Inside lining with polyethylene is successful and convenient; use clear, with a thickness up to 6 mil. A77-995
Space between layers = the space between the outer and inner liner should be about 1/2 inch, or the internal convection of air will greatly reduce the insulation value. A70-977
Surface-mounted air-inflated plastic is suggested as an alternative method to reduce high fuel costs. A77-712
The best is two layers of polyethylene to restrict heat loss by up to 40%; the two layers must remain separated by about three inches of dead air; the greatest efficiency is fiberglass panels with an inner layer of polyethylene film with no holes or tears. OD75-122