NEW Basement Studio Design in New York. Looking for advice

[richroyc]

hey i've been making some more progress with the framing! even got some of the ceiling joists up! - I'll post some photos soon.

I have a question regarding wood typer for door jambs.

rod's basic detail calls for "5/4 stock wood jamb"

DOOR OPTION 1.jpg

i was thinking about using a 2x10 lumber for this. (1-1/2" thick) - Then I'm thinking regular lumber may not really be sound proof as it may have knots or split over time and not be airtight.

Can I use (2) layers of 3/4" OSB to make us the "5/4 stock wood jamb"?

[richroyc]

Here are some more construction photo updates!! Things are moving along pretty steadily!

I had one question regarding the wall construction. I'm pretty sure I'm doing it right but just wanted to double check since I don't think I specifically detailed how I wold be building the inside out walls.

You can see from these photos how I did it. Basically same exact method as the existing ceiling "beef-up". Just added the gypsum in between the studs, with copious amount of acoustic caulk. The inner leaf is made of (1) layer of 3/4" OSB and (1) Layer of 5/8" Gypsum.

partial gypsum installation.jpg

partial gypsum installation 2.jpg

partial gypsum installation 3.jpg

partial gypsum installation and ceiling frame.jpg

looking into control room.jpg

this looking okay?

The ceiling will be done in a similar way, I'll post some photos of that as I already installed (1) ceiling module into the ceiling framing grid.

[Gregwor]

this looking okay?

I'd throw some sealant around the beef up drywall as well.

I can't make out where we are looking. But the beef up looks right!

The ceiling skeleton looks like normal dimensional lumber. How far is the span? Did you get an engineers stamp or at least an email confirming that the dimensional lumber will hold the dead load?

Greg

[richroyc]

Hey Gregwor Thanks for the reply!!

I'd throw some sealant around the beef up drywall as well.

check the photos below - plenty-o-sealant

The ceiling skeleton looks like normal dimensional lumber. How far is the span? Did you get an engineers stamp or at least an email confirming that the dimensional lumber will hold the dead load?

The ceiling grid is 2x6. the span is almost but not quite 11'-0" with the joists 16" oc. Although I'm not planning on adding too much weight to the ceiling (no hanging cloud in the booth etc..) the ceiling would be able to handle quite a bit of extra weight. I have the calculations somewhere.

Yes my architect/engineer gave me the size for those joists in writing, they helped me design my whole ceiling plan actually because there were steel beams involved which were basically impossible to calculate on my own what size to use. The formulas for determining steel beam size had bunch of terms I couldn't quite wrap my head around ("moment on inertia", "modulus of elasticity"). lol. so I needed an engineer.

Here's some photos of the completed ceiling framing/grid. I also installed ONE of the ceiling modules. I believe this similar to the method that Stuart uses. I basically built the module on the ground, 2x3 frame with 3/4" osb on top, forming a box with the bottom open. Then 5/8" gypsum is added to the inside of the box up against the osb, actually this is a "beef-up" method as well. I am referring to anything where the gypsum goes in between the framing as "beef-up".


Here you can see the inside out wall - now with the gypsum complete (beef-up style)

walls and ceiling gypsum.jpg

Cut outs for the A/C and conduit for electric.

walls and ceiling gypsum 2.jpg

One thing that slightly concerns me is the bottom plate of the wall. The 3/4" OSB that is behind goes all the way to the floor. But then in front, the bottom plate is becoming part of the second layer. Is this okay? In other words, I guess I'm asking is a 2x4 lumber laid on its side soundproof (<---LOL please don't laugh) Lumber tends to have knots and can split, etc... this is treated lumber with no knots. Or can any lumber or framing parts be part of the LEAF mass? Let me se if this sketch helps clarify...

hand sketch wall.jpg

Do I need to cover that bottom plate in gypsum?

a little tricky to get the vapor barrier in the right spot

ceiling grid with module.jpg

close ups of the installed module (still needs to be caulked around the edges) - you can sort of see the additional mineral wool on top of the module filling the remaining 2" airspace.

ceiling grid with module close up.jpg

ceiling grid with module close up 2.jpg

Doing the ceiling this way is pretty much the only way I can imagine to MAXIMIZE the ceiling height - the "acoustical" ceiling is only 1-3/4" lower than the existing floor joists.

The bottom of the grid joists is a bit lower around 3-3/4" lower than the existing floor joists.

[richroyc]

my sketchup is not working on this computer so forgive the rough hand sketch but this is the basic idea of the ceiling grid + module.

hand sketch.jpg

[Soundman2020]

forgive the rough hand sketch but this is the basic idea of the ceiling grid + module.

Are you sure that 2x6s are rated to span the distance you have between walls, with that load, and the typical deflection used for studio ceilings?

Also, I'd leave more than 1/4" gap between the existing joists and the top of the new sheathing: Firstly, there's no space for insulation there 7betwen existing joist and new sheathing), and secondly, even a tiny error in construction would create a flanking path and trash your isolation. As would slight swelling in the OSB / drywall due to humidity changes, temperature changes, or structural stresses over time. Even a slight warp that happens to develop in one of the joists could force the sheathing to bend up slightly by that 1/4"...

Yes, it's a good idea to save as much space as you can, but not at the risk of losing all you isolation. Structures are not static: they move, settle, and change shape over time for many reasons, and you need to take that into account in your design. Leave enough space for structural movement, and also for insulation.

Finally, you don't show how you plan to hold the drywall in place to the underside of the OSB... That's important. You can't just nail it there...


- Stuart -

[richroyc]

Hey Stuart. Thank for the reply!

Yes I'm sure that's the size my architect/engineer told me to use for that span. I can probably dig up the email and post it n here I'm mobile right now. I remember because I wanted to use 2x4 and he said no to that. I guess it's possible that I gave them the wrong weights for my load but I'm pretty certain on that as well I remember over estimating everything g to be on the safe side

Just looking on my own at joist span calculators it looks like it's ok as well even when I push the load up past what I will have it still says I'm ok. I guess it's not too late to go with 2x8's but it would be a pain. I'll triple check everything again but I know I double checked before I ordered the material.

As for how the gypsum is fastened to the osb, yes it's just screwed in with drywall screws. I figured this is ok since this is how the rest of my house and other houses I've seen built in NY and elsewhere is done? They just screw the gypsum to the bottom of the joists which are usually 16" oc Same as Walls. Maybe I'm missing something? I could always use cleats the same way I did the beef up if that is the suggested method? Are you thinking that the osb isn't meant to hold weight in that direction? That I can understand. The piece of osb is very narrow only 16" wide and the span between the frame 3" less - 13". This particular detail of the plan I did not get approval from my engineer. I could always redesign the modules so that the Gypsum is the same size as the osb and they both sit on top of the frame. That would mean I would have to use wood smaller that 2x3. Maybe 1x2? Or use cleats similar to the existing ceiling beef up?

As for the 1/4" gap, that is actually the measurement in the tightest spot. I guess either my floor is not 100% level, or my existing ceiling joists are not 100% level. Because in some areas the gap is actually greater than 1/4" probably 1/2" in some areas.

I think it would be a great pain to re-do all of this now so please let me know if what I described is ok or if I'm headed for disaster? Is there's any way to monitor the non-static nature of the structure as I'm building to see if it's heading in a bad direction? When I do the larger control room, I can make sure to leave a larger gap...

As for not being able to insulate the 1/4" gap you are correct I am not putting any insisting in that small space. I think 1/2" seems too small to try and fill as well no? I was not thinking. I would achieve 100% perfect full with the cavity? Is this small area a big issue?

[Gregwor]

That would mean I would have to use wood smaller that 2x3. Maybe 1x2?

Part of the awesomeness of inside out ceilings is their ability to have insulation in them. even with your 2x3 and drywall beef up eating up another 5/8" of that, it's leaving you with very little space for insulation. You should have 4" of insulation up there. You maybe should have used 2x8 for your skeleton and then 2x6 for your module frame to allow for a nice thick layer of insulation in the modules.
Something to consider for your other room anyway!

Greg

[Soundman2020]

Yes I'm sure that's the size my architect/engineer told me to use for that span.

What is the span there? From the photos, it seems to be quite a few feet...

As for how the gypsum is fastened to the osb, yes it's just screwed in with drywall screws. I figured this is ok since this is how the rest of my house and other houses I've seen built in NY and elsewhere is done? They just screw the gypsum to the bottom of the joists

In other words, it's not the same! Think about it: As you say, all those "other places" attach their drywall to the bottom of the JOISTS... (which have 15" of empty air between them....) you want to attach yours to a flat piece of OSB... Not the same at all. If that was a valid way of doing it, then you could have done the same thing as you did for the "beefing up" between the joists above you. This situation is exactly the same as the "beefing up" situation: you have two small panels of sheathing that are in direct contact with each other but still need to act independently. If you screw them together, they can't act independently.

I could always use cleats the same way I did the beef up if that is the suggested method?

Are you thinking that the osb isn't meant to hold weight in that direction?

Nope. Not a problem. It's about acoustics, not structures.

As for the 1/4" gap, that is actually the measurement in the tightest spot. I guess either my floor is not 100% level, or my existing ceiling joists are not 100% level. Because in some areas the gap is actually greater than 1/4" probably 1/2" in some areas.

What is up above you? That's the floor of a room above, right? So what happens when somebody walks across that floor? Or moves a heavy piece of furniture? Did you take into account the downward flexing of those joists under the load? For example, if that's an 8 foot span above you, and is designed for 1/360, then that's already more than 1/4" flexure, just from normal live loads...

Plus, if you don't have a gap with insulation in it, then you don't have much of a damped MSM spring up there! Even if the joist never touches the OSB, there's only that 1/4" air gap there with zero damping in it, so any vibration in the joist would be easily transmitted across that gap and into the sheathing below...

As for not being able to insulate the 1/4" gap you are correct I am not putting any insisting in that small space.

See above comment...

I was not thinking. I would achieve 100% perfect full with the cavity? Is this small area a big issue?

Do the math: What is the surface area of the bottom face of all those joists, vs. the surface area of the drywall in between? I'm betting those joists represent at least 10% of the entire surface area, probably more. Is 10% a large percentage? Will there even be much transmission across that gap? If you happen to have an old-school tuning fork laying around some place, you can test this yourself: Tap the tuning fork on something hard, and hold it 8" away from your ear, with a pillow in between (to represent the absorption). Now tap the tuning fork again and this time hold it 1/4" away from your ear, with nothing in between: only air. That's the difference in how much sound would be transmitted across the tiny uninsulated 1/4" gap, compared to how much sound is transmitted across the rest of the ceiling cavity (and this is not even considering MSM: just plain direct transmission). Draw your own conclusion about the difference in surface areas! Is loud transmission across 10% of the ceiling a big deal, compared to very quiet transmission across the other 90%? Do the tuning fork test, and you be the judge... It's your ceiling, so you are the only one that can decide if this will be acceptable for YOUR situation.

Personally, if that was my place and I was shooting for high isolation, I'd want at least a 1" gap there, with insulation in it. But if you don't need high isolation in this room (going in both directions: in and also out), then you could leave it like that, as long as you are OK with the reduced isolation.


- Stuart -

[richroyc]

Stuart! Thank you for the response!!

Ok so firstly I dug up the email from the architect and engineer. See photo below: He was actually the one who boldedthe 2x6, not me lol.

Engineer email.jpg

What is the span there? From the photos, it seems to be quite a few feet...

The span is a little less than 11'-0". About 10'-8".

I believe the number I came up with was around 13psf for the load - I used 3 psf for gypsum 3 psf for OSB, 2 psf for 2x6's, 2 psf for 2x3's, 2 psf for insulation, 1 psf for fabric/fastners. (all which were overestimates)
I was also unsure if i was getting my load calculations right (or didn't want to be responsible) so I just told the engineer what materials I was using and how, and he came up with the everything.

You maybe should have used 2x8 for your skeleton and then 2x6 for your module frame to allow for a nice thick layer of insulation in the modules.

I would have loved to put 4" of insulation/treatment in the inside out ceilings for sure! However, even if i went/go with 2x8's (or 2x10 for that matter) the bottom height would still be the same. See my beams go up inside the space between the existing floor joists. So I am always limited by the bottom of the existing floor joists. Upping the size of the 2x6 to 2x8 would not change anything acoustically, only add structural support. If I had more height to work with - I would have 100% done it the way you described. If I was to re-do ALL of the walls, and lower the ceiling by 2 or 3 inches, then yes I could do it this way. However, you'd barely be able to stand up straight without hunching over . Then once you get into door openings with 2x6 headers below the double top plate, you basically have a door that you have to crawl through to get into the room. I had to choose lesser of two evils here.

This situation is exactly the same as the "beefing up" situation: you have two small panels of sheathing that are in direct contact with each other but still need to act independently. If you screw them together, they can't act independently.

Independently?

I don't think I am following? Did I do ALL MY WALLS WRONG TOO? I screwed the gypsum to the OSB? they are 100% directly fastened to eachother rigidly. I did NOT use green glue. Is green glue REQIURED? I know green glue is recommended often but I thought it was optional?? Why can I not have the gypsum screwed into the osb? I though the whole point was to make a massive leaf?

I've been on here for 3 years and I dont even know how to build a wall!

I thought the reason I used cleats for the "beef-up" was so that I didn't risk putting a nail through the sub-floor or pushing up any floor finishes above?

Stuart, to be clear - I will 100% do this the right way as you described with the cleats, I just want to know if I missed something major and I'm doing all my walls wrong too?

Do the math: What is the surface area of the bottom face of all those joists, vs. the surface area of the drywall in between? I'm betting those joists represent at least 10% of the entire surface area, probably more. Is 10% a large percentage?

Ok yes 10% is a significant amount, I guess I was thinking the MSM system had more to do with Volume of the cavity. So yes, 10% if you are only looking at the surface area, but when you look at the cubic volume of the overhead cavity, we are talking like less than 1%? Just a visual glance at one of the bays you can see that it's about 99% filled, not 90% filled. Either way, this point makes a lot of sense so I think I will be increasing the gap in the control room.

I will also post some photos tonight of the actual gap with a tape measure, I don't think it is as tight as I made it seem. I think maybe in one spot the gap is 1/4", the rest is way closer to 1/2" and in some spots its more than 1/2".

then you could leave it like that, as long as you are OK with the reduced isolation.

As far as the reduced isolation I'm starting to lose track of all the many many many places where I have settled for (slight loss in isolation) over and over again. Single doors, less MSM distance than I would have liked, AC openings, electrical conduits, no green glue...... I'm wondering if I'll have any isolation at all once these slight losses add up to a big loss in isolation

What is up above you? That's the floor of a room above, right? So what happens when somebody walks across that floor? Or moves a heavy piece of furniture? Did you take into account the downward flexing of those joists under the load? For example, if that's an 8 foot span above you, and is designed for 1/360, then that's already more than 1/4" flexure, just from normal live loads...

So the existing floor joists are not 2x6. They are a bit thicker/wider, I think they are true size 2" x 8" - I'll double check that tonight as well. They don't deflect as much as a 2x6 would. I did not do any formal tests but I did have about 5 people standing in the living room (which is fully furnished) directly over this room and did not see any thing.The gap seemed to be roughly the same, with 5 people standing in the room above. I realize this is not something you can really see without specifically checking for it so I will do a more formal test.

[Soundman2020]

The span is a little less than 11'-0". About 10'-8". I believe the number I came up with was around 13psf for the load

OK, that sounds about right. It looked like a longer span in the photos. Optical illusion, I guess.

Independently?

Right. The layers of a leaf act both together, as a single mass, and also independently.

I don't think I am following? Did I do ALL MY WALLS WRONG TOO? I screwed the gypsum to the OSB? they are 100% directly fastened to eachother rigidly.

You should only use screws (or nails) into the STUDS (or joists). Yes, the screws on the drywall will also go through the OSB, then into the studs, but you don't ever screw only into the OSB where there is no stud behind. If you screwed the drywall into the OSB away from the studs, then that's not so good. It's the same as gluing the layers together.

Screwing or gluing two panels solidly together will make them act as one, with one single coincidence dip frequency taht will be LOWER than the individual coincidence dips for each panel by itself, and the bend/flex characteristics will also be different from having the panels separate. In almost all cases, this is not good for sound attenuation. When the panels are only attached to the studs, with broad open areas between them, even though the faces are in contact they can still move independently, with the layers sort of "sliding past" each other if necessary, especially with the case of bending waves, but also with plain old flexing waves. To understand with a simple practical illustration: pick up any paperback book that you happen to have on hand, and firmly grip the spine in one hand and the opposite edge in the other hand, pressing the pages together tightly. Now bend the entire book, so that the front bends towards you, then away from you, then towards you again. Not what happens to the pages in the middle of the book: they slide past each other as the "flexing wave" moves through the book. In effect, that's what happens to the layers in a leaf on the wall as sound waves cause it to flex, just on a smaller scale. The friction between layers adds to the isolation, since it removes some of the flexing and bending energy. And that's what Green Glue helps with: it damps that "sliding" action, since it is viscous, so it removes even more of the energy than friction alone would. But if you rigidly attach the layers to each other with screws out in the "field" of the sheathing, then that action cannot happen, and the two layers act more like a single, solid, rigid block, with a lower coincidence dip. This is the same reason why windows should be laminated glass, which is two thinner panes of glass with a flexible PVB bonding layer between them. That allows the two panes to flex and bend independently, as well as together, and the interlayer absorbs some of the flexing/bending energy. That's why laminate glass has superior isolation to what you'd get from a single solid glass pane of the same thickness.

I did NOT use green glue. Is green glue REQIURED?

No, it's not required. Not at all. It helps, yes, since it puts a damper on several types of resonance that happen in the wall sheathing, but it is not NECESSARY to get good isolation from a wall. It's just an extra boost for low frequency isolation. It works, yes, but it's expensive. So it's a cost/benefit thing: If you absolutely must have the best possible low frequency isolation, then it is worthwhile using Green Glue. But if you don't absolutely need that extra boost, and can't afford the extra cost, then that's OK. The wall won't isolate quite as well as it would have WITH the Green Glue, but it also won't be terrible! You can get roughly the same boost as Green Glue would give you, by simply adding one more layer of drywall.

So basically: It's nice to have if you can afford it and can justify the cost, but is not required. Not by any means.

Why can I not have the gypsum screwed into the osb? I though the whole point was to make a massive leaf?

Yes, you certainly do need the mass! But mass is only one aspect of how walls isolate. There are four main aspects: Mass, spring, resilience, and damping. And there are five regions of the spectrum where each of those has a different role, with some being dominant in one area and others dominant in other area. For frequencies BELOW the MSM resonance of the wall, resilience rules. Down that low, it is the key aspect. The walls "breath", like a balloon inflating and deflating, in sympathy with the sound pressure. You want the wall to be flexible and resilient but also damped for very those low frequencies, since that's the major mechanism that is removing energy from the waves. Around MSM resonance, mass, spring and damping matter most, with resilience being less important. Between MSM and coincidence, mass matters most (although the others do still play a role: just reduced). At coincidence, we get back to resilience and damping. Above coincidence, it's mostly mass again. There's a LOT going on in a wall to get you good isolation at all frequencies. Imagine how hard that wall is working when you have a deep sub-sonic thump from the kick, plus the bass guitar doing things around MSM, the keboards fleshing out the middle, the electric guitar smashing into coincidence, and the cymbals screeching through the top end... all at once! The wall is doing all sorts of stuff together to isolate all that, and of course, there's also interaction between them: the ultra-low flexing of the wall has the bending wave from the bass riding on it, with the mids and highs superimposed on that... so each panel of the wall is vibrating in several different modes at the same time... It gets complex!

I've been on here for 3 years and I dont even know how to build a wall!

It's not that bad! You have built them quite well... maybe not optimally, but certainly not bad!

I thought the reason I used cleats for the "beef-up" was so that I didn't risk putting a nail through the sub-floor or pushing up any floor finishes above?

Not really: it's mostly to keep the panels acting separately. If it weren't for that, you could just use shorter screws...

I just want to know if I missed something major and I'm doing all my walls wrong too?

I'm not 100% clear on how you did those: if you ONLY put screws through the drywall where the studs are, then that's great! But if you ALSO put ADDITINAL screws through the drywall and into the OSB at locations where there are NO studs behind, then that might be a problem... The magnitude of that potential problem is hard to predict...

Ok yes 10% is a significant amount, I guess I was thinking the MSM system had more to do with Volume of the cavity.

Actually, not really! there's no parameter in the MSM equations for volume. Only for depth. There's only three variables that you can change in the equation: the mass of the leaves, the distance between them (depth of the cavity), and the damping factor (given by the insulation). The volume of the cavity does not come into it, for MSM. Here's the actual equation:

f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5

Where:
C=constant (60 if the cavity is empty, 43 if you fill it with suitable insulation)
m1=mass of first leaf (kg/m^2)
m2 mass of second leaf (kg/m^2)
d=depth of cavity (m)

That's it. No volume.

but when you look at the cubic volume of the overhead cavity, we are talking like less than 1%?

Right, but not relevant!

Just a visual glance at one of the bays you can see that it's about 99% filled, not 90% filled.

And that's a good thing! But that is "damping factor" in the equation, not "cavity depth"...

Either way, this point makes a lot of sense so I think I will be increasing the gap in the control room.

One way you could do that is to simple drop the "modules" down an inch. Instead of having the bottom face of your module flush with the bottom of the joists, they would be an inch lower. That still gives you an inch and a half of contact between module frame sides and the joists, which is fine. Yeah, that means the bottom edge would be an inch lower, which might not be so pretty... but I'm assuming you'll be covering those modules with fabric of some type, so you could just paint the bottom face of the joists black, and that can look nice up there.

As far as the reduced isolation I'm starting to lose track of all the many many many places where I have settled for (slight loss in isolation) over and over again. Single doors, less MSM distance than I would have liked, AC openings, electrical conduits, no green glue...... I'm wondering if I'll have any isolation at all once these slight losses add up to a big loss in isolation

How much isolation are you shooting for? What's your "number"? (in decibels) If you really suspect that you aren't going to get there, then you can always add another layer of drywall to all of your inner leaf. If you do that on your ceiling you should re-check with your engineer, but from what you said above, there seems to be enough margin to handle the extra load (the walls are not a problem from that point of view).

There are solutions...


- Stuart -

[richroyc]

Stuart! Thank you immensely for the in-depth and extremely informative reply!

Totally makes sense now about the wall panels being independent and "sliding" past each other as part of their isolation. I shall use the cleats!

For the walls, yes I landed the screws on the studs (or at least that was the goal, there was no real way to check behind to see if I missed slightly). I didn't put any screws in between the studs just to the OSB (at least not intentionally).

Dropping the modules seems like a great solution!

here's no parameter in the MSM equations for volume. Only for depth.

Gotcha - I was always wondering about that - In a previous post I even had proposed a sealed "chimney" to increase the cavity volume, but now I see that would not make any difference.

I have a few more questions I am fleshing out before I post on here, but just wanted to say thanks right away since you replied so fast!

[Soundman2020]

For the walls, yes I landed the screws on the studs (or at least that was the goal, there was no real way to check behind to see if I missed slightly). I didn't put any screws in between the studs just to the OSB (at least not intentionally).

Then you should be good!


- Stuart -

[richroyc]

Just a slight update regarding the ceiling modules. So I took some measurements.

Here is the gap between existing and new ceiling. It's actually about 3/4" in almost all areas. There however definitely are some spots that are as narrow as 1/4".

gap close up.jpg

gap close up 2.jpg

I think in addition to the floor joist not being 100% level, the new ceiling grid is actually slightly deflecting downwards under its own weight, making the gap larger the farther away from the wall. If I do a pull-up on the ceiling joists, I can visually see the ceiling deflect down even more. I'm thinking that once all the modules are in, there would be similar weight deflecting the ceiling downwards, making the gap larger than it currently is. Does this make sense? Obvious, around the perimeter, the gap would stay where it is, but towards the middle of the span it would be increased?

Also, see photos of how the (1) module is currently installed (still need to redo/add cleats). In order to have the top flush, I actually recessed the module into the framing slightly about 1/4"

module far away.jpg

module close up.jpg

So if i was to bring the module down flush with the bottom, that should give me the overall 1" gap. At least this would be the case in MOST areas. Then, factoring in the deflection that will happen on the new ceiling, that gap may even be more than 1"

Let me know if this makes sense. Flush bottoms for the modules and framing would be the easiest to deal with anyway.

Side note: The existing floor joists are 1-3/4" wide by 7-1/2" tall.

[Soundman2020]

the new ceiling grid is actually slightly deflecting downwards under its own weight,

By how much? Is it the same amount as you factored in when doing the span calculations? If it is greater than the factor you got with the span calculations, then that could bs a sign of overloading. If it is less, then you are fine. But it's important to check.

If I do a pull-up on the ceiling joists, I can visually see the ceiling deflect down even more. I'm thinking that once all the modules are in, there would be similar weight deflecting the ceiling downwards, making the gap larger than it currently is. Does this make sense?

Yes, but once again, the deflection must be what it is supposed to be! If you are seeing a large deflection with just you hanging on it, that might be a cause for concern. Your weight is negligible compared to the weight of the entire ceiling...

So if i was to bring the module down flush with the bottom, that should give me the overall 1" gap.

I would still bring the modules down more. The bigger the gap, the better, and you do need to get insulation in the gap without having to compress it so much that it bridges.


- Stuart -