I had a "courtesy" fan installed in our half-bath that is switched with the light.  Since it isn't removing humidity, I thought I could just vent it into the unconditioned attic.  It has a flap in the unit to shut when the fan isn't running.  The energy audit I just had said it should be vented to the outside, but couldn't enunciate a specific reason why (it wouldn't be wasting any more heat/cooling than the status quo that I can think of).

So, experts, is there a reason that I should vent this to the outside?  The unconditioned attic is well vented - soffit and gable vents, so air pressure shouldn't be an issue.  It's just to get odors out of the bathroom.

Thanks!

Intentional or not, there is significantly more humidity in the bathroom than there is in the unconditioned attic during the winter- it can't HELP be removing humidity into the attic.

When the temperature of the attic is below the dew-point of the bathroom air, liquid water from that vent exhaust WILL condense on whatever surface it comes in contact with.

How likely is this?

Take a look: http://upload.wikimedia.org/wikipedia/commons/0/05/PsychrometricChart-SeaLevel-SI.jpg

When it's a comfortable 50%RH at 20C/68F in the bathroom, the dew point (temperature at which the RH=100%) is about 8C/46F. Any temps below that and you'll get significant condensation. This is not an uncommon temperature & humidity condition even in cooling dominated climates, abd EXTREMELY common in heating dominated climates. (To find the dew point temp at the 20C/50% RH condition on the chart, find the intersection of the 20C and 50%RH lines, then move horizontally to the saturation line, and read the temperature directly below that for the dew point.)

The ventilated attic provides a drying path for that moisture to later excape, but using it as your moist-air sewer isn't good practice.

sastexan;

regardless of humidity issues it is a code requirement to vent to outside

Oh, yes, well there is THAT too, I s'pose... ;-)

I figured this was a bootleg anyhow, but sometimes it's useful to understand the rationale behind certain code line-items.  There are MANY instances of this sort of bathroom vent installation that predate current code, and the houses haven't all run into issues ( but...   )

And code isn't always rational or "right" for a particular situation.  Attic ventilation specs of x-feet of vent per y feet of roof/floor area were never based on evidence or actual climate conditions, but remained/remain on the books forever. There are similar issues with crawlspace ventilation specs, where crawlspace ventilation can sometimes be counterproductive in terms of structural timber rot issues, even if it mitigates soil-gas issues in the above conditioned space somewhat.  (And it's nearly always less energy efficient, cooling or heating.)  There are now-known better methodologies surrounding crawl space issues based on measured studies, not WAGs,  prior-art, or "standard practices".

So I don't necessarily fault sastexan for asking.

Anyone that trys to sell a home with a non-compliant issue will need to address it at that time, why not do it right the first time and not worry about it?

Dana1, thanks for the insight. I knew if I saw humidity coming in the fan that something needed to be done, but thought if not, there was no problem.

Chris, the full bath fan was not vented outside (well, sort of - it was directed at the gable vent from inside the attic and held to the rafters with duct tape). Of course, our home inspector (who missed other things but we won't go into that now) said all looked good when he checked out the attic, and that was an obvious error (I should have also stuck my head in the attic during inspection but didn't). And the prior owner did an addition without pulling any permits and LOTS of things are not done to code (I've been trying to fix them as I find them). So, yes, technically, when we sell the house everything should be done to code but reality is with a 60-year old house, there are going to be lots of things that are not done to the current code.

Based on the suggestions here, I'll probably put a Y connection in the duct from the full bath fan to the roof vent and extend the vent from the half bath to it so that fan will vent to the outside. No reason to tempt fate. I'll only have to extend the current half-bath vent pipe another 6-10 feet to reach. Thanks to all for your input.

Steven

Steven;

No inspector would expect a 60 year old house to be up to current codes , however when inprovements are made such as adding vented fans, it will be expected to comply with the code enforced at the time of installation.
Don't think the "Y" meets code either, should be vented independent.....nobody said it would be easy

Yup- a Y would partially vent each bathroom into the other- definitely sub-code. (And I concur completely with doing it right in the first place, whether selling the house in a few years or living in it 'til ya drop!)

Anytime you deviate from code it's best to have a solid technical reason for doing so, and get an inspector to sign off on it. (I recently had to push a bit beyond code limits for locating the side-venting of a tankless HW heater, but it's now a done-deal, with inspection approvals.)