The short answer is that both HAC and CAC are nearly equally effective because they prevent supply and exhaust air from mixing. Cold aisle containment (CAC) provides excellent control over supply temperatures to optimize CRAC set points. Hot aisle containment (HAC) returns warmer air to CRAC units which can increase cooling capacity. The best approach for your data center will be dictated by your site conditions. At most existing sites, it is easier to implement a CAC solution because there are typically cable trays and other obstructions above the hot aisle. It is also very difficult to implement hot aisle containment if there is not a return plenum. Our Project Managers can help you determine which approach would be most effective in your data center.