can it be used to insulate between floor joists that are exposed to the outside?

The better plan would be to fill the areas between joists with fiberglass or other insulation and then staple this along the bottom directly on the joists. That way, it acts as insulation, a vapor barrier and a vermin barrier all in one!

It will act as a vapor barrier ... an expensive vapor barrier. However, its unique insulation qualities will be lost if you install the reflective material underneath this wool batting. The reflective surface needs to face toward an open airspace in order to be effective. Aluminum does not radiate heat but it will CONDUCT heat from one surface to another. The aluminum material has the same tempature as the house floor or the the wool insulation if it is in physical contact with one of these . However, when the reflective surface faces the open air in your basement it will not radiate the heat which has migrated from the living space Consider an aluminum cooking pan in your kitchen. If you touch it with your finger when it is hot you will get burned through heat conduction! However, the aluminum does not release heat by radiation. The air which surrounds the hot metal is less likely to "feel" hot when you are standing in the kitchen. Aluminum does not lose its radiant heat (more or less) unless it is in contact with another material. Use the aluminum insulation by itself or place it on the outside surface after you have installed the batting.. You might also create air space on the inside of the floor cavity between the aluminum sheathing and the wool batting but do not compact the wool insulation. The reflective surface will sill face toward the basement floor but this will protect you from radiant heat loss on both sides of the insulation material (provided this insulation has a reflective surface on both sides of the material). The heat will stay within the floor cavity and keep your floors warmer. Consider reflective insulation in your attic as an example. Sometimes the wood surface on your roof will have aluminum insulation which faces inward toward the attic space. The solar heat from the outside radiates into the asphalt shingles and heats the wood surface inside of the attic - the aluminum surface inside the house also gets hot from contact with the wood but the heat does not radiate into your attic. The SAME PRINCIPLE will apply to your basement project. Install the batting first unless you are sure you will not want to do this later. The reflective insulation can always be installed at a later time if you want to improve your insulation system. Reflective aluminum sheathing can prevent the loss of heat which has accumulated within the wool batting.This will keep your house floor warmer in the winter. It will stop heat convection and reduce the circulation of air around the wool batting. In this way you prevent two forms of heat loss which are convection (i.e. convection ovens) and radiation (i.e. aluminum cookware). If you have issues with moisture you might try installing a moisture barrier directly on the basement floor and investing in a dehumidifier for this purpose. You do not need a moisture barrier on the floor surface, in my opinion. The house floor and basement are less affected by the higher tempatures variables at work in the wall cavities. I hope this is helpful to you as I have done a bit of research on energy conservation at government related websites and at an educational website that is related to the state of Florida.

I think in your climate zone you should be careful about installing a vapor retarding material in the location you're suggesting. Maybe ask a code official or a certified energy rater or BPI analyst in your area if a vapor retarder would appropriate under the subfloor or if it would cause a moisture issue. If you're floor is over a crawlspace that is ventilated (exposed to open air) I would say that any type of vapor barrier would go on the ground. There are different classes of vapor barriers, all based off the permeability of the product. See if you can find that "Perm Rating" for this product. That would help a code official type understand what you had going on. Good luck!