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Old 22nd October 2019
Lives for gear
mutetourettes's Avatar

Originally Posted by Soundman2020 View Post
One thing you should be aware of, is that you won't be able to accurately predict the isolation of your system: the equations for doing that assume that the mass of each leaf is fairly consistent, and that the mass on the inner leaf is roughly similar to the mass of the outer leaf. If there's a large difference in surface density, the equations won't give you the correct answer. In other words, if one mass is a 6-inch thick concrete block and the other is a piece of paper, obviously the equations won't produce correct results. If they are both 2 layers of drywall, they will be quite accurate. In your case, if you put huge amounts of mass on your inner-leaf ceiling and just a thin, light deck for the roof, that's skewed quite a bit.

First, seal the brick. Use a quality masonry sealer all over the inner surfaces. Then, once the roof desk is completed, use backer rod to fill in the gaps as much as possible, and a really good quality caulk that sticks very well to both masonry and also wood (assuming you do an OSB roof deck). Buy several different brands of caulk, and test them all on samples of the same brick and wood, so see which one gives you the best result. Use a caulk that remains flexible and soft after it has cured. And seal the hell out of it! Don't be shy with the quantity. But build it up in layers: first put one thick bead as deep as you can in the gap, then wait for it to cure. Add another bead, ditto. It needs to be thick to build up the mass. The density of caulk is nearly three times the density of OSB, so as long as you get it at least one third the thickness of your OSB roof deck, that will keep the mass consistent. In other words, if the OSB is 3/4", then you need at least 1/4" deep caulk in the gap, to get the same surface density. That's the way I would do it: others might have a different take.

- Stuart -
Hi Stuart - thanks for both parts of your answer - I wasn't aware that skew between the two masses would break the calculations or that the equations 'assume' 'similar' mass.. ho hum... well... I don't imagine i'll actually be doing many equations but I suppose if a thin leaf can't be helped then still the benefits of increased gap and increased mass on the other leaf aren't entirely removed.

Thanks very much for the practical tips around the brick-wood caulking!