Egg Substrate Preferences - Whats yours?

[MarkS]

If you use substratless like above it dont matter what you use as long as it holds humidity well.

Actually, you'd want something that will 'release' moisture well. Perlite is hard and won't compact down so you get more actual surface area then you will with vermiculite which will clump together. Greater surface area will give you a higher evaporation rate which equals higher humidity. Though in actual practice it probably doesn't make that big of a difference.