It looks like an army issue goretex bivvy. I use one a lot. It's good kit. You shouldn't have any trouble with condensation inside it.
Moisture from your body is driven out through the bivvy bag and may condense on the outside of it, but that doesn't matter.
If it's in good condition you can use it in more or less any weather. I've woken up lying in an inch of water in mine and it's still kept me dry.
Very cold temperatures, where water will freeze outside your sleeping bag but inside the bivvy bag will prevent the water vapour from leaving through the bag.
A hot water bottle can help to force moisture out of the bag, and one inside my sleeping bag is my idea of luxury.
A second modestly-priced sleeping bag will give you lots of flexibility, you can use just one, or the other, or both inside the bivvy.
Even if I'm using a tent I still use a bivvy bag, it keeps you much drier and warmer.