Poorly ventilated homes can have high core, or a rotary wheel core (Figure 2). Parallel-plate cores, whether counterflow or crossflow, are made of aluminum, plastic, or, in the case of an energy-recovery ventilator, a moisturepermeable membrane (see “Energy- Recovery Ventilators,” next page). Some manufacturers tout the theoretical superiority of counterflow cores over crossflow cores, or aluminum components over polypropylene. In practice, however, HRV efficiency depends upon many design factors, not just core geometry or material type. The best resource for comparing HRV efficiencies is the Product Directory published by the Home Ventilating Institute (available online at www.hvi.org, or by calling 847/394-0150). When choosing an HRV, small differences of efficiency may be less important than the level of service provided by a