Some homes don’t have attics, they are flat roofed, and some homes have 2-stories with floors in-between. In either case, this is a situation where there is usually 2×12 wood framing with drywall attached to the bottom and some plywood attached to the top. There is no crawl-space, it’s literally like a horizontal wall. In this case we treat it just like a wall: we drill holes and inject insulation into the empty cavities. We use cellulose because it has a higher R-value per inch and it’s the only material that can be effectively blown into a cavity.