I bought a 1920s house wwit original windows. I can feel a draft coming from the sides of the window frame and there is about a pinky finger size gap on the side of the windows. I am not sure if I can caulk this gap or use spray foam from the inside or if I should do it from the outside. Any tips or ideas appreciated.

 

Not if you ever want them to open.

and personally, I think there should be a 30 day waiting period for people to buy expanding foam.

 

Couldn't agree more.

To the OP...Do you plan on restoring this home? For the current season use fiberglass bat insulation to gap the holes and then use 3M or Frost King window film to help keep out the cold. In the meantime decide what you want to do to this house.

 

Please post a picture so we can see what your seeing, there should only be tiny gaps around the sashes.
The only way you would see a gap that big is if the trim was removed.
Houses that old are always going to drafty until a lot of work gets done.
Installing replacement windows and removing the side trim so that huge empty space where the weights are now and insulating would cut down your heating and cooling cost by as much as 25%.
A house that old is also likely balloon framed so the cold air can be drawn in through the basement all the way to the attic unless it's been fire blocked at the top and bottom of the walls and may not even have insulation in the walls.
Also check how much insulation is in the attic.
https://www.energystar.gov/index.cfm...sulation_table

 

Owning a house of this vintage can be very challenging. While insulating usually cannot be done unless the wiring has been upgraded. The age of knob and tube electrical wiring, the sheathing of the wires becomes dry and brittle and is a fire hazard when insulation is installed.

As far as the drafts around your windows, again the vintage of the home is what makes so challenging. In those years there was little or no regulations concerning what one could bring into their home. Builders knew this and specifically designed the house to be drafty. Two to three more air exchange than a house designed today. Considering the age of the house it is probably 5 times the amount of air exchange an hour than today's standards. That's the equivalent of leaving your front door open in a new home.

There are many of us that would like to advise you on possible courses of action you could take on this forum. However we require more information from you. Such as, are you looking for a temporary measure to get you through this winter or a more permanent solution? And probably a lot more questions. So don't feel offended by our inquiries, we're just trying to help.