I’m a homeowner, not a builder, though I’ve been a subscriber to Fine Homebuilding since 1995.
Our home was built in two parts 1996 and 1999. The Soffits are vented as well as the ridges. We also have gable vents in the roofs. There are actually 3 attic spaces in our house all with the above setup. Two of the attic spaces also have whole house fans blowing into them. We live in Vermont.
I’ve read various thoughts on venting roofs over the years. At least one “expert” says you shouldn’t have both ridge and gable vents. What is the current thought? Are the gable vents “short circuiting” the movement of air from the soffit vents to the ridge? Do I need the gable vents to allow sufficient air to move with the whole house fans?
I started thinking about this the other day since we are staining the house and I was working in front of one of the gable vents which was pouring hot air out at me.
Thanks for any thoughts,
Jerry
Replies
Are the gable vents "short circuiting" the movement of air from the soffit vents to the ridge?
That's always a possibility, but not a given. That hot air you felt coming out of the gable vent could have been being supplied by the ridge vent and not the soffit vents. Was this one of the gable vents being fed by the WHF and was it running?
Do I need the gable vents to allow sufficient air to move with the whole house fans?
That's a good question and one that can't be answered without a lot of specific numbers and details.
http://www.quittintime.com/ View Image
The vent was not one with the WHF. If the air was from the ridge aren't I adding hot air to the house?
If there is sufficient ridge venting area and a balanced or greater soffit venting area, then the WHF is not needed for venting the attic. The fan may be useful at night (summer) for drawing cooler night air through the house, but counter-productive during the hot day. Who wants to draw 80+ summer air through a house that has been cooled to 68 overnight? For a Vermont winter, I hope that fan opening can be sealed up and insulated well.The gable end vents normally should be closed off when ridge vents are installed, since they can result in short-circuiting, with reduced inflow at the soffit. However, since you felt hot air coming out at you, then short-circuiting is not happening at least at that vent. It could mean that you have considerably more soffit vent area than ridge vent area, and hot air is being vented out at both gable and ridge vents. With the ridge higher than the gable vent, I can't imagine cooler outside air being drawn down through the ridge to go out the gable vent. Without a fan, the air flow would be induced by density difference, up through the soffit, out through ridge and (in your case) the gable vent.What happens in winter, with a pile of snow on the roof, is another matter. Then the gable vents might be producing shortcircuiting. Then you want more vent flow up through the soffit to minimize ice damming.
Dick,Thanks for the extensive, and thoughtful, reply. I guess like most things in life there is no easy answer. It's the old "it depends" situation.The WHF are insulated in winter. Good point though.Fortunately ice damming has never been a problem for us. I guess that is one good reason to leave things as they are.On the other hand I question whether in summer time we are venting the hot air as efficiently as we can. In one section of the house the second floor, below the attic, is like an oven towards evening. This section does have a whole house fan from the second floor to the attic. Though I've never been convinced that it lowers the temperature much. We always make a point not to run the fans if the outside temp is above the inside temp.
This section does have a whole house fan from the second floor to the attic. Though I've never been convinced that it lowers the temperature much.
I'm not an HVAC guy so my opinion is only based on observation. The best WHF systems I've seen have been ducted out the gable ends. The fan is installed in the gable end. In addition to very positive air flow, this makes the system quiet.
The direct ducting out the gable end is probably a good idea. Would take a major restructuring to make that happen. Have to contemplate that. Would mean eliminating the gable vent as a "vent" and just using the soffit/ridge vent system to cool the attic space. In a way back to my original question. Appreciate the suggestion.
Yeah, it does make sense, doesn't it. It's a fan that's meant to move air, not pressurize an attic space and hope that the roof vents will work in its favor.
Of course the duct work has to be made to measure and installed, along with cutting and reframing a hole in the gable end so those costs may make the project less attractive.
I must say that the first time I was in a home with that system it really impressed me. The evaporative effect plus the cool fresh air from the shady side of the house was surprisingly refreshing, more pleasant than AC.
That system is particularly effective for night time cooling, lowering the temperature of the thermal mass so that it can absorb hours of heat the following day.
That's the way that convection is supposed to work with double-hung windows, opened top and bottom, but when night time temps change slowly, it doesn't happen fast enough to be very effective.
Edited 7/1/2009 10:25 am by Hudson Valley Carpenter
With the ridge higher than the gable vent, I can't imagine cooler outside air being drawn down through the ridge to go out the gable vent.
I've actually tested for and seen that happen on a warm breezy day. Probably doesn't happen often, though. In this case, the gable vent on the windward side was intaking and the ridge vent was outletting. On the leeward end, the ridge vent was intaking and the gable vent was outletting. It was pretty stagnant in the middle with slight outward movement thru the ridge vent. There was minimal soffit venting and it was not determinded if it was actually functioning or not. Didn't appear to be.
There was only the height of 2x4 truss members or so difference for the drop between the ridge vent opening and the top of the gable vent.http://www.quittintime.com/ View Image
aren't I adding hot air to the house?
No - to the attic. But the attic is never gonna cool below the outside air temp, no matter how well it's vented.
http://www.quittintime.com/ View Image
Jerry - I have a 'flow-through' design with a gable end vent (actually it's a small fixed Marvin window with muntins that's screened, not glazed) and at the other end there are two quarter-round screened louvers with Azek muntins/frames mounted over them (again these read as windows, not louvers). There is a 1,000 cfm fan mounted at that end (to the windward side of the house, generally). This flow-through design is so effective at cooling the attic that I really don't think that ridge vents or soffit vents would make any real difference one way or the other - we don't have them and don't need them.
Jeff
Jeff,Thanks. Interesting setup. Have to decide on best course. Don't know if will change anything at this point. We are finding that staining the house is a major undertaking.Jerry
GAble vents are a bad idea with ridge vents.
I read a study many years ago that proved that. They built a house specifically to test the airflow and temps and set up an identical house with different systems. The researchers proved that gable vents were a bad idea.
In Northern climates, they found that snow was actually pulled into the gable vents, from atop the roof when the wind blew in certain directions. Wind was a major factor because it tended to create positive and negative pressure on the gable walls.
There was far more information and I wish I had kept that report but alas, you'll have to trust my memory.
Jim,Thanks. This is the sort info I need. Too bad you don't remember the reference. Always a question as to whether it is a sometimes problem specific to certain homes, or always a no-no.Jerry