If you want to spray open cell in your metal building, there are a few things to consider because it can sound like a good idea until it’s not. Open cell is a perfect option for many application scenarios; however, metal buildings have their own unique set of characteristics that make it a risky application if you don’t have a full understanding on what’s going on behind the scenes.
The main concern is moisture. Open cell does not have a vapor barrier. With that said, if it is sprayed onto metal that collects condensation, you could be setting the foam up for long term failure. This isn’t a theory or speculation. This is what building science, real world observation and field failures tell us. There are one off situations where open cell on metal might be acceptable, but these are exceptions not the rule.
Section 1: What You Need to Know About Open Cell Foam
Open cell is lightweight and breathable. It is a light density foam that is known for high expansion and sound dampening. It is traditionally more cost effective and provides excellent thermal control specifically in interior applications.
The key is that open cell is vapor permeable. This means moisture can move through it. Unlike closed cell foam, open cell is not resistant to moisture which matters when dealing with metal a substrate known to naturally condensate when temperatures swing.
In climate zones with high humidity or in buildings that create internal moisture like barns, grow rooms or workshops, that condensation can hit cold metal and soak into the foam from behind.
Over time that trapped moisture can:
- Deteriorate the foam structure
- Generate mold or mildew
- Cause rust and damage the metal panels
This isn’t about the open cell being a good product, which it is. It’s about what it is designed to do, and moisture resistance is not what it is made for. That is why the climate zone, building purpose, and interior humidity levels all play into the determination of its application.
Open cell foam should be considered in low humidity environments with careful attention paid to dew point and building code compliance.
Open cell can still be used in colder climates, but only if the appropriate vapor retarder is installed between the foam and the interior living space. Without it, you risk condensation and moisture damage.
Section 2: What Makes Metal a Challenging Substrate
Metal has its own set of challenges apart from other substrates like wood or drywall. It’s more durable, quick to install, and most used in structures like pole barns, warehouses, and commercial buildings. However, when it comes to insulation, it comes with a characteristic that requires the right foam product. It sweats.
Metal is a great conductor of heat and cold, which can cause its surface temperature to swing rapidly. In humid climate zones or in spaces with subpar ventilation, the interior side of a metal wall can easily drop below the dew point. When this happens, it causes condensation like the condensation on your drink on a hot day.
Imagine you sprayed foam to this condensation prone substrate, and the temperatures start greatly varying. This causes a situation where the condensation contacts the foam which can cause:
- The foam to degrade from the inside out
- The metal to start to rust or corrode
- An expensive problem that may not show up for years to come
Worse, many metal structures are not conditioned or fully sealed. They can lack mechanical HVAC, which can cause temperature and humidity to constantly fluctuate.
To make it work, you need more than just foam. You need to condition the space, the vapor, and the insulation system. This can include vapor retarders, coatings, or a different foam system.
This brings us to closed cell foam.
Note on adhesion: One more thing—check your adhesion. Some sheet metal panels come pre-primed with a factory-applied white paint, which can cause issues—especially for closed cell foam. As the foam cures and contracts, it may delaminate from surfaces with poor adhesion. Galvanized panels also often come with a thin oily film from manufacturing, which must be removed before spraying.
Whether it’s new or existing construction, metal should be pressure washed or cleaned thoroughly. A simple pull-off adhesion test before full installation can save you a major headache down the road.
Section 3: What to Do Instead – Understanding Vapor Drive and Why Closed Cell Foam Is the Safer Choice
When it comes to insulating your metal building, you can’t put vapor drive aside. The way moisture moves through your wall or roof comes down to temperature and humidity. The colder it is, vapor typically drives outward. The hotter it is it drives inward. Either way you want to keep that moisture from forming condensation that can’t escape.
Building codes in colder climates (Zones 5 and up) require a Class II vapor retarder (1 perm or less) for assemblies with normal interior conditions. This includes many homes and commercial buildings. However, if the building has high interior humidity—like car washes, hot tubs, grow rooms, or livestock barns—a Class I vapor barrier may be required instead.
These vapor retarders can be installed as a plastic membrane (like Intello or MemBrain), or applied as a coating or primer. Some paint manufacturers (like Sherwin-Williams or Benjamin Moore) offer vapor-retarding primers. Some thermal or ignition barriers also provide Class II vapor resistance, but always verify performance.
At the end of the day, open cell foam isn’t designed to stop moisture. It lacks the vapor barrier and unless you add one in the right place, you’re taking a gamble.
The risks increase in:
- Colder climates (Zones 5 and up)
- High interior humidity conditions (like livestock barns or grow rooms)
- Metal structures without HVAC
If you’re still set on open cell, you’ll need to do the math:
- Perform a dew point and hygrothermal analysis
- The hot side will need a dedicated vapor retarder installed
- Ensure the building is conditioned year-round.
However, for most jobs the better and safer option is closed cell spray foam.
Closed cell foam has a higher R-value per inch, adds structural integrity, and creates an air and vapor barrier all in one pass. Moisture can’t pass through this system like open cell and is more durable in demanding conditions. If you spray to metal, specifically on exterior walls or roofs, closed cell just makes more sense and will save you money in the long run.
Note on deformation: Thinner metal panels can deform during closed cell foam application—especially over large, unsupported spans. To reduce this risk, use a “picture framing” technique and apply a thin first pass (flash pass) to stabilize the panel before building thickness.
Have questions about vapor barriers, climate zones, or the best foam for your job? Give us a call. We’re here to help you get it right the first time.
