DesignGuy wrote:

Actually, a positive pressure inside your home relative to the
outside
environment would be a good thing in case of a chemical/biological
incident.
Keeps the nasty stuff out.

That "extra" air resulting in the positive pressure comes from outside.
You could have one source of incoming air and run that through
filters, but it wasn't clear that's what you meant. But at that point
you're talking saferoom with a separate air supply. If you're house is
that tight, you'd probably have a stale air concern more than
biohazards.

R