Sleeping bag under tarp vs tent

[whitestone]

As a general guide how much difference do you find between a tent and a tarp with regards to how warm your sleeping bag feels? I.E. if a 5C rated bag is fine in a tent, does it struggle when used under a tarp? A small(ish) tent is going to have a build up of warm air that a tarp isn't. Also if there's a breeze/wind then a tarp is going to be cooler.

Just curious rather than anything else.

[ianfitz]

There is a difference. But I'd struggle to put a figure on it. Depends on shelter, pitch, weather, and bivy bag too.

[AlasdairMc]

Wind is the main issue here I find. A tent will generally keep it out whereas a tarp with bivi less so. If your sleeping bag is right against your bivi, you'll feel the cold through it badly, whereas a tent has the air gap to keep you warm.

I've been 10 degrees under the comfort limit in my tent and felt fine, but on the same night with just a bivi I reckon I'd have been properly suffering...

[jBay]

Height of the tarp above you is also a factor

[gentjim]

I usually (though not always) use a bivy bag under a tarp, so find the bag is at least as warm from the bivy.