|Originally posted by uxbridge
<snip>HRVs and ERVs only remove moisture in the winter. They tend to bring it in, in our summer climate. By the way whats the humidity like in the rest of the house. Is the floor sealed?
I think uxbridge is on the right track here. I'm not an hvac technician but I've contracted a few large projects and learned a few things (ie don't put trane economizers in a library, doh!)
If you have a heat exchanger I'm assuming you have a relatively new home, which means the living space should [should!] have an effective vapor barrier (ie foil or paper faced insulation, or even better sheet plastic over unfaced insul). If you ever cut a hole if your drywall or had reason to remove it you probably have noticed one of these. Of course it's easy to cut corners on this type of thing, but the quality of the rest of your home should be indicative of how well insulated it is.
The reasoning behind the heat exchanger is your living space is so well sealed that all the building materials (plywood, osb, carpet, etc) that are off-gassing into the home cannot dissipate via the drafts common in older construction. Thus the heat exchanger pulls fresh air into the home and removes the "contaminated" air.
As far as your living space goes, there are only two ways humidity can be introduced 1) your vapor barrier is inadequate and humidity is coming in through the builiding envelop (walls, ceilings, etc) or 2) the heat exchanger is drawing moist air into your hvac system, which is then pumped throughout your house. Aside from an open window or the occasional open door, there really is no other way humid air can enter (well, fireplace is another one- brick and mortar that is).
My bet is your heat exchanger is introducing humidity. I'm not sure of the mechanics (do these things have motorized dampers?)-- would unplugging it actually disconnect the outside air, or would it merely cause the air exchange to be passive?
Next is your basement itself. I know you did the "foil test", but I would wager you are getting at least some moisture in through the walls and floor. My suggestion would be to lay down some plastic over the floor before you do the floor covering (heavy plastic- HD carries it in boxes like 20x100 and other sizes). For the cost, it's well worth the benefit.
For the walls you could go with a drylok type sealer, but that would be my least favored stand-alone option. However it is a great compliment to any of the others I will mention.
In my own home I would drylok, stud out (metal studs), insulate with r13 and cover the studded walls with plastic and whatever covering you are using. Another option would be applying a foil-faced rigid insulation directly to the walls (homosote makes a product "ultra-r" in various r values)-- make sure to tape the seams. Last would be good old plastic sheeting over the walls.
Next is the ceiling, or the bottom of the first floor. Is the floor insulated? If it's not insulated with foil or paper faced insulation, I would bet money there is no vapor barrier between the basement and first floor.
The question is, do you want to isolate the basement from the rest of the home with a vapor barrier. That is going to depend on how you solve your humidity problem.
It would be nice to zone your basement, in which case I would install that vapor barrier. If you don't ever intend on doing that, then I probably would not. Another solution would be an air conditioner for the basement (mitsubishi makes some nice units) with it's own supplies/returns (thus a completely independent system- no exhange of air between the living space and basement). Impractical, I know, but it is an option.
Lastly, where is the condensate from the coil going? Do you have a condensate pump to pump it outside?
Again, I'm not an hvac guru, but I hope this information is helpful.