Thread: Is this enough attic ventilation?

1. ps
Professional Member
Join Date
Feb 2005
Location
MA
Posts
150
Post Likes
Here we go again!

2. Regular Guest
Join Date
Jun 2005
Posts
4
Post Likes

crappy fans not doing their job

uktra.
This is my first reply. Uktra your not making sense. Are you saying that a 100 cfm fan pulls more than what it is rated for? If so then I need to go back to school and complain to my teachers that I was taught incorrectly. I want my money back!! The fan will pull the air that is closest to it, and if that vent allows for 100 cfm or greater, then thats where the fan is going to pull the air..period. Thats not my opinion, thats the laws of physics. You can't argue with reality.

3. ps
Professional Member
Join Date
Feb 2005
Location
MA
Posts
150
Post Likes

4. Regular Guest
Join Date
Jul 2004
Posts
253
Post Likes

Re: crappy fans not doing their job

Originally posted by whos on call
uktra.
This is my first reply. Uktra your not making sense. Are you saying that a 100 cfm fan pulls more than what it is rated for? If so then I need to go back to school and complain to my teachers that I was taught incorrectly. I want my money back!! The fan will pull the air that is closest to it, and if that vent allows for 100 cfm or greater, then thats where the fan is going to pull the air..period. Thats not my opinion, thats the laws of physics. You can't argue with reality.
If you have a PV rated at 100 CFM, and it is indeed exhausting 100 CFM, then 100 CFM will be drawn into the attic from any source available. You cant assume it will only drawn from gable vents - it may, and probably will, draw from the conditioned space of the house. The ratio of air drawn from the outside via vents to air drawn from the conditioned space of the house will vary from house to house depending on how well sealed the conditioned space is from the house. Air moves in response to a pressure gradient force created by the exhausted air and will move to reduce the pressure gradient force.

5. Regular Guest
Join Date
Jul 2004
Posts
253
Post Likes
Originally posted by docholiday
Respectfully, I must disagree, a sealed attic is not a good idea. They should indeed be well ventilated. Gable ends are better than soffit vents for power exhaust. Soffit vents and either gables or ridge vents are good for natural ventilation. The house should not be under positive pressure and the attic should not be under a negative pressure. Otherwise, this is when you leak conditioned air.

The drop in attic temp in a ranch home by 10-20 degrees can be a significant reduction in load and cooling requirements and the amperage to run the fan is likely less than the additional compressor requirements to accomplish the same thing.

Remember the equipment is already installed and the duct is done, I offer an exhaust fan as a viable option at this point.

Ventilated attics may make sense (and actually be needed) in cold climates but there is a growing body of evidence against it in warm climates. I have a non-ventilated attic - sprayfoam insulation on the underside of the roof sheathing. My attic temperature is between 80 and 90 degrees on the hottest days. We religously use bathroom fans to exhaust moisture from showers and the AC takes care of the rest. I am sure the absolute mosisture content of the air in our attic is less than the absolute moisture in the outside air and it is certainly cooler. There is no need for ventilation.

6. ps
Professional Member
Join Date
Feb 2005
Location
MA
Posts
150
Post Likes
Tuccillo - you are a kinder and more patient person than I.
The observed tone and lack of basic knowledge of some posts are usually a sign of someone looking to stir it up and hijack the thread. I like a good fight when opponents are evenly matched - but the previous poster was obviously way out of his league and just spoiling for some name-calling.

7. Regular Guest
Join Date
Jul 2004
Posts
253
Post Likes
Originally posted by ps
Tuccillo - you are a kinder and more patient person than I.
The observed tone and lack of basic knowledge of some posts are usually a sign of someone looking to stir it up and hijack the thread. I like a good fight when opponents are evenly matched - but the previous poster was obviously way out of his league and just spoiling for some name-calling.
PS, thank you for your kind words, although my wife would argue with you ;-). The problem with a lot of these discussions is that they are not black and white - just a uniform shade of grey. There is a mixture of people's realife experiences and controlled, scientific experiments. My own experiences with PVs was not a good one. It was obvious to me that significant amounts of conditioned air were being drawn into the attic from the PVs, although some of it could also be the stack affect. The reason I know this is that I could see black dust accumulate between the floor molding and the carpet. Clearly the carpet was filtering dust out the air as it got sucked past the floor molding and into the wall cavities leading to the attic. My attic had a lot of soffit vents and a large gable vent - perhaps more were needed - hard to say - the only thing I know for sure was that there was air motion into the wall cavities. Also, the attic was still plenty warm - I regret never putting a thermometer up there.

In my new house we have a sealed (non-ventilated attic) - it is quite comfortable up there in the middle of the day.

8. Regular Guest
Join Date
Jun 2005
Posts
37
Post Likes
Ok!!

All this argueing over ventilation an Pv's. All I want to know is giving the pic I posted, are three of theses enough to keep a 1500-1700 foot attic cool with sofet vent's all the way around:

9. Banned
Join Date
Aug 2002
Location
Office and warehouse in both Crystal River & New Port Richey ,FL
Posts
18,836
Post Likes
Sure ,

if they pull enough air from the home.LOL

Seriously,what you have been told is worth checking,if the walls ,etc. aren't tightly sealed,and/or the soffit vents are too small,it's more of a problem then not.

10. Regular Guest
Join Date
Jul 2004
Posts
253
Post Likes
Originally posted by smp7015
Ok!!

All this argueing over ventilation an Pv's. All I want to know is giving the pic I posted, are three of theses enough to keep a 1500-1700 foot attic cool with sofet vent's all the way around:

Define what you mean by cool.

11. ps
Professional Member
Join Date
Feb 2005
Location
MA
Posts
150
Post Likes
smp - unfortunately, there is no way anyone here can give you the thumbs up or down. There are too many factors to consider and in the end, the only way to know for sure is via actual measurements over an extended period of time taking into account all the contributing (and varying) influences on your measurements.

I don't know your situation - but I would suggest that you have the installer "prove" to YOU that it is sufficient, along with the criteria he is using as the basis for his installation/design. Good luck.

12. Regular Guest
Join Date
Jun 2005
Posts
4
Post Likes
Tuccillo
I am not assuming. Everything in nature always attempts to take the easiest route in order to accomplish it's task.
In the case of replacing exhausted air, the air will be replaced by air that is closest in proximity, and in this case it will come from the nearest vent opening. Like I said , this is not my opinion, but it is a fact. Your not going to get negative pressure in a room that has equal inlet and outlet pressures, nor is the space under the attic going to try to compensate for exhausted air that is being replaced on a countinuous basis, by inlet air that is readily available close by.

Join Date
Aug 2003
Posts
626
Post Likes
Whos on call--we are not talking about nature, we are talking about pressure differentials. If you pull air from an attic you will have a pressure differential on all holes no matter where they lead. If it is a 100 cfm fan, 100 cfm is not coming from one hole if there is more than one hole. You get a whole house fan in the old days and you can feel wind from many rooms if the windows are open. Again--buy a smoke pencil, and put it up to a non-sealed canned light and see what the smoke does when the pav is on vs when its off.

Page 2 of 5 First 12345 Last

Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

Related Forums

The place where Electrical professionals meet.