Can you spray foam a house from the outside?
I live in Calgary, Alberta. My house is a 1940s bungalow with a second storey added in 1988. We are replacing the vinyl siding with Hardie Board, and want to improve the insulation at the same time. Right now there is 1/2" rigid foam with foil directly under the siding. Someone recommended removing the sheathing and doing spray foam from the outside. It seems we could also drill holes and blow spray foam in without removing the sheathing. Now that the blowing agents are more eco-friendly, are either of these a good option?
How will it affect existing insulation in the walls? What are the risks for vapour build up if we have a mix of plaster and lathe in the older parts of the house, with fiberglass and interior vapour barrier in the new parts?