I have a beach house near the water on pilings. It is completely open underneath and not closed in. The floor will get R19 insulation. How do you keep the insulation from blowing out from under the house during storms and high winds? Do I cover the whole underneath of the house with Luan or some type of thin plywood? What is the best construction practice for this type of situation? Thank you in advance..

I think the most responsible answer is for you to ask other home owners in your area, or the people at your local city / county offices (building dept.). Actually, ask both. This way you'll find out what has worked well for others (your neighbors) as well as what's legal (the fine folks at the building dept.)!

A Couple Of Different Options Here. 1 You Can Use Viynl Soffet Which Makes A Clean Look But More Costly Than Ply Wood, You Can Use Black Board Which Just An Insulation Board I Think It Runs About 4 To 5 Dollars A Board
The Viynl Soffet Usually Runs About 55 60 Dollars A Square Which Is A 10' By 10' Area

If It Was Me Id Try And Use Viynl Siding. Hope This Helps , Nichols Trim And Carpentry