Will A Tiny Third Leaf Matter?

Hi All, I've started a second floor build above a garage. I have a question I was hoping to get some opinions on. The architect has placed some furring strips on the exterior of the building to be papered/stuccoed over to give the bldg a look of floating out passed the beams. I'm wondering if having this small airspace (1 1/2") between the exterior plywood and the stucco will constitute a third leaf or if the small size will make the effects negligible. I've attached 2 jpgs that may help explain what i mean. Also seriously considering adding a layer of drywall between the exterior studs to get some more mass. Opinions on whether this is necessary are welcome as well. From inside to outside it looks like this: 5/8" Drywall | Green Glue | 5/8" Drywall | 1/2" OSB | Soundboard | 2x4 studs | 1 1/2" airspace | 2x4 studs | structural ply |1 1/2" furring strips | felt/stucco. I hope I've explained this clearly enough. Thanks!

Teh first question in my mind is how much sound isolation do you need? Yes, the third leaf will mater AND make things worse for music isolation. Add more mass to the inner or outer leaf if you want. The current plans show a mess of material in the inner leaf. Using multiple layers of 5/8" gypsum will be better acoustically and cheaper. Good luck! Andre

HI Avare- Thanks for the reply. I live in proximity to a small airport so the more isolation the better. You're saying that even tho the airspace between the 2nd and 3rd leaf is so small (1 1/2") compared to the space between the 1st and 2nd (11 1/2") in your experience there will still be a substantial negative effect? I realize there's a mix of materials in the interior lamination. The OSB is there to give a solid nailing surface for interior treatments. Rather than having to use drywall anchors everywhere. Many Thanks, dd

You're saying that even tho the airspace between the 2nd and 3rd leaf is so small (1 1/2") compared to the space between the 1st and 2nd...

To my mind, it isn't a matter of "comparing" the air spaces, so much as the fact that the thin one exists at all. You have a reasonably massive leaf over a thin airspace: that implies a rather high resonant frequency for that leaf. However, I don't even see where you get that 1 1/2" figure from. Your diagram shows the "stucco lath and felt" over "structural ply" attached to a 2x6 frame, but doesn't show the "1 1/2" gap. Where is it exactly? Also, you say that the studs are 2x4, but the plans show them as 2x6. Which is it? Also, the "mess of materials on the inner leaf" that Andre refers to sure is a mess, and the OSB nailing surface isn't the problem. You show it as "5/8" Drywall | Green Glue | 5/8" Drywall | 1/2" OSB | Soundboard | 2x4 studs", but the plans show it as: "5/8 drywall - 1/2" sound board - 1/2" OSB - 5/8 drywall - 1/2" OSB". No mention of GG. I'd say forget the sound board: it isn't doing much for you. Forget the extra layer of OSB: you only need one nailing surface, and it should be right up against the studs, not in between two layers of drywall. I'd go with something like: "5/8 - GG - 5/8 - OSB - studs". You might also need more mass on your outer leaf. My 0.02 - Stuart -

HI Stuart- Thanks for the reply! Yeah, I know there are some discrepancies in the plans. The project has become a bit liquid because of some unexpected survey results and it's difficult to keep the drawings from the architect and the engineer in sync. The 1 1/2" air space I was referring to is in the Construction details pick. The felt and stucco would be applied over the 2x4 blocking on that image thereby creating an airspace of 1 1 1/2" inches between the structural ply and the stucco. I hear ya on the soundboard as well. I've been consulting with a respected studio builder and he requested that it be in the lamination schedule. I realize it's kind of old school but figured it couldn't hurt. I'm exploring some options with the contractor now on eliminating that airspace somehow. Thanks again for the reply, dd

dannyd wrote:

Thanks for the reply. I live in proximity to a small airport so the more isolation the better. You're saying that even tho the airspace between the 2nd and 3rd leaf is so small (1 1/2") compared to the space between the 1st and 2nd (11 1/2") in your experience there will still be a substantial negative effect?

Stuart gave the good answer. The size of the small space compared to the large space is not significant. The space itself is. Andre

Soundman2020 wrote:

You're saying that even tho the airspace between the 2nd and 3rd leaf is so small (1 1/2") compared to the space between the 1st and 2nd...

To my mind, it isn't a matter of "comparing" the air spaces, so much as the fact that the thin one exists at all. You have a reasonably massive leaf over a thin airspace: that implies a rather high resonant frequency for that leaf. However, I don't even see where you get that 1 1/2" figure from. Your diagram shows the "stucco lath and felt" over "structural ply" attached to a 2x6 frame, but doesn't show the "1 1/2" gap. Where is it exactly? Also, you say that the studs are 2x4, but the plans show them as 2x6. Which is it?

Apparently there are 2 very different conditions - the lower pic indicates the details he is referring to - which is an easy fix.... it can stay just the way it is and install 1 1/2" dow board insulation prior to the exterior wall finish - don't gain mass - but it does solve the 3rd leaf issue.. To the OP - this is a total cluster-f**k......... where in the world is your architect getting his design info from? There are a slew of things screwy with all of this..... starting with the base of the iner wall. If he is making you go through all of the expense to create an isolated slab assembly - then why in the world would you need to try to isolate the wall from that assembly? Either it works (in which case the wall can couple directly to it) or it doesn't - in which case floating the wall and creating structural stability via the clips can offer some assistance......... but this would also mean that you were throwing your money away on the floor assembly...... The wall assembly itself is (as mentioned above by both Sound and Andre) a total waste of money and energy and will do little to help your isolation......... To begin with - I would need to understand this a little better - like why is the architect building exterior walls using 2x4 in some locations and 2x6 in others (or is this an addition to an existing older home?) Inside I would increase the 1/2" space between walls to 1"......... I would add mass (as explained above as well) to the outside wall leaf - and I would be using (again as mentioned above) multiple layers of 5/8" gyp DW with GG over the OSB....... I would also point out that if you were using OSB on all surfaces inside the inner room there would be no need for the iso-clips because the room would be structurally sound in and of itself - which is much better isolation that you can achieve with the clips alone..... But the is a lot of speculation here because I cannot "get" the entire picture....... I would need to see much MUCH more of the plans before I could completely work through this - there may well be other even larger problems that exist that you are not aware of - personally I am not impressed with your architect's ability to design isolating systems - probably be even less impressed with his studio design........ but on that I might be wrong - never know unless I see it......... Rod

HI Rod- Thanks so much for chiming in. I'll ask the contractor about the 1 1/2" dow board insulation which seems like a great solution! As far as the wall/floor assembly goes I apologize for all the discrepancies in the drawings. This project, as with any build I would assume, has been a moving target for a number of reasons. The attached pic is the closest I have to what the actual assembly will be. However the airspace between the studs will be 1 1/2" to accommodate the Mason Industries clips required by the engineer, and there will be no drywall on the interior of the 2x6's of course. Hopefully this looks a little better to you. The reason for 2x4's in some places and 2x6's in others has to do with the property setbacks and some issues I had with the engineer involving him creating the whole north wall of the bldg with the interior wall as the load bearing wall. Many Thanks For Your Input- dd

hi fellas- ok so attached is a drawing of the assembly as it stands now. this particular drawing does not contain the afore mentioned furring strips that were the reason for my original post. but it does show the exact wall/floor connection and lamination. any advice is appreciated. the clip connection between the studs is a neoprene clasp that was required by the structural engineer. as a side note, Rod i looked into the Dow board you referred to. Dow makes so many different versions of this stuff. Is there a particular one you would recommend specifically? Thanks Again, dd

dannyd wrote:

hi fellas- ok so attached is a drawing of the assembly as it stands now. this particular drawing does not contain the afore mentioned furring strips that were the reason for my original post. but it does show the exact wall/floor connection and lamination. any advice is appreciated. the clip connection between the studs is a neoprene clasp that was required by the structural engineer. as a side note, Rod i looked into the Dow board you referred to. Dow makes so many different versions of this stuff. Is there a particular one you would recommend specifically? Thanks Again, dd

Danny, whatever is the least expensive will be fine - the idea here is to damp the surface - one will serve as good as any other. Rod

OK - looked at your new detail...... another question - is the ceiling independent and being carrier by the inner wall? Have a very uneasy feeling about the Machine Rubber........ what (exactly) are they specifying? Loose the soundboard - it is an added expense that will gain you pretty much nothing........ apply the OSB directly to the studs - and then gyp dw over that..... Lose the 2x4's inside and switch to 2x6 24" oc - you get much better LF isolation with the stiffer frame - which is pretty important. and answer the questions I asked above - very important questions those are...... Rod

hi rod- happy monday. thanks for the dow board answer. makes sense. the roof is bearing on the exterior studs only and the ceiling will be same lamination as the interior wall hung from IsoMax clips. I know the machine rubber came from acme but i'll find out exactly what it is today. i'd love to go 2x6 on the interior as well but the space has been shrinking everyday because of setbacks and surveys and at this point i'm a little leary of losing another 4" all the way around. but i do understand what you're saying. rubber details to follow... dd

DD, Seeing are you are trying to isolate yourself from small planes leaving an airfield - you might be well suited to lose that additional 4".......... in years to come you will never miss it - but the airport will probably get bigger as time goes on....... Rod