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Why is Knauf loft roll 1140mm and Rockwool 1200mm wide? I gather either 400mm or 600mm gaps between joists are standard, so if you split the Knauf in three that seems slightly too narrow.

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In North America, standard joist spacing is 16 or 24" and joist thickness is 1½". This means that the void is 14½ or 22½" wide, and those are the sizes of most batt insulation and some roll insulation (with a little oversize for safety and a snug fit).

However, some roll insulation is a full 16 or 24". This is because it's intended to be used crosswise in attics, for example, or in other places where round nominal sizes corresponding to standard sheathing dimensions are more convenient. Another example is with steel framing, where the negligible thickness of the perpendicular joist member isn't deducted from the center-to-center dimension. I suspect that this is the intent of your 1200mm variants.

I also suspect that sizes aren't as rigid between types as you imply, but that both types are available in both sizes.

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