how much R value does a bivy bag add to a sleeping bag?

[oldgoat]

So I picked up an OR Molecule bivy bag.. Ventia fabric etc.. I have a 20° bag, how much on average do bivy bags increase the rating of a sleeping bag. I know it's not a cut and dry amount, but more like what's your experience? I have a heavy canvas cowboy bedroll that does an amazing job increasing the temp rating of a sleeping bag. Curious what this thing will do for me?

 

[Ramshead]

Over the years the number I have heard most often quoted is 5-10 degrees warmer, but I can't say if this has been scientifically tested and proven. Of course every fabric is different (breathability), so results will vary.

 

[BruinPoint]

I've heard 5-10 as well and I imagine I get 10+ deg in an old Gore Tex military bivy. Hard to say but it definitely feels a lot warmer. It probably depends on whether you're in a shelter too.

 

[Kevin_t]

There are a lot of dependencies such as fabrics and types of usage. Heavier fabrics that are waterproof breathable can add a fair amount of warmth when nearly fully enclosed. Sometimes, simple things can add a lot of apparent warmth , such as covering your face. I had a night in Nov, near tree line, in which my quilt was under powered for the conditions. I zipped up the full mesh bivy, and it made my face warmer, and then my complete body warmed up to an acceptable point. I've measured , inside / outside temps before with a waterproof breathable bivy zipped up most of the way and the difference could be as high as 15 -18 degrees. I've also used the face covering "trick" before, when I was cold without a bivy, by just grabbing a shell and putting it loosely over my face and I usually warmed up.