Regular Truss with partially vaulted ceiling?

[JZG85]

Hi all,
Just found this place and thought maybe someone has been in my situation before or could come up with some ideas to help me proceed.

I recently got my first house about 6 months ago which has a 500 sq ft detached garage behind it that I want to use as a shop. The interior is not finished at all but due to summers here consistently exceeding 110F, I have to insulate it. I'm not sure how hot it gets in there but it's got to be close to 130. This is in California and moisture is not a huge problem here.
The shop was built in 88 or 89, all 2x4, with regular trusses. 4 soffit vents total and 1 gable vent on each end. I added a solar attic fan to one of the gable vents. Currently there is no ridge vent.
On the interior it's regular trusses (not sure of the pitch), 24 OC with ceiling joists at 8 ft. Not many people are understanding why I don't just drywall the ceiling at 8' and call it done but I really, really, do not want a shop closed in that low. I know that raising the structural ceiling, you might as well rip the roof off and start over with a different truss type.
What I'm thinking of doing, and I know it's not ideal or great aesthetics, is to work around the existing trusses and vault the ceiling up to at least 10 ft or so, and then flatten the ceiling across a 2x3 or 2x4 similar to a collar tie. As far as I understand, I won't be compromising the structural strength or doing anything that cant be reversed.

For insulation, I would use 1" polyiso foam board (R-6) in the rafter bays, with a 1.5" air gap between the decking from the frieze/bird blocking to the mini attic space near the ridge. Behind that I would use either R-15 or R-23 mineral wool. This requires some sistering, furring, or something of the rafters to add some depth. The last layer would be galvanized corrugated paneling because it's light, not flammable, I can get it up there between the trusses, and I don't think it will put much stress on the rafters. I think I can make the corrugated paneling look OK in the end with some creative thinking. The ventilation does require cutting a 1.5" slot into each frieze/bird block but I did try it on one with an oscillating tool and it's doable.
From the exterior wall truss on each end to the adjacent truss, I would finish the ceiling at 8' ft with drywall. This would connect both gable vents and the mini attic so the entire roof is vented... who knows what the airflow would ultimately do though. A ridge vent might make more sense and could possibly do that down the road and seal up the gable vents.

So lots of work, maybe a crazy idea, and not sure what the best way would be to increase the remaining rafter bay depth. 5.5" is needed for R-23 mineral wool, I can almost get there by (daughtering?) a 2x4 to the rafters.

Just put the corrugated panel up there to visualize it, not fastened.

Idea of about where the flat ceiling would be

 

[Toyomech]

2x4 and lack of depth is where closed cell foam shines because of much R it packs per inch. Also being only 500 sq ft it shouldn't be too painful price wise. By the time you factor in the iso board, rockwool, and added lumber and labor, my opinion you would be ahead calling out a spray foam contractor.

 

[JZG85]

I have briefly thought about and should revisit the idea but there were 2 things that made me keep looking. We are seeing significant price increases locally for pretty much any contractor because of construction demand after all the fires in 2018. I expect that to go on for years, they still have hundreds of houses to rebuild.
The other is, even though lots of people are saying you don't need to vent your roof anymore, I just cant wrap my head around that being a good idea here with how extreme the heat is. My parents lost their house in one of the recent fires and it was rebuilt with a vented roof (and embershield soffit vents). I haven't heard of anyone doing it here and I'm not sure who to believe if it's a good idea.

 

[JZG85]

Just got a rough estimate over the phone for closed cell spray foam.

2" ceiling only, $2000ish
2" whole shop, $4000ish

 

[Toyomech]

At what R value? Then I can tell you if price is reasonable. Price doubles every inch so this matters.

 

[The Cobbler]

note to thread starter. for imgur pictures to show up on here, you have to remove the "S" from the "HTTPS" in your picture links

 

[JZG85]

I believe he said R7 per inch, so R14. He also said it's not really an apples to apples comparison to compare with R values of batting. If I went out to 3.5" he said it would be quite a bit more because I would want it scraped level for whatever kind of finished ceiling I eventually attach and it takes a couple guys several hours to do scrape it level with the rafters.
I'm not sold so far.

 

[JZG85]

note to thread starter. for imgur pictures to show up on here, you have to remove the "S" from the "HTTPS" in your picture links

Thanks, it also just said I needed 5 posts before posting images when I went to edit it, this should get me there.

 

[JZG85]

Not sure if this picture is going to cause more confusion than they helps... it's very rough, don't focus on any of the details (dimensions, pieces protruding into others, material thickness). This was just something I had saved that shows how I was thinking of increasing the rafter bay depth and a collar-tie like ceiling.

 

[JZG85]

Reading your post again, were you going to put in a ceiling 2 feet above the bottom chord of the trusses and then leave the trusses in place? I don't see the utility of that, the trusses would still be in your way at 8' high. Something I don't understand?

Edit:

Okay, now I can see in your pictures you have trusses. What advantage to you see in having a second ceiling level above the bottom of the trusses? Make you "feel" like there's more height? It'll sure be a LOT harder and more expensive to do, and won't give any advantage I can think of, unless it helps you not be claustrophobic.

Well it would be nowhere near as useful as a structure built for a 10' or higher roof.
Some of my reasoning:
-I've already rotated some large lumber between the trusses that would hit a 8' ft ceiling (partly because there is a bunch of **** dumped in there from the move).
-Part of the reason I wanted the shop was someday I want to start building a kitplane and would it would give more options for moving around or resting large components like wings, stabilizers, etc.
-I think it will make an easily accessible space to move, add, or reconfigure service like air, dust collection, exhaust, data, power, and lighting.
-Anything like sawdust, fumes, gases has that much more volume above my head to fill up before it's annoying.
-Yeah, some of it is the feeling of less closed in. My attached garage is higher but not really an option to do everything I want to in there.

 

[LX-Markham]

If I understand correctly: you are adding 2x4 framing at the 10’ level to install a drywall ceiling. The existing roof trusses remaining intact and protruding down through your ceiling?

edit: I should of read the reply directly above my post, d’oh!

I guess that will feel a bit loftier, but that’s going to be a drywalling nightmare!

edit: end result should look pretty cool!

I vaulted my ceiling and spray foamed. Was totally worth it. Can see the build in my sig