roof vents

Status
Not open for further replies.

skeeter

New User
Charles
I was talking to some men last night and I mentioned that I was thinking about installing thermistat controlled electric vent fan on my roof. One of them said I shouldn't do it if I have ridge vents, which I do. I had never heard this and would like to know what you think!

According to him, in rainy weather the fan would create a vacuum, pulling moisture in through the ridge vents, causing mold and mildew to form.

Thanks for any opinions!
 

jmauldin

New User
Jim
It is my understanding that if you install fans along with ridge vents, the fans will pull air through the vents rather than pulling the hot air out of the attic. In other words you would get air circulation from the ridge vents to the fans but not pull the hot air out of the lower part of the attic. At least that is what was told to me when I had the new roof put on my house. I have the fans and not the ridge vents
Jim in Mayberry
 

Trent Mason

New User
Trent Mason
Hi Charles! :eek:ccasion1 I'm very interested to see the replies to this question. I would think that if you have ridge vents that the humidity would already be the same as it is outside. :eusa_thin But the fan may cool it off in there a little. Then if the relative humidity is greater than the air temperature you might end up with moisture..... I don't think it's really a case of "sucking moisture in" but more lowering the air temperature with respect to RH. But it's been a while since I've done that type of work. My psychrometrics is a little fuzzy. :wconfused: Definitely looking forward to other replies.... :icon_thum
 

Bas

Recovering tool addict
Bas
Corporate Member
From http://www.airvent.com/homeowner/resources/troubleshooting.shtml:
Can I install a power fan if I have a ridge vent already on my roof?
Mixing a power vent with a ridge vent can short-circuit the attic ventilation system just as a gable vent can as discussed above. This happens because air follows the path of least resistance. When the power vent turns on, it can pull air from the ridge vent, which could lead to weather infiltration and unbalanced airflow along the underside of the roof deck. When the power vent turns off, it acts like a roof louver — an opening on the roof without a motor. In this scenario, the ridge vent pulls its intake air from the power fan leading to possible weather infiltration and less than optimal ventilation along the underside of the roof deck.
Basically, mixing two different types of exhaust vents on the same roof that has a common attic is not recommended because it can lead to short-circuiting of the attic ventilation system.
 

Glennbear

Moderator
Glenn
Boy can I relate to this thread !!! When I moved into my ranch type house 3 years ago I had the traditional ridge and soffit vents. Judging from the temps in the attic this system was useless. I had read about the "short circuit" airflow effect when installing a roof fan with a ridge vent so I stapled foam foundation seal over the ridge vent opening inside the roof. The roof fan works great and sucks air from the soffit vents all the way up to where it is mounted near the ridge. I realize that it meets BOCA codes but IMHO a static ventilation system down here in Dixie is an exercise in futility. :wsmile:
 

mquan01

Mike
Corporate Member
I installed a pair of solar attic vents when my roof was reshingled. I also had and still have a ridge vent. Prior to the attic fans, it was not uncommon to have 150+ degree temps in the attic. Since I added the solar fans, it has never gooten abour 85 degrees. These fans come with an 85 +/- 5 degree switch, so they waon't activate in the cooler weather. Seeing the temperature come down over 50 degrees tells me the fans are working.
 

skeeter

New User
Charles
Hey guys!

Thanks for all the info. I don't know what I'm going to do yet. What "Glenbear" said about sealing the ridge vents from the attic side and installing two attic vents instead of one may be the best choice.
 

woodworker2000

Christopher
Corporate Member
My house was built around 8 years ago with a ridge vent along with (continuous) soffitt vents. I have a high pitch hip roof (12:12 pitch) and discovered that it would get extremely hot in the (unfinished) attic during the summer. Several years ago, I had a powered attic fan installed but did not seal up the ridge vent. I can tell you that since installing the fan, there is a very noticeable difference in temperature during the summer months in the attic. If I had to guess, I would say it is 20 degrees cooler on average.

I agree with Glennbear about the ineffectiveness of a passive ventilation system in this area of the country.
 

dave "dhi"

New User
Dave
Ridge vents are all you need and want! I've heard inspectors say roof/attic fans will be disallowed soon on new construction!
 

Gofor

Mark
Corporate Member
I went just the opposite in FL (gulf coast), where I had a powered fan that kept burning up the motor. I put in ridge vents and saw a decrease in AC bills as well as it was noticeably cooler in the attic (150 down to 110).

If your attic is getting really hot, then ensure your soffitt vents are all free and clear to let the air flow. Sometimes the insulation guys get carried away and block the vents. If and when you replace the shingles, especially on the high peak roofs (which I have) ,go to a lighter colored shingle. That alone will drop the temp as much as 30 degrees, depending on how light a color you go. I can't figure out why the most common shingles on new construction here are black, when we have 100 degree summers. I can see it in the mountains, but not here in the piedmont/sand hills. When our house was built 5 years ago, the lightest I could get without paying a really ridiculous extra cost, was a medium gray. With them, even in 90 degrees temps, my attic stays about 110 to 115 and it is a south-facing roof.

However, if you are getting 150 temps, I too would be looking at an alternative. If you have a hip roof, the peak run may not be long enough for a ridge vent to adequately ventilate.

Go
 

dave "dhi"

New User
Dave
Also the most important thing with ridge vents is that you have soffit vents along with you Must have baffles between your plywood roof and insulation. Without one of these, your ridge vent is useless! :help::dontknow:
 

Gotcha6

Dennis
Corporate Member
Many years ago, Duke Power Company did a study that concluded power vents were actually counterproductive. The finding was that although your attic may be hotter, the air in and near the insulation stratifies and doesn't allow the heat to move to the conditioned space. Think of it as a 'negative wind chill effect'. The movement of the air to get cooler air in place actually moves some of the heated/cooled air energy as well. Ridge & soffit vents that allow gravity ventilation, convection movement, and normal expansion should be all that are needed.
JMTCW
 

woodworker2000

Christopher
Corporate Member
Many years ago, Duke Power Company did a study that concluded power vents were actually counterproductive. The finding was that although your attic may be hotter, the air in and near the insulation stratifies and doesn't allow the heat to move to the conditioned space. Think of it as a 'negative wind chill effect'. The movement of the air to get cooler air in place actually moves some of the heated/cooled air energy as well. Ridge & soffit vents that allow gravity ventilation, convection movement, and normal expansion should be all that are needed.
JMTCW
I would have to see that study as my actual experience has been quite different. My walk-up attic (plenty of headroom and square footage up there....I am considering finishing it off into livable space) is unfinished and there is absolutely no blockage of the soffit vent that runs the entire perimeter of my house. Before I installed the power roof vent, it was so hot up in the attic that I became concerned about things I had stored up there melting. Since installing the power roof vent, it is noticeably cooler in the attic during the summer. I set the thermostat on the power roof vent to only come on somewhere around 100 degrees so it does not run all of the time. If the soffit/ridge vents would work as they are supposed to, it wouldn't ever come on (or would only come on infrequently...during very hot days). I can't see the power roof vent pulling conditioned air from my finished living area (below the attic). Instead, it should (and seems to) be pulling air through the soffit vents.
 

Gotcha6

Dennis
Corporate Member
The study didn't say it pulled conditioned air out of the space, but rather it created a movement of air that allowed heat transfer to take place between the conditioned space and the attic more rapidly, much akin to a radiator with the fan running versus no fan at all.
 
Status
Not open for further replies.

Our Sponsors

LATEST FOR SALE LISTINGS

Top