My recent experience in the nice, toasty attic of my humble abode inspired me to investigate adding more vents to my attic.
It was hot up there and it was only 10 AM in the morning.
Little did I know that there is a serious debate going on among contractors and home improvement types about the "best" kind of attic ventilation.
My internet research revealed the main arguments as these:
Some say passive convection systems are best (wind turbans, ridge vents, soffit vents).
Some say powered systems are best (gable fans, and/or "flying saucer" roof fans, and soffit vents).
One guy even said none is necessary if the purpose is to reduce heat transfer into the living space - just good insulation will do the job.
Gee, I thought this was all pretty simple and settled long ago until I read all this stuff.
Mmm, guess not.
My take on the whole subject -
When the sun shines on a house roof, the material of the roof - shingles, wood decking, and structural members - absorb heat from the sunlight.
These transfer their heat to the air inside the attic.
This body of hot air transfers its heat to the ceiling of the house, which, in turn, transfers its heat to the air inside the house, starting at the top of the rooms and drifting downward.
To counteract this natural process, houses have had small holes (vents) cut in the roof to allow the hot air to escape and cooler air to come in and take its place.
Natural convection takes place as heated air begins to rise while cooler air moves in behind/below the warm air.
To work properly, the vents in houses need to be cut in the lowest parts of the roof - along the soffit, and in the highest part of the roof - the gables (if any) or the ridge(s).
The vents near the ridges allow the hot air to escape while the vents in the soffit allow cooler outside air to enter the attic area.
As the cooler air from outside moves through the attic it is heated and thus moves higher and then out the top vents, which draws in more cooler air from outside.... well, you get the idea.
You need both sets of vents for proper attic ventilation to work.
And the area of both sets of vents needs to be about equal.
The good news about this system is that it is relatively cheap, works quietly, uses no energy and when weather conditions are right, can move a lot of air.
"Right" weather conditions is, basically, some wind.
As little as 5 mph is enough.
And, in the case of ridge vents, it helps if the wind is coming from the right direction.
The "right" direction in the case of ridge vents is perpendicular to the vent.
But several years ago, some professional types questioned whether this passive system is enough.
To "make sure" that "enough" air is moving through the attic, they recommended that vents with electric fans in them be installed to "make sure" that enough air is moved through the attic.
Wind or no wind.
Enter - "active" vents.
The reasoning for this is that the surface area of a roof is large.
On a single-story house it is typically larger than the floor area by 10 - 20 percent.
This roof area can (an will!) absorb a LOT of heat energy.
It will take a LOT of air moving through the attic to remove/displace all the hot air produced by this heat build up.
Most passive vent systems cannot move enough air fast enough to offset the heat build up.
So they need some help.
Enter the powered attic ventilator.
These, along with enough soffit vents can force more air into/out of the attic and move all that evil hot air out of the attic as fast as it heats up.
The fans are relatively quiet, they do not use very much electricity, are durable and reliable, they don't depend on whether the wind is blowing or from which direction it is coming, and they are not too expensive ($50-200).
Then some people found a problem with the new solution.
The fans sucked too much air.
They found that in some houses, the attic fans were actually sucking the nice, cool, air-conditioned air, that they paid so dearly to cool, right out of the house.
In some cases, it was even sucking smoke and carbon monoxide gas back down chimneys and/or vents into the house.
The attic fan does not care where it gets its air from.
It just knows that it needs x cubic feet of air every minute to match its designed ability.
If it cannot get enough air from the vents in the soffits, it will pull air from inside the house.
The typical house has dozens of small cracks and holes in it.
They are around electrical outlets, lights in the ceilings, stove and bathroom vents, chimneys, the attic access door, etc.
Like all processes in nature, air likes to be at the same pressure all the time, everywhere.
The technical term is equilibrium.
When/if things are not equal, it tries to get that way.
That is what wind is; nature trying to equalize the pressure between two adjacent extremes.
So, when the fan in the attic causes the area in the attic to be a lower pressure than the house below it, the air in the higher-pressure area tries to get to the area of lower pressure next to it to make the air pressure of the two areas equal.
So, now there are some "experts" and contractors recommending against powered attic ventilation.
So, back to the good old methods.
DO YOU BUY A NEW CAR IF YOU GET A FLAT TIRE?
And if you find that your attic ventilator fan is sucking out the nice, expensive air conditioned air out of your house, you need to adjust something.
Either add more low (soffit) vents to your attic or reduce the number or capacity of your attic fan(s).
You are not trying to make a helicopter.
You don't need to suck your roof off.
You need just enough suckage to change the air in your attic every two or three minutes.
Too much air movement in your attic does not gain you anything more.
Diminishing returns and all that.
THE CONCLUSION OF THE WHOLE MATTER
So, here is where I am on this.
I already have some passive vents in my attic.
They seem to be insufficient to the task - meaning, my attic gets reelee hot.
(I measured the temp up there today and it was 121.7 degrees at 4:28 PM).
(The outside temperature was about 97 degrees.)
(That is not as hot as some houses become.)
And one more thing - the location of the inlet and outlet vents is important.
In fact, it is critical to proper attic cooling - whether active or passive.
Air, like many elements in nature (e.g. water), is lazy.
It will take the path of least resistance to do its work.
So if an outlet vent can get its air from some other vent closer than the more-distant soffit vents, it will.
Thus, if one locates passive turban vents too close to passive gable vents (as is the case with my house), the gable vents stop being outlet vents and become inlet vents, or vice versa - thus changing the balance of inlet to outlet vent area.
This short circuits the design and "shuts off" the intended cooling mechanism.
So instead of moving cooler air across six hundred square feet of hot roof surface, you are only moving cooler air across ten square feet of hot surface.
I think I need more ventage in my attic and I want a powered vent to make sure that some air is moving in my attic even when the outside air is still, as it often is on hot summer days in north Alabama.
If, by installing a powered vent fan, I cause interior house suckage, I will add more soffit vents until I get it balanced.
Wee shall see....
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