Thursday, August 07, 2008


Who knew?
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).
Nice solution.


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.
Not good.

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.
Or not?


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.


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.
Not good.

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....


Bob Lunchbox said...

It is October now. How about a progress report??

Paw said...

Well, since you asked, here tiz....

As we type, the fan is running.
It is a nice warm day - about 80+.
With a light breeze and only moderate humidity.
Since the thermostat on the fan is set at about 95 degrees, and the fan is running, it must be 95 degrees or more in the attic.
It is 77 in the house now. (comfortable for me.)
The a/c has not turned on in days/weeks.

My gut assessment is that it is working as planned/hoped.
Some days the attic fan does not come on at all.
I have yet to determine what the outside turn-on point is, but I am guessing it is about 80 degrees with sunny weather.

MarkFromTexas said...

Interesting and makes a lot of sense. I am in the process of adding an active ventilator to my attic. It has a passive system with soffits and three square outlets (flat, passive vents) near the top.

But replacing one of the passive vents with an active turbine may lead to short-circuiting the airflow between the two remaining passive vents and the new active vent, rather then inducing more flow from the soffits.

Deane said...

Thanks Paw you have given me something to think about in my quest to cool the house down without aircon here in Queensland Australia

Jeremy said...

Just came accross your post. As an Architect, I feel I should offer some additional clarifications. Attic ventalation is required not only to keep the attic from getting too hot, but also to provide a way for any moisture that finds it's way into the attic to get out. This is particularly important in humid environments, such as the south east. It has been my experience that for most roofs with adequate insulation (R-38) soffit & ridge vents are enough to get the heat & moisture out. I typically only recommend mechanical ventalation for low-slope roofs (flat roofs), or if the soffits are to narrow to provide good ventalation. If you do need to provide mechanical ventalation, I recommend against using that in conjunction with a ridge vent, and make sure that the mechanical vent is located as high on the roof line as feasable. the intakes should be located as far away from the vent as possible as well. I would also recommend the mechanical vent be hooked up to a humidistat as well as a thermostat. Finally if you are installing a soffit/ridge system. make sure that you install insulation baffels at the eaves to ensure adequate airflow. often this if overlooked, and the insulation blocks the air path, therefore negating the usefullness of the vents, in the soffit/ridge system, if the soffits are blocked, you are getting no ventalation at all.

Just my $0.02

Roof Ventilators said...

Roofing vents help to make your roof last longer also. Since the hot air escapes the attic area, there's less chance of snow thawing and refreezing on the shingles of the house. This causes damage and eventually leaks in the roof.


Chantay Smithingell said...

For me, there is no single ventilation system that is best for your home because a lot of factors come to play (i.e., roof area, climate, power consumption, preference). That’s why I want to focus on the principle of equilibrium. A good principle to follow is that the air taken in must be equal to the air expelled. Some houses might only require passive ventilation, while others might need a combination of both active and passive vents.

Anonymous said...

Somewhat well-written discussion here.
But it still doesn't encompass a couple "facts" that presented themselves in my situation.

First of all, lets be clear to state that not all soffit interior space is open to attic space. While a contractor cut EIGHT holes in my soffit for this purpose , it did nothing, ( that's spelled N-O-T-T-I-N-G !!!) to cool my attic. So while I now have 8 holes in my soffit, and FOUR more additional holes in my roof. This contractor was even alligned with the infamous QuieterHome Project in my city, but all they was destroy the look of my soffit, and open up new possibilities of rain getting in past my roof.

Secondly, it is clear that the type of roof one has is ever-important to the type of ventilation one needs. If you have a hip roof on your house, you're sunk to start with! . . .wind turbines and / or power ventilation are your only options. But if you have a gable roof, do not , ( that's spelled D-O N-O-T !!!)...let a contractor cut holes in your roof to install ANY type of vent in your roof!!! By far, the best design is one used over 100 years ago, with louvered vents installed in the gable ends of your house. Just make sure they are large enough to create good CROSS VENTILATION . . . a term not even mentioned in this article! Geeeez!

NeanderthalsRising said...

Turbine vents do not need wind, heat will spin them quite nicely. We just bought a house with louvered vents with fans at north & south ends, and 8 passive square vents. There are no soffit vents. (??) the only other vent is a high louvered vent where the house meets the garage roof. I plan on replacing two to four square vents with turbines. I think the squares are almost useless. The attic is about 2000 square feet.