What’s the toughest thing that goes into the building envelope? Steel? Concrete? How about a soft, fluffy cloud of goo that weighs next to nothing and comes in a can?
That’s right. When it comes to tough, durable materials, nothing packs more punch per ounce than foam sealant. It’s a stable, flexible, water, vapor and thermal barrier that conforms to any space and tolerates a wide range of conditions.
What is foam?
Foam is a gas trapped in either a liquid or a solid. It’s not much different than a loaf of bread. When you add yeast to flour and knead it, it releases a gas that expands the flour. You bake it to cure it, and all that gas and expanded flour become stable (at least until lunchtime).
Just like bread, you actually make the foam yourself when you shake the can—all that’s actually in there is a mixture of polyol, MDI (polyurethane) and propellant. As it’s exposed to air (either through the straw or the tip of a gun), it reacts, then expands and cures.
How to use spray foam insulation
How you make the foam determines the quality of the foam. You shake the can for a minute to create a stable foam, with a uniform, even distribution of gas bubbles. Perfect conditions are room temp (70°) with high humidity. All foams are triggered by moisture. Of course, you can’t control moisture completely on a jobsite, but in a dry environment, you can spritz a little water first. Or apply in the afternoon, when humidity is higher and the air is warmer, instead of first thing in the morning.
As soon as the ingredients hit the air, they become foam. In a straw, that means it’s foam all the way through the straw. In a gun, it stays liquid all the way through the nozzle and only becomes foam as it leaves the tip. So applying with a gun gives you more control over the foam.
The smaller the straw, the lower the post expansion. The larger the straw, the more the post expansion. The foam also acclimates to moisture. Apply it on a rainy day and it expands a little more. Later, as it adjusts to the relative humidity, it will shrink a little bit. The opposite is true, too: apply it on a dry day, and when humidity rises, the foam will expand a little.
Why is foam used for insulation?
Foam’s ability to expand and its unique properties upon curing makes it versatile and hardworking. In one product, you can secure large gaps against heat, cold and water. Caulking won’t cure in a large gap. Fiberglass batt insulation, when compressed into a space, is a less effective insulator and air and water pass right through. Dry cellulose settles over time. And wet cellulose has been shown in some studies to cause moisture to wick into the OSB. Some contractors will fill gaps with backer rod and caulk around it, which seals, but doesn’t insulate. So foam is the most complete solution. About the only thing you have to watch out for is making sure they’re protected from the sun because they don’t hold up well in UV light. But in a properly sealed interior cavity, foam will outperform anything.
Not just for windows anymore?
Beyond the low expansion, low pressure closed cell foams used for window and door installation, today there are many types of foams. They’re being engineered for a wide range of insulation properties, temperature variations and durability. There are fire inhibitors and pest inhibitors. There are foams that are used to seal concrete, that can be cut and drilled. There are even foams that are engineered for fence posts—below ground, burial grade foams that come in a bag, and you knead them to activate and then pour into a post hole to cure.
