<packages>
  <package id="Dapper" version="1.50.2" targetFramework="net45" />
  <package id="Npgsql" version="3.1.7" targetFramework="net45" />
</packages>

var sqlBuilder = new NpgsqlConnectionStringBuilder
                {
                    Host = host,
                    Database = database,
                    Username = user,
                    Password = password,
                    Pooling = false,
                    ClientEncoding = "SQL_ASCII"
                    //Tentei "UNICODE", "utf8", "win-1252"
                };

// Tentei: SQL_ASCII, win-1252, unicode
 connection.Execute("set client_encoding = 'SQL_ASCII'");

 var data = connection.Query<T>(strSQL);