Are these SIP panels with tongue and grove? Are there wiring channels (sometimes there are little troughs cut through the insulation for running wire)?

I don't know what a SIP panel is. They are thick cedar T&G boards (long boards), not panels. There are no channels in the back.

Re: In your lower photograph, what is the material on top of the T&G wood, and what's on top of that? Compressed dense insulation is what is on top the thin layer directly on the T&G is foam, the other some kind of dense compressed insulation that is in blocks

Do they cover the entire ceiling? Yes and it's a huge area (open floor plan)

This is why the ancient Greek gods gave us fish bits. I've never used one to go through foam insulation, but I don't see why it wouldn't work. (No endorsement intended, link is for information only.)

I used my fish bit to go through insulation. It works. I was actually running refrigerant lines for a split system through some spray foam. Since you have an open end, you can try it.

Yeah, that and lousy aim. The first time I used one, I ended up in the next room over.

Are the foam layers applied to eachother or the t&g? Could a guy jam some pvc as a raceway in there?

Are you trying to add more fixtures in the same areas or between the same joists, or are you looking to add more circuits and need to run all new cable?

I don't think the foam layers are "applied" (e.g., glued) to anything, I think they're just sitting there. But I only briefly was able to capture this pic a few years ago when they were installing new skylights when roof was being redone, thought it may come in handy one day to have a pic of what is going on between roof and T&G. Re: jamming some PVC in there I'm not sure if it would get hung up on that foam as you're trying to push it and the opening to push the PVC through would only be the size of a junction box which would be a severe angle. I'm thinking new junction boxes would probably go about 6'-8' from existing junction boxes so perhaps not a long span.

Adding more fixtures in the same area is what is needed. There is a 1/2 bath and coat closet floating in the room (goes up to ceiling) that I plan on removing as part of a kitchen remodel which means those areas will need light fixtures which don't exist, can be tied into existing circuit running up there. It's just that there are far too few lights in the giant open concept.

I'm just thinking someone else must have crossed this bridge who has a vaulted T&G ceiling with no attic and needed to add more junction boxes/lights in the ceiling.

Thanks for the added info. The more of that we have, the better we can help. Is it just a straight shot or do they need to go up and over the peak? If a straight shot, a lucky guy could maybe use fish tape through a hole in the junction box to the new location? And are their rules to staple the romex to the joists along the way? I just dont know how you’d properly fashion the staples, or the junction block clamps in the kd hole. Perhaps others can assist? Cutting the holes for the new junction boxes will be less than ideal too unless a voltage detector tool can be used through t&g for existing wires.

Maybe where you live. I've seen lots of vaulted ceilings, but not T&G.

If you fish, you won't need staples. You don't have to secure the NM when fishing, as long as you use old work boxes with clamps.

Some guys can fish through KOs. I've never been able to make it work. When extending a circuit, I always have to rip out the original box and replace it with an old work box.

One of the tricks of working with T&G is to use a sharpened putty knife to chisel off the tongue of one board, and then remove a board or two. Of course you still have to deal with the nails. But depending on which way you need to go, that might make it easier. If you notch the ceiling joists for the NM, you'll have to use nail plates over it for protection.

You may want an electrician to do the final hookup, but most of the work involved is simply grunt work with fishing and running the wires. To find the path(s) to fish the wire will require a lot of patient analysis, and then a lot of careful drilling and fishing. You may even need to remove the existing junction boxes or one of the tongue and groove boards temporarily to give you more space to work; however, the foam layer appears to be thick enough to provide plenty of space to fish new wire. If you can do most of that work yourself, or hire a handyman that won't screw it up, you will save a lot of money. Then just pay the electrician to hook it up and ensure its safe.

I have a 1940 house and have done a lot of work with running new electrical wire. In some cases it has taken me several days to run a new circuit, since there is a lot of trial and error. My wife got used to finding me standing in the middle of the floor just staring at a ceiling or wall and thinking about how to make a run of wire. And all you need is a couple unexpected 2x4s that are in the way, and all of a sudden an apparently good path gets a lot more complicated.

Bruce

In most cases conduit is not going to be required.

I most cities, conduit in private homes is only required for physical protection of wiring, and that usually applies only to areas that are lower than about 6 to 6 1/2 feet in height. However, if you do run exposed NM across an open ceiling (e. g., across an unfinished basement), there may also be a requirement for a board or other support for the wire if its not drilled through the joists. The general idea is to not provide a convenient place to hang things where the wire itself is the only support.

If your city allows NM cable for home wiring (which is very common), then you can run it across through the space above your ceiling. You only need to have wire retainers/clamps at the junction boxes, since there is no way to get into the cavity and add any staples.

Bruce

I'm not sure I really understand what is going on here, but if the goal is to get cable into the space behind the T&G ceiling, I would consider hiring a finish carpenter to carefully remove whatever boards need to come down. Then have the electrician do his thing. Then have the carpenter come back and re install the boards. He would have to do it carefully and strategically but I think that's your best bet. Maybe once the ceiling is put back together give it another coat of stain or paint or whatever you have now to help hide the work.