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Consider cutting one down as a 1 inch filler and putting it lengthwise? :) (It's funny how I'm missing carpentry vocabulary in English...)
Yeah I can just fill it with more lumber, which might be the best move. Apparently these things only really work if it's an airtight seal, so more pieces will create more cracks but it'll be better than nothing.
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Maybe also fill it with foam? That would imo add to sound isolation
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Yeah going to fill it with mineral wool insulation. Basically I’m building a wall on my window sill.
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