FHA Manufactured Housing Loan Modernization Act of 2005 - Amends the National Housing Act with respect to Federal Housing Administration (FHA) housing loan insurance for manufactured homes (or lots for such homes) to: (1) exempt such loans from certain financial institution portfolio limits; (2) establish loan-to-value ratio and downpayment requirements; (3) provide that any such contract shall be conclusive evidence of an institution's insurance eligibility; (4) increase loan limits; (5) set forth borrower premium charges; and (6) direct the Secretary of Housing and Urban Development to establish underwriting criteria that will ensure the manufactured housing program's financial soundness.