We made a blunder last year and packed the compost layers in too tight. Oh, and the main brown material was straw.
I used to read about people making icehouses in this area with straw packed around the ice blocks and still having ice in July. Now I believe it.
The compost has been mostly pulled apart now, and spread out to warm up in the sun, but there's still a few very large frozen chunks. Even after yesterday's 20 degrees and sunny.