Many means have been devised to confine heat. Generally speaking, their effectiveness varies in direct proportion to their cost. They range from just leaving the forms in place to elaborate setups in which protective shelters are built and steam heated. In the simpler methods, there is much dependence on the "built-in" heat of the concrete. More expensive protection relies upon artificial heat. Many different insulation materials exist. Ordinary work forms offer some insulating value. Wood form thicker than standard offer somewhat greater insulating protection but not enough to justify the increased cost. Another material is straw. Straw, by itself, is not sufficiently air proof to act as an efficient thermal insulator but when used in conjunction with other materials, it often is useful. It is low cost and serviceable during relatively mild weather, although not as effective as commercial form insulation. More effective are plastic and polyethylene sheets, building paper, sisalkraft, tar paper and like material which can prove helpful at temperatures down to 18 degrees. They are especially effective when laid so that a small dead air space is left between them and the concrete. With the advent of improved, more rugged backing, form insulation is finding an increasing group of users. This material is easy to apply, can be reused often and provides fine thermal insulation. The most effective and also most expensive form of protecting concrete during winter is enclosures. They generally used with artificial heat, either salamanders or steam.