I need to add a vent in a block wall and I'm not sure how to start

So here's my situation, attached single car garage with about 10 feet of space in front of the car for workspace.  No windows, no vents, no moving air of any sort.  I would like to install a portable AC unit to help with the summer heat and humidity.  The issue I am having is the entire garage is concrete block and is underneath my family room.  I can't go up with the vent so I need to go out the side, but I think it's pretty foolish to cut a 4x10 opening in the rim joist.  I think my best option is to cut/drill/hammer out a half or full block and install a proper 6" vent through the hole.  If I owned the home I would have already drilled the mortar around a block and hammered until I had a block sized hole in the wall and sealed it back up.  Since I rent, and rent from family, I want to make sure it's done "properly" and in the cleanest, most "building improvement" way possible.  Even in the winter (I have hot water heat in the garage and the rest of the basement) I'd like to be able to stick a fan in the hole and vent paint fumes, car exhaust, etc.

4x10 isn’t very big. Why can’t you cut that in the rim joist?

If you need a 6” dia hole in the block, why remove a full block?  Why not cut a 6” hole??

...and what is this venting?  That’s really small for AC duct, and also very small for foundation venting. 

6” would be fine for a hose connected to the car exhaust, but it would be too small a fan to move enough air to vent paint fumes or ambient exhaust. 

In reply to SVreX :

I have a portable AC unit, the hose on the back is 6" (really 5.9x) and it came with a window adapter that is essentially 4x10 on the outlet side.  My feeling was that I could ditch that adapter and run straight 6" out through the block and put a cap on it like a bathroom vent with some screening to keep out rodents.  4x10 would exceed the 1/3 rule I've always been taught when cutting a floor joist for plumbing or electrical, and it's also not reversible the way knocking out a block or a half block is.  I could easily put a new block in, paint it with drylock and make it look like nothing ever happened, that's not possible with dead tree carcass.  

As far as the vent in the winter anything is going to be better than what I have now.  I bedded in brakes last night and when I woke up that was all I could smell upstairs.  A squirrel cage fan on a switch just to create negative pressure in the garage to keep the air moving out instead of in when working would be a vast improvement.

6” masonry hole saw.  Not cheap but the “right way” or you can do a 6” round hole in the rim joist with a much cheaper wood hole saw

In reply to bbaker480 :

Ok. 

There is no 1/3 rule for a rim joist. It’s not spanning anything- it’s 100% supported. 

The simple way to go through block is a series of small holes with a small diameter masonry bit, then just whack out the center with a hammer. 

In reply to bbaker480 :

The rim joist shouldn't be spanning anything and should be fully supported along the top of the foundation (or wall below, in the case of platform framing).  It's just there to give consistent backup to the siding, and to securely anchor the ends of the joists framed into it so they stay vertical.  You can put a 6" hole saw through it with no problem.

The important thing to determine is the layout of the cinderblock.  

Some are 2-core, and some are three core.  You want to make sure you have the space for a 6" hole, that's tight.  4" is the norm for dryer vents and such. 

The bit you want is a Core Bit, and they are available for drills and angle grinders.  2" is common for the DIY, 4" is OK if you have a strong tool, 6"is usually best left to the pros.  They use special drills made for core drilling.  

Solid concrete is a different animal of course.  I cut a 4" hole through cinder block to vent a bathroom fan, similar to a dryer vent.  I used masonry bits, an impact drill, and a hammer .  The hole was a bit rough, but there are plenty of vent kits available to finish the hole professionally.  

One out of the box thought however...what about the garage door?  I mean, it's replaceable.  You could swap out two windows for mesh screens, then hang a $20 box fan over one of them whenever you needed to vent the space.  They also make garage door vents. 

Oh, and this: http://ventmygarage.com