Q: If I run a bathroom fan for 15 minutes a day in my house, will I be wasting a lot of warm air in the winter?

A: Often indoor air quality and energy efficiency goals are intertwined, but sometimes they are in conflict.  You will lose some heat energy from the warm air being carried away by the fan, but it is not a huge amount.
Consider an "average" 3 bedroom house in New Hampshire with a 2,500 CFM50 air leakage rate as measured by a blower door.  This house is losing about 250 cubic feet of warm air through uncontrolled leakage every minute 24/7!  A bathroom fan may be exhausting a quarter as much warm air, and at 15 minutes only about 1% of the time each day.  The annual heat-loss for strategically used bathroom fans is usually less than $75.

Heat recovery ventilators (HRVs) that exchange stale indoor air for fresh pre-heated outside air will reduce the lost heat.  But HRVs can use a lot of electricity, particularly if their defroster cycle must run.

The advantages of exhausting potentially damaging moist air with a bathroom fan after showering/bathing outweigh the disadvantages of wasting the heated air.  See below.

Q: My bathroom fan is vented directly into my attic.  Should I be concerned about this?

A: YES!  Building Energy Technologies, LLC regularly encounters bathroom fan ducts that either: (a) vent directly into the attic, or (b) have holes in the vent duct, or (c) have poorly installed duct systems.  All of these conditions are problematic.

The problem is due to moisture.  Bath fans vent moisture.  That is a good thing.  But they need to vent moisture to the OUTSIDE, not into the attic.  The accompanying photo shows what happens when a bath fan duct has a hole in it.  The hole in the metalized duct is in the attic near the roof sheathing.  This small hole is venting a lot of warm, moist air into the attic.  Notice the dark staining on the sheathing.  This is mold coming from the moisture of air leaking from the bathroom.  (There was also a lot of other warm air leakage into this attic, which B.E.T. drastically reduced through careful attic air sealing.) 

In the wintertime, warm air cools when it escapes into the attic, and as it cools it cannot hold as much water vapor in the air (remember Meteorology 101?).  The excess water condenses or freezes often on the coldest surface: the underside of the roof sheathing.  Viola a mini rain forest in your attic!  It is amazing the number of people with attic moisture problems who think they have roof problems or attic ventilation problems when they really have a warm air leakage problem.

When faulty bath fan vents, poorly sealed attic hatches, and other defects leak all this warm air into the attic, the problem can become severe as in this house.  Attics can warm up sufficiently during the day to cause mold to form in these moist conditions.  And in really severe situations, the moisture and mold can destroy the roof sheathing and/or leak into the rest of the house.  Water vapor can also condense inside uninsulated bathroom vent hoses.  B.E.T. has emptied about 2 gallons of condensed water from inside a long run of bathroom vent hose in an attic!

In these situations, the only real solution is to stop the sources warm air leakage from the conditioned part of the house into the attic.  Installing additional attic ventilation is akin to attempting to solve a chronic headache with aspirin: it doesn't get at the root of the problem.  At Building Energy Technologies, LLC we focus on real solutions based on building science -- insulated bathroom venting to the outside, blower door testing, extensive attic air sealing, and then appropriate levels of attic insulation.  Contact B.E.T. if you suspect your attic has moisture problems.

                                  B.E.T. -- Solving your building energy problems