HVAC for Seattle Studio Build

[silky smoove]

First post here, so thanks in advance for any help those of you more knowledgeable can throw at me.

I'm building a small studio at my place in Seattle. It's a detached 14'x24' structure. There will be one main room for tracking and mixing, and a smaller entry/vestibule that will be used for gear storage, the building's electrical panel, etc. It will also be the "start" of my sound isolation perimeter. See the schematic layout below for clarity. The layout doesn't show that the ceilings are vaulted (approximately 3:12 interior ceiling pitch at the bottom of the scissor trusses). The vault starts 8'-1" up from the subfloor.

I've been planning on using a mini split to heat and cool the structure. It's a small enough structure that this should easily be achievable. My initial plan was to have one unit in the main room (interior volume around 1,520cf), and then a smaller unit in the vestibule area (interior volume around 410cf). Both interior units would go to a single exterior unit.

As with everyone else attempting to use mini splits for their studio's HVAC goals, the problem is fresh air supply and getting the stale air out after the behemoth drummer from the local metal band lays down a once in a life time take. Currently I'm planning on venting the main room into the vestibule, which will have its own vent to the outside air. My concern is that this won't allow for sufficient transfer of air and interior humidity from the aforementioned sweaty drummer. Would I be better off venting into the vestibule, but containing that in a dead vent built into a duct chase that connects directly to the outside world? I would then have to do a second vent just for the vestibule area. I'll be building with baffles, silencers, etc. to minimize sound escaping through the vent/duct, but at this point my biggest concern is getting sufficient fresh air in and out of the structure.

So, to sum up:
1. Vent the main room into the vestibule which then vents to the outside world, or...
2. Vent the main room and the vestibule separately to the outside world, or...
3. Some other solution that I've not yet considered.

Thanks everyone!

studio layout.png

[Soundman2020]

Hi. Please read the forum rules for posting (click here). You seem to be missing a couple of things!

Currently I'm planning on venting the main room into the vestibule,

That would be illegal, or at least it would not pass inspection, or meet code. You cannot source the air from one room out of another room, which is what you would be doing with that scheme.

My concern is that this won't allow for sufficient transfer of air and interior humidity from the aforementioned sweaty drummer.

The ventilation system does not deal with humidity. The air conditioning unit does. The cooling coil inside the AHU removes humidity from the air simply because it cools the air, so the moisture is dumped out as condensation, which you then have to drain away to the outside somehow.

Would I be better off venting into the vestibule,

Nope! As above, that would not meet code. You have to ventilate each room separately to the outside world. You can combine the return ducts from several rooms into one central plenum if you want, and send that outside, but you cannot move stale air from one habitable room into another habitable room. Big no-no.

but at this point my biggest concern is getting sufficient fresh air in and out of the structure.

There are equations and tables for figuring that out. There are two concepts here: air changes per hour, and exhaust/make-up air. You need to replace all of the air in each room several times per hour (at least 4, probably 6 or more) and/or you need to supply the correct amount of fresh air for the number of occupants in your room (while simultaneously removing the same amount of stale air) and you have to do all of that at a speed of less than 300 FPM at the registers. In other words, the air flow velocity at the registers needs to be slower tan 300 FPM, and the registers need to be large enough (cross-sectional area) to move the required volume of air (CFM) in order to achieve the correct air change rate for your room, while also adding the correct volume of make-up air (fresh air) and removing the same volume of stale air.

That's the ventilation side of things: just moving air around, without doing anything to change the temperature or humidity. That job is done by your mini-split AHU. You also need to do the math here, to determine what your sensible heat load and latent heat load will be, so that you can calculate what cooling capacity you need, in BTU/hr (or tons).

So there are two very different aspects that you need to work on for your HVAC system. One is just moving the air around, and the other is cooling/heating/dehumidifying it. There's a lot of math that you need to do for both of those.

2. Vent the main room and the vestibule separately to the outside world,

Yes. Each room has it's own air supply duct(s) and register(s), and it's own air return duct(s) and register(s). They might both bring in fresh air from the same external inlet point, and they might both deliver stale air to the same outlet point, but each room must have it's own separate supply and exhaust.


- Stuart -

[silky smoove]

Thanks Stuart.

As for the forum rules:
1. Did that, still a little unclear as to the actual construction.
2. Same as #1
3. Done, and had it in the original post for clarity.
4. Stated the general goals of the project in the original post, but to cover the other aspects: The building is already constructed. It is built from 2x6 framing with 7/16 OSB and Hardie Plank siding, scissor trusses to vault the ceiling, and a standard plywood, felt paper and asphalt shingles roof. The building is on top of a 3-sided concrete wall (needed to create a level building pad) that's backfilled with multiple lifts of 1-1/4" minus, and then 3/8" minus crushed rock that's been heavily compacted. The framing is built onto a galvanized steel substructure (think floor joists), which I've filled with Roxul to eliminate any drum head-like resonance when walking around in the structure. On top of that is a 3/4" TG plywood product. The interior wall dividing the two rooms is not yet constructed, nor is the flooring above the subfloor, insulation, drywall, etc. It's basically just a shell at this point.

5. The loudest source of noise in the structure will be a full drum kit. I'm confident that my overall construction plan will create enough isolation for my needs (see my schematic in the first post which covers everything except for the ceiling which matches the wall construction, and the floor which will be done with Densdecking per Rod Gervais' recommendation in another thread, Green Glue, flooring underlayment and then either a laminate wood product or a novilon type linoleum product).

6. Explained above
7. Haven't planned on floating the floor
8. The building is 14'x24' at the outside of the 2x6 framing. The roof pitch is 5:12 with an interior ceiling pitch at the bottom of the scissor trusses of 3:12. The vaulted ceiling begins at 8'-1" above the subfloor..

9. See my original post. I'll get my current SketchUp model uploaded when I'm back at my office.
10. Done
11. Done
12. Check
13. Done
14. Budget is... Well... I'm already approaching $30,000 on this project given all of the site construction, demolition and removal of an old garage that was in the way, and construction of the building shell. I don't have a hard and fast figure for budget at this point for the overall construction. I'm trying to be economical while still doing things correctly. I'm well aware that this will not be cheap.

15. Check

----------------------------

Now on to your responses!

I need to re-read Rod's book to get a better handle on how HVAC works in general. That said, am I understanding you correctly in that this is going to lead to a total of six penetrations in the structure?
1. Mini split for the main room
2. Fresh air supply for the main room
3. Exhaust for the main room
4. Mini split for the vestibule
5. Fresh air supply for the vest use
6. Exhaust for the vestibule

To state it another way; Each room using a mini split will have three penetrations. Am I following that correctly?

[Soundman2020]

To state it another way; Each room using a mini split will have three penetrations. Am I following that correctly?

Right, plus the penetration for electrical system, plus the penetration for signals.

The penetration for the mini-split is probably the most complicated, since it is a bunch of things in a small package. You have the two copper pipes that carry the refrigerant, one of which is wrapped in thick insulation, plus the condensate drain, plus the electrical cable. When they are all bunched together, it's an irregular shape that is hard to seal at the penetration point. You need to take care with that. Plus, if you need high isolation, you can't just run them directly across the MSM air gap: you need to make a resilient path. I usually do that by curving them 90° where they pass through one leaf, then running them a few feet over (up, down, whichever) inside the wall cavity, and curving them 90° again to pass through the other leaf.

The silencer box sleeve penetrations are not so bad. Even though they are huge holes in the drywall, the rectangular MDF sleeve makes it fairly easy to get them through, and using backer rod plus abundant caulk keeps them well sealed and hopefully also decoupled.

- Stuart -

[silky smoove]

I can see how that grouping of penetrations could get complicated.

I'm wondering if dedicating one side of the vestibule area as a mechanical "room" with a central HVAC unit might make more sense. I could then duct over to the main room through the ceiling. Am I right in thinking that a central unit, unlike a mini split, could provide the fresh air supply and exhaust as well?

[Soundman2020]

Am I right in thinking that a central unit, unlike a mini split, could provide the fresh air supply and exhaust as well?

Right. Well, you still have to do the ducting to bring in the fresh air and add it to the recirculating air, and another duct to extract some of the stale air, but you only need one single AHU for the entire place like that.

I'm just wondering, though: Will the are that you are calling the "vestibule" be part of the actual isolated area of the studio? Or is it just an external area that does not need to be isolated acoustically? What is the purpose of that area?

- Stuart -

[silky smoove]

The vestibule is purely a storage area for cases, cables, my mic locker, etc. It will not be acoustically isolated like the main room. The idea was to be able to keep gear and clutter out of the main room until it's needed.

[silky smoove]

Just got the quote back from the first HVAC contractor I've gotten an estimate from. Their quote doesn't include some details that I asked for, which is frustrating, but the bigger problem is the cost. Does this seem high to anyone else (all prices are USD)?

----------------------------------------------------------------------------------
Ductless install at Recording Studio
Install 1 ^ FVXS09 Floor Mount in the Studio
Install 1 ^ CTXS07 Wall Mount in the Entry
Install 1 ^ 2MXS18 Outdoor unit
We will run the line sets and comm line as needed
We will cover the line set with vinyl, paintable line cover
We will install Condensate lines as needed
We will install piping for a Dead Vent makeup air on the supply side
We will install piping for a Sound Box Assembly on the return side
Test, check and purge the system
Permits and Inspections included
Includes all labor and materials: Total: $11,931 + tax
Warranty: 2 year Fox workmanship
Daikin: 12 year parts, 12 year Compressor
Electrical by others
Subtotal: $11,931.00
Tax: $1,145.38
Total: $13,076.38
----------------------------------------------------------------------------------

EDIT: I should note that I've not yet looked at those model numbers to see if they're sufficient for my needs.

[silky smoove]

In addition to my question above (does this quote seem high?) I'm now wondering about this HVAC system in general. Let me explain: Here in Seattle almost no one has AC in their homes. We get very few days that are warm enough to bother with the expense. With that in mind; Would it be feasible to only provide heat and fresh air ventilation to the building without including the AC component?

[Soundman2020]

Does this seem high to anyone else (all prices are USD)?

To me, yes it does!

A pair of 18,000 BTU mini-split systems would most certainly not cost me US$ 13,000.

Here's a link to a local large hardware chain (similar to Home Depot), to their page for 18,000 BTU mini-split systems:

http://www.easy.cl/SearchDisplay?catego ... :&pageSize:&

It's in Spanish, but you can see the capacities and prices. The exchange rate is roughly 660 pesos = 1 US$, so the system listed for 379,990 works out to US$ 575. Even the most expensive "high efficiency" system, at 764,990 is still only about US$ 1,150. Two of those would cost me US$ 2,300. Installation would add another US$ 150 each, roughly. Total installed cost: US$ 2,600. And if you bought the cheaper units, total installed cost would be under US$ 1,500, all taxes included, for the pair of them.

That's what I would expect to pay here in Chile.

Yes, prices can vary greatly in other parts of the world, and we are not talking about the same brands of equipment either, but the two more expensive brands listed on that website are both prestigious and well-known here, and comparable in quality to any good system elsewhere in the world.

So I can't see how it would cost ten times as much where you live. I know things are expensive in the USA, but TEN TIMES more expensive? Hmmmm.... I don't know how that could be justified.

I'm now wondering about this HVAC system in general. Let me explain: Here in Seattle almost no one has AC in their homes. We get very few days that are warm enough to bother with the expense. With that in mind; Would it be feasible to only provide heat and fresh air ventilation to the building without including the AC component?

How many people that live where you do have their houses perfectly sealed, hermetically, twice over, with extremely thick, very massive leaves for their walls, and multiple layers of very thick, high efficiency thermal insulation inside and out, plus a large air gap that completely removes all thermal bridging between the inner wall and outer wall? Those are the houses you should be looking at, to see how they cool those tightly-sealed rooms in winter, and how they heat them in summer, and how they control the humidity year-round. I'm betting that "heat pump" is what you'll see. ie, = "mini-split HVAC system".

Your room will have many "things" in it that put out heat: your DAW, interfaces, outboard gear, console, lights, amps, and people. Unlike typical homes, the room will be hermetically sealed, twice over, so all the normal air paths that you find in typical houses will not be there. With nowhere to go, the heat will build up. So will the humidity. ventilation alone won't do the job of keeping it comfortable in there.

But to me, it sure looks like HVAC is a hell of a lot more expensive than it should be where you live. For that price, you could buy a half dozen units here in Chile, and get them shipped to you along with the installation guy and all his gear, flying Business Class, put him up in a good hotel for a couple of days, fly him home again, and still save several thousand dollars, plus have a couple of spare units laying around, in case one fails...


- Stuart -

[silky smoove]

I've been doing some research and I'm confident that I can install both the mini split and the HRVs myself. The mini split will require a contractor to come in at the end and pressure test, purge, and vacuum the precharged refrigerant into the lines, but everything else on the installation is well within my abilities as a builder. The HRV looks like a relatively straightforward installation. That should knock the cost down from the contractor's $13,000+ estimate to around $3,500 (making some assumptions about the size/cost of the HRVs and how much a contractor will charge me to come in at the end of the mini split installation). Now onto my questions!

I don't really have the floor space to build room-in-a-room style, so I was going to rely on Whisper Clips, hat channel, and doubled-up drywall with Green Glue between layers to serve as my isolation (there are no windows and only one door into the isolation perimeter that I was going to source from someone like Overly). I've been doing some digging, but I can't find examples of how people wall-mounted their mini splits on a Whisper Clip and hat channel wall section. I could hang the mini split on the exterior wall studs and then bring the drywall layers up to it, but that would leave a mini split sized hole in my isolation perimeter. That would also create problems if I ever needed to take the mini split down for repair or maintenance purposes. Alternatively, the folks over at TM Soundproofing have assured me that Whisper Clips and hat channel are capable of supporting the weight of many layers of drywall, which would certainly outweigh a 22lbs mini split, but I don't really feel safe about that approach. How do people wall mount their mini split through the type of wall section I'm describing?

Regarding the HRVs I'll be using: It appears that the relative humidity here in Seattle is low enough that an ERV won't be necessary, so I'll be using two HRVs for the space (one for the entryway/vestibule and one for the main room, or possibly a central unit with vents that supply/return from a common plenum for both rooms). Where I'm still a little confused is the sizing of the HRV. I've calculated the interior volume at 2,215 cubic feet, and I know I want to have a minimum of four room changes per hour, if not six. Doing some simple math that means I need a net rate of 148cfm for four changes per hour, or 222cfm for six changes per hour. There are plenty of HRVs capable of those rates, but I see lots of references to only needing 1/3 (sometimes listed as 35%) of the rooms air to be handled by the fresh air side. Does that mean that the conditioned air from the mini split accounts for 2/3 of the air volume change while the HRV only handles 1/3 of it, or should I simply size the HRV for 4-6 times the room volume per hour?

[silky smoove]

Bumping this up for my previous question.

[Soundman2020]

I've been doing some research and I'm confident that I can install both the mini split and the HRVs myself. ... hat should knock the cost down from the contractor's $13,000+ estimate to around $3,500

10 grand cheaper? That's a lot more reasonable! that makes sense, for sure.

I don't really have the floor space to build room-in-a-room style,

Are you SURE about that? Have you considered the size of the acoustic treatment that you'd have to hang on the walls if you do it your way, vs. the space you'd save by doing an inside-out wall for your inner-leaf?

so I was going to rely on Whisper Clips, hat channel, and doubled-up drywall with Green Glue between layers to serve as my isolation

That certainly is an option, but I don't think you'd be saving much space like that. Perhaps an inch or two at best. And your isolation would not be as good...

I could hang the mini split on the exterior wall studs and then bring the drywall layers up to it, but that would leave a mini split sized hole in my isolation perimeter.

yup! Not an option.

Alternatively, the folks over at TM Soundproofing have assured me that Whisper Clips and hat channel are capable of supporting the weight of many layers of drywall, which would certainly outweigh a 22lbs mini split,

Yup! That works.

but I don't really feel safe about that approach.

Why not? What's wrong with that? 22lbs is about the same weight of a piece of drywall measuring 4' by 3'... why would that be a problem? Your biggest issue would be locating the hat channel at the right places to match the mounting template for the mini-split, but that could easily be solved by running one extra short piece of hat channel on a couple of clips at the correct position. Apart from that, I'm not sure what other concerns you have. What's the difference between that and mounting a mini-split conventionally?

so I'll be using two HRVs for the space (one for the entryway/vestibule and one for the main room, or possibly a central unit with vents that supply/return from a common plenum for both rooms).

I would go with the second option. I can't see a need to have a separate HRV for the vestibule, when it isn't even part of the studio.

but I see lots of references to only needing 1/3 (sometimes listed as 35%) of the rooms air to be handled by the fresh air side. Does that mean that the conditioned air from the mini split accounts for 2/3 of the air volume change while the HRV only handles 1/3 of it, or should I simply size the HRV for 4-6 times the room volume per hour?

You seem to be confusing the re-circulation of air through the room, with the fresh air/stale air issue. Only the incoming fresh air / outgoing stale air needs to go through the HRV. The rest of the air just goes back into the room, right where the mini-split system is. Only a small amount of fresh air is needed, compared to the re-circulation.

- Stuart -

[silky smoove]

Are you SURE about that? Have you considered the size of the acoustic treatment that you'd have to hang on the walls if you do it your way, vs. the space you'd save by doing an inside-out wall for your inner-leaf?

I've never even heard of an inside-out wall for the inner leaf. I'll need to do some research on it, but am I right in assuming that you would simply spin the inner leaf 180-degrees so that the insulated portion is facing the interior of the room? In my case, working from the exterior or the structure to the interior of the room I'd have the following:
Siding (Hardie Plank in my case which is a wood/cement hybrid)
Building wrap (a product like Tyvek here in the US)
7/16" OSB sheathing
2x6 exterior wall framing (stud bays filled with fiberglass insulation)
Air gap
Outer layer of 5/8" type X drywall
Green Glue
Inner layer of 5/8" type X drywall
2x4 or 2x6 interior wall framing (stud bays filled acoustic treatments as needed)
Fabric

Is that correct?

If so does the doubled-up drywall constitute one leaf, while the exterior sheathing and siding constitute a second leaf?

How deep should the air gap be in this type of arrangement? With Whisper Clips and hat channel you lose roughly 1.75" of floor space on each wall. Would a 1" air gap suffice? If so that would actually save me some floor space which would be wonderful.

That certainly is an option, but I don't think you'd be saving much space like that. Perhaps an inch or two at best. And your isolation would not be as good...

As I mentioned previously an inside-out wall assembly is not something I was even aware of, so thanks for the suggestion. I'm going to do more research on it and am looking forward to your response to my questions above.

Why not? What's wrong with that? 22lbs is about the same weight of a piece of drywall measuring 4' by 3'... why would that be a problem? Your biggest issue would be locating the hat channel at the right places to match the mounting template for the mini-split, but that could easily be solved by running one extra short piece of hat channel on a couple of clips at the correct position. Apart from that, I'm not sure what other concerns you have. What's the difference between that and mounting a mini-split conventionally?

What's wrong with that? Haha... Well... Irrational paranoia! I've never worked with hat channel before, so I don't have a good feeling for how much it can support. That should be obvious given the amount of drywall it supports, but... Well... Paranoia, haha. None of this matters if the inside-out wall assembly you've suggested makes more sense for the application.

You seem to be confusing the re-circulation of air through the room, with the fresh air/stale air issue. Only the incoming fresh air / outgoing stale air needs to go through the HRV. The rest of the air just goes back into the room, right where the mini-split system is. Only a small amount of fresh air is needed, compared to the re-circulation.

[/quote]
So to be clear, the HRV/ERV sizing calcs that basically equate to room volume multiplied by air changes required per hour multiplied by 30-35% are a good way to go for studio purposes? If that's the case, it will save me quite a bit of cost.

Thanks once again for all your help. It has been invaluable.

[silky smoove]

Bump