Vapor Diffusion Ports: How to Keep An Unvented Roof Dry (7 Minutes of BS)

In this episode of 7 Minutes of BS, Dr. Joe Lstiburek explains a relatively new concept in roof construction—and a new addition to the IRC: vapor diffusion ports along the highest part of a roof.

“ Dan, I never get a call to say, Joe, things are going great. Let's have a beer. I don't get that call. I get the call. The vampires are coming. It's the end of the world. My roof is on fire. My roof is rotting; my roof blew away. My neighbor stole the roof. I have homeless animals living in my roof." 
Those are the calls I get.”

Hey Joe, things are great. Let’s have a beer while we listen to the Vapor Diffusion Port episode I cobbled together from leftovers of a previous episode.

What a vapor diffusion port is

In a recent episode about hot roofs and cold roofs, Dr. Joe touched on the topic of vapor diffusion ports as a way to deeply mitigate moisture problems in unvented roof assemblies. Because vented roofs are becoming harder and harder to accomplish effectively.

And they do not help during hurricanes and wildfires.

Well, all right, believe it or not, the impetus for this came from the people in California yelling and screaming, saying our buildings are burning.

Look, we just lost a thousand houses around Boulder, Colorado, right?

And a lot of them burned because the attics were vented, and you have to vent there because they have snow to deal with ice damage.

To recap the Hot Roof Cold Roof episode, venting a roof works all the time, everywhere, except for most places, most of the time.

It works great if you have a poorly insulated attic in a cold climate, and lucky for us; there are a LOT of poorly insulated attics in cold climates.

That’s kind of a tongue-in-cheek way of saying that roof venting is hard to do well with heavily insulated attics.

We also have the issue of fire wildfire.

We also have the issue of wildfire.

Vented attics are burned because we get the embers into them.

And I don’t know if you noticed, but there are a lot more wildfires lately.

And guess what? They blow off more during high-wind events.

Yeah. Because the venting path provides a place for the sky hook to slip its straps into. Listen to the episode about wind-resistant construction for more info about sky hooks.

So, you know, if you wanna, you know, reduce your risk of, hurricane blow off. Don't vent your attic in Florida.

If you want to reduce the risk of wildfire, don’t vent your attic.

You wanna reduce your risk of wildfire. Don't vent your attic.

See?

Why vapor diffusion ports matter

Now here's the problem. There are places where we have to have a vented over-roof to deal with ice damming that are also in wildfire areas.

And so the approach is, is that you have an operable vent. So you have a vent that's open to handle the ice damming, but when a fire event occurs, the vent is closed.

Like foundation vents that can be opened or closed, hire ski bums to open the vents in the fall and close them in spring.

But we’re not here to talk about ski bums.

As climate change ramps up with more heat, more sideways rain, and more wildfire—it’s gonna mean doing things a little differently. Like adding a lot more insulation to attics.

I've come to the conclusion that I can't make vented attics work typically in areas where I have to have lots of thermal insulation, and I've got a complicated roof.

Complicated with hips, valleys, dormers, skylights…

I can't make it work.

I mean, if he can’t make it work…

And so I don't, I don't bother. And then I, you know, candidly, the unvented roofs actually work better anyway, and they're easier to construct.

They are easier to construct. All of those attic baffles are a pain in the neck. But it re-introduces the problem of moisture not being able to escape.

And that’s where vapor diffusion ports come in. “Port” is maybe kind of a misnomer, but nobody asks editors what to name stuff before naming it. While a vapor diffusion port could be a series of circular holes they are more likely going to take the form of a continuous ridge vent that is open to vapor movement and closed to air movement.

How do we know if it’ll work? Science and experience.

Science-wise, Building Science Corp built a test hut and tried different assemblies in each rafter cavity. They placed monitors for temperature and humidity and monitored them for multiple years.

That science was based on judgment, and Joe likes to say, “Good judgment comes from experience. Experience comes from bad judgment.”

Well, I, yes. Well, I'd like to point out that failure has made me the man that I am today. 
Dan, I never get a call to say, Joe. Things are going great. Let's have a beer. I don't get that call. I get the call. The vampires are coming. It's the end of the world. My roof is on fire. My roof is rotting. My roof blew away. My neighbor stole the roof. I have homeless animals living in my roof. 
Those are the calls I get.

Pretty sure I opened this episode with an invitation to drink beer, but anyway, experience and good judgment lead to building code updates, and you’ll find vapor diffusion ports in section R806.5 Unvented attic and unvented enclosed rafter assemblies.

How a vapor diffusion port works

The concept is to stop air leaks while allowing vapor to escape. It’s sort of the same thing we do with walls when we use house wrap that allows vapor movement but stops air and liquid water.

So air can't leave, but water molecules and diffusion can work. So it's called a vapor diffusion port.

The cool part about the physics is that air holding more water is more buoyant than air with less water. I mean, water is heavy. You only need a vapor diffusion port at the peak of the roof.

And one of the things that make them work is that when you add water vapor to air, you lower the density, and so what ends up happening is, is moisture-laden air. It's less dense and more buoyant.

We all know that warm air rises and that warm air holds more moisture than cool air. But when we think of hot, humid air, we do not think of lightness. We think of heavy air. 
When we think about cold crisp air, we think of skiing, lightness.

Physics doesn’t care what we think.

Some moisture-laden air always floats up. So you end up with the moisture because of what we call hygric buoyancy, ending up at the ridge. So the code allows you to use vapor diffusion ports, but it specifies that the roof has to have a slope. And the reason it has to have a slope is to allow for the water molecules to end up at a higher point.

How to make a vapor diffusion port

Fortunately, all framers and roofers already know how to do the first and last parts of vapor diffusion port installation Because it begins and ends like a ridge vent.

It's, you know, a two-inch airspace on either side. So you've got four inches of missing sheathing, and normally you put a ridge vent over it. Well, instead of putting the ridge vent down right away, you cover that four-inch airspace at the ridge with a vapor-permeable membrane.

And so when people say membrane, I think of illustrations of lipid bilayers, and I usually stop listening to them—unless we are talking about lipid bilayers.

So think of a high-temperature Tyvek or a high-temperature Typar.

Got it. High-temperature material is important because roofs see the most direct sunlight of any part of the house. And as you know, the sun is hot.

And then you put the ridge vent on top of it. So you basically have a strip that's very, very vapor open but airtight.

Aside from the high-temperature membrane, the material must be at least 20 perms. Why 20?

Well, I'm responsible for the code language—a minimum of 20 perms. Now, one of the reasons that I like 20 perms is that we could use gypsum sheathing, like, you know, Georgia Pacific DensGlass Gold and the USG product.

Securock. You know who else doesn’t consult editors before naming stuff?

We could use gypsum sheathing As the barrier. Now, what's nice about that is that they don't burn. In other words, I can have a vapor diffusion port membrane layer that doesn't burn.

So you can use dense glass gold as the membrane layer. So you just fill in that two inches on either side of the ridge with that and run a bead of sealant. And that’s the crux of the biscuit, so to speak. This assembly needs to be airtight. If you’re using a sheet membrane, like Typar, you need to seal the edges into the roof deck with high-temperature caulk or sealant.

If you’re using a solid sheet material like DensGlass, you still need to seal the edges into the existing roof deck. I’m just gonna go out on a limb and say that I think it would be smart to use a sealant AND a flashing tape on those joints.

And cover that with a normal ridge vent. With a ridge vent. And, and guess what? ou don't have to worry about wildfires, right?

I don’t have to worry about wildfires?

Yeah. Whoa. Who knew?

I didn’t, but I do now. And now you know what a vapor diffusion port is, where to find it in the code, how it works, and how to build one. I’d like to thank Dr. Joe for joining us on 7 Minutes of BS again—a production of the SGC Horizon Media Network.

More on unvented roofs:

• buildingscience.com on vapor diffusion ports: Guide to Unvented Roofs
• Unvented Roof Assemblies: 7 Minutes of Building Science
• Exterior Roof Insulation Retrofit (Unvented Roof)
• Hot Roof, Cold Roof: Where Did the Terms Come From? (7 Minutes of BS)

Image credit: BuildingScience.com