When possible, it's a good idea to make every layer flame resistant.
When it comes to layering FR clothing, both the outerwear and the regular work clothes worn underneath it need to be flame resistant. The purpose of the outerwear, however, is not to improve the FR level or arc rating of the entire ensemble, but rather to provide protection from the cold and the elements without adding a flammable layer on top of the work uniform. In other words, if the overcoat or outer layer is removed, the garments underneath must still provide sufficient and adequate FR protection.
(See these 6 Key Fire Resistant Protective Clothing Options to Consider for more details)
A base layer worn underneath an FR outfit does not itself need to be flame resistant. However,
it's important to note that according to OSHA standard
CFR1910.269, employers must ensure that employees exposed to flames or
electric arcs do not wear clothing that could increase the extent of an injury. This means that the base layers, while not necessarily FR, must still be made of materials that cannot melt or drip. In other words, they must not be made of synthetic fabric or synthetic blends. All base layers must be 100% natural, such as wool, cotton, or silk.
The best winter layering system should include the following:
- A base layer intended for cold climates (to ensure it wicks away moisture and transports it to the next layer)
- An FR layer that is breathable, offers stretch, and has high warmth-to-weight performance
- FR outerwear that is breathable, wind resistant, and water repellent