Do You Need A Solar Powered Attic Fan? (7 Minutes of BS—Building Science)

" Solar-powered attic fans boost airflow in attics"

This is Seven Minutes of BS—building science with a beat.

I'm Dan Morrison, editor of Pro Tradecraft. 

And they're really cool because if you wanna move air through the attic when the sun is shining, when it's also hot up there, it's like, hey, you get the energy when you need it.

And that's Peter Yost, building enclosure consultant with Building-Wright, and my old partner in crime at Green Building Advisor.

Peter and I launched GBA with Martin Holladay and Rob Wotzak way back in 2009, and one of Peter's first blogs there was about solar-powered attic fans. He wasn't necessarily a fan, you might say.

And they're pretty simple because you just cut a hole in the roof, and then if you're not wiring it because it's solar powered, you just sort of let it do its thing, and maybe add a thermostat so that it only runs when the attic gets hot. 

And attics get hot.

Especially in hot climates like Houston. Peter says he's measured temperatures as high as 185 degrees on the roof, and over 170 in the attic 

What are solar attic fans?

You try to locate it on the roof so that the PV panel that's driving it sees the most sun, and it takes quite a bit of power to turn a fan. But since the electrical energy produced by the PV panel is direct current, you just use a DC motor, and DC motors are really efficient.

So that's what's slick about it 

How does a solar attic fan work?

The fan turns on when the sun comes up, and because a DC fan is super efficient, the little solar panels create enough energy to get the job done 

And most of them are controlled by a thermostat because, again, if the attic gets hot, the sun is shining, a good match with that.

But some of them are also controlled by a humidistat, which turns them on and off based on humidity. And in that way, if you have both a thermostat and a humidistat, it will operate depending upon whether it's managing for temperature or managing for humidity, or both.

It could be that it's both humid in the attic and it's pretty warm.

Most people install them because they're trying to solve an energy problem in the summer with their bills for air conditioning, or their top floor is uncomfortable because the ceiling is warm because their attic has gotten really hot. And the idea is if the outside air is 100 degrees but the temperature in the attic is 170, that's still bringing in air that's a lot cooler than what's in the attic.

And on paper, that makes sense. Because on paper, life is simple. If you want a wall somewhere, you just draw a line. If you want air to go somewhere, you draw an arrow. And it works on paper.

Why do attic fans matter?

Unfortunately, reality is different than theory for all kinds of reasons, but in this case, for at least two. 

The first thing is that any exhaust fan, everybody says, pulls air according to the path of least resistance.

But only in Germany does air actually follow the arrows drawn on the paper. Everywhere else 

It pulls proportional to the resistance of individual pathways. 

You can always count on Peter to give nuance. He's like a diplomat and a scholar. 

So that means if you've got a big hole, yeah, most of the air that it's pulling is coming from there.

But there are other pathways, and they don't just shut off. 

Especially when you add a fan to the attic, which actively sucks air out. 

If they are smaller and they represent more resistance, they just pull less air. So if I have places in the ceiling of the top floor or the attic floor that represent small leaks 

Like can lights, ceiling fans, or even along the tops of walls.

Attic fans can suck conditioned air out of the living space

Even though the arrows show that it's only going in the desired direction from the vent ports out the fan, that's not the way it works. And so they don't pull to the path of least resistance. They pull proportionally based on the resistance of each path. Some paths are wider than others.

And an example of that is let's take a chimney chase, right?

Well, the code says you have to have a two-inch gap between the framing and the chimney chase. Most chimneys are not sealed with non-flammable metal in that gap. So if you're turning that fan on, it's going to pull from that hole just like it's pulling from the place that you want it to. And the air coming through that shaft is coming from inside the conditioned house.

So that's one thing. You're sucking air from the living space into the attic. And if you're sucking air from the basement or crawl space, you may well be sucking a lot of moisture up through the house, perhaps moistening the framing all along the way. 

Moisture isn't easy to control with an automatic fan

Here's the second problem. And remember, we're just talking about fans in general now.

We'll come back to solar power in a second. We vent attics not only to keep them from getting too warm in summer, but the code is mainly about venting to control moisture. Okay, so why do a lot of fans only have a thermostat when if too much moisture gets into the attic, I wanna pull in outside air to, to reduce that moisture?

So we really need fans in attics for two reasons: for an energy perspective to make it not so hot up there in the summer, but also to reduce the amount of moisture that's in the attic. All right, so now I need both a thermostat and a humidistat, which measures the moisture content of the air. Well, what if the air outside has more moisture in it than inside 

Such as in an air-conditioned house, where moisture is pulled out of the air and can then leak into the attic.

The humidistat is in the attic, so now you're pulling in air that has more moisture. 

Think about a hundred-degree day in New Orleans with 70% relative humidity. 

Maybe what I need is two humidistats, like one outside and one inside, and then write the control logic. All of a sudden, you've taken this really simple standalone solution, and you've made it a lot more complicated.

What was once a self-contained, solar-powered fan is now more of a multidimensional, solar-powered mechanical logic center. 

How to do attic fans right: Seal the attic

Okay, is there a way to do it the right way? And the answer is yes, but if you don't want it to pull from unintended locations like the top floor ceiling/the attic floor, you better have an airtight lid.

Seal the attic first.

And then the second thing is, if you're gonna install a fan, you'll probably need more complex controls. You will need to include a battery or wire it for current, because there will be times when you want it to run when the sun isn't shining.

And here's the basic moral of this to me, which is, at face value, this is an elegant solution. But most things that are about building science are never just about energy. The physics involved with building science is about hydrothermal. It's about how- 

How heat and moisture react to each other when one or the other changes.

You can't manage heat without affecting moisture, because of physics

Every time you try to separate those two, like, "Hey, I'm just gonna attack energy," because the physics won't let you disconnect them, you'll have an unintended consequence when you manage energy in terms of how the moisture is behaving.

So the right way is to first Air seal and insulate your attic 

Attic-equately 

And then you probably don't need an attic fan, regardless of whether it's wired or solar powered. 

I just want to point out that in another Seven Minutes of BS episode, our old friend Dr. Lstiburek talks about lowering the temperature of roofs by using lighter colored shingles.

Okay, this is really simple. Just make your shingles white. 

Which is another pretty simple solution.

I want to thank Peter Yost of Building-Wright—spelled with a W—for joining me on this trip down memory lane. And thank you for tuning in.

Remember, you get paid for what you do and what you know. You're already doing way too much, but now you know more. 

Seven Minutes of BS is a production of Endeavor Business Media, a division of Endeavor B2B.