This may have been a mistake....but, when I build a window header I used a 6x2 on it's flat and two 10x2 standing on their sides, however I put one 10x2 to the outside and one to the inside. Therefore there is a 2" space between them. The top is sealed off with the 6x2 top board.
So I have a void approx 2" wide 9.5" deep and 36" long.
Can I insulate this void using spray foam cans (Great stuff)?
I can drill a series of holes in the 10x2 to put the "straw" from the can through. Should I drill top and bottom? How far apart? Shoud I use the expanding foam or the minimal expanding foam?
Is there a better option?
The foam is a good idea. Curious: What was the point of the 2x6 on its side? Convenience for something? I don't see how it was a "mistake", unless it is for structural reasons. If you put it there for more strength, next time use 2x12s instead. The load carrying capacity of a beam increases as the square of the height, so higher is always better. On its side, a 2x6 is almost useless structurally. Just an FYI; if you want to reinforce a header and don't have a ton of room (depth), add some plywood panels to the 2x10s or 12s.