I cannot fine homosote board on the HD website any larger than 2' x 4'...which makes this > $42 a sheet. Lowe's lists 2 in 4'x8'sheets. Apparently one is twice the density of the other (based on weight), but the low-density is unavailable (priced in $16 range, the high density is twice that). I'm wondering about putting up Reflectix (foil faced double thickness bubble wrap). Lowe's has that at about $135/400' in 4' x 100' rolls. Either WILL burn, but the foil face on the Reflectix will not ignite from an errant spark or ember.