These are "nutshell" answers since envelope issues are tough to diagnose over the internet. The HRV is an exchanger, but if your house is tight and your winter interior humidity is high, it'll exchange stale humid interior air for fresh dry exterior air while minimizing heat loss during the exchange. A blower door test is usually part of a more comprehensive analysis which could run a few hundred dollars. $200-$400 depending on the thoroughness of the inspection. If your house is excessively tight then it might make sense to install make-up air duct for the furnace. If you have an HVAC company out there, often times they'll do envelope and blower door tests. With all the "green" garbage going on, there are a lot of folks jumping on the bandwagon just as "energy consultants." If you're going to have work done, many companies will credit the cost of the blower door test and other analysis towards the cost of future modifications. Backtracking and peeking at the larger picture, basements can be a source of moisture vapor. Moisture vapor and maybe even liquid through rubble foundations, or vapor through poured concrete or block foundations. Think of it this way: You tightened up your house, which lessens moisture removal, but if your basement is a source of moisture vapor, it's still adding moisture vapor to the house. If it applies to you, a good thing to do is to minimize as best as you can air movement from the unconditioned basement into the conditioned living space above. Go in the basement and look up. Any gaps in the basement ceiling that lead into the house above (plumbing penetrations, electrical penetrations, hvac penetrations, etc) can be sealed with canned foam. It makes sense to do the same thing in the attic floor if the attic is unconditioned. The goal is to keep the conditioned air that you're paying to condition within the thermal envelope of the house. It's not a sexy job, mostly elbow grease, but walking and crawling around your house and shooting gaps with foam can have great effect. With those DIY cans of foam like Great Stuff, it's best to uncover the gaps first, then fill them all at once. Ling delays between trigger actuations can allow foam to cure in the nozzle.

Building Code Requirement: Minimum ventilation for the bathroom is to be a window of at least 3 sq. ft. of which 50% is operable, or a mechanical ventilation system of at least 50 cubic feet per minute (cfm) ducted to the outside. (IRC R 303.3, IRC M 1506.3) Heating Ventilation Institute (HVI) Recommendations: HVI is an industry association of heating and air conditioning product manufacturers that, among other things, tests and certifies ventilation products and publishes consumer guides on ventilation issues. These are the HVI recommendations for venting bathrooms: Small Bathrooms: HVI recommends that in small bathrooms, up to 100 square feet of floor space, the exhaust fan be sized to provide a ventilation rate of 1 cfm per square foot (about eight air changers per hour). This usually results in a ventilation requirement of less than 50 cfm, so the code minimum fan size must be used. Example 1: The bathroom is 5' x 9' (with 8' ceilings). Multiply 5x9 = bathroom area of 45 sq. ft. At 1 cfm per square foot the minimum recommendation is a fan rated at 45 cfm. This is less than the code minimum 50 cfm, which must be used. Medium and Large Bathrooms: For bathrooms larger than 100 square feet in area, HVI recommends a ventilation rate based on the number and type of fixtures present, according to the following table: Toilet 50 cfm Shower 50 cfm Bathtub 50 cfm Tub/Shower Combination 50 cfm Whirlpool Tub 100 cfm To calculate the minimum fan rating, add the cfm for each fixture in the room to arrive at a total cfm. Example 2: The bathroom is 20'x12'. There is a tub, a separate shower enclosure and a toilet. Toilet 50 cfm Shower 50 cfm Bathtub 50 cfm Minimum Fan Rating 150 cfm Toilet Compartment: If a toilet is in an enclosed stall with a door, then the toilet enclosure is considered by most code authorities to be a separate room. Likewise, if a shower is enclosed right to the ceiling, it will be considered a separate room. These guidelines recommend a separate mechanical exhaust system for "each enclosure", which means a fully enclosed toilet or shower must have its own exhaust fan separate from that of the rest of the bathroom. In most localities, this is also the building code requirement. A steam shower always needs its own ventilation. Strictly follow the manufacturer's recommendation for venting a steam shower. Otherwise you will have mold growing in places you would not think mold could grow. Example 3: The bathroom is 15'x15'. There is a tub, a separate, fully enclosed, shower and a fully enclosed toilet in a stall with a door. The fully enclosed toilet and fully enclosed shower should have their own exhaust fans rated at 50 cfm each. The bathroom fan will exhaust only the bathtub and should also be rated at the code minimum 50 cfm. How Long to Ventilate: When house air is exhausted to the outside, heat goes with it, meaning that your furnace has to work harder to make up for the lost heat. You want to exhaust moist air to reduce the risk of mold and mildew, but once the moist air is gone, you want to stop ventilating. HVI recommends that the fan be left on for a minimum of 20 minutes after use of the bathroom. A timer is a good solution, allowing the fan to turn off automatically at the proper time. There are also fans on the market now that sense the level of humidity in the air and automatically turn on when it is too high, and turn off when it is back to normal. Unfortunately, there is no fan that senses odor, so a manual switch is still necessary.

The formula for sizing a bathroom exhaust fan is as follows: Step 1: Calculate the cubic feet of your bathroom (length x width x height) Step 2: Divide by 60 (the number of minutes in an hour) Step 3: Multiply the result by 8 (the number of recommended air changes per hour to prevent mildew and exhaust odors). Greg

I consider that a severe mold problem. It can be DIY if you have the skills needed to demolish and replace the drywall - including the ceiling. If the studs are infected as well, that may require more specialized skill than the average DIY homeowner. You'll need to call in a professional to assess and resolve the ventilation issues - for sure. Health risks of living with mold include migraines, nausea, respiratory duress and rashes, to name a few. Those symptoms can be amplified to dangerous levels if one has or develops an allergy to mold. It's not acceptable to live this way. Do you have another bathroom you can use until this issue is fixed?