Beat the Heat with Light-Colored Roofing (7 Minutes of BS)

In this episode of 7 Minutes of BS, we grab a section of a side conversation with Dr. Joe Lstiburek about the best way to alleviate heat in houses in the southern part of North America without tripling the air conditioning load. His answer: Light-colored roofing. Yeah.

Official Transcript:

Keeping Houses Cooler

This is 7 minutes of BS. I’m Dan Morrison, editor of ProTradeCraft.

Okay, this is really simple: just make your shingles white. Make your freaking roof white. I'm serious; this is a really big deal.

And that is Dr. Joe Lstiburek, the chief troublemaker at buildingscience.com

Make your freaking roof white. I'm serious; this is a really big deal. Now, there’s more. I don’t freaking like white roofs and white shingles. You can get a dark shingle and a dark roof that we’ve fooled into thinking is a white roof because we’ve changed its emissivity. These things are called cool roofs.

They’re cool not because of their fashion sense or entourage but because light colors reflect heat better than dark colors.

There’s also an emissivity angle and we’ll get to that in a minute, but we’re gonna begin by talking about commercial roofs. And what we found out when we started using while roof membranes instead of black membranes. 

Black membrane roofs were running at 180 degrees in the middle of the day in the summer in places like, you know, the middle of the United States. When you change them to a white roof, they’re 120, 110, and that’s a big deal.

The roofs absorbed less energy. Like 60 or 70 degrees, or a 40% reduction

But what happened was that the white roof membranes lasted longer than the black membranes, but the black assembly roofs lasted longer than the white assembly roofs. 

Because…

It’s because the black assembly roofs had a very high drying potential.

In other words, they were hotter, and heat dries stuff faster than cool.

So when we changed the color, we had to introduce air barriers and vapor barriers to compensate for the fact that the roof wouldn’t dry as much because it didn’t get as hot.

Unfortunately, it also means that the so-called test was being done on leaky roofs that had a lot of moisture running through them. Eliminating 40% of the drying potential dramatically increased the wetting potential.

In a new building, that’s easy to deal with because you can design in your air control, your vapor control, your thermal resistance, and everything else.

When Joe says vapor barriers, he is not telling you to add plastic to your building assemblies. He is telling you to control the vapor, which is almost always best done by controlling airflow.

In an existing building, changing the emissivity without changing the wetting potential is very risky. 

Such as a house in Texas that is getting too hot in the summer, (and spring, and fall). One thing houses have in common with commercial buildings is that they are leaky.

Because there is no longer way too much heat in the attic, there is also no longer the big beach towel taking care of the moisture from showers, people, pets, plants, and evaporating toilets leaking into the attic.

And we see this problem all the time when somebody puts a radiant barrier in their attic. It actually works a little bit, and everything else sweats.

Worse, radiant barriers tend to also to trap moisture. Now, one way to equalize the temperature fluctuations in a building is a little off-topic, but which circuitously weaves back to roofing is drywall.

Distributed thermal mass—and the key word is distributed—you could double up your gypsum board on your interior partition walls and have a tile floor. You can ventilate at night and charge that with cool air, then not ventilate during the day. Or you run your air conditioning at night, stop running the air conditioning during the day, and you’ve increased the time constant of your house.

That sounds like it could be a big deal for load-shifting required with an aging grid or one tied to renewables. 

This is going to be a big deal because of the way utility pricing is going. We’re paying a lot through peak load billing, and the idea is to shift the load off the peaks. You can do that, and it’s got to be distributed thermal investment. It’s got to have a large surface area.

Like a Trombe wall.

No, building a Trombe wall is kind of a dumb idea.

My mistake. Bringing the basement concrete into the thermal envelope? 

The trouble is that concrete isn’t coupled to upstairs very effectively, and only the first half-inch to inch of thickness is effective for getting heat in and out quickly. And so that’s why, believe it or not, the drywall people could do really well with this. You basically shift the load to the night.

One way to sell twice as much drywall. Double up the drywall on interior walls, and you’ve doubled the capacity of those walls to store heat and cool.

That, and floors. The problem with floors is you cover them with carpet, and that’s not a good idea. So, if you have a tile floor with spot carpets, believe it or not, that’s energy efficiency, baby.

So that’s less of a retrofit situation, where the trim is already nailed to the walls.

Well, that’s one way of doing it. Another incredibly easy way is to close the vents, change the shingle color, blow a load of fluffy insulation on the attic floor,

Before you add a shitload of fluffy stuff to the attic floor, it is worth sealing up the giant holes in the floor, which the people below it call a ceiling.

 and then add a diffusion vent. That’s a great retrofit. Are you kidding me? 

I am not kidding you, but I am pleased that you feel so comfortable swearing on my podcast.

And you can bury your ductwork 

Not because the ductwork is dead, but because…

…because you’re not going to have condensation on the ductwork since you don’t have any moisture in there because you’re not venting. But here’s the risk—are you ready? In Texas,

Where the hypothetical house is that began this exercise 

...they put their gas furnaces and gas water heaters in the attic. You close your vents, and you’re going to kill Texans.

I am sure there is an Oklahoma punch line here, but I am going to skip it, and keep us focused on the topic at hand: keeping people inside houses in Texas, Oklahoma, Arizona, or any other hot climate cool—especially during power outages.

The biggest energy gain and loss is where the building touches the sky, 

The roof…

followed by where the building touches the sides, 

The walls…

and then where the building touches the ground. 

The foundation…

Deal with the roof first, then deal with the holes in your walls, which are windows, right? Then, deal with where the building touches the ground.

So start with the ground…

Don’t start with the ground, you f*cking moron—that’s not the big one.

The big kahuna is the roof.

I want to thank Dr. Joe for tolerating my salvaging this podcast from an earlier conversation and for making my language choices sound a shitload less offensive.

Remember, you get paid for what you do and what you know. Now, you know more about how to cool a hot building using nothing but a light-colored shingle. And some other stuff.

7 Minutes of BS is a production of ProTradeCraft.