I'm about to start remodeling my 1940s cape here in Maine.  We're adding two shed dormers and I'm convinced close cell spray foam is the way to insulate.  This will create a sealed envelope in my upstairs.  We heat with a woodstove in the winter so I'm not concerned about moisture during the cold season but from what I'm reading I should be strongly considering an ERV unit.  We have no central air system.  Heat is provided by wood and hot water via baseboards.

The upstairs will have two bathrooms and laundry so there will be moisture around.  I was going to proceed with good exhaust fans in both bathrooms but should I be putting an ERV in the limited crawl space I'll have in my attic?

If so where can a DIYer get an ERV?  Recommended brands?  Anyone have experience with a similar application? 

The downstairs space will not be as tight as upstairs so this might help with moisture issues but I am concerned about air quality but more importantly moisture issues.

Any advice would be much appreciated!

Don't confuse ERV/HRV with make-up or combustion air.

ERV/HRV are ventilation and indoor air quality devices that EXCHANGE air.  Some of the units with 2 fans give you the opportunity to create slightly different pressure environments by varying the speed of the supply/relief fans accordingly. 

ERV/HRV do not (at least in my opinion) take the place of exhaust fans in bathrooms or kitchens.  These exhaust fans are typically low volume that do not run for long periods of time and have little effect on the overall indoor air pressure environment.

If you have a combustion appliance, you really need a direct source of outside combustion air into the plant room.

If you have a fireplace that is used a lot or have a BIG gas range with BIG kitchen hood, you probably need additional make up air.  Conditioning this make up air costs money, however.

What you want to avoid is negative pressure environment that backdrafts combustion appliances and sucks in outside air and moisture through the building envelope.  I think a slightly positive pressure environment is beneficial, so when I do this in a couple years, I plan to use a 2 fan ERV and dial in more supply than relief.

The upside of the direct duct connection from outside to the return air plenum of the furnace/air conditioner is that it creates a positive pressure indoors, at least while the unit is running.  The downside is that if you don't at least run the fan, you get no outside air and there is no heat exchanger or additional filtration.

Bruce