Search the forums

Loading
Home › Forums › Breaktime › Energy, Heating & Insulation

Wise to block gable vents in lieu of soffit vents?

SwissCarpenter's picture

Wise to block gable vents in lieu of soffit vents? (post #205402)

SwissCarpenter on Sun, 11/27/2011 - 17:29
in

 

I've got a 1900 square foot 50's era ranch house and I'm getting ready to insulate the attic.  The house currently has fairly large gable vents (triangles roughly 3x3x5) at the apex of the gable.  When the house was reroofed by the prior owner, a number of roof vents were installed on the rear/west side of the house.  I'm going to install soffit vents and baffles prior to blowing in cellulose.  I've come across a few people that advocate sealing up the gable vents since there’s a chance that they'll disrupt air movement from the soffit vents to the roof vents.  It kind of makes sense since the air will take the path of least resistance as it rises. Could someone in the biz give me some concrete advice on this one?

-A +A

Take a look at this thread (post #205402, reply #1 of 4)

by KDESIGN in reply to SwissCarpenter [original] on Sun, 11/27/2011 - 18:22

Take a look at this thread dealing with the same question:

http://forums.finehomebuilding.com/breaktime/general-discussion/it-necessary-close-gable-vents-when-roof-ridge-vent-added

Generally from what you describe, I think there is a good chance that it would be okay to leave your gable vents in. 

If you're going to (post #205402, reply #2 of 4)

by davidmeiland in reply to SwissCarpenter [original] on Mon, 11/28/2011 - 00:58

install new insulation, make sure you air-seal the entire attic floor very carefully before the insulation goes in. A good air-sealing job will dramatically reduce the amount of humidity from the interior that makes it into the attic, and will help make the venting details a lot less important. After you do the insulation work, check the attic periodically, and if there are signs of trouble you can decide how to rectify.

I didn't read the above (post #205402, reply #3 of 4)

by fassbinder in reply to SwissCarpenter [original] on Sun, 12/11/2011 - 09:54

I didn't read the above mentioned thread, but I did see a recent episode of This Old House where this was mentioned. It might be available on line to view.

What Tommy said was, in a nutshell, If you have ridge venting and soffits the gable vents would be a problem. He claimed that the Ridge Vent would draw air using the path of least resistance, which would be the Gable Vents as they are the closest.

That would severely decrease the draw through the attic baffles from the soffits, which would move more air along a more widely distributed area.

Tommy is wrong.  It is not (post #205402, reply #4 of 4)

by KDESIGN in reply to fassbinder [original] on Sun, 12/11/2011 - 16:33

Tommy is wrong.  It is not just the ridge vent alone that creates the draw of air.  The draw of air is created by the chimney effect extending all the way from the sofit vent intakes to the ridge vent outlet.  The longer that run, the more draw is created.  It is not like putting a suction directly onto the ridge vent outlet.  The suction starts as soon as the air enters the sofit vent and is increased with every foot of rise as it heads for the outlet. 

The ridge vent will not short circuit the sofit vents and draw only from the gable vents.  Most of the suction is developed below both the ridge vent and gable vents, so it will draw air into the sofit vents regardless of what kind of outlets are above.

But there is an advantage in having the total ventilation outlet area smaller than the total inlet area.  That way the air being drawn into the sofit vents will tend to pressurize the attic or roof insulation cavity.  Whereas, if the outlet is larger than the inlet, the air will tend to pull a vacuum on the attic or insulation cavity. 

That vacuum will then draw air in from the conditioned space through any available holes.  So achieving an outlet smaller than the inlet may be one reason to close off the gable vents.  But if you have no leaks into the space from the conditioned space, it won’t matter.