Part II: Conservation FirstChapter 2. Find the Leaks
Smoke-Pencil and Thermal-Camera Leak Hunting
How to follow drafts with smoke and how to read a thermal camera without fooling yourself.
- Difficulty
- Intermediate
- Cost
- $15–300
Leaks hide because air is invisible and because a cold spot on a wall can mean three different things. Smoke makes the air visible. A thermal camera makes surface temperature visible. Neither one, by itself, tells you how many cubic feet per minute you are losing. Used on a cold day, or with the box-fan door running, they tell you where to spend the caulk.
Buy the incense before you buy the camera. A camera is useful after you already know what a leak looks like and you have a temperature difference to photograph. A camera on a mild afternoon, aimed at a window, mostly photographs reflections.
Difficulty
- Level
- Intermediate
- Time
- An hour on a cold or windy day
- Skills
- Moving slowly and doubting a bright color on a screen
Cost estimate
$15–300
Incense is the low end. A phone thermal dongle or a basic camera is the high end, and the price does not buy judgment. A tissue is free and finds the large holes.
Materials
| Item | Qty | Notes |
|---|---|---|
| Incense stick or a commercial smoke pencil | A few | Incense is the gentler source. Smoke pencils can irritate lungs and eyes. Ventilate. |
| Tissue or a thin plastic strip | 1 | For drafts strong enough to move paper. Use this before smoke in dusty attics. |
| Thermal camera | Optional | Only useful with a real indoor-outdoor temperature difference, often 15 degrees F or more. |
| Notebook | 1 | Write the outdoor temperature and whether the fan was on. A picture without that is hard to trust later. |
Smoke
Light the incense in a holder, away from insulation and away from anything that can catch. On a calm day you need the blower-door fan or a windy exterior to make a direction. Hold the stick near a suspected gap and watch the trail, not the glowing tip. A trail that bends and disappears into a crack is a leak. A trail that rises straight is not, at that moment.
Move slowly. Your own breathing and a quick walk will swirl the smoke and invent a leak. Work the attic hatch, the perimeter of a pull-down stair, electrical and plumbing penetrations at the top plates, the rim joist, and the meeting rails of old windows. Mark the hits with a piece of tape and seal them in the air-sealing weekend, then look again.
Do not use a candle or a lighter as the indicator. You want a cool trail, not a flame near cellulose or dust.
What a camera actually shows
A thermal camera reports the temperature of the surface it sees, modified by emissivity and by whatever the surface reflects. Bare metal and glass are poor subjects. A window often shows the reflection of you, the sky, or the opposite wall, and that reflection is not the glass temperature. Matte drywall and wood are easier.
A cold streak on a ceiling during a cold night can be an air leak, missing insulation, or a wood joist that bridges the insulation. The shapes help. A straight line on the rafter spacing is often a thermal bridge. A bloom around a hatch, a can light, or a wire is often air. To tell which one you have, check with your hand, a smoke stick, or a look from the attic.
Palettes that turn a one-degree difference into a rainbow make a sound wall look sick. Set the scale so the range matches the room, and compare with a surface you can explain, such as an interior partition that should be near room temperature.
Phone attachments with coarse sensors will miss narrow cracks and will still see a missing insulation bay. Treat them as a survey tool; they do not replace an energy audit.
Running a session you can trust
Go out when the house is at least about 15 degrees F warmer than outdoors, or run the fan. Write the temperatures down. Photograph or sketch only the spots you confirmed with smoke or with a trip to the attic. Dramatic colors alone do not tell you what to fix.
If the whole ceiling looks cold, you may be looking at thin insulation everywhere, not at a single leak. That finding belongs to the insulation guide, after the holes are sealed. Air moving through even a thick blanket still carries the heat away.
Create a direction for the air
A windy day or the box fan blowing out. Combustion appliances are not firing.
Sweep the ceiling plane with smoke or tissue
Hatch, stairs, top plates, cans. Mark movement. Ignore drama you cannot repeat.
Only then use a camera
Cold day, matte surfaces, a scale that is not screaming. Confirm every spot you might spend money on.
Seal and look again
The second pass is the one that tells you whether the mark was real.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/Conservation/conservation.htm
- https://www.builditsolar.com/Projects/Conservation/BlowerDoor/BlowerDoor.htm
- https://www.builditsolar.com/Projects/Conservation/BlowerDoor/BuildingBlowerDoor.htm
- https://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
- https://www.builditsolar.com/Experimental/IRCamera/IRCamera.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.