Mad Mike
Well-known member
Kerdi, Permnase, & Weidi type backer boards contain foam, but they are not suitable fireblocks. Lightweight cement boards use vermiculite and foaming agent that suds up to air entrain the cement in manufacturing. There is no residual foam, and vermiculite is fire-resistant.Be careful using ‘cement board ‘ that is designed for bathrooms around fire . Some brands have foam bubbles in it to reduce wieght and mesh scrim that is also not fire retardant. You could use mega board , but it’s $160 for a 4x8 so a bit silly .
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I've never seen cement boards sold in Canada that are not fire rated (but I could be educated).
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