If your home is missing foundation vents, I recommend having a qualified contractor come out and evaluate/install foundation vents around the perimeter of the home to ensure that the foundation and underside of the home are properly cross-ventilated.  Ventilation is CRITICAL to homes in the Pacific NW; lack of ventilation can lead to myriad issues including, but not limited to, attracting wood destroying insect (i.e., carpenter ants, termites and anobiid beetles) and mold.  Truth be told, mold is VERY RARE in crawl spaces under homes, but insect activity is not.  So, keep the underside of the home well ventilated and they’ll have no interest in your home.  It’s all about moisture!  If it’s impractical to cut in or install foundation vents, a mechanical vent fan on a humidistat can be easily installed.  Personally, I prefer foundation vents because they don’t rely on electricity or anything fancy to operate.  Keep them open and let Mother Nature keep your home’s crawl space ventilated!