The tighter the home the better, but you can get a home too tight, some air change is benefical to the occupants, plus if you run any gas appliances, you may need some air change for proper venting.
Jake
In most climates you cannot (reasonably) get a home too tight. You can get it tight enough to require mechanical ventilation, but that really is something most homes (even leaky ones) probably need anyway, and something the vast majority of homes already have (I'll get to that in a second). The reasoning is this indoor air in homes can be more than 5 times more polluted than outdoor air. Studies have consistently shown that in work places adequate mechanical ventilation reduces the number of sick days people take. Now homes are not work places (for one they are often less densely occupied) but there are some parallels. For one homes that have moisture issues do benefit from mechanical ventilation, except during hot humid weather. Secondly, just because a house is leaky does not mean it is getting adequate ventilation passively. It can even be getting adequate ventilation but having it inadequately distributed.
For human health there are a few things we need to make sure are addressed, in regards to ventilation. #1 is oxygen levels, in this case it is normally derived from CO2 concentrations because CO2 is easy to measure. Even in leaky homes it is easy enough to get above minimal CO2 concentration levels. I've seen readings about 1000ppm CO2 in leaky homes, while that is not an immediate health threat I'd like to see it less than 600ppm. #2 is carbon monoxide, this is easy enough to control, only use non combustion or sealed combustion appliances. #3 Radon, again mitigation not ventilation. #4 NO2, which is a combustion by-product again eliminate the combustion eliminate the risk. #5 Acrolein, again a combustion by-product (wood, tobacco, candles, food) for the most part not burning stuff in the house eliminates most of the risk, the rest is from cooking. #5 Formaldehyde; this off gasses from glues and binders used in cabinets, furniture, insulation, etc... elimination of sources is the most effective way of dealing with this but it is often not feasible to always eliminate it completely. This one is tricky because you need to get the levels down to 0.10 ppm and there is not a set ventilation rate that will guarantee to get it down to that level (if it is elevated). #6 Particulate Matter, particularly below 2.5 micrometers in diameter. For the OP this is the major concern since he is drawing air from unsealed quasi ducts. PM is typically controlled by recirculating air filtration. #7 VOCs in general, point source control is most effective (not using insecticides, air fresheners, solvents to clean with, etc...).
Most homes have effective mechanical ventilation already, though they may not be used effectively. Those are Most clothes dryers, bathroom exhaust fans and range hoods. If ensuring ventilation is a concern the cheapest option is to put the bath fans on a timer so that they provide 3 cfm per 100 sqft of living space and 7.5 cfm per person while the home is occupied (ASHRAE standard 62.2), or in accordance with Building Science Corp's ventilation standard (
http://www.buildingscience.com/docu...ationforNewLowRiseResidentialBuildings_v2.pdf). Those are minimum rates (BSC's is typically the lower of the two), If you have pollutant sources you are not able to control (typically Formaldehyde) then you need to ventilate at a rate that brings levels down to what is acceptable. There are situations when I do not consider bath fans an acceptable solution. Here the most typical is likely houses with attached garages, for those houses I would prefer to see a supply ventilation system ducted to each of the bedrooms (15 cfm to each) and the remainder to the living (family) room, or have it connected to the HVAC system. A balanced system would also be acceptable but I would prefer if the attached garage also had a separate exhaust ventilation system. The other instance I do not find exhaust ventilation systems acceptable is where there is not adequate air mixing in the house. Typically this is actually an issue with the builder not allowing for adequate return air to the HVAC's return from various rooms, or only using something like a wood stove to heat with.
Works!