I think I'll have a look behind the skirting boards to check the wall / floor joins as well, and seal with acoustic caulk if required.
Acoustic caulk is great, but its often expensive. Ordinary bathroom/kitchen caulk works very well for this, and is cheaper. Get the type that sticks like crazy to all surfaces, and never hardens: remains soft and rubbery, even when fully cured. I have had good results with Sika brand "Sikalfex 11FC", which seems to be commonly available in many places around the world. But any good quality caulk will do the job.
In truth, I had no idea a mini split was ductless and installed in addition to a ventilation system.
There's two types of mini-split. One is ductless, the other ducted.
The one on the left is the ductless or non-ducted type (what you typically see on the wall im houses, shops, etc.), and the one on the right is the ducted type, often also called an AHU for "Air Handler Unit" (which would be mounted in the ceiling space or wall space, normally, or maybe in the attic or basement, connect to the ducts that go to the room(s) ). Both types are mini-split, and in both cases there's some type of compressor unit outside, such as one of these two types:
The reason these systems are called "split", is because they work the exact same way as the typical old-fashioned "through the wall" or "through the window" type of air conditioner, but "split" into two parts, instead of all being inside one box. The compressor section is mounted in one unit that goes outdoors, and the condenser is mounted in the other unit that goes indoors (either on the wall if it is a ductless, or in the attic space if it is ducted/AHU). There's a bundle of pipes and wires that connect the two units. Principle of operation is simple: it is a closed loop system, filled with a pressurized refrigerant gas that circulates between the two units. The compressor (outdoors) compresses the gas into a liquid, which makes it hot, then passes it through a radiator with a big fan behind it to cool it down. One of the pipes then conducts that cool liquefied refrigerant to the indoor unit, which is basically just another radiator (coil of copper pipe with fins attached). Just before the refrigerant gets to that "radiator", it passes through an expansion valve that suddenly drops the pressure and turns it back into gas again... with the result that it get very cold, very fast. That icy cold gas then passes through the coils of the radiator, picking up heat from the room air, before finally being sent back down another pipe, back to the compressor, where the whole operation is repeated, ad infinitum
In the indoor unit radiator, that very cold refrigerant cools down the coils and fins, and there a small fan in the same box that passes room air through the radiator, then back into the room. And two things happen: Because the room air is warm and humid, as soon as it touches the cold surface of the fins, the humidity condenses out as liquid water (hold a metal spoon in the vapor rising from a boiling kettle, and watch how this happens: the vapor condenses on the cold metal and drips off. The same with a cold windscreen in a car, misting up from the vapor condensing on it). So that action removes the excess humidity from the air passing through the radiator... and it also warms up the fins and copper coils WITHOUT cooling the temperature of the air! This is an important point to understand! The simple fact of the water changing phase from gas to liquid releases a lot of heat into the fins and the air. Just like evaporation is a cooling process (which is why you feel cooler when you sweat), so too condensation is a warming process: it releases the latent heat in the water vapor, as the water condenses. Thus you end up with liquid water on the fins, and they also get warmer... which uses up some of the cooling capacity of the mini-splt system! Important point! If the fins are still colder than the air AFTER the condensation has taken place, then the fins can also cool down the air, so the unit blows cool dehumidified air back into the room.
However, if all of the cooling capacity was used up just in condensing the water, then the air does NOT get cooler: it just gets dehumidified. In fact, if the condensation process released more heat than the capacity of the unit, then the air can come out warmer, not cooler! This is something that people don't expect... and it's the reason why you need to do the math carefully, when dimensioning the cooling capacity of the unit: it needs to have enough capacity to deal with the "latent heat load" of dehumidifying the air, AND ALSO the heat in the room air itself, which is called the "sensible heat load". The sensible heat comes from things like your equipment, lights, your nicely steaming coffee maker, that huge piping-hot pizza that everyone is enjoying, sunlight through the windows, heat coming through the walls, floor and ceiling, ... and from another biggie that most people forget about: body heat. Half a dozen musicians jamming hard can put out as much body heat between them as a decent size electric space heater... A person at rest produces about 100 watts of waste heat, and an Olympic athlete in full exertion at maximum potential can put out nearly ten times that. A musician playing hard can be producing maybe 250 to 350 watts (give or take a bit). So half a dozen of those, is between 1 and 2 kilowatts.... 1 watt is 3.4 BTU/Hour of heat, where "BTU/Hr" is the way HVAC engineers measure heat movement. So just to cool down that bunch of musicians, you would need a mini-split capacity of between (1000 x 3.4) = 3,400 BTU/Hr and (2000 x 3.4) = 6,800 BTU/Hr. Add in the gear and lights, coffee and pizza, sunlight, walls, floor nad ceiling, and you get to maybe 8,000 or 9,000 BTU. If you just did that math, and forgot about the humidity problem of latent heat, then you went out an bought a 9,0000 BTU mini-split system thinking it would be fine, you'd have a problem! On a humid day, with all those people huffing and puffing humidity out with every single breath, there could easily be 3,000 BTU of latent heat to deal with in that humidity... and since the mini-split condenser uses up its capacity FIRST in dehumidifying, if it has 9,000 BTU capacity and 3,000 of that is used up just to remove the moisture from the air, then only 6,000 is left over to actually cool the air... but there's 9,000 units of heat to deal with, and only 6,000 units of "cold"... soooo.... the room does not get cooler, even with the poor thing running full out at maximum capacity.
Thus, the importance of considering BOTH sensible heat AND ALSO latent heat when you dimension your mini-split system. In this hypothetical case above, you would need a 12,000 BTU/Hr unit to do the job...
In other words, you need to dimension your system to be able to handle the worst-case: the room packed full of hot sweaty hard-working musicians with their amps, instruments, and other gear, as well as your own gear, on the hottest day in mid-summer with high humidity outside... and it also needs to be able to handle the opposite extreme: just you sitting quietly alone on a freezing cold day in mid-winter.
I forgot to mention that the majority of mini-split systems are not just "air coolers": if you reverse the flow of refrigerant through the tow units, then the indoor unit become an "air heater": the compressor sends hot refrigerant to the indoor unit, so it heats up the air, instead of cooling it. And in fact, this is the most efficient method for heating a room. A modern HVAC system can produce many times more heat that the electrical power it consumes, because it is the phase-change of the refrigerant that does the heating or cooling: the electricity just runs the compressor and fans, but the hard work is done by the refrigerant changing from gas to liquid and back again as it circulates around the system. So a mini-split system that uses 2 kilowatts of electricity can provide 4, 5, 6, 8, or maybe even 10 kilowatts of heat, depending on the unit and the design. Very high efficiency. Great power saver!
OK, long rant here, to say basically: take care when defining the capacity of your system!
Other points to take into account: the "bundle" that connect the indoor unit to the outdoor unit has two copper pipes in it (usually one is small diameter, the other a bit larger), plus a cable with several wires inside, plus a condensate drain: that water that condensed on the fins of the indoor unit has to go somewhere! So there's a drain pipe to carry that away, and dump it some place. All of those have to go through your isolation walls! Oops! Potentially, that bundle of pipes and wires can create a flanking path through the wall, carrying sound and vibrations in both directions. There's a method for dealing with that. And of course, you need to figure out where the condensate drain can go to dump its water... Maybe just on the grass outside, or the garden, or into a drain if there's one nearby.
OK, I'll stop now, before I wear out my keyboard...
Here's a design that came into my head when I looked up at the house and noticed a bathroom extractor fan vent coming out of the roof soffit;
That looks like a good plan. The vents under your eaves are to vent the roof deck itself, so don't interfere with those too much, but adding your intake and exhaust registers under there would be fine. Do make sure that the intake and outlet are far apart from each other, as you don't just want the intake to be sucking the same old exhaust air right back in again! Keep them at least 2m apart.
The only potential issue I am aware of with soffit extracter fans is the danger of expelled moist air rising back into actual soffit roof vents. I think that directing the outlet duct at a bit of an angle outwards rather than straight down, and having the fan close to it will solve this.
If the HVAC mini-split is doing its job, the exhaust air should not be too humid. Or you could consider adding an ERV unit, which recovers the humidity and waste heat in the outgoing air, and adds it back into the incoming air... but that's extra expense, and extra complexity, and you likely don't need that.
Here's the tentative rough design sketch. Very much at the idea stage - nothing to scale, no calculations done yet, and the duct routing needs work. The fan I'm thinking of is an inline centrifugal quiet type designed for mounting in loft spaces. I'm assuming oversized slow running ones can be used here, and floated on the rafters to prevent vibration transfer if required.
Looks like a good plan to me! However, if you are planning to have a mini-split at some point, then I would suggest putting it high in the rear wall/ceiling, and bringing in your fresh air just above it, so the incoming air gets sucked through the mini-split before going into the room.
I am also seriously contemplating having the radiator behind the left speaker soffit removed.
I think that's a good idea!
I would also consider improving the insulation in your walls: you say you have access to do that, so if you can make sure that your wall cavities are completely filled with good insulation, that would help with both sound isolation and also with thermal issues: keep you warmer in winter, cooler in summer.
- Stuart -