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Exhaust fans and attic ventilation questions ChrisW 06-27-2007
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Posted by ChrisW on June 27, 2007, 5:44 am

Presently, my two bathrooms have exhaust fans venting straight into
the roof attic space. I do not believe that my roof has any
ventilation system at all. I am concerned that this may promote
unhealthy air which filters down into the living areas below. Are
these concerns valid?

I am considering installing ducted versions of the exhausts fans which
exhaust air outside through the roof. Or I could install a draught
stopper version of the exhaust fan to prevent air from coming down
from the roof. I would also like to install some sort of roofing
ventilation like those twirling whirlybird ventilators. Would these
address the issues above?


Chris


Posted by Bill on June 27, 2007, 2:51 pm
I vented my bath fans through the roof. (Used Galvanized dryer vent.
Galvanize pipe from fan thru to vent).
It shows moisture on the roof just below where it vents. So you definitely
are getting moisture in your attic. Not good.


>
> Presently, my two bathrooms have exhaust fans venting straight into
> the roof attic space. I do not believe that my roof has any
> ventilation system at all. I am concerned that this may promote
> unhealthy air which filters down into the living areas below. Are
> these concerns valid?
>
> I am considering installing ducted versions of the exhausts fans which
> exhaust air outside through the roof. Or I could install a draught
> stopper version of the exhaust fan to prevent air from coming down
> from the roof. I would also like to install some sort of roofing
> ventilation like those twirling whirlybird ventilators. Would these
> address the issues above?
>
>
> Chris
>



Posted by Glenn on June 27, 2007, 4:30 pm
Most residential bath fans are vented into the attic.
Every attic I've been in has been dryer than a popcorn
fart so the minimum amount of bathroom steam you will
push up there won't hurt anything and that is just one
less future roof leak which WILL hurt something. Bath
yes, kitchen stove no, vent it out.


>I vented my bath fans through the roof. (Used
>Galvanized dryer vent. Galvanize pipe from fan thru to
>vent).
> It shows moisture on the roof just below where it
> vents. So you definitely are getting moisture in your
> attic. Not good.
>
>
>>
>> Presently, my two bathrooms have exhaust fans
>> venting straight into
>> the roof attic space. I do not believe that my roof
>> has any
>> ventilation system at all. I am concerned that this
>> may promote
>> unhealthy air which filters down into the living
>> areas below. Are
>> these concerns valid?
>>
>> I am considering installing ducted versions of the
>> exhausts fans which
>> exhaust air outside through the roof. Or I could
>> install a draught
>> stopper version of the exhaust fan to prevent air
>> from coming down
>> from the roof. I would also like to install some
>> sort of roofing
>> ventilation like those twirling whirlybird
>> ventilators. Would these
>> address the issues above?
>>
>>
>> Chris
>>
>
>


Posted by Dennis on June 27, 2007, 7:56 pm
> Presently, my two bathrooms have exhaust fans venting straight into
> the roof attic space. I do not believe that my roof has any
> ventilation system at all. I am concerned that this may promote
> unhealthy air which filters down into the living areas below. Are
> these concerns valid?

This is a good place to start.
http://www.askthebuilder.com/546_Bathroom_Fan_Ventilation.shtml

Terminating a mechanical vent fan into an attic is against all building
codes; and has been for quite some time. The code requirements for attic
ventilation are there specifically to rid the attic space of natural
moisture due to infiltration from the conditioned spaces below. Dumping
mechanical ventilation into the attic is far more than the code minimums
were ever designed to take and have the potential to cause all kinds of
problems. (Attics and eaves, are not considered part of the "exterior").

Mold and mildew aside, it will cause a dramatic decrease in insulation
value. The amount of moisture going into the attic may not be visible, but
it is there nonetheless. As the warm moist air hits the cold air of the
attic, it condenses into a water film and clings to individual insulation
fibers (insulation works by trapping a tiny amount of air film around each
fiber). A small amount of moisture will dramatically lower the ability of
the fibers to resist heat transfer.

If you have loose-file insulation in your attic, you start out with one
strike against you already. In tests conducted by the Oak Ridge National
Laboratory, loose fill insulation can loose as much as 1/2 its R value do to
conduction in very cold weather (when you need it most). This is especially
true of insulations with a high air permeability like fiberglass and to a
lesser degree cellulose. Adding excess moisture from a bath or kitchen vent
fan will cause insulation R value to drop even further. (Much further).

http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/92/920510.html

Condensation occurs when a surface is cooler than the dew point temperature
of the air next to the surface. Any time condensation occurs, mold and decay
are possible. Mold can grow when the air is damper than 80% rh.

Surprising little moisture is needed to condensate. Consider that it only
takes from 4 to 6 pints of water to raise the rh inside a 1000 sq ft home
from 15% to 60%. Now turn on your bath fan and dump that moisture laden air
at the rate of 50 cfm (usually more like 80-100) into your cold attic and
the warm, moisture laden are from a bath or shower will begin condensate and
fall back onto the insulation.

It's very inexpensive to install vents (with dampers) on the exterior roof.
Some metal flex connected to the fan outlet is all it takes. (You may have
to pull a few shingles, but it shouldn't take more than a couple of hours.
Your home will be warmer next winter and you'll use less fuel.



Posted by DanG on June 27, 2007, 8:58 pm
Code typically demands vented attic spaces. This can be handled
many ways. Look at the soffits to see if you have screens or a
perforated strip at the eave. Google <continuous ridge vent>
<gable end vent> . The wind turbines were all the rage a few
years ago, and they make powered vents. Each has its pros and
cons.

I doubt you will suffer any ill effects if your bathroom fan isn't
vented to the outside if there is any kind of attic ventilation,
but venting to the outside eliminates any possibility of increased
humidity, etc.

--
______________________________
Keep the whole world singing . . . .
DanG (remove the sevens)
dgriff237@7cox.net



>
> Presently, my two bathrooms have exhaust fans venting straight
> into
> the roof attic space. I do not believe that my roof has any
> ventilation system at all. I am concerned that this may promote
> unhealthy air which filters down into the living areas below.
> Are
> these concerns valid?
>
> I am considering installing ducted versions of the exhausts fans
> which
> exhaust air outside through the roof. Or I could install a
> draught
> stopper version of the exhaust fan to prevent air from coming
> down
> from the roof. I would also like to install some sort of roofing
> ventilation like those twirling whirlybird ventilators. Would
> these
> address the issues above?
>
>
> Chris
>



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