According to NFPA® 70E section 130.7 (C) (12) (a), non-melting flammable garments (i.e., non-FR) are permitted to be worn under FR garments for added protection. In fact, putting an extra layer of clothing between your skin and your outer FR layers can provide an added level of protection and has the further benefit of absorbing perspiration to keep you drier and more comfortable in warmer months, and insulate you against the elements in cold weather.
So, what does that mean for you? For starters, meltable fibers such as acetate, nylon, polyester, polypropylene, and spandex cannot be used in the layer closest to your skin. Why? Because in the event of a flash fire or electric arc, some heat will inevitably pass through the outer layer of FR and cause a T-shirt to melt, if it is made from synthetic materials like those fabrics mentioned above. The melting of these materials can significantly increase the burn injury. Undergarments made with natural fibers are permitted by the standards as they will not add to the injury. Assuming there is no break open of the outer layer and the outer layer is worn correctly (tucked-in), both of these scenarios could allow for the base layer made with natural fibers to ignite.
When it comes to layering your FR, the best solution to maximize both safety and comfort is to opt for an FR base layer, like those offered by Bulwark. Our FR base layers are designed to wick moisture and keep you comfortable, while increasing your overall protection by eliminating ignition.