Fresh air again!

(Moved from the DIY HVAC topic to this new one.) I'm putting in honest research effort, but this whole HVAC thing is wayyyy too overwhelming for me. Nonetheless, I have to work out my fresh air situation, and soon. Background: I'm converting a 2-car garage to a studio. Dimensions are 20 ft W by 20 ft L by 8.5 ft H, PLUS a vaulted ceiling that goes up another 6.5 vertical feet. So, complete vertical height is 15 ft. and volume of entire studio is 4700 cu ft. Studio is located in Vallejo, CA behind my house. Closest neighbors are maybe 20 ft away on either side. Behind the studio is better: a shed, 15 ft of yard, a fence, then probably 20 feet or so before the neighbors' house. This is good because I'm a drummer. The outer walls are plain ol' 1/2 inch plywood, to which I am adding 3 inches of mineral wool and two layers of 5/8 inch drywall floated on hat channel in Kinetix IsoMax sound iso clips. For reasons of logistics and cost I am *not* putting in a ceiling, but the roof is insulated with mineral wool held in with radiant barrier. The radiant barrier will be covered with burlap, both for aesthetics and to hopefully absorb whatever HF reflections the radiant barrier might produce. Obviously, the lack of a ceiling is my Achilles heel in terms of isolation; I just have to hope I can get away with it and live within the limitations it imposes. Current status: I've insulated all the walls with mineral wool and am ready to start installing the iso clips, but I want to work out my HVAC plans before I hang drywall. The current notion is to install a mini-split for AC and heat (Sanyo 18KHS72), but my struggle now is fresh air ventilation. There are two 18 inch by 18 inch vents located near the bottom of one exterior wall that will supply intake and the current notion is to put an exhaust fan high in the gable on the opposite wall for outflow. The fan will serve to vent the roof as well as suck air out (thereby drawing in fresh air from the intake vents). At this moment, I am thinking of getting a Panasonic FV-13VKS2 WhisperGreen. It moves 130 cfm at .5 sones and can run continuously as low as 50 cfm. That should be more than enough fresh air for people, given that I will be working alone the vast majority of the time and anticipate never having more than five people in there at a time. Questions: The intake vents. I think the two vents will provide enough air, the question is how I can maintain some isolation. The vents are low in the outside wall and my thought is to put about four or five feet of lined duct in between the studs and then having a vent about five or six feet high for the air to enter the room. Although JM SuperDuct seems to be recommended, the minimum quantity one has to buy is much more than the 10 or 12 feet of length I will need. I could get a small quantity of JM Permacote Linacoustic R-300, but this is a Rigid Fiber Glass Plenum Liner Board, which doesn't seem like the right thing. Again, I don't know nothin' from nothin', but my impression is that duct board is a flat board you buy and form into a duct, whereas duct liner seems like a flat board you use to line a duct. So the questions are: 1) Does it make sense to put duct in the wall to have air enter from outside low and enter the room high? 2) If so, what do I want to use for ducting? 3) Is there something additional I can do, on either the interior of the room or the exterior of the building, to help maintain isolation? 4) Does this make scheme make any sense at all? (I guess that should be my first question, eh?) vent fan 1) Anybody have a better suggestion than the Panasonic? It will cost me $250, and I'd be hard-pressed to come up with much more than that. 2) 130 cfm seems more than adequate to me. Does this make sense? 3) Any suggestions for mounting the fan? It's actually intended for ceiling mounting, but I don't see why I couldn't flip it vertical for wall mounting. I am guessing I need to isolate it with rubber to keep it vibration-isolated, but I'm not certain how to handle the depth (nearly 8 inches). 4) Anything else I can do either in the room or on the exterior to reduce noise and maintain isolation? 5) Will passive intake work if I use an exhaust fan like this? Thanks! Larry the O

Dan Fitzpatrick wrote:

Hi everybody! Been a while since I stopped by. Hi Larry, Don't be shy about starting your own thread! Sounds like you need one. Passive ventilation does work. However it's an open question whether your roof is airtight enough so that intake air will be forced to come through your lower vents. It may be that all of your circulation is just happening in the roof area. I like your idea of ducting around your lower vents. You could just use wood for this. Leaving two 18x18 inch vents is going to be a big hole in your isolation. I don't know if you will have enough isolation, but I guess you can always add more layers of drywall to the inside. If you were just starting I would have suggested adding more mass to the outer wall. I have much better isolation that what you state (average of 2 layers 5/8 on each leaf separated by 12") and you can still faintly hear drums at 15 feet away. Your fan capacity seems a little small to me. I use a fantech fx8 which maxes out at 500 cfm. But I'm a big believer in overkill. http://www.fantech.net/fg.htm Dan

Thanks, Dan! I did not caulk the sheathing on the roof. Just couldn't do it. As I stated, there is mineral wool (and vent chutes under it) all the way from the base of the roof up to the top (with a little space at the top for airflow), held in place by radiant barrier. I think the mini-split system will probably circulate the air, it's getting fresh air into the room that is my current concern. Do you think that if it enters at 6 feet that's too high? The vents don't have to be 18" x 18", that's just the openings that were already in the wall, but I can close them off partially from the inside. That's exactly the sort of thing I'm trying to figure out before I put the inner walls in. I would have loved to have added more mass on the inner walls, but there is cross-bracing that means I would have to cut many small pieces of drywall and attach each individually. Unfortunately, I am under some time pressure to get online and just couldn't spare the time and effort for that level of detail. I fully understand that I am compromising my isolation in doing so, as I am without a ceiling. Can't be helped, and it might just mean that I'm restricted to daytime and evening drumming, though I'm hoping less noisy activities can take place over more extended hours. I did not mention this before, but this studio will not be used just for music rehearsal, and it will almost never be used for a full band. Most of its use will be for sound design, music composition, and playing or recording one or two instruments at a time. Of course, with drums, it only takes one instrument to be loud! I am really in a quandary about fan size. I've read lots of threads here and over at recording.org, as well as reading through Rod's book. Unless I am misunderstanding (which is distinctly possible), the Panasonic should be adequate to provide fresh air for people, but I don't know if it is adequate for venting the roof, especially in summer. How loud is the Fantech? I'm not yet committed to anything, that's why I'm asking these questions now. Man, I wish I had the time and money to really do this right, but we work with what we have, eh? Larry

If your whole ceiling is rockwooled that should stop airflow pretty well. I have both my intake and outlet on my ventilation in the ceiling, but on opposite sides of the 16 foot length of the room. But I have the inlet duct pointed toward the part of the room not between the vents -- if that makes sense. Actually, pointed right at the drum set which is nice when you start to get hot (assuming it's cool outside). So if your air comes in at six feet, you can make the end so it points in a direction that makes sense for maximum air circulation. Away from the outlet duct I guess. The way I figured it my ducts were 8 inch diameter so my cross section was 4x4x3.14 or about 50 square inches. I just tried to keep my entire length of air travel at least that dimension. An 18x18 duct is 324 square inches. If your fan pushes air through a 6" hole (as does the Panasonic FV-13VKS2 WhisperGreen) that's obviously way, way too big. You only need about 30 square inches inlet not 324 square inches. For an inlet duct you can use the space between your exterior wall studs, cover that with plywood ... if that is a 24 inch by 3.5 inch space that's more than enough cross section. Make a sealed box (sealed except for your 6x6 inch inlet). You would have enough extra room there to put in some rockwool for extra sound absorption in the duct (cover rockwool with cloth). Then cut a six inch hole in the plywood at the top and use a 6" elbow to point the air that comes in wherever you want. I would do the same duct type thing on the other side where the fan is, for isolation. The ductwork will put resistance against your fan that's why it's nice to have extra capacity. The fantech site has a graph showing how the longer the ductwork is, the less airflow you will get. You have to count all the duct including the inlet duct even though it doesn't seem connected. My fan is not noisy since it is located outside my leaves. It would be too noisy to have exposed in the studio though. Good luck it seems you are making calculated decisions.