Looking for vents that snow/ice can't block
#1
07-13-15, 09:43 AM
Looking for vents that snow/ice can't block
Hello,
I asked a question looking for recommendations on ridge ventilation and the subject turned to concerns for ridge vents being blocked. Naturally, that is something to be avoided. Right now I am imagining that something like gable or taller tube shaped vents to be the way to go if this is an issue to be concerned with.
Active Ventilation 10 in. x 15 in. Aluminum Flat Roof Exhaust Static Vent in Mill Finish-KV-10 - The Home Depot I know that these are for flat roofs, and mine is certainly not, but that is what I have in mind for taller ventilation in design.
Any thoughts on this subject?
I asked a question looking for recommendations on ridge ventilation and the subject turned to concerns for ridge vents being blocked. Naturally, that is something to be avoided. Right now I am imagining that something like gable or taller tube shaped vents to be the way to go if this is an issue to be concerned with.
Active Ventilation 10 in. x 15 in. Aluminum Flat Roof Exhaust Static Vent in Mill Finish-KV-10 - The Home Depot I know that these are for flat roofs, and mine is certainly not, but that is what I have in mind for taller ventilation in design.
Any thoughts on this subject?
#2
07-13-15, 10:00 AM
It is more of an issue to be aware of as opposed to designing around. I'm in Maine and they certainly do not shy away from ridge vents around here. If you used both ridge and gable vents, then in the rare event that snow did cover the ridge, the gables would serve as back-up.
The other point is that venting in the winter, or lack there of, is not a short term problem. If all venting was blocked for a week you would never know it. If blocked for month and there were sources of moisture, that attic could experience condensation, but once the vents were open it would quickly dry. Close them all winter and you could have problems.
The high vent you linked to would certainly work, but so will a ridge vent. And, if you look up after a lot of snow, time to get the roof rake out and pull it down, which is necessary with many storms anyway.
And just in case you run into the advice that ridge vents and gable vent do not work well together, the roofing industry has had that wrong for decades. Once you see how far off they are it makes you wonder about all of the other advice they provide.
Bud
The other point is that venting in the winter, or lack there of, is not a short term problem. If all venting was blocked for a week you would never know it. If blocked for month and there were sources of moisture, that attic could experience condensation, but once the vents were open it would quickly dry. Close them all winter and you could have problems.
The high vent you linked to would certainly work, but so will a ridge vent. And, if you look up after a lot of snow, time to get the roof rake out and pull it down, which is necessary with many storms anyway.
And just in case you run into the advice that ridge vents and gable vent do not work well together, the roofing industry has had that wrong for decades. Once you see how far off they are it makes you wonder about all of the other advice they provide.
Bud
#6
07-13-15, 09:56 PM
Group Moderator
I totally get you Furd, and I'm rarely one to argue semantics... but since we are talking expressly about an attic, look at it another way.
Yes, radiant heat moves equally in all directions away from the heat source. No one disputes that fact.
And I agree that people often mistakenly use the phrase "heat rises" when referring to CONVECTION. And that as you stated, a better phrase is "hot air rises."
But since we are talking about an attic in this thread, and a potential ridge vent... in the wintertime... the HOUSE is the source of most of the heat. The laws of thermodynamics state that heat moves from areas of higher temperature to areas of lower temperature. In an attic, you have a difference in temperature- the heat from the house ceiling is trying to leave... and radiates away from its source toward the roof, where the snow is sitting.
So having said that, the HEAT rises in the sense that in the wintertime, radiant heat is leaving the top of the house. In that sense, heat does indeed rise. It also goes in all directions leading away from the heat source (the heated portion of the house) due to both conduction and radiation.
So the "heat" in an attic is not really wanting to travel equally in all directions. It may be radiating in all directions, but where is the heat actually going/moving? What direction is heat being actively transferred? Generally speaking... vertically, albeit in a wide arc! If we were talking about the exterior walls, heat would be radiating horizontally to reach the cold.
I know I must really be bored when I ramble like this... LOL! And maybe I'm all washed up. Just wait, some physics teacher will show up and school us.
Yes, radiant heat moves equally in all directions away from the heat source. No one disputes that fact.
And I agree that people often mistakenly use the phrase "heat rises" when referring to CONVECTION. And that as you stated, a better phrase is "hot air rises."
But since we are talking about an attic in this thread, and a potential ridge vent... in the wintertime... the HOUSE is the source of most of the heat. The laws of thermodynamics state that heat moves from areas of higher temperature to areas of lower temperature. In an attic, you have a difference in temperature- the heat from the house ceiling is trying to leave... and radiates away from its source toward the roof, where the snow is sitting.
So having said that, the HEAT rises in the sense that in the wintertime, radiant heat is leaving the top of the house. In that sense, heat does indeed rise. It also goes in all directions leading away from the heat source (the heated portion of the house) due to both conduction and radiation.
So the "heat" in an attic is not really wanting to travel equally in all directions. It may be radiating in all directions, but where is the heat actually going/moving? What direction is heat being actively transferred? Generally speaking... vertically, albeit in a wide arc! If we were talking about the exterior walls, heat would be radiating horizontally to reach the cold.
I know I must really be bored when I ramble like this... LOL! And maybe I'm all washed up. Just wait, some physics teacher will show up and school us.
#7
07-14-15, 05:12 PM
And just in case you run into the advice that ridge vents and gable vent do not work well together, the roofing industry has had that wrong for decades.
#8
07-14-15, 05:58 PM
The short circuit theory has been out there for generations and it is so wrong. The explanation is very long because other issues have to be explained to support the death of the short circuit theory. But briefly:
The warm air exits the top vents because the entire attic has been pressurized by the heavier cold air pushing in the low vents. Warm air does not rise by itself, it is being pushed up by the cold air flowing below it. Just like the water below a boat holds the boat up, the cold air below holds the warm air up.
So let's take the three major points of the short circuit theory one at a time.
1. The warm air rises and exits the ridge vent. Wrong, warm air does not rise by itself and it exits the ridge vent because there is a pressure difference across that opening.
2. If the gable vents are left open they will short circuit the desired air flow from soffit to ridge because air always follows the path of least resistance. Wrong again. The air entering those soffits doesn't care where it exits, it enters based upon the pressure difference across the soffits which actually increases slightly when the gable vents are left in place.
3. The path of least resistance mentioned above is the other mistake. Air flow through any opening requires a path and a pressure. If you created an example with a large and small opening, both with the same pressure across them, more air would flow through the larger opening, the path of lesser resistance. But our gable vents not only have a different pressure across them than the soffits, in most cases that pressure is pointed out.
I apologize for the long post, but that is actually a short explanation.
The bottom line is, yes there are reasons one might want or need to close off the gable vents, wind blown rain or snow being the primary concerns, but ye old short circuit theory isn't one of them.
As you can tell, this is a topic I have spent considerable time researching and the short circuit theory will be fully explained in a forth coming blog on a range of related issues. Word of my efforts has been slowly seeping out and several of the
Bud
The warm air exits the top vents because the entire attic has been pressurized by the heavier cold air pushing in the low vents. Warm air does not rise by itself, it is being pushed up by the cold air flowing below it. Just like the water below a boat holds the boat up, the cold air below holds the warm air up.
So let's take the three major points of the short circuit theory one at a time.
1. The warm air rises and exits the ridge vent. Wrong, warm air does not rise by itself and it exits the ridge vent because there is a pressure difference across that opening.
2. If the gable vents are left open they will short circuit the desired air flow from soffit to ridge because air always follows the path of least resistance. Wrong again. The air entering those soffits doesn't care where it exits, it enters based upon the pressure difference across the soffits which actually increases slightly when the gable vents are left in place.
3. The path of least resistance mentioned above is the other mistake. Air flow through any opening requires a path and a pressure. If you created an example with a large and small opening, both with the same pressure across them, more air would flow through the larger opening, the path of lesser resistance. But our gable vents not only have a different pressure across them than the soffits, in most cases that pressure is pointed out.
I apologize for the long post, but that is actually a short explanation.
The bottom line is, yes there are reasons one might want or need to close off the gable vents, wind blown rain or snow being the primary concerns, but ye old short circuit theory isn't one of them.
As you can tell, this is a topic I have spent considerable time researching and the short circuit theory will be fully explained in a forth coming blog on a range of related issues. Word of my efforts has been slowly seeping out and several of the
Bud
#9
07-14-15, 06:18 PM
Group Moderator
warm air does not rise by itself
Convection occurs even inside a sealed insulated glass unit. Warm air rises, cold air falls. Wider the space, the more convection occurs. Same thing is true if framing a dead air space wall. 1/2" is the optimum spacing for a dead air space because if it's any wider, convection negates the "dead air"... due to hot air rising and cold air falling.
#10
07-14-15, 07:34 PM
X, you are not the only one to make that argument and as I stated in my post, there are many other issues that each have to be covered to fully accept my explanations, buoyancy is just one of them. Buoyancy is an external force. A boat floats because of the external buoyant forces. Warm air rises because of external buoyant forces and it is the (heavier) cold air that is creating those external forces that push the warm air up.
Less dense just means it is lighter, not that its weight is negative.
"Warm air rises" is a half truth. Of course we see warm air moving up and because we see that and because we have heard that phrase repeated so often we have come to believe it must be doing so by itself.
The phrase "warm air rises" is like phrase "the sun rises in the east". Neither is correct, but both provide a short simple description of what we see and for the most part they do no harm. It was this "short circuit theory" where I discovered there has been harm. People are being told they must remove those gable vents when in fact there may be no problem leaving them and in some cases there may be benefits.
I won't get into convection as it is a chain of events where the weight of the cold air is greater than the warm air. But both have weight and both are trying to move down.
Bud
Less dense just means it is lighter, not that its weight is negative.
"Warm air rises" is a half truth. Of course we see warm air moving up and because we see that and because we have heard that phrase repeated so often we have come to believe it must be doing so by itself.
The phrase "warm air rises" is like phrase "the sun rises in the east". Neither is correct, but both provide a short simple description of what we see and for the most part they do no harm. It was this "short circuit theory" where I discovered there has been harm. People are being told they must remove those gable vents when in fact there may be no problem leaving them and in some cases there may be benefits.
I won't get into convection as it is a chain of events where the weight of the cold air is greater than the warm air. But both have weight and both are trying to move down.
Bud
#11
07-14-15, 08:28 PM
Group Moderator
I get what you are saying about the half truth since hot air and cold air are interrelated. I won't say that I'm with you 100% on the gable-ridge stuff, but I suppose that's why there is such a difference of opinion even among the "experts".
#12
07-14-15, 09:35 PM
Our attics are like short houses and when the inside air is hotter than the outside air, the pressures inside come to a balance where there is a positive pressure at the ridge and a positive pressure below the soffits. This is an attic stack effect and it all developes because of the heavier air outside pushing in through the soffits.
If our house has all three vents, ridge, gable, and soffit and we temporarily cover those gable openings, on a hot day there should be air flowing in the soffits and out the ridge. And, if the NFA for the ridge and soffit are equal and those gable vents are mounted high, then there will also be a positive pressure at the gable level. Remove the cover and warm attic air will exit the gable vents and the additional air exiting will actually increase the desired air flow coming in the soffits. Keeping the gable vents has a list of considerations, hot climate, cold climate, dominant winds, moist air, and more, but short circuiting the air flow from the soffits is just an invention to simplify the venting explanation. You see, they don't know how to add the gable vents into their high to low calculations. See if you can find any web site that calculates for all three.
Bud
If our house has all three vents, ridge, gable, and soffit and we temporarily cover those gable openings, on a hot day there should be air flowing in the soffits and out the ridge. And, if the NFA for the ridge and soffit are equal and those gable vents are mounted high, then there will also be a positive pressure at the gable level. Remove the cover and warm attic air will exit the gable vents and the additional air exiting will actually increase the desired air flow coming in the soffits. Keeping the gable vents has a list of considerations, hot climate, cold climate, dominant winds, moist air, and more, but short circuiting the air flow from the soffits is just an invention to simplify the venting explanation. You see, they don't know how to add the gable vents into their high to low calculations. See if you can find any web site that calculates for all three.
Bud
