How to Use a Smoke Pencil to Read Air Direction

Why this matters

A smoke pencil is the cheapest diagnostic tool in the truck and the one most often misread. A tech puffs at an outlet, sees the smoke drift, and declares a leak; another puffs at the same outlet after the dryer stops and sees nothing. Both are right about what they saw and wrong about what it meant. A smoke pencil tells you one thing: which way air is moving through a gap at the moment you puffed, under the house condition at that moment. It does not tell you how much air, and it says nothing at all about a gap when the house is not pushing anything through it. Used that way, it finds air barrier holes, confirms a seal you just made, shows a closed room pushing air under its door, and shows a draft hood spilling. This card is about getting a reading you can stand behind.

The tool and its hazards

Smoke pencils and puffers come in two broad families, and the difference matters indoors.

• Chemical smoke tubes make their smoke by reacting with moisture in the air, and the smoke they release is acidic and corrosive. It irritates eyes and lungs and attacks electronics and metal. Keep it for places away from people, electronics and burners, such as the outside of a duct run in an unoccupied crawlspace.
• Heated-fluid puffers vaporize a glycol-type fluid into a fog, the same family as theatrical fog. It is far milder, which makes it the right choice in occupied rooms, but it is still an aerosol you should not breathe in a cloud.

Read the safety data sheet for the tube or fluid you actually carry, and match the precaution to its route: a cloud in your face is an inhalation exposure, so puff small, puff away from your face, and keep it away from anyone with asthma or other breathing trouble.

Smoke alarms. Both kinds can trip a smoke alarm, and a monitored alarm system may dispatch the fire department. Ask the customer whether the system is monitored before you work near ceilings, keep puffs small, stay well away from alarm heads, and never puff at an alarm to test it. If one sounds, tell the customer at once and, for a monitored system, have them call the monitoring company with their passcode before dispatch.

Burners and flame sensors. At a fuel-burning appliance, puff only at the draft hood's relief opening or at a fireplace opening, never into a burner compartment or combustion chamber. Corrosive smoke on a flame sensor or a burner shortens its life, and any aerosol drawn into a flame changes what you are looking at.

Reading the plume

The whole reading happens in the first second, before the room's own convection and your body's movement take over. Watch the puff at the instant it leaves the tip.

A lazy plume is the reading most often over-interpreted. It does not mean the gap is sealed. It means nothing is being pushed through it under this condition. Change the condition and check again before you call a hole closed.

The technique

1. Set the house condition and write it down. Exterior doors and windows closed, and note which exhaust devices, air handler and interior doors are on, off, open or shut. The reading is meaningless without this, because the same gap reads pulled, lazy or pushed depending on what is running.

2. Stand still and get your body out of the path. Put the gap between you and any supply register, hold your breath for the puff or breathe away from it, and wait for the air around you to stop moving. Walking up to an outlet and puffing as you arrive reads your own wake.

3. Hold the tip about an inch from the gap, at its edge. Too far and the room's convection carries the puff before the gap can act on it. Pressed against the gap, you see the puff squeezing out of the tip, not the gap.

4. Make one small puff. A large cloud spreads in every direction at once and buries the first-second movement. A thin wisp is easier to read.

5. Watch the first second and repeat twice. Three puffs that agree are a reading. Three that disagree mean the difference across that gap is near zero or gusting, and the honest record is "lazy or variable".

6. Change the house condition and repeat at the same gaps. Turn the exhaust devices on, or the air handler, or close an interior door, and return to each gap. The change between conditions is where most of the information is.

Where to puff, and what each spot tells you

• Outlets and switches on exterior walls. The wall cavity behind is connected to the outdoors through the siding and sheathing. Smoke pushed back into the room means outdoor air is entering; pulled in means room air is leaving. Interior-wall outlets read the wall cavity, which may connect to the attic or basement through top and bottom plates.
• Baseboards and the bottom of exterior walls. Gaps under baseboard trim often open into the rim joist area and the basement or crawlspace.
• Top plates, ceiling fixtures and recessed lights. On the top floor these open into the attic. See related: How to Air Seal Penetrations Into the Attic.
• The attic hatch or pull-down stair. Check the gasket line all the way around, including the hinge side.
• Under interior doors. A closed room with supply and no return pushes air out under the door; puff on the hallway side and the smoke is pushed away from the door into the hall. See related: How to Check a Closed Room for Pressurization.
• The door to an attached garage. Puff on the house side at the sweep and the latch edge; smoke pushed into the house means garage air is coming in. See related: The Attached Garage as a Pressure Neighbor.
• A draft hood or fireplace. Smoke pulled up into the draft hood or flue means the vent is drawing; smoke pushed out into the room means it is spilling. Treat a spill as a stop, and follow How to Screen for Spillage After You Add an Exhaust Fan.

Worked example: one house, two conditions

A two-story house in January, outdoor 20 F, indoor 68 F. In heating season the stack effect makes the upper part of the house positive to the outdoors and the lower part negative, with a neutral pressure plane between where indoor and outdoor pressure match. See related: Stack Effect and the Neutral Pressure Plane.

Condition A, everything off.

That pattern is exactly what the stack effect predicts: out at the top, in at the bottom, near neutral somewhere around the second floor outlets.

Condition B, range hood on high and both bath fans on. Exhaust pulls air out of the whole house, so every point in it shifts toward negative. The neutral plane, the height at which the house matches outdoor pressure, has to move up to find a place that is still at zero.

What the comparison tells you. The attic hatch gaps are real holes; they read pulled under A and lazy under B because the pressure across them changed, not because they closed. A tech who tested the hatch only with the fans on would have called it tight. The bedroom outlet looked fine under A and leaks under B. Both deserve sealing, and the two conditions together are what showed it.

What it does not tell you. Nothing here says how much air any of these gaps passes, or how negative the house got. If the question is whether the exhaust drives a combustion appliance to spill, puffs at outlets are not the answer; the draft hood is, under the procedure built for it, and a manometer reading (see related: How to Read House Pressure With a Manometer) is what puts a number on it.

Common misreads

• Puffing near a supply register. The jet from the register blows the smoke around and reads as a leak anywhere nearby. Turn the air handler off or move to the other side of the gap.
• Trusting a reading taken in a gust. If the plume is pulled on one puff and pushed on the next, record it as variable and recheck on a calmer moment rather than picking the one you expected.
• Calling a seal good from a lazy plume. Check the sealed gap under a condition that pushes air through it. A seal you made at the attic hatch should be checked with the house in winter-stack condition or with an exhaust condition that brings the hatch back to a clear pull or push.
• Reading interior-wall outlets as exterior leaks. An interior wall cavity can carry attic or basement air, which is a real bypass, but it is a different finding from an exterior wall leak and it gets sealed in a different place.

References

• Safety data sheet for the smoke tube or puffer fluid you carry
• See related: The House as One Pressure System; Stack Effect and the Neutral Pressure Plane
• See related: How to Find Air Leaks While the House Is Depressurized; Where the Air Barrier Actually Is in a House
• See related: How to Screen for Spillage After You Add an Exhaust Fan