Again, my dad taught this exact form of survival. This was his life's work. When he created his "Survive" game as a means of teaching people about this stuff, it meant I also thought about this a lot as a kid (as we played this game alot).
He used to say any home with a basement and not directly in the path of a nuclear fallout cone could be turned into a fallout shelter. It doesn't need to last forever, as the radiations involved will degrade to livable levels within a few weeks. My dad used to talk about ways to fairly quickly modify an existing home to withstand radiation. Granted, I don't think this applies to homes directly in the path of the fallout cloud (then I feel you're effed if you don't GTFO immediately after say 2-3 weeks).
Some of the strategies involved creating pockets of air, dirt and gravel around basement openings. Dad would diagram up windows having plywood leaned up against lower level windows, which creates an air pocket. Then he would shovel (or end-load) layers of dirt and gravel (maybe 1' thick) to cover the plywood. I think my dad's plan involved using a tractor to scrape off top layers of soil from around the windows and pile that up.
Another aspect is air. He would say you don't need a crazy filter system, other than having a non-direct path for the airflow. And a light filter or screen preventing the light airborne particulates from entering the house. He would argue that radiation doesn't travel around corners, it travels in a straight path, through stuff.. And air doesn't become radioactive... So as long as the particulates aren't getting in, then you're probably good. Remember, these aren't Covid & ebola sized viral particles, it's larger dust, so long as you have a dust covering in front of your air intake, you might be good? I'm wondering is a t-shirt or light weight breathable cloth would work? Again, we're only talking about it needing to last 2 weeks before you can come out. I might do some more research on ventilation systems, but I know if you're having a LONG term bunker built, it likely requires a much more complicated system.
I hadn't really thought about it until now, but I imagine IF THE BOMBS DROPPED, I'd do the plywood/dirt trick to my few basement openings, then run to the HVAC air intake and tack up some "cloth" over the openings...? I think I'd have the kids start charging up ALL the LED emergency lights (depending on EMP functionality). I figure we have an hour or so to get that all done before the radiation winds start rolling in, and fallout becomes an issue (totally a guess)... Once the outside shell of the house is secure, we can worry about situating the house interior... I don't live in a military bunker, and my basement has what we need, but it's set up for life and entertainment right now... I know we'd need to re-organize a ton of stuff inside, from sleeping quarters, hygiene, security, etc...
