In most cases, when the temperatures fall way below zero, the sun is usually available to heat the house. The solar collectors will be positioned to take advantage of the winter sun and should be able to heat the house adequately. Water is by far one of the best materials to store heat. Quite a few radiant systems use large tanks of water to store excess heat. The disadvantages to using a tank are that they take up space, need occasional maintenance, and sometimes leak.
Water is also corrosive to certain materials.
Sand does not nearly have the same capacity to store heat as water, but it can be placed under the house to store heat. The advantages are it's a material that is easy to use and doesn't require awkward tanks to store it. The disadvantage is that you need lots and lots of it, and it is not as efficient in transferring heat. There is some debate as to how well sand performs especially when it is dry. The dryer the sand, the less efficient it becomes.