Unless you can say what you're drilling into, it's all wild guessing. You are drilling into the exterior wall from the exterior, and you expect sawdust but are getting masonry? Need some more info there. Pics are always nice.

There's no other way into the joist space other than through a structural wall?

Bits that will drill through rock concrete and masonry and not get thrown for a loop when hitting metal are only a little expensive but they generally need an expensive drillmotor to run them. The real problem is what is that metal, is it rebar or gas pipe or electrical conduit, and so what possibly exciting stuff are you going to hit after going through or into that metal?

I'd remove the siding. Those 5" hole plugs are going to look horrible on a hundred-year-old house, no matter how you try to disguise them.

Those are great questions, grubby_me!

It never occurred to me it could be a conduit based on how smoothly it seemed to transition from one material to the next, but it's definitely worth looking into in case it actually is a conduit.

Other ways I can think of getting into the joist space are:
1. Through the roof. I'd rather not try this, especially since I'm relatively inexperienced with this type of work.
2. Through the ceiling of the room beneath.

I opted for the external option because I figured those chambers could use ventilation. Cutting holes that lead into the room beneath would just ventilate the hot "attic" air into the bedroom.

I'll see if I can get some pics. Hard to get good pics of the bottom of a narrow hole. :oP I'll see if there's anything else I can take pics of that might give clues of what I'm drilling into...

If it is a flat roof like you say. 99% chance you are hitting a piece channel (L shape metal).

Cant you just drill the holes up through the ceiling between the trusses to blow the insulation into? A lot less work repairing and no damage to the outside walls.

Thanks for the suggestion, hippy! I'll look into that.

Meanwhile, you're right, I could drill up through the ceiling to get into the space. My thinking was that most of those spaces have no ventilation (completely enclosed on all sides), so by drilling through the outside wall, I would create ventilation as well as create an insertion point for the blown insulation. My guess is that not having ventilation causes the temperatures to be much hotter, so ventilation+insulation would be cooler than just insulation.

However, I am by no means a contractor or even a frequent home-DIY-er... just a guy who's used some tools before and is trying to cool down a room. :oP Okay, I also took some engineering classes in college but I'm sure there's lots of stuff they don't cover in those classes that's important for designing and building houses ha...

Soffits and roof's need to be vented, not walls. All your doing is opening up avenues, for water, bug and potential rodent infiltration.

Years ago, tenants at a 19th Century rental house I owned called me--on an icy wintry day, natch--about a portion of a water-soaked ceiling that had fallen into their bathroom knocking the sink off the wall.

Turns out a p.o. had insulated the few inches of the flat roof with numerous "ventilation" holes, blew in some insulation and patched. But not very well. I ended up re-roofing the whole thing, after knocking off an unused chimney so there were no pesky flashings to worry about.

You're better off forgetting the whole thing. Or at the least, add the insulation topside to the flat roof and re-roof.

Before drilling into things you should be sure what they are.

Good luck!

My 1940 house has a 2" galvanized vent pipe for the plumbing that runs in an exterior wall. Based on what I can see of the pipe in the basement and attic, it would be very tough to drill a hole if I picked the wrong spot in the wall and hit that pipe instead of open space.

Bruce

Thanks everyone for the advice so far!

Finally took some pics... might not be very helpful. This is an attempt at zooming into where the hole saw cut...
I think I'll try drilling somewhere nearby in a diagonal direction to see if it might be a metal pipe or conduit...

You can see where I attempted to drill the hole, above the right side of the ladder. To the left is an existing vent. To the right is a lamp, so I'm thinking there's a possibility I could be running into a conduit or something for that lamp. Above the lamp is another vent.

The hole drilled by the arbor