for under a tarp ( i am assuming full coverage tarp) you don't want waterproof. breathable is the answer. Momentum as Chris answered is the A#1 choice. However if the $14yd price is too steep for you check OWFINC.com they have breathable 1.1 rip for a lot less. I like the red, it's cheaper becuase they think it's a 2nd. I can't figure out why in the 20 some yds i've ordered of it. In fact Chris has a bag i made using this exact fabric. I order bulk and pay under $4yd for this. Always find a way to order 20yd of 1 type of fabric from them to get wholesale on your entire order. A great savings, you just have to plan all your projects at once, and fund them too.