Are you saying your registers are high on the wall and the return is low on the wall? If so then you would need to run up through like your saying. I added one return vent through the floor and opened up the sill plate on all the rest in my home since the 'installer/builder' only cut 3x6" max openings in the floor. It sucks and it can be the most tedious part of the job. I used a tiny drill bit and drilled through the floor at the base of the trim at an angle so I stayed out of the carpet so I wouldn't cause a run in the carpet. I then went downstairs and located and double checked my measurements from the bit before cutting. I had to use a hole saw in one area and drill holes through the subfloor and 2x and then use a coping saw to finish the hole which coping saws are not designed for but it worked.

I have seen people who have decent sheet rock repair skills pull the trim and cut the sheetrock and do it right there and repair the sheetrock after. If your putting a floor register off the wall you will already need a hole there anyway but that type of register has pretty poor airflow compared to just putting a 4x10 register right in the floor.

I would vote against putting a supply duct physically in an outside wall cavity as the insulation factor will be near nothing. I would consider building out from the outside walls and keep the conditioned air on the inside of the building envelope. I know of one that the owner built and used for family pictures. I know of someone else that built a coat closet by the front door that has shelves for shoes and places to hang coats and accessories that also houses a return vent duct. I have seen photos where someone cut a hole in the living room floor and built a column to the ceiling in a ranch style home that houses the return vent and put a flat screen tv mount on it with other items. It sounds horrid but it really blended well and looked like it had always been part of the house. I put some boots in the basement wall as I built it and I can't say it was enjoyable. I tried to boot the return I added and gave up but was able to seal the sheetrock to the studs which wasn't fun either. Use your imagination.

More information like where you live would give some hints as to how harsh the climate is which can be a factor also. Ranch style or multi level?

Cheap solution would be a ceiling fan or just a standalone fan(s).

Adding vents are very labor intensive(they have to cut up alot of stuff to install the new vents and then you have to patch it back up again.) It is a very involved remodeling job and so it will cost a lot of $$$$$ to do.

before adding ducts to existing system,
you need to have the capacity to serve them.

call a pro to give you guidence.
its more than just cutting holes and
running duct.

best of luck.

Thanks for the replies.

It is a ranch house in a northern climate running a heat pump.

These are not for high wall grills, but for supply registers only a few inches up the wall. In the existing ones I can reach through the cutouts in the floor into the basement. I just can't get the register head sheet metal shoe-horned into that space because of the angle. Not to mention a saw to cut holes for new ones.

I'm going to an HVAC place today that has a good assortment of sheet metal to see what they might have that will work.

Any other suggestions appreciated.

Thanks!

David

Have someone come look at and run some air flow tests and calculate how much air is needed in each room. I was able to boost the airflow in one room by increasing the return air volume which made for more supply air without adding another supply duct and vent.