I use flannel, however, I do think that fleece might be better. Its hot a lot here and my sewing room is upstairs so I probably use the A/C 10 months out of the year. Where my design wall sits, there tends to be a lot of air from the A/C unit blowing and it does blow the blocks off of the flannel. At some point, I'll probably recover both pieces of insulation with fleece, but for now I just stick a pin in the ones that seem to fall off.