"Question: If I made the bivy, and then painted up the bathtub bottom…"

That just gave me a great idea on a cheap bivy. I don't know how it would hold up sewing wise and not sure of the actual weight(probably kinda heavy), but how would a shower curtain work as the bottom of a bivy? Then run to a Cabelas, Gander Mnt. or alike store and buy a frogg toggs poncho, cut it down and use that as the top of a bivy.

Might not be the lightest, but it would be a cheap alternative.

Good luck

Thanks for so much feedback already…

Cam – Yeah normally I'm in a tent because I have the wife, kid, and dog with me. Because I'm gonna be able to fit under a friend's tarp, my main concern is oversplash and any ground water channels that emerge if it starts raining. Blame the economy or whatever, but I honestly cannot think of a construction site I've seen in months, except for highway construction. Does Lowe's sell the stuff in small quantity?

Michael – I considered the heatsheets bivy at first, but am concerned about overheating in the cheap model and water protection in the pricier model

John – The neat sheet is an interesting option… would I just duct tape it or sew it?

Keith – thanks, if I go with Tyvek then I may paint the bottom

Andy – weight aside, that does give some unique style options :)

I used a neatsheet as my first bivy, but did use a 3'x7' dropcloth under it since the neatsheet is not waterproof. I simply took out seams I didn't want, left in those I did want and then cut it to size (hard to explain). The top remained open. Weight of finished "product" was about 6 oz. of only the modified neatsheet.

If it was me (and I had no money to spend and short timeframe), I'd see how large the tarp is and just bring an oversized sheet of something waterproof, sleep on a portion and pull over the remainder if I needed it. That will depend on expected low temps, how long trip is(possible condensation on down bag), bag temp rating.

The 30th place I've called locally has some regular 1.9oz rip-stop in stock for $6/yd! As this is my preferred material, does anyone else wanna take a guess as to whether or not painting on the silicon will work? Or shall I be the guinea pig? :)

I'd imagine it will work, but will be messy maybe. It may also attract dirt like crazy.

Thanks John. I thought about just pulling stuff over me, but I toss and turn, so I'm sure I'd kick it off.

if I did something like a neatsheet with a ground cloth, wouldn't I run the risk of water trapping between the ground cloth and my bag?

"… just bring an oversized sheet of something waterproof, sleep on a portion and pull over the remainder if I needed it." I've often done this on short trips instead of a tent when the weather forecast is good (no tarp at all). It works in New England summer/fall conditions. I'd hate to get caught in a long downpour with this setup.

A better solution would be to use plastic for the bottom and something breathable for the top. A good plastic to use is heat shrink film for temporary storm windows. Tape in a tub floor arrangement. If you can get a driducks poncho use that for the top; I've got that but haven't put it together yet. I'll try taping the driducks to the plastic with the double sided tape that comes with the storm window. If you can't get the driducks, use tyvek for the top. Or, use the 1.9 nylon and spray it with a water repellent (scotchguard or something).

Only if the tarp is small for two people and you have blowing rain or your groundsheet is too big or you pick a bad campsite for a tarp (lots of variables). Plenty of people tarp with an 8'x10' for one or two and only use a groundcloth which would stick out on all sides of the bag, but never sticking out past the tarp boundaries.

Frogg Toggs is offering free shipping. $13 for the poncho right now. Could button two together like Bill from TX on here did.

good ideas guys – thanks for the help!

If all you want is a weather cover that will fit over a bag or quilt, Jacks R Better will retrofit a driducks poncho with drawcords and omni tape (velcro) for you.

OK, now I know I'm going off the deep end here, but I was just thinking – I have a can of spray polyurethane wood coating left over in the garage. polyurethane coated nylon is what's typically used in dry bags. you don't suppose I could do something as stupid-simple has spray down the inside bottom of the bivy with polyurethane, do ya? I mean, it couldn't be that easy, right?

I'd guess that's the wrong kind of PU. It dries hard to protect the wood so it would not be flexible for use on fabric. Just a guess though – test a piece and see.

Yeah, it'll help keep it watertight for a while. Try to roll the bivy up with your sleep pad rather than folding it.

Before you get to enthusiastic with the quantity you put on, feel the weight of the spray can. How much of that do you want to carry?

I turned a XXL tyvek overall into a bivy in about an hour and a half with Kath's help on the sewing machine. Works fine. Just cut up the leg inseams and sew together. Then cut the arms off and make them into the hood.

Kier- yes the silicon would work if you were able to get full coverage (not an easy task to say the least). The weight to get full coverage would be way out of line- the cost of the silicon and fabric (you would be ruining the fabric,IMO) would be more than sewing 2 cheap poncho's together. There are a number of issues you haven't discussed:
Condensation
Full rain coverage or just wind blown
etc.

If full rain coverage isn't an issue then use the 1.9 ripstop as the top ant a cheap poncho ($15 at the outdoor store), use some small velcro to attach the rip stop to the tarp (like snaps), overlapping the ripstop over the poncho and you will be set.
But for a little more you can get a Equinox bivy that is 6 oz for about $50- buy it!

I full rain coverage is an issue then there are a number of issues (like condensation) that come into play and a cheap fix will not help.
Use the tarp and the Equinox bivy!