David,
I assume you're talking about in your fixed residence? If so, you might want to search for what they use in Australia for the filters in their off road RV trailers. This same material should work in your residence. They probably have it pretty much nailed on what works best.
As for in RV's, we use unfiltered positive pressure quite often to keep dust out of the interior when we're traveling on dirt roads. We close everything up (including the cab's ventilation system) except for one vent in the roof, which is way up above the dust from our tires. We set a fan in this vent to blow IN and hence create a positive pressure so no dust can enter from down low where the road dust is. This seems to work pretty good in our RV - assuming we're not following another vehicle on these kind of roads - in which case there would be dust up high, too, where the roof vent is.
