Swamp Cooler Covers: How to Stop Winter Drafts and Heat Loss Through Your Cooler
Choose and fit swamp cooler covers: breathable exterior covers, interior ceiling vent covers, duct dampers and up-duct insulation to stop drafts and heat loss.
Short answer
A winterized swamp cooler needs two seals. Seal the warm side first with an interior ceiling vent cover or closed damper, which stops heated, moist house air from rising into the cold duct. Then fit a breathable exterior cover to block wind, snow and debris. Insulating the up-duct adds a third layer in cold climates.
Key takeaways
- The interior vent cover or damper does most of the work: it stops warm, moist air before it reaches the cold duct and condenses.
- Use a breathable, UV-rated exterior cover strapped under the cabinet. A tight plastic tarp traps moisture and speeds rust.
- An unsealed cooler opening can leak tens of CFM in winter, which on a cold night adds up to a few thousand BTU per hour of heat loss.
- In cold climates, add rigid foam or an insulated plug in the up-duct, and label it so nobody starts the cooler with it in place.
- Never run the cooler with any cover on. Remove interior and exterior covers before spring startup.
On this page
A shut-down swamp cooler is a big hole in your roof. The up-duct that carries cool air down in summer is typically 16 to 22 inches square, uninsulated or lightly insulated, and connected to a cabinet with three or four open louvered sides. In winter, warm house air rises into it, loses heat, and in many homes condenses and drips. Covering a cooler properly is about sealing that hole from both ends: inside at the ceiling vent or damper, and outside at the cabinet.
This page goes into the details of choosing and fitting each layer. For the full fall shutdown sequence, see the winterizing overview, and drain the water first using how to drain a swamp cooler.
Why the inside seal matters more than the outside cover
Most people buy an exterior cover first. It helps, but the physics favors the interior seal. Warm air in a heated house is buoyant, and the cooler duct sits at the highest point of the ceiling. With only an exterior cover, warm air still rises into the duct and fills it. The duct metal is near outdoor temperature, so the air cools, and the moisture in it condenses on the duct walls.
Indoor air at 70 F and 35% relative humidity has a dew point of about 41 F. Any duct surface colder than that collects condensation. On a 25 F night, that is the entire duct. Over a winter, the result is rust in the duct seams, water stains around the ceiling diffuser, and sometimes mold on the drywall around the register. The mold and swamp coolers guide covers cleanup if you already see it.
The rule is the same one used for vapor barriers in walls: seal the warm side. An interior cover or closed damper stops the moist air before it meets cold metal. The exterior cover then handles wind, snow, debris and UV.
The four layers of protection
| Layer | What it stops | Typical cost | Install time | Who needs it |
|---|---|---|---|---|
| Interior ceiling vent cover | Warm, moist air entering the duct; drafts into the room | $10 to $40 | 5 to 10 minutes | Everyone with a ducted cooler |
| Duct damper (manual or motorized) | Airflow through the duct at the damper point | $40 to $120 manual; $150 to $400+ motorized installed | Minutes to close; 1 to 3 hours to install | Homes that want a quick seasonal seal or seal without ceiling access |
| Up-duct insulation or foam plug | Conductive heat loss and condensation inside the duct | $15 to $60 in materials | 30 to 60 minutes | Cold-winter regions (high desert, mountain, plains) |
| Exterior cabinet cover | Wind, snow, rain, debris, UV, nesting animals | $25 to $90 | 10 to 20 minutes | Everyone with a roof or wall unit |
Interior ceiling vent covers
The diffuser where cooler air enters the house is usually a large square ceiling grille, often 16 x 16 to 24 x 24 inches, sometimes with adjustable louvers. Options for sealing it:
Clip-on or magnetic insulated covers
These are rigid or semi-rigid panels, often foam-cored vinyl, that fit over or behind the grille. Magnetic versions stick to steel grilles; clip versions hook behind the louvers. They are the easiest choice for most homes. Measure the outside of the grille frame and buy one that overlaps by at least 1/2 inch on every side.
Closable diffusers
Some ceiling diffusers have louvers that close fully with a lever or chain. Closed louvers cut drafts but rarely seal well; there are always gaps. Treat a closed diffuser as a first layer and add a cover over it in cold climates.
Plug-style covers above the grille
Where the grille is removable, you can fit a piece of 1 or 2 inch rigid foam board cut to the inside of the duct collar, set it on the grille, then reinstall the grille. This is nearly invisible and seals well. Use foil-faced polyisocyanurate or XPS board, and tape a pull tab to it.
Pro tip: Run a strip of painter's tape or weatherstripping foam around the edge of an interior cover where it meets the ceiling. Air will find a 1/8 inch gap. If you feel a draft at the edges with your hand on a cold night, the cover is not sealing.
Dampers in the cooler duct
Many installations have a damper in the drop or up-duct, sometimes called a winter damper or cooler damper. Three types are common:
- Manual slide or blade damper. A sheet metal blade you close in fall, often reached from the attic or through the ceiling grille. Cheap and reliable, but rarely airtight.
- Motorized damper. Opens when the cooler runs and closes when it stops, controlled by the cooler thermostat or a relay. Convenient year round, since it also keeps hot attic air out on summer nights when the cooler is off. Install by a qualified HVAC or electrical contractor; it needs a low-voltage or line-voltage connection to the cooler controls.
- Barometric or gravity damper. Opens under air pressure from the blower and falls shut when it stops. Leaks more than the others and is best combined with a cover.
A damper, even a good one, is not a replacement for an interior cover in a cold climate. It is a backup that cuts airflow while the cover handles sealing.
Sealing and insulating the up-duct
The up-duct is the vertical or angled section between the roof penetration and the cooler cabinet. In cold climates it is worth sealing inside the duct, not just at the ends.
- Measure the inside of the duct. Most residential cooler ducts are square, commonly 16, 18, 20 or 22 inches inside. Measure at the point you will plug, typically just above the roof deck or at the cooler discharge.
- Cut rigid foam to fit snugly. Use 1 to 2 inch foam board, cut 1/8 inch oversized so it friction fits. Two layers with staggered joints seal better than one thick piece.
- Install from the cooler side. With the cooler drained and powered off, remove a side panel and the pads if needed, and press the foam into the discharge opening or the top of the duct. On downdraft units this sits right below the blower.
- Seal the edges. Run foil tape around the perimeter to the duct metal. Do not use spray foam, which you will not be able to remove.
- Label it. Write "REMOVE BEFORE STARTUP" on the foam and tape a tag to the cooler thermostat or switch. Running the blower against a plug strains the motor and can tear the foam into the duct.
Commercial insulated duct pillows (inflatable or batt-filled bags) are another option and are easier to remove. They are sold for chimney and duct sealing and work in square cooler ducts if sized correctly.
Warning: Do not insulate or plug any opening that is part of a combustion or exhaust system. If the cooler duct shares a chase with a flue or vent, or you are unsure what a duct connects to, have an HVAC professional look before sealing.
Choosing and fitting an exterior cover
Material
Look for breathable, UV-stabilized polyester or vinyl-coated fabric. Breathable fabric lets moisture escape while shedding rain and snow. Solid plastic sheeting and blue tarps trap condensation against the galvanized cabinet, which is how pinhole rust starts. If your cabinet already shows rust, see cabinet rust and pan repair before you cover it for the winter.
Sizing
Covers are sold by cabinet dimensions. Measure width, depth and height. Common residential downdraft cabinets range from roughly 28 x 28 x 30 inches for small units to about 40 x 40 x 40 inches for large ones, but cabinets vary by brand, so always measure. A cover 1 to 2 inches larger than the cabinet in each direction slides on easily without excess fabric to flap.
Securing
Wind is what destroys covers. Use the built-in drawstring or straps, and run a strap or bungee under the cabinet stand, not just around the sides. On windy sites, add a second strap around the middle. Check the cover after the first big storm.
Where pads fit in
Many technicians remove aspen pads for winter, which lets the cabinet dry and prevents pad rot. If you leave rigid media in place, make sure it is dry before covering. See how often to replace cooler pads to decide whether to just replace them in spring.
How much heat an unsealed cooler loses
Air leakage through an uncovered cooler is easy to underestimate because you do not see it. The heat loss formula is the same one used for cooling capacity: BTU/h = 1.08 x CFM x temperature difference.
Worked example: one winter of leakage
Assume an unsealed 18 inch duct leaks 50 CFM on average (an illustrative figure; actual leakage depends on wind, stack height and how open the grille and cabinet are). With 70 F inside and a winter-average 35 F outside, the difference is 35 F.
Heat loss = 1.08 x 50 x 35 = 1,890 BTU/h. Over 24 hours that is about 45,000 BTU, and over a 120 day heating season about 5.4 million BTU, or 54 therms.
With a 90% efficient gas furnace, that is about 60 therms purchased. At typical 2026 residential prices of roughly $1.20 to $2.00 per therm (varies widely by utility), the leak costs on the order of $70 to $120 per winter. A $20 interior cover and a $50 exterior cover pay for themselves in the first season, before you count comfort and condensation damage.
If your house also has a dedicated relief vent or up-duct relief damper for summer airflow, close or cover it too. Those are another path for warm air to escape.
Spring removal checklist
- Remove the exterior cover first. Shake it out, let it dry completely, fold and store it out of the sun.
- Remove any foam plug or duct pillow. Check the duct for rust or debris while it is open.
- Open the damper. Confirm a motorized damper cycles with the cooler control.
- Remove the interior vent cover. Store it with the exterior cover so you find both next fall.
- Continue with the startup. The spring startup checklist covers water, pads, belt and pump.
Frequently asked questions
Do I need both an inside and outside swamp cooler cover?
In any area with real winters, yes. The inside cover stops warm, humid air from reaching the cold duct, and the outside cover keeps wind and snow out of the cabinet. In mild desert climates, an interior cover and closed damper alone are often enough.
Can I use a plastic tarp to cover my swamp cooler?
It is not recommended. Non-breathable plastic traps condensation against the cabinet, flaps in the wind and degrades in UV. A purpose-made breathable cover that is sized to the cabinet lasts longer and protects better.
How do I measure for a swamp cooler cover?
Measure the cabinet width, depth and height in inches, not the nominal CFM. Covers are sold by cabinet dimensions, and a cover that is 1 to 2 inches oversized each way fits more easily than one that is snug.
Why is water dripping from my ceiling cooler vent in winter?
Warm, moist room air is rising into the cold duct and condensing, then running back down. Close the damper, fit a sealed interior cover, and check that the exterior cover and cabinet are not letting snow melt in.
Can I leave the cooler covers on during a warm spell in winter?
Yes. Leave them in place until you do your spring startup. Removing and refitting covers for a few warm days risks forgetting to reseal before the next cold snap.
Sources and further reading
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- Air Sealing Your Home, ENERGY STAR
- ASHRAE Handbook: Fundamentals (air leakage and heat loss), ASHRAE
- Evaporative Coolers (fact sheet), Colorado State University Extension