A tonneau cover is an excellent way to protect the contents of your truck bed from the elements. But will it really keep your cargo dry if you leave it outside overnight when dew forms? Let’s take a closer look at how tonneau covers work and what level of protection they can provide.
What Exactly is a Tonneau Cover?
A tonneau cover is a protective cover that goes over the bed of your pickup truck. Tonneau covers come in two main styles:
Hard Tonneau Covers
- Made out of aluminum, fiberglass, or vinyl materials.
- Fold, retract, or tilt up for easy access to the bed.
- Offer the most rugged protection for your truck bed.
Soft Tonneau Covers
- Made from vinyl, canvas, or other fabric materials.
- Roll up or fold away when not needed.
- Less expensive but still offer good protection.
The main purpose of a tonneau cover is to shield the contents of the truck bed from the elements – like rain, snow, wind, and dust. Tonneau covers also help improve security by concealing whatever you are hauling in the back.
How Does a Tonneau Cover Protect Against Moisture and Dew?
Tonneau covers form a protective barrier over the truck bed, sealing out exterior moisture and dust. The edges of the cover usually have a weather stripping material that helps create a tight seal along the sides and rear of the bed. This prevents most rain and snow from getting in.
But what about lighter moisture like dew and humidity that can condense overnight? While not completely watertight, a quality tonneau cover will still provide a good level of protection against these:
- Hard covers made from aluminum or reinforced vinyl are more impenetrable to moisture.
- Soft covers of vinyl or canvas material can repel lighter moisture if properly sealed along the edges.
- The insulation factor also helps regulate temperature and reduce condensation.
So while some humidity may still penetrate, a tonneau cover can significantly minimize the amount of dew that forms on cargo left in an open truck bed overnight.
What Are the Limitations for Protection Against Dew?
It’s important to understand that tonneau covers have limitations in the moisture protection they can provide:
Gaps Along Edges
- Even well-fitted covers can have slight gaps along the tailgate, cab, and sides.
- These small gaps leave openings for humidity and dew to sneak into the bed.
- Proper maintenance and resealing is needed to minimize gaps over time.
Condensation Under the Cover
- The cover traps existing humidity already in the truck bed at sundown.
- This can lead to overnight condensation on cargo from trapped moisture.
- Leaving the tailgate open allows ventilation to reduce under-cover condensation.
Material Type and Quality
- Vinyl and canvas covers are naturally less watertight than solid covers like fiberglass.
- Cheaper covers often don’t seal as well along edges and may use lower-grade materials.
- Paying more for a higher quality tonneau cover means better moisture protection.
Items Extending Above Bed Walls
- Cargo piled higher than the bed walls won’t be protected by the cover.
- Tall items can collect moisture from humidity and dew overnight.
- Keep cargo below the bed rails to maximize moisture protection.
Tips for Guarding Against Dew with a Tonneau
- Fully seal all edges using provided weather stripping or tape if needed.
- Open the tailgate overnight for ventilation to minimize interior condensation.
- Use moisture-absorbent cargo blankets or covers as an added barrier.
- Store delicate items in waterproof cases or containers.
- Park in a garage or carport to avoid exterior moisture altogether.
- Consider adding a bed rug for further insulation under the tonneau.
- Maintain the cover properly over time to prevent gap formation along edges.
- Buy the highest quality cover you can afford for the best sealing and materials.
Are Other Truck Bed Covers Better Than Tonneaus for Dew Protection?
Tonneau covers are a popular choice for protecting pickup truck beds. But are other types of covers potentially better at keeping moisture from forming on cargo?
Truck Bed Cap
- Fiberglass canopy sealed tightly to the bed for maximum moisture protection.
- More expensive but great for weather-sensitive cargo.
- Reduced accessibility compared to folding tonneaus.
Weather Resistant Tarps
- Thick vinyl tarps can be bungee-corded over the bed.
- Cheap and easy full bed coverage.
- Less secure and prone to flapping in winds.
Indoor Vehicle Storage
- Eliminates exterior moisture concerns altogether.
- Not always feasible and can be expensive.
- Garages, carports, and shelters are good options when possible.
While truck bed caps rank highest for keeping dew away, the convenience and balance of protection and access with tonneaus make them an attractive choice for most truck owners.
Will a Higher Quality Tonneau Cover Keep My Cargo Bone Dry?
It’s important to have realistic expectations about any tonneau cover’s moisture protection capabilities. No truck bed cover will act like a vacuum sealed, waterproof vault – there are always vulnerabilities that persistent dew can exploit. Higher cost does bring better materials, construction quality, and weather seals over bargain tonneaus. But even premium covers have slight moisture permeability.
The key is taking additional protective measures:
- Use waterproof cargo bags or cases for sensitive items.
- Elevate cargo above the truck bed floor on pallets or racks.
- Add absorbent blankets on top of cargo as an extra barrier.
- Keep cargo below side rails so it’s fully under the cover.
Anticipating vulnerabilities like interior condensation, edge gaps, and material limits allows you to supplement your tonneau’s protection. This provides the most secure defense against truck bed moisture.
How Much Condensation Should I Expect Under a Tonneau Cover Overnight?
The amount of interior condensation that develops on your cargo will depend on a few environmental factors:
Higher relative humidity means more moisture in the air to condense when temperatures drop overnight. Coastal and lakeside areas see higher humidity levels.
The greater the temperature swing from daytime highs to overnight lows, the higher the resultant condensation.
Persistent winds prevent air stagnation and moisture buildup inside the truck bed. Calm winds increase trapped humidity.
Leaving the tailgate cracked open allows airflow to minimize trapped moisture at night. A closed cover traps humidity.
Better insulated covers regulate bed temperature to reduce condensation. Reflective underside liners also help.
Taking preventive measures will keep condensation to a minimum. But some interior humidity is unavoidable. The good news is that the cover still protects from the much heavier external dew that would otherwise soak your cargo.
What Are the Best Tonneau Cover Materials for Dew and Moisture Protection?
Tonneau cover materials range from canvas to aluminum in terms of price, durability, and water protection. Consider these material options:
- Very affordable and decent water resistance.
- Can be layered or coated for higher durability and moisture blocking.
- May degrade faster than canvas in harsh weather conditions.
- Most impenetrable to moisture but also the most expensive.
- Heavy duty but prone to dents.
- Can feel very hot or cold to the touch.
- Lower cost but still durable when treated properly.
- Not as moisture resistant as vinyl but repels light dew.
- Superior breathability helps reduce condensation.
- Hard shells that provide excellent humidity protection.
- Cost is higher but great durability.
- Can crack or chip if not cared for properly.
- Combines materials like aluminum panels with vinyl edges.
- Balances strength, weight, cost, and water protection factors.
- Allows customizing materials to need.
Vinyl and fiberglass finishes generally offer the best moisture protection per dollar spent. But proper sealing and adhesion along the cover edges is just as crucial as the cover material itself.
How Else Can I Prevent Moisture in My Truck Bed Besides a Tonneau?
Adding a tonneau cover is the first line of defense against a wet truck bed. But you can take additional measures to reinforce moisture protection:
Spray-on or drop-in bedliners protect against humidity and condensation from below. The textured material also helps channel away any moisture.
Install Drain Holes
Drilling holes towards the bottom of bed walls allows water drainage before it saturates your cargo. Just beware of debris also getting in.
When possible, back your truck so the tailgate faces the prevailing winds. This blows moisture away before it penetrates the bed.
Moisture-wicking cargo blankets and desiccant packs help actively draw humidity away from sensitive gear.
Use Protective Cases
Pelican cases or other waterproof cargo boxes keep individual items separated from truck bed moisture.
Check and redo any deteriorating seals seasonally. Be sure to close gaps that let in humidity and dew.
Limit Cargo Height
Stacking items above bed walls leaves them exposed. Keep cargo lower to stay under the tonneau.
Tonneau covers are very effective at protecting your pickup truck’s cargo from getting soaked by rain, snow, and other exterior moisture. When it comes to lighter interior humidity and condensation, tonneaus do help minimize this nuisance as well. Although dew may still form to some degree inside the bed, a tonneau prevents the much heavier external dew that would otherwise drench unprotected cargo.
Paying attention to quality, materials, sizing, seals, and taking additional moisture precautions gives tonneaus the best chance of keeping cargo dry. Being realistic about their limitations for interior humidity will lead to effective solutions. With reasonable expectations and sound preventive measures, a tonneau cover is an excellent way to keep your truck bed cargo guarded from the elements.