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Comment by sanj | Hacker News Reader

Any idea why you’d want two layers of drywall?

We’ve done it throughout many parts of our house for sound insulation. Apparently it works best when you use two slightly different pieces of thickness. Not sure if that’s why it was done in this case though. It doesn’t seem like the best choice for thermal insulation.

Two layers of 9mm used to be quite common in UK if you are aiming for a robust, heavy, quality feeling wall. One layer of 12mm feels cheap, not that hard to put a hole in it while moving furniture etc. One layer of 15mm board is common now though.

It is generally recommended for sound insulation and strength (ability to hang heavier things onto walls).

I was wondering if perhaps drywall isn't the right word or perhaps it's a different material?

In the US we have drywall and something similar called blue board that as far as I can tell is only used in the North East. They seem similar at first but they're very different.

Blue board is designed to accept a skim coat of plaster well. Which makes it easier to get nice looking flat walls without having to texture like is common with drywall.

My understanding is that blue board is also much stronger than drywall and has better sound deadening properties.

Most of the texture in drywall comes from the paint and is intentional to hide imperfections. Joint compound is used at the seams to hide and smooth out imperfections, it's of a different texture than the drywall itself and a truly flat paint job would reveal all the seams. Blue board avoids this by coating the entire surface and joints in a skim coat of plaster.

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