Beefing up interior of roof in a garage.

Hi. I'm building a room within a room style studio in a 2 car garage. Wondering about beefing up the exiting roof below the asphalt shingles and raw ply wood. I'm concerned that the heat from the sun would trap moisture underneath the beef up layer. For example, trying to avoid mold underneath a layer of drywall. Maybe it's better to just add insulation in the ceiling joists instead? More airflow but still some sound abatement? The room inside will be pretty soundproof hopefully so the existing roof is just additional precaution. Thoughts?

The room inside will be pretty soundproof hopefully so the existing roof is just additional precaution.

How can you make the inner-leaf room "pretty soundproof" if the outer-leaf of your MSM system is not sealed and doesn't have much mass on it? :) Isolation (not "soundproofing") is best accomplished by building a fully-decoupled 2-leaf MSM system, which seems to be what you are trying to do. But it is a SYSTEM: if any of the parts are not done correctly, then the system doesn't work and the isolation will be poor. If your roof is the outer-leaf, and is not sealed airtight, or doesn't have much mass on it, then you don't have a functioning isolation system, regardless of how well you build the inner-leaf. So you have two choices here for your isolation: Either you use the roof as the outer-leaf of the system, in which case it must be beefed up and sealed air-tight, or you forget about the roof (leave it exactly as it is), and add another leaf down lower to do the job of isolating. If you go with the second choice, then you will have a three-leaf system in some respects, so you need to compensate for that by adding extra mass to the middle leaf, and keeping your air gaps large.

Maybe it's better to just add insulation in the ceiling joists instead? More airflow but still some sound abatement?

Insulation does not stop sound. Or rather, it is very poor at stopping sound. You need an awful lot of it to have any appreciable effect. 4" of pink fluffy will only get you a couple of dB extra, if you are lucky. That's not what insulation is used for in acoustic isolation systems. The insulation in the air gap of a 2-leaf MSM wall is there mostly to damp resonance, not to abate sound. When installed correctly in a properly designed 2-leaf wall, those same 4" of insulation will add 10 to 15 dB of isolation, mostly for this reason. In other words, you'll need the insulation in the joist bays anyway, regardless of what you do to your roof, since it is there as part of the MSM system. But it's not there to abate sound (and indeed won't do much of that): it is there as part of the tuned MSM system, so if the system is not tuned properly, then the insulation wont be doing anything useful either. All parts of the system must be present and designed correctly, if you want to have good isolation. - Stuart -

OK. That makes sense. Thank you. I think the ideal thing to do is to make the outer leaf be within the interior of the garage, rather than messing around with the roof. By doing this the outer leaf would be sealed where the existing pre fab truss system is currently in place. I'm a little nervous putting too much mass on it for fear of it collapsing. Perhaps a single layer of drywall would not be too much. I'll have to calculate it some way. Sealing it here versus the roof above seems preferable in a lot of ways. I just need to figure out how much extra weight the existing structure can withstand.

I just need to figure out how much extra weight the existing structure can withstand.

Right. You'll need to get a structural engineer for that. He's the only person qualified to tell you that. He'll need to visit your place, inspect what you have, look at your plans for what you want to do, then tell you if it is safe or not, and if not he'll also be able to tell you how you can add extra support to do what you need to do.

Perhaps a single layer of drywall would not be too much.

Probably that would be OK, since most trusses are designed to have both a roof on top and a ceiling underneath, but "a single layer of drywall" likely won't be enough to get good isolation! In my previous post, I mentioned that for a three-leaf ceiling, you need extra mass on the middle leaf... It all depends on how much isolation you need: Do you have a number for that (decibels)? If not, then that's the first things you should be figuring out: if you don't know how many decibels you need to reduce the sound levels by, then you can't know how to build your isolation system. - Stuart -