Do you mean the AC fan or a fan in a window?

As a house is made more and more tight, for energy efficiency, you get to the point where you really need mechanical ventilation for human health reasons. You really shouldn't rely on air leakage to provide fresh air. A table in ASHRAE 62.2 tells you how much continuous air you need as a function of size and number of bedrooms. Here is a link:

http://ths.gardenweb.com/forums/load/hvac/msg0912413831345.html?1CHu6n

A blower door test is the only way to see just how leaky the house is. You'd divide the CFM at the standard 50 Pascals depressurization by a local factor to approximate natural leakage in cold windy weather. The tester would tell you what that factor would be for your area.

The problem with depending on leakage for fresh air in a tight house is that most of the time the leakage is nowhere near adequate. Further, you have no control over it. You can install passive air inlets, or if you are interested in energy savings, you can install a heat recovery ventilator.

That URL for the ASHRAE table should be:
http://www.mainegreenbuilding.com/files/file/Panasonic/ASHRAE%2062_2%20doc.pdf

I agree with dickrhn,
you may need fresh air.

with fan on constantly the humdity
that is evaporated while a/c is running
is picked up and redistributed thru the
house when only fan is running.

if you test your house with a blower door
it will determine air changes per hour.
if your house is .35 ach or less fresh air
is required.
you've invested in the foam, saved a bundle
on the heatpump..so spend a little to make it
all work together.
there are many methods of adding fresh air to a home
erv's aircyclers, and my favorite...barometric damper
with duct to return ( return grill with filter outside).

you want to have comfort. so RH @ 50% is the goal.
dehumidifiers help to keep this level of r/h

best of luck.

Before you do all this blower door stuff. You need to look into the reason your Return Air is pulling the filters into the return grill "Pretty Hard" Your foam has nothing to do with your filters pulling hard! The air handler is self balancing. Air in equals Air out. This is assuming there are NO air leaks in the duct system. What happens is these filters are not matched to the proper cfm at the intakes. What is happening is...the speed of air [velocity] is not correct now. And the amount of air maybe too little. Choked off. You want air speed approx. 400-600 feet per minute at the return air intakes. high density filter should match the capabilities of the air handler and requirements of the hvac unit. 2.5 ton, approx, 560 cfm at both intakes if even. Or min. 20 X 25 each. this is my opinion.

If you foam your house you need a Energy Recovery Ventilator.