No app turns your phone into a thermal camera

Quick answer: A regular phone can't see heat, so an app alone can't find a leak. You need a thermal sensor (a plug-in attachment or a standalone imager), a cold day, and about an hour. For drafts, a stick of incense does most of the job for nothing.

The app problem

Search "thermal camera" in any app store and you'll find dozens of hits. Most of them take your normal camera feed and paint it in orange and purple. It looks like the movies. It tells you nothing.

Your phone's camera reads visible light. Heat at room temperature shows up in a part of the infrared spectrum that sensor can't capture. Seeing it takes a different piece of hardware, a thermal sensor that reads temperature at every pixel. That hardware isn't hiding in your phone, and no update will put it there.

So here's the rule. If the product is an app and nothing else, it's a toy. If the app is the screen for a sensor you plug in, it's a tool. FLIR and Seek Thermal both make attachments that plug into a phone and use an app to display what they see. Check which phone ports and models the attachment supports before you buy. Connectors change, and a new phone can strand an old accessory, which is one more reason to read when a new phone is worth it and when to skip a year before you upgrade.

Start with the free tool

Before you spend anything, walk the house on a windy day with a stick of incense or a candle. Hold it near the places air likes to sneak in. Smoke that leans sideways or a flame that flickers is a leak you've just confirmed with your own eyes.

Check these first:

• The attic hatch or pull-down stairs
• Around the outlets and switch plates on exterior walls
• The gap under the front door
• Window sashes, especially the old ones
• The fireplace damper
• The dryer vent and any pipe that goes through a wall

I've seen a man spend a weekend caulking windows that were fine, while the attic hatch above his kids' bedroom sat there like an open fridge door. The smoke would have told him in five minutes.

The limit of this method is that it only finds moving air. It can't tell you whether the wall behind the drywall has insulation in it.

The cheap middle step

An infrared thermometer, the point-and-shoot "temp gun," reads the surface temperature of one spot at a time. It's not a camera. You aim it, pull the trigger, and read a number.

It's useful for confirming what you suspect. Point it at the wall by the window, then at the wall six feet away. If one is several degrees colder, something's going on behind it. It's slow, because you're scanning a house one dot at a time. But it costs a fraction of an imager, and it's honest about what it does.

When a real thermal imager earns its keep

A thermal imager shows the whole wall at once, so patterns jump out. A missing batt of insulation looks like a cold rectangle. A stud bay with no insulation in it looks different from its neighbor. A leaking duct or a wet patch near a pipe stands out in a way no thermometer would catch.

If you own an older home, a big house, or you're about to spend real money on insulation or windows, the imager can pay for itself by telling you where the money should go. If you live in a newer, tight apartment, skip it. Borrow one from a friend, or hire it out for a day.

Resolution matters more than the color palette. A low-resolution imager gives you blurry blobs, and you'll spend your time guessing. Ask what the sensor resolution is before you buy, and be skeptical of any listing that hides it. Standalone handheld imagers and phone attachments both exist, and the attachments usually give you a sharper picture for the money, if your phone fits.

How to scan so the picture means something

The tool is only half of it. Bad technique gives you a pretty, useless image.

Wait for the right day. You need a real difference between inside and outside, generally 15 to 20 degrees Fahrenheit or more. A mild fall afternoon won't show you much. A 25-degree morning with the heat on will.

Scan from the inside. Exterior walls in sunshine heat up and muddy the picture. Go early in the morning or after dark.

Let the house settle. Don't scan right after you've been running the oven, the dryer or a space heater. Give it a couple of hours.

Know what shiny does. Glass, polished metal and mirrors reflect heat from other things in the room, so they can read wrong. If you're checking a metal frame, stick a strip of masking tape on it and aim at the tape. Tape reads true.

Take two pictures of anything strange. One close, one from across the room. A cold corner that follows the shape of a cabinet is probably a reflection or an air pocket. A cold corner that follows the line of a stud is probably a gap.

Check it with something else. If the imager shows a cold stripe along the baseboard, hold the incense there. If the smoke moves, you've found the leak. If it doesn't, you've found missing insulation or a thermal bridge, and the fix is different.

What a thermal image can't tell you

It reads surface temperature. It doesn't see through drywall and it doesn't measure how well your insulation performs. It shows you where something is different, and you still have to work out why.

It also can't replace a proper energy audit. A pro with a blower door test can pressurize the house and measure total air leakage. If you're planning a serious upgrade, that's the person to talk to, and many utilities offer audits or can point you to someone local. Use your own scan to go in informed, and so you can't be sold work you don't need.

What you do with it

Most leaks are cheap to fix, and you can do them yourself:

• Foam gaskets behind outlet covers on exterior walls
• Weatherstripping on doors and the attic hatch
• Caulk on the gaps around window trim
• A can of spray foam in small gaps around pipes and wires (not near a flue or a flame)
• A door sweep, which costs very little and stops the gap under the door

Do the cheapest one first, then re-scan. That's the real use of the imager: proof. You'll see the cold stripe disappear.

And leave the house better than you found it. If you borrowed a friend's imager, return it with a fresh battery and a full charge, and tell him what you found. If you rent, fix the little leaks anyway and tell the landlord about the big ones in writing. Your kids' bedroom will be warmer and your heating bill will thank you for it.

If you do buy an attachment, it will live on your phone, so back it up before you start taking pictures of every wall in the house. Back up everything so a lost phone is just a bad afternoon covers that. And once the leaks are sealed, smart home kit that earns its place is where a thermostat or a cheap temperature sensor can keep an eye on things for you.

A candle costs nothing, and a good imager costs real money. The mistake is paying for the toy.