One topic that often creates confusion during insulation inspections is the requirement for drain holes in cold insulation cladding. On many projects, inspectors and contractors automatically drill drain holes because the method statement or inspection checklist requires them. But have you ever stopped to ask why they are there?

For hot insulation, the answer is quite clear. Water can enter the cladding through joints, damaged areas, or during heavy rain. Drain holes allow this water to escape and help reduce the risk of corrosion under insulation (CUI).

Cold insulation, however, is a different story.

The main purpose of a cold insulation system is to keep warm, moist air away from the cold pipe or equipment. This is achieved by using a continuous vapor barrier. If moisture reaches the cold surface, it condenses into water, reducing insulation performance and potentially causing corrosion.

This is where the debate begins.

Some insulation standards focus on maintaining a completely vapor-tight system, while many project method statements specify drain holes in the outer metal cladding. At first glance, these two ideas seem to contradict each other.

So, are drain holes on cold insulation wrong?

Not necessarily.

In many projects, the drain holes are intended only for the outer metal cladding. Their purpose is to allow any rainwater or moisture trapped between the weather jacket and the vapor barrier to drain away. The vapor barrier itself remains continuous and should not be damaged. In this case, the drain holes do not compromise the insulation system because they are not penetrating the vapor barrier.

On the other hand, if someone drills holes through the entire insulation system, including the vapor barrier, the situation is completely different. That can allow moist air to enter the insulation, which is exactly what the vapor barrier is designed to prevent.

As inspectors, we sometimes focus on checking whether drain holes have been provided, but we rarely ask an equally important question:

Where do those drain holes actually end?

Are they only in the metal cladding, or do they pass through the vapor barrier as well?

This small detail can make a significant difference to the long-term performance of the insulation system.

Rather than assuming that drain holes are always required—or never required—it is worth reviewing the approved insulation details, project specifications, and method statement together. Understanding the design intent is just as important as checking compliance.

What is your experience?

• Have you seen drain holes specified for cold insulation on your projects?
• Were they drilled only in the cladding, or through the complete insulation system?
• Which project specifications or standards do you follow for this requirement?

I would be interested to hear how different projects and companies approach this topic.