I have had great experiences with bivvy sacks, using one both for emergencies and for routine use. I carried one for years as part of my SAR gear, since you never knew how long the operation would last.

I recently slept in one for seven successive nights on a paleontologicl dig. Worked out fine.

A bivvy sack provides about ten degrees of warmth to your bag and a good one will keep the rain and dew out. I have wakened to find pools of standing water on mine on occasion.

'Biy sack' todaay includes the traditional ones, meant for a bivouac on a mountain ledge -essentially a large envelope into which you insert your bod. Also marketed as bivvy sacks these days are rigs with poles and mosquito netting -essentially they intergrade into one person tents, being somewhat heavier than the trad models.

My sack, purchased in to 70s, weighs one pound. You can also get twp person sacks, which are equally useful'

Bivvy sacks fill a useful niche.