So you not only need to be concerned about how much air goes through which tiles, but you also need to be very knowledgeable about exactly where you install grate-type tiles. If you put them in a place where air volume and pressure are already low, they're going to do little for you and will probably make things worse somewhere else. Other then trial and error, the best way to make these determinations is through CFD modeling.
Also keep in mind that, while the GrateAire tile offers 58% open area, this is only if the tile does not have a damper. Dampered tiles allow you to control the air flow at each location so that you don't waste air in places where less cooling is needed. However, the dampered GrateAire tile has a maximum open area of only about 33%. This is not a whole lot better than a fully open standard perforated tile, so we don't specify it very much. But since the 58% open tiles affect under-floor air volume and pressure, we use them very carefully and only after performing a thorough CFD analysis.
This was first published in November 2005