Roof venting

Posted: Wed Jul 01, 2009 3:55 pm

I have finally gotten underway converting the 20' x 20' garage behind my house into a studio, albeit in a way vastly compromised by time and money from my original room-within-a-room plan.

The goal is to build a single-room facility capable of recording up to three musicians at a time, maybe four in a pinch, but typically just myself. I play drums, so isolation is a real issue. While the back of the room will be for instruments, the front will be the production area, which will be outfitted for surround production.

Frankly, time and money limitations and the urgency of getting the facility online are forcing me to make some pretty big compromises. I will simply have to live within the constraints that result.

I have already turned the garage doors into a real wall, built with 2 x 6s covered by two layers of 5/8" drywall (taped and caulked), with 3/4" siding on the outside. A birch solid-core door was installed. The rest of the existing walls are just 1/2" plywood boards on 2 by 4 studs, 18" (!) OC.

The interior walls will be two layers of 5/8" drywall on furring strips mounted in Kinetics Isomax sound isolation clips. Before installing those walls, I needed to caulk every seam in the exterior walls with OSI SC-175 acoustical caulk. I caulked every opening I could find, from the eaves under the roof to the base under the bottom plates. (The garage was moved from its original location about 15 feet from its current location and set on top of some blocks and plates.)

Having gotten all of that caulked up tight to the point that I think I've damaged by hands from all the caulking, I came across comments from Rod in a few places about venting under the roof to prevent buildup of condensation, and became concerned that maybe I sealed up too much.

Alas, the cathedral roof, which peaks at 16 feet, will be my biggest compromise. The existing roof support system is rather arcane and complex, if effective. I had a structural engineer and a carpenter friend who has built studios look at it, and both suggested solutions that would simply be more expensive and time-consuming than I can manage, so, for the time being, at least, the roof will remain essentially untreated, other than perhaps tacking some insulation up to it. I know that will pretty well undermine all of the wonderful isolation I'm working for in the rest of the studio, but it's a force play. I'll try to put some absorption on top of the rafters, though the amount of mass I can just throw up there is obviously limited. It's dumb, but simply the best I can do. C'est la guerre.

I do intend to put in an HVAC system, of course (with fresh air intake, but HVAC is for a different topic), which I imagine will take some moisture out of the air, but now I want to find out more about whether or not I need to be trying to put some sort of venting in for the roof, and, if so, what it should consist of and how I can do it without further compromising my isolation.

So, finally, to my question: roof venting - what's the deal?

Posted: Wed Jul 01, 2009 9:09 pm

"though the amount of mass I can just throw up there is obviously limited."

Actually...I can't see it or even get a handle on it. Do you have a picture or two of this for those like myself that are challenged in this area?

I;m thinking you are talking about a typical framed structure that you have either removed the joists from or severely hindered the buildings ability to stay upright by removing almost too many joists.

And then having a direct opening to the rafters but no way to support any additional load due to this modification.

But I could be way off.

Posted: Thu Jul 02, 2009 12:39 am

Well, it's kind of tough to get a decent angle in there for photos of the roof. I might be very wrong about how much weight the structure could hold, but a lot of the supports are 1 x 10s or some other kind of hinky thing (it was, after all, only a garage), so I am choosing to err on the side of assuming more weakness, rather than assuming strength. I have done nothing to alter what is in place.

I'll post links to a couple of shots I took before I did all of the caulking and such as soon as I can work out somewhere I can upload to. Neither one shows the eaves or gives a good angle, but maybe they will help convey the idea.

Does the weight capacity somehow bear on the venting issue?

Posted: Thu Jul 02, 2009 6:09 pm

No sir, not at all. But the venting issue is unclear to me.

Posted: Fri Jul 03, 2009 10:17 am

Larry,

the venting of the space directly below the roof sheathing (in wood framed construction) is a matter of code - and is directly tied to the possibility of moisture making it to that surface - and condensing in colder winter months...... thus destroying the structural integrity of the decking - and possibly of the roof structure itself.

This is required under the UBC without exception. The only exception being the amount of free air required per bay (in a closed rafter application - such as with a typical cathederal ceiling) when a vapor retarder is applied directly between the sheathing and the bottom of the rafter.

Applying gyp-board directly to the underside of the plywood is a no-no.

Rod

Posted: Fri Jul 03, 2009 11:21 am

OK, first, I figured out where I could upload photos, so here are two:

Second, thanks, Rod, for your reply. Being a complete construction cretin, I had to read it over several times and do some research to understand it at all.

I have a cathedral ceiling with panel sheathing. I think I must have just sealed up the venting: there were small rectangular openings formed between the rafters and the angle 2x4 you can see going up the roof to the peak. For the time being, the only thing I'm thinking of putting up under the sheathing is fluffy insulation or rock wool, covered by fabric, because of the difficulty of getting gyp board installed around the crazy support system you can see in the pictures.

For an isolation standpoint things will be bad enough in this situation; how do I enable venting without entirely compromising isolation? Or do I just scrape out the SC-175 from those openings and live with the leakage?

Posted: Mon Jul 06, 2009 10:05 am

you build a room within a room - with a new ceiling - made with structural ceiling joist (i.e.: 2 x 12) and you sheath that top and bottom - and live with the quasi 3 leaf asembly -

I would sheath the top with 1 layer 3/4" T&G plywood - then G.G. w/ 1 layer of 5/8" gyp - then GG - then one more layer of 5/8" gyp......

Rod

Posted: Mon Jul 06, 2009 6:32 pm

"Alas, the cathedral roof, which peaks at 16 feet" Subtract "9 feet from top plate of wall to apex."= existing walls are 7 feet tall?

Just for reference larrytheo, this isn't a cathedral. It is stick built trusses with no ceiling. The difference and your tech guys are already making you aware, is that a cathedral is built in such a way to vault up and remain open(no joists that tie exterior walls together) with the structural ability to support the loads placed on it.

Posted: Tue Jul 07, 2009 4:06 am

Rod Gervais wrote:

you build a room within a room - with a new ceiling - made with structural ceiling joist (i.e.: 2 x 12) and you sheath that top and bottom - and live with the quasi 3 leaf asembly -

Rod

Now this would have been a good answer for my project, Rod, instead of the drubbing I received! My "quasi 3 leaf assembly" sounds pretty good too, by the way.

Posted: Tue Jul 07, 2009 10:28 pm

Do tell.... Wink

Posted: Fri Jul 10, 2009 10:15 am

Well, Rod, room-within-a-room was the original plan, but that was foiled by time, money, and the difficulty of working with or replacing the current roof support system. So, I need to figure out what I can do to satisfy venting needs with things as they are. Sad, but the facts of life right now.

What about those roof vents that sit atop the apex? Any thoughts on those? Does the dehumidifying provided by an AC system help the condensation situation at all?

Space, I think that 9 foot measurement is probably a bit of an overestimate made a year or so ago before I started the project in earnest. I've got some better measurements I'll dig up and post.

Thanks for all the help, folks! I'm good at sound and music, not very good at building.

Posted: Fri Jul 10, 2009 8:39 pm

"I caulked every opening I could find, from the eaves under the roof to the base under the bottom plates."

OK, I think I have an idea of what you are trying to achieve. The amount of roof ventilation is your concern versus isolation, is that correct?

The build of your garage with an open air exchange from the roof area to the ground level did not require gable vents, which is why they do not exist.

When you install a barrier on the ceiling this lack of ventilation will have had required your attention in order to maintain a properly vented roof.

In general one square foot of vent area per 300 square foot of attic or area to be vented is required.

Ridge vents on a roof is always a good idea in conjunction with gable and soffit vents to allow heat to escape and to mitigate moisture entrapment, but this is a convection system that also requires air flow to be the most effective.

Any closing up of air space access under the eaves (read: soffit vents) need to be opened back up if air flow has been restricted.

Then install your ceiling.

Posted: Mon Jul 20, 2009 2:52 pm

Thanks, Space. Between what you wrote, what Rod wrote, and the reading I've been doing, I'm starting to get a glimmer of what's going on.

>>The amount of roof ventilation is your concern versus isolation, is that correct?

Ding ding ding ding!! That is, indeed, the question I have so awkwardly tried to ask.

Looking closer at my situation, I don't think what I closed up were vents, in fact, I don't believe there is any venting at all currently. If you look at the top photo I posted, just above the inaccurate notation "9 foot to top plate..." you can see the 2 x 4 that goes from the base up to the peak. You can also see the 1 x4s under the sheathing. The openings I closed up were the ones formed by the space between 1x4s where they pass over the 2x4, as indicated by the oval in this photo:

They were not sealed to the outside, but neither were they entirely open. I think they were not intended to be vents, which means I need to add some venting. Now I have two questions: what is the most affordable venting method that provides the necessary amount of venting, and my original question: how do I handle isolation?

Note that, at this time, I am not able to put ANY ceiling in. I'm planning to put mineral wool under the sheathing (though it is my understanding that I want to stand the insulation off of the sheathing itself by an inch or so), then cover that with something to hold the insulation in and provide a bit more thermal insulation (a friend suggested Reflectix reflective radiant barrier), and then some fabric over that for aesthetics.

This topic sponsored by: