What happens here?

What happens here?

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Tuesday, April 9, 2013

What does Infrared actually tell you about your home?

Infrared Imaging, commonly referred to as IR, is a very useful tool for anyone practicing building science. It provides visual images that offer clues to what is happening, temperature wise, in the wall (or floor, ceiling, roof...).

Wait, that last sentence is rather, well, limiting. Doesn't IR see through walls?

No, IR isn't magic. It's simply making a best guess about the temperatures on the surface of the walls based on a lot of assumptions. The operator of the equipment has supplied some of those assumptions as has the manufacturer of the imager. The manufacturer made his assumptions based on solid physics and years of testing. The operator is making his assumptions based on what he remembers from his training and how he is interpreting the conditions of the objects he's imaging. This can be a weak link, and it's where most bad IR interpretation starts to go wrong.

That's the other thing. IR doesn't tell you anything, it gives you information that you need to interpret. The operator needs to understand the workings of the system that's being imaged or the images make no sense. I work with buildings and I'm comfortable interpreting IR images of buildings. Ask me to evaluate an image of a horse and you might as well flip a coin, I'd be way in over my head on that.

Still, IR is a powerful tool to have. Because of the relative speed of acquiring information and because it can present that information graphically it makes diagnosing problems quicker and better. Homeowners get a better understanding as well. Seeing the effect of missing insulation really brings the point home quicker than the proverbial thousand words of explanation.

Let's take a look at a few images. The first one is simple. We're looking at a ceiling on the top floor of a building. The attic above is cooler than the room and the sun hasn't warmed it up yet. This attic had no access so I had to base my interpretation solely on this image. Fortunately when the contractor made an access to insulate the area properly I was proven correct.

The fiberglass insulation is in batt form. One end of a batt is being held up by the can light in the picture and the cold air in the attic is getting under it to the drywall ceiling. This is a classic situation and all too common. But this had been in place for 12 years and until I came in with a thermal imager no one knew about it. If you read my post on gaps in insulation you know how bad this can be.

One of the most useful things you can do with IR is find air leaks. With a reasonable temperature difference between inside and out I can use the blower door to create airflow. Once the door has run a few minutes you can get images like the one below. The cold air flowing from the attic creates the cold (blue) streaks on the warmer surfaces. Not only are these powerful for illustrating the effect, an air sealing contractor can use this as a guide to what needs to be done.

Important note: Ask anyone offering air sealing if they use a blower door and IR to find leaks and confirm results. Working without these tools is like tying your hands behind your back.

Once in a while you run into airflow strong enough that you can see it without a blower door. Every building science professional I have shown this image to assumes I had the blower door running on a cold day. The temperature difference was 20 - 25 degrees between in and out. My blower door was still in the van, so what happened?

I was in the lowest level of a former barn converted to child care. The building was a tall 4 stories and a really good example of the stack effect in a building that is leaky at both the top and the bottom. Think what the airflow would be like on a really cold day!

Sometimes unexpected patterns show up. Below is a small section of wall in a half basement. At the very top right is the edge of the board that caps the wall that is against the concrete wall. (BTW, this should not be done the way it was, that deserves a separate post.) The lower left is just a chair that intruded into the image. The orange-ish section on the right is mostly undisturbed fiberglass insulation under drywall. The center area, with those dark splotches are tunnels and nesting cavities created by mice. We confirmed this by removing the drywall (wearing protective masks and clothing) and cleaning out the mess and damaged insulation. Excluding vermin is really important in buildings both for health reasons and because they can cause damage to insulation and structure.

All of the above information was inferred from the temperature of the surfaces I looked at. With the IR and knowledge and experience with buildings I got the right answers. Ask your prospective energy auditor how much experience they have with buildings. If they were selling cars last month will they be able to adequately interpret the information they gather? Having the tools doesn't give you the skills and knowledge to use them.

Sometimes you can have a little fun with IR. I worked on several child care centers over the past few years. It was the most fun I've had on any project I've done. IR is not ideal for portraits but I got a lot of requests from the kids (Take my picture! Me too! You took her picture. Take mine). This little girl had just been playing outside and she was warming her cold hands on her forehead.

I'm told that IR is commonly being used in some countries as a medical diagnostic tool. The only interpretation I was able to make was that she had been having fun.

When you get an energy audit don't forget to have your picture taken as part of the process!


  1. Hi Bill,

    What camera are you using for the infrared photos above?


    1. John, I'm using a Fluke Ti25. There are newer units but this serves my purposes nicely.