I'm about to buy my first bivy (for a PCT thru-hike next year). I'll be using it with a tarp, so I'm thinking about something with a sil-nylon floor and a water-resistant/breathable top. I've got 2 quick questions:

1) How reasonable is it to use a bivy without a groundcloth? If I get one with a sil-nylon floor, I was thinking of treating it with a silicone sealant (see http://jwbasecamp.com/Articles/Silnylon1/index.html) to toughen it up a bit.

2) Should I get a bivy with a side opening? If I go without a groundcloth, I'll be using my bivy every night, including a lot of warmish nights. The side opening seems like it would make sense since I could leave the bivy wide open on warmer nights. But, never having used a bivy, I don't know if this really would make a difference. Any thoughts?

A side zip option, as on the MLD Soul side zip, allows for easier entry/exit and provides added ventilation in warm weather. Keep in mind that the side zip, when opened, is not protected with bug netting which can allow ants and other insects access into your sleep system. Tigoat will also add a side zip as an upgrade, for a cost.

I elect to use a ground cover to protect my gear but that's just me. Most bivies will function fine without. You could opt for a tougher bottom fabric on a MLD bivy if you decide to leave the ground cover at home.

I don't think a groundcloth or sealant is necessary. A silnylon bottom on a bivy is very durable and completely waterproof, it already has silicone impregnated into the fabric.

I would recommend a side zip bivy, it makes for easy bed entry and exit. I have slept in my bivy in temps from low 20's to high 50's and never had a problem getting too hot, however the nights in the high 50's I have pushed my quilt down to the bottom of the bivy and slept in only a T-shirt and shorts. A Momentum top bivy is remarkably breathable. It will add some warmth, which can be good if your trying to push a quilt thru colder temps. Again, dealing with the warm temps, while maintaining your bug protection and ground protection it not a big deal, you just adjust the amount of clothing and use of quilt accordingly.

I would recommend the MLD Superlight Bivy among any other. It's proformance, weight, and price are all a 10! It is extermely roomy, so when you hang it from the your tarp there is lots of head room, and no claustrophobic feeling.

Check it out here- http://www.mountainlaureldesigns.com/shop/product_info.php?cPath=22&products_id=30&osCsid=93d6c626a8ce79b4e2a4c301d35ef05e

I find zips on a bivy overkill. There are many bivies on the market that provide a large enough opening to get in easily and not having a zip means less chance of zipper failure, a snag on your sleeping bag, or letting in moisture. Plus it adds weight.

My main thought with regards to bivies is that I highly suggest you make your own. Find someone you know who owns a sewing machine, borrow it for a day, and build a bivy to your own specifications for greatly reduced cost over buying one retail.

I have used a lot of different bivies and I prefer the eVENT models from Integral Designs, my usual choice being my South Col with an ID Silwing. Mine are in OD for hunting.

My "emergency" bivy for dayhiking solo, is an ID Bugaboo in yellow eVENT and used with a light bright coloured tarp, such as my ID Silshelter, this will save your life if you go down with an ankle fracture in cold, wet conditions.

For short camps and emergency uses, a good bivy, bag, pad combo is the best option I know of and I NEVER go away from my vehicle without a pack containing these plus other emerg. gear. Hyper-cautious???, Yeah, but, I am still here after 44+ years of wilderness activities.

Scott S

In response to your thought about additional waterproofing with the JW method, I heartily endorse it. 3:1 for seams, and 4:1 for general extra layer of waterproof. It also removes the slickness of sil nylon, which is a bonus for some applications IMO, it is a good idea for any piece of sil you want truly waterproof. Adds weight, but I don't care, I want to stay dry. My sil nylon never "mists", no matter how hard the rain, and it used to, so I am beliver.