Query on definition of one leaf using varied masses

[gobberjohn]

My weak point in my studio design is the ceiling and roof.

Currently there is drywall, then insulation then corrugated iron on my shed. My research suggests this is 2 leaves. No matter what I do with walls this is my weak point.

I have recently found a product called Stratco Cooldek which is a polystyrene core that is permanently bonded to corrugated iron. If I took off the existing iron and placed particle board on top of the trusses, then placed Cooldek on this - I would get some mass into the roofing system, BUT would this truly be one leaf?

If this were the case then I could build a decoupled room within the shed with drywall layers to get mass on the 2nd leaf.

My query is - would the sandwich of particle board with Cooldek placed on it be 1 leaf ?

[Soundman2020]

Hi there, "gobberjohn", and welcome to the forum!

Currently there is drywall, then insulation then corrugated iron on my shed. My research suggests this is 2 leaves. No matter what I do with walls this is my weak point

Correct! So the solution is to make it one leaf again: take off the drywall!.

... which is a polystyrene core ...

Polystyrene is closed-cell, at the microscopic level. Which means that air cannot pass through it, the way it does with normal "open cell" porous insulation such as fiberglass and mineral wool. Closed-cell insulation has practically no uses in acoustics. It is great thermally, but pretty much useless acoustically.

If I took off the existing iron and placed particle board on top of the trusses,

Depending on the type of "particle board", that might be an option. If you are talking about OSB, then yes, that would make a good roof deck. So would plywood. In both cases, you'd want it to be thick: 3/4" would be good. For good measure, you could put two layers up there (assuming the trusses can handle the load). But instead of putting corrugated iron on top, just put normal roofing felt, then asphalt shingles. That makes for a pretty decent outer-leaf roof.

BUT would this truly be one leaf?

No. Not from the sound of what you are saying. The polystyrene is not a porous absorber, but it is still a spring, of sorts, so you are still creating a tuned MSM system. I've never looked into the properties of such a system, so I can't tell you how well it will isolate, but considering the resilience of polystyrene, my guess is that the answer would be: not very well!

So, since that will be a 2-leaf system by itself, and your existing drywall is still there, that makes 3 leaves. Then you plan to build your "room-in-a-room" below that, with it's own ceiling, so that makes FOUR leaves...

If that were my place, and I was planning on taking off the roof anyway, I'd proceed as outlined above: take off the drywall below (and leave it off), lay an OSB or plywood roof deck on top of the studs, and put asphalt shingles on top of that (with the suitable underlay, of course!). Assuming that is sealed to the walls, and that the roof deck does not need ventilation in your climate (you didn't say where you live, so I have no idea what your climate is), then you'd have your outer-leaf.

- Stuart -

[gobberjohn]

Hi Stuart,
Thanks for your advice. I'm doing some research on asphalt shingles, they sound excellent. I'm located at Woodend, Victoria - elevation of 1900 feet , so I'm thinking I may have to ventilate the roof cavity using vents with appropriate sound treatments.

The above sounds ideal but I'm not sure what the council will think of the change in roofing material. Is the roof deck of plywood with corrugated iron placed on it a single leaf also or does the air space involved in the corrugations affect its suitability?

Thanks John

[Soundman2020]

Is the roof deck of plywood with corrugated iron placed on it a single leaf also or does the air space involved in the corrugations affect its suitability?

It probably wouldn't make a lot of difference from that point of view, but tin roofing can rattle in the wind if not tied down well, makes a huge noise in a hail storm (or even heavy rain), and can "creak" as it warms and cools. That might or might not be a problem in your situation. Hard to say.


- Stuart -