Soundman2020 - Studio Design Forum

as long as the outer mass it heavier than the roof sheathing then you should be ok. fill the inner and outer space with a thick layer of pink slightly compress to damp the mass layers.

btw for those who don't know what a collar (or rafter) tie does, here's a nice easy to grasp link. https://www.finehomebuilding.com/projec ... after-ties

Thanks to a little help from Glenn for helping me with preventing a 3 leaf system, I have decided not to use a rafter system for my inner leaf. Now that the collar ties have lowered the ceiling height, I am going to reclaim it by using isoclips instead. Note that I have not updated the sketchup but I wont use those decoupling brackets to the rafters, I will use isomax clips that will time them to the exterior walls instead.

Also the gap between the roof sheathing and the 1" pink rigid foam insulation is to properly vent the cathedral ceiling from the soffits to the ridge vent.

Never sent this update, but we now have a roof, sheathing, and tyvek! It's officially weather resistant. I ordered a door but it takes several weeks.

Sorry to be rather late with this!

As Glenn pointed out, there are things you can do to help deal with it. Which it looks like you already did!

It will create a 3-leaf system, yes... but the real questions are then: How bad that would be? and Can you compensate?
There are equations for calculating resonant frequencies of 3-leaf systems, but they are a little complicated ( ), so maybe not really useful.
If you have a large air path above that "middle-leaf" and below the roof deck itself, up through the eaves then out through a ridge vent (possibly required by code, but a good idea anyway), that would help to mitigate the situation, a little. If the roof deck itself "leaks" air, such as through an old-fashioned tiled, shingled, or slate roof, that helps too. Insulation in that gap (but not filling it, due to the need for the air path) also helps a little. Making that gap as large as possible (oversize rafters, or spacers under the rafter for the middle-leaf) also helps. A large air gap between the middle and inner leaf, completely filled with porous insulation (eg, "pink fluffy" fiberglass insulation, or mineral wool, in both cases of suitable density) will also make a difference. Adding extra mass to the middle leaf and/or inner leaf also helps.
So there's several things you can do to improve isolation if you are forced to use a 3-leaf system. You can do the math if you want to figure out f it is enough, but be warned: it's complicated!
Reasonable rules of thumb: increase your air total gaps by at least 50% above what it would have been for the 2-leaf, and double the total mass on the leaves if possible (or at least increase it as much as the structure can safely handle).
Basic question underlying all this: How much isolation do you need (in decibels), and what's the lowest frequency you need to isolate? Knowing those two can help you make better decisions.


- Stuart -

First load of drywall has arrived for the outer leaf. I’ve also learned that my 10 gallons of green glue is out for delivery today! A good day.

Question: If I use iso clips and furring channel on the ceiling, how do I attach a cloud to it? The instructions say that those clips arent rated for heavy loads. Certainly, 2 layers of 5/8" is already pretty massive. Is it reasonable to to be able to also hang a fairly heavy cloud to it as well? I dont feel like it is.... One thought I had was to pre hang some high tension wire from the rafters and put it through the drywall, then attach the cloud to that.

Thoughts?

generally speaking - depends on the supporting frame and the distribution of clips and hat channel. if your framing is 16" instead of 24", and likewise the iso clips and hat, you'll be able to put much more than the 5.2lb/ft2 than the drywall weight. check with your iso clip manufacturer for load recommendations. also - its good idea to add a layer of plywood to the hat channel as a first layer - this helps distribute the loading better and attaching stuff is more secure (and your drywall attachment doesn't get compromised by missing a hat channel) etc. so minimum 1/2" ply or osb first, then the drywall layers.

The outer door arrived, so figured I'd put it in. Perfect fit.

Soundman2020 wrote:Source of the post ou have a large air path above that "middle-leaf" and below the roof deck itself, up through the eaves then out through a ridge vent (possibly required by code, but a good idea anyway), that would help to mitigate the situation, a little. If the roof deck itself "leaks" air, such as through an old-fashioned tiled, shingled, or slate roof, that helps too. Insulation in that gap (but not filling it, due to the need for the air path) also helps a little. Making that gap as large as possible (oversize rafters, or spacers under the rafter for the middle-leaf) also helps. A large air gap between the middle and inner leaf, completely filled with porous insulation (eg, "pink fluffy" fiberglass insulation, or mineral wool, in both cases of suitable density) will also make a difference. Adding extra mass to the middle leaf and/or inner leaf also helps.

Thanks for the response Stuart! I actually took some measurements today while my BRMM BRMMMMM lawnmowing neighbor was out. It looks like I need 50-55DB of isolation at 40hz-100hz. Its a fairly long tail that decreases over the full range up until 35DB in the upper register above 4000khz

The cavity depth of the rafters with the hat channel would be about 7" between the sheetrock layers. The distance from the dry wall to the exterior sheathing would be 3". 2" of it is rigid pink insulation, 1" air.

The walls would have a 9.375" cavity, so losing some low end in the ceiling with the shallower cavity already. Is that 2nd 1" air gap going to make it worse?

Hi All! Im about to start packing out the wall cavities with drywall and rockwool. Do I need to glue the drywall to the exterior sheathing? I was just going to place it up against it, green glue 2 layers together, cleat that in to hold it tight to the sheathing, and caulk the perimeter. Is this the right process or does something have to happen between the drywall and the exterior sheathing?

Don't glue it! The procedure you outline is fine.
What sheathing do you have there, and is it bare (eg, plywood, MDF, OSB...)? If so, you could consider using Green Glue in there. That would be fine.

- Stuart -

Soundman2020 wrote:Source of the post Don't glue it! The procedure you outline is fine.
What sheathing do you have there, and is it bare (eg, plywood, MDF, OSB...)? If so, you could consider using Green Glue in there. That would be fine.

- Stuart -

Thanks Stuart, its bare 1/2" plywood on the outside sheathing. I'm 16"OC for the layout. so it will be a lot of cutting and packing this weekend!

Soundman2020 wrote:Source of the post Don't glue it! The procedure you outline is fine.
What sheathing do you have there, and is it bare (eg, plywood, MDF, OSB...)? If so, you could consider using Green Glue in there. That would be fine.

- Stuart -

One more question for you, I used sill seal (the cheap poly foam kind, not the rubber kind) all the way around the perimeter of the bottom plate. Do I still need to caulk where the PT base plate meets the concrete? I think I know the answer, but the sill seal is already pretty air tight. I don't know if the additional caulk helps or I'm just spending money.

from the perspective of the frame seal - the rubber should do it. from the perspective of potential gaps once the drywall is going up - i'd use the backer rod and caulk on each layer on the floor (as well as as corners -- walls and ceiling-wall joins) to ensure there is no bypassing into the frame (which constitutes your air boundary).

I'll second Glenn's advice on the backer rod and caulk, and stress that's for EACH layer, just like Glenn said. As each layer goes up, put the backer rod and caulk. Put a shim under the drywall sheet to raise it off the floor by the right amount, then pull the shim once you have the drywall attached. Use backer rod that fills the gap depth about half way, with a good fit, and fill the rest with caulk. Colored caulk is a little more dense than transparent, so go with the colored stuff. Actual color doesn't matter: Use Hot Pink or Lime Green if you want! (And if you can find such stuff... ). White is also a color here.

- Stuart -