Curable compositions are prepared using polyisocyanates, hydrophobic hydroxyl-terminated polymers (such as hydroxyl-terminated polyfarnesenes and hydroxyl-terminated polydienes) and organically-modified nanoclays, optionally in combination with chain extenders and/or urethane catalysts. When cured, the compositions yield polyurethane-based sealants useful in insulating glass units which have improved (lowered) moisture vapor transmission rate values as a consequence of the inclusion of the organically-modified nanoclays.