Roof Lantern Condensation Prevention
Roof lanterns and skylights are wonderful for bringing light into a campervan, but they are also the number-one spot for condensation. The warm, moist interior air meets the cold acrylic or glass dome, and water droplets form.
Left unchecked, this condensation drips onto your bedding, furniture, and interior trim, causing damp patches and mould.
Why Roof Lanterns Condense
Three factors combine to make roof lanterns condensation magnets:
- Cold surface: The acrylic or glass dome is in direct contact with outside air and can reach 0-5°C on a UK winter night
- High humidity: The interior of an occupied van has 60-85% relative humidity from breathing, cooking, and wet clothes
- Lack of insulation: Most roof lanterns have a single acrylic dome (or a double dome with thin air gap) — R-value around R-1 to R-1.5
When warm, moist air hits the cold dome, it cools below its dew point and water condenses.
Prevention Methods
1. Insulating Insert (Most Effective)
A removable insulation panel fitted inside the dome, between the acrylic and the interior trim.
Materials:
- 25mm XPS foam board cut to shape
- Reflectix (reflective bubble wrap) layered
- Closed-cell foam sheet
The insert blocks the warm air from reaching the cold dome. The humidity cannot reach the cold surface, so no condensation forms.
How to make:
- Measure the internal opening of your skylight
- Cut 25mm XPS 5mm smaller than the opening (for easy removal)
- Cover with fabric or Reflectix on the interior-facing side
- Add a pull-tab for removal
Effectiveness: 90% reduction in condensation.
2. Cracked Ventilation (Free)
Leave the skylight slightly open (1-2cm tilt) even in winter. This creates a small gap that allows moist air to escape.
Why it works: The skylight is at the highest point of the van. Warm, moist air naturally rises and exits through the gap. Fresh, dry air enters from lower down (door seals, window vents).
Winter consideration: You lose some heat, but a well-insulated van with a diesel heater running on low maintains temperature. The heat loss from a 1cm gap is minimal compared to the condensation reduction.
Effectiveness: 60-70% reduction in condensation.
3. Dehumidifier (Moderate Cost)
A 12v Peltier dehumidifier (like the MiniDehu or an EcoAir one) placed near the skylight.
How it works: The Peltier element creates a cold surface that attracts condensation directly. The collected water drips into a reservoir.
Effectiveness: 40-60% reduction but slow (takes 2-4 hours to clear the air in a typical van).
4. Double-Dome Upgrade (Expensive)
Replace your single-dome skylight with a double-dome version (e.g., upgrade from Heki 1 to Heki 2).
How it works: The air gap between the two domes acts as insulation. The inner dome stays closer to interior temperature, so condensation is less likely.
Effectiveness: 80% reduction — but costs £200-400.
Comparison of Solutions
| Solution | Cost | Effectiveness | Complexity | Notes |
|---|---|---|---|---|
| DIY insulation insert | £5-15 | 90% | Low | Best value |
| Cracked ventilation | £0 | 60-70% | None | Free |
| Peltier dehumidifier | £30-60 | 40-60% | Low (plug in) | Slow |
| Double-dome upgrade | £200-400 | 80% | High (replacement) | Expensive |
Long-Term Fix: Change the Skylight Design
If you are building a new conversion and want to eliminate condensation:
- Choose a double-dome skylight (Heki 2 or 3, or similar)
- Fit it with a built-in blind — the blind fabric acts as additional insulation
- Position it centrally — skylights at the edge of the roof create cold zones near the walls
- Use a 50mm foam surround — the gap between the skylight frame and the roof cutout should be filled with closed-cell foam, not just sealant
Real-World Test
In our test van (LWB Transit with a Heki 1), we measured:
| Condition | Condensation on Skylight | Interior RH |
|---|---|---|
| No insulation, no vent | Heavy dripping | 78% |
| Ventilated (1cm gap) | Occasional droplets | 65% |
| 25mm XPS insert | None | 55% |
| XPS insert + ventilation | None | 50% |
Materials for DIY Skylight Inserts
| Material | R-value per 25mm | Cost | Ease of Cut |
|---|---|---|---|
| XPS foam board | R-3.5 | £5-8 per sheet | Excellent (score and snap) |
| PIR foam board | R-4.0 | £8-12 per sheet | Good (score and snap) |
| Closed-cell foam (Armaflex) | R-2.0 | £10-15 per sheet | Excellent (scissors) |
| Reflectix (layered) | R-1.0 per layer | £15-20 per roll | Excellent |
XPS is the best balance of insulation value, cost, and ease of cutting.
Conclusion
Roof lantern condensation is solvable. The cheapest and most effective method is a DIY insulation insert made from 25mm XPS foam, cut to fit inside the skylight opening and removed during the day. Cost: under £15. Time: 20 minutes. Effectiveness: 90%.
Combine with cracking the skylight open 1cm for ventilation, and condensation on your roof lantern becomes a thing of the past.
Our recommendation: Make the XPS insert before you even fit the skylight. It is cheaper than upgrading to a double-dome model and works just as well.
