How to Prevent Condensation Under a Metal Roof: The Enemy Within
July 28, 2017
If you’re wondering how to prevent condensation under a metal roof, you’re not alone. Owners of agricultural buildings, man caves and horse barns all are worried about the corrosive effects of condensation. Here’s how to fight back against the enemy within.
The impact of condensation in post-frame buildings is very real. From livestock to equipment, condensation can take a financial and structural toll.
But condensation can be defeated, or at least, kept at bay, through the proper building techniques. While we often spend a lot of time working to keep water out (which is a good thing), we sometimes neglect the problem of the water within.
How Does Condensation Impact the Things Inside Your Pole Barn Building?
The old saying, “It’s not the heat, it’s the humidity,” applies to livestock and equipment, albeit indirectly.
For livestock, excessive humidity can become a big problem. According to University of Kentucky Agricultural Meteorologist Corey Pieper, livestock will become uncomfortable when the heat index reaches about 90 degrees, but the real issue is humidity.
“It could be 120 degrees outside, but if the dew point is low enough, animals will be fine.”
Keeping equipment dry is important as well. According to research by the National Physical Laboratory (NPL), “Galvanised steel does not satisfactorily withstand…the severe condensation that is not uncommon in some farm buildings.”
This condensation can result in corrosion, which the NPL states “can result in a variety of defects, seizure of moving parts, distortion of metering equipment, blocked outlets, etc, which can seriously affect the function of the machine (e.g. cause uneven distribution of fertiliser).”
What Causes Condensation Problems on the Underside of Metal Roofs?
To understand what causes condensation, we reached out to Chris Davis, President of DripStop. Chris explains that there are two important things to remember with condensation, and they may seem somewhat counterintuitive:
- Warm air holds more moisture than cold air
- Warm air is actually lighter than cold air (known as Avogadro’s Law)
Essentially, condensation occurs at night, when the temperature inside a building is warmer than outside. Warm, wet air rises and meets the cold roof and cools. As it cools, it can’t hold as much moisture. That excess moisture ends up as drops of condensation on the roof: Picture a cloud of moisture floating overhead between the rafters.
This video shows what a significant condensation problem can look like.
There are essentially three factors to be considered in the fight against condensation – let’s take a closer look at each of them.
Fighting Condensation Factor #1 – Keep Moisture From Getting In
The first factor focuses primarily on the site preparation. If you have a smart architect and/or builder who is attuned to your building’s intended usage, they can situate your building and provide the correct features to prevent moisture from entering in the first place. These include:
- Build your structure (where possible) in an elevated position with good natural drainage
- Place a vapor retarder under floor (gravel or concrete)
- Ventilate enclosed building when concrete is being poured, and continue to ventilate until it is cured
- Use heaters that are vented to the outside (in heated buildings)
- Provide adequate drainage of surface water away from the building, and provide underground tile to lower the water table or remove water from gutters draining into the tile
- Be aware of other potential sources of moisture
In-Depth Look: What are Some Additional Sources of Moisture Inside a Building?
The last bullet point in the previous section mentioned other sources of moisture. People don’t account for the amount of moisture that can be produced inside a building. In the video of the barn above, some of the following elements are likely causing the significant condensation:
- Fill dirt
- Poor drainage
- High groundwater table
- Use of unvented heaters
- Snow melt from vehicles or washing vehicles
- Washing down floor areas
- Storage of firewood, green lumber of fresh cut timber
- Hay storage or other farm commodities
- Open grate pits or sumps with water
Chris notes that space heaters (unvented heaters) can contribute to the moisture issue, increasing air temperature and the air’s ability to hold moisture.
Fighting Condensation Factor #2 – Move Wet Air Out Through Natural Ventilation
Natural ventilation uses local wind and temperature differences between the inside and outside of the building. This is an area that falls within the realm of the architect and the builder, and its effectiveness is based on their design.
Most of us count on using natural ventilation in agricultural and suburban buildings, but it’s often misunderstood. A well-designed system will take into account the sources of moisture present in the building; local wind and climate conditions throughout the year, air exchange, control and flexibility.
It requires properly calculating ridge and eave venting, as well as placement. Chris says quite often the need for venting is underestimated, most typically at the eaves. The sum of the openings at the eaves should equal that at the ridge.
Unfortunately, this isn’t always done correctly. Because “a ventilation system is only as good as its weakest point,” as Chris says, the wet air is not moved effectively.
Fighting Condensation Factor #3 – Cheating the System
Even a properly ventilated building can find itself in a situation where sources of moisture and weather conditions overwhelm the system. When that occurs, or in the event we can’t optimize methods 1 and 2, we need to cheat the system. There are primarily two ways this happens:
- Insulate and try to keep from reaching the dew point
- Add a preinstalled membrane to absorb and release the condensation as it forms with the natural weather cycle
Traditionally, we’ve insulated the roof panel to try and keep the temperature on the panel FROM reaching the dew point. We’ll add a vapor barrier so the moist air does not flow up through the insulation and condense on the roof. But a preinstalled membrane may be even more effective at removing condensation.
The Problem With Using Insulation Alone
The goal of insulation is to keep the inside-to-outside temperature difference low enough at a specific humidity level, to prevent it from reaching the dew point.
But the greater the temperature difference, the less humidity the system can handle before dripping. Conversely, the greater the humidity level, the less temperature the system can handle before dripping.
Vapor barriers can also tear, compromising the system. Ultimately, “Insulation is no guarantee against condensation.”
Using a Preinstalled Membrane
Chris’s company produces DripStop, a preinstalled membrane, which catches condensation droplets as they form and holds them in the membrane until the conditions change, and the moisture can evaporate back into the natural humidity of the air. It is a simpler way of dealing with an age-old problem and shortens the construction process, saving time and money.
Ten square feet of area holds more than 1 quart of water, which is a “whole lot of moisture,” according to Chris. “If you have more than that, we believe it’s a ventilation issue, not a condensation issue.” (This might be the case in the farm video.)
The DripStop is actually part of the roofing material. It comes preinstalled to the building site. (The membrane cannot be added to an existing structure.) It’s also made of polyester and rubber, so it’s resilient to aging and corrosion.
Here’s a video that shows the DripStop being applied to the roofing material.
Prevent Condensation Before it Occurs
Condensation is not a natural force that’s beyond your control. It’s a matter of science, and understanding what factors are causing excessive humidity within your structure.
These factors can be influenced before the construction process begins, and some can be altered once the building is in place. It’s a balancing act, and you will likely “cheat the system” using methods like insulation and membranes like DripStop.
Whatever you do, for the well-being of what’s inside your structure, be careful not to underestimate the corrosive powers of condensation. Water is the stuff of life, but left to its own devices, it can also be the stuff of drained bank accounts. Treat any condensation problems with the respect it deserves.