I don't imagine any homeowner has the knowledge or resources to pressurize their home with filtered air. It seems to me that a face mask would be the most practical protection. What is the smallest particle size of radioactive fallout? Is there any public information regarding approved masks and where to buy them?
Normal surgical masks are good, if not perfect. Normal pollen, dust, etc. will be radioactive and it is these particles, when taken into the body, that do the most harm. Also, I'd cease eating for as long as possible until well clear of the area.
Actually, with the cheap cost of really good furnace filters, a good fan, duct tape and drop plastic it's pretty easy to create a positive pressure in a room; assuming you want to go all MacGyver and all :-)
As far as masks go, any surgical grade mask will work in the short term. The radioactivity isn't necessarily floating around in the air. After the initial thermal (big fire ball) and Gamma burst (which nothing short of lead underwear or three feet of dirt will stop) it's the residual radioactive particles of dust and soot that pose the most biggest danger.