10 Tips on How to Spray Foam Insulation

Updated: Mar. 20, 2024

Never worked with two-part spray foam insulation before? Here's a guide on how to spray foam insulation with no mess and no stress!

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Froth-Pak 200 Spray Foam Kit | Construction Pro Tips
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What is spray foam insulation?

Closed-cell polyurethane spray foam has some pretty awesome qualities. It has a high R-value. It’s resistant to water, mold and insects. It expands into hard-to-reach nooks and crannies. Heck, it can even add structural support to walls and roofs.

However, it is expensive. So we spoke with a couple manufacturers and some pro insulators about where and when to use it, when to call in a pro with a truck-mounted unit, and when a portable kit is the best option. We also dug up some great installation tips to help you spray safely and get the most bang for your precious buck

We used the Touch ’n Foam System 200 two-component, low-pressure kit. It has an R-value of 5.4 per inch.

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Man wearing proper spray-foaming gear | Construction Pro Tips
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Wear spray foam insulation protective gear

Protect your lungs with a respirator, and be sure to wear a full protective suit, chemical-resistant gloves and wrap-around goggles. Tape your gloves to your sleeves and wear your junkiest shoes. Acetone will dissolve fresh foam right away, but once this stuff cures, it has to be physically scraped off. In general, if you don’t want it covered with foam spatter, keep it away from the action.

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Poking wood with a moisture meter | Construction Pro Tips
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Never spray a wet surface

Never spray foam on a surface with a moisture level of more than 20 percent—the foam won’t stick. If the surface you’re spraying hasn’t been exposed to water for a few weeks, it should be OK to spray. But if you’re patching a wall that recently sustained water damage, or was just assembled with new, wet lumber, you may want to test the moisture level before you spray. You can get a moisture meter here.

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Spraying foam into a box to remove build-up | Construction Pro Tips
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Make a plan before you pull the trigger

Before installing the first tip/mixing nozzle, point the gun into a garbage bag or empty box, and pull the trigger until both chemicals are flowing evenly. But long before you do that, plan your work.

You’ll have to install a new tip every time you stop spraying for more than 30 seconds because the foam will set inside the nozzle and clog it. Spray foam kits come with extra tips, but it’s still important to (1) clear the room of everything that could get in the way, (2) decide which areas you’ll spray first, (3) put ladders wherever you need them, and (4) set up a fan or two to vent the fumes outdoors.

Before you start, shake the cans to mix the contents, and then shake every few minutes after that.

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Covering windows with plastic to protect from overspray | Construction Pro Tips
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Cover windows with plastic

Staple plastic to the framing around the windows (and doors). Don’t worry about covering the gap between the window and the framing because you’ll want to fill those with low-expanding canned foam specifically designed for windows. Cover all the switch and outlet boxes with masking tape so you don’t fill them with foam. You don’t need to use gasketed electrical boxes when you insulate exterior walls like you would if you were using fiberglass—the foam will air-seal around the box.

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Lubing the tip of a spray foam gun | Construction Pro Tips
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Lube the gun every tip change

The end of the gun needs to be lubricated before the first tip is installed, and again before the tanks are stored after use. Many pros prefer to lube the tip of the gun every time they install a new tip. Kits will come with some lubricant (petroleum jelly). Always follow the manufacturer’s instructions on operation, tip changes, storage and disposal.

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Spraying in a picture frame pattern first | Construction Pro Tips
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Picture-frame first

Before filling the wall cavity, spray the perimeter with a 1/2-in. layer, which will expand to a 1-in. layer. These layers are called “lifts.” This will ensure that the foam penetrates any voids where the studs meet the sheathing. This also reduces the risk of bulges in the sheathing. If foam is sprayed into a large gap between the stud and the sheathing and is immediately covered by a thick layer, the pressure of the expanding foam can actually push out any sheathing that isn’t securely fastened. Wait a few minutes after picture-framing the cavity before you go back and lay down the first layer in the center of the wall.

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Spraying two inch lifts | Construction Pro Tips
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Spray two-inch lifts and wait

Always wait a few minutes before spraying additional lifts, and never spray a lift more than 2 in. thick. The foam needs time to cure before being covered with another lift. Uncured foam results in reduced R-values. The allowable thickness of lifts varies by manufacturer, but all have a limit. The two most common spray foam installation mistakes are failing to wait for lifts to cure and spraying lifts that are too thick.

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Spraying the Rim and the Bottom Plate | Construction Pro Tips
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Spray the rim and the bottom plate

There should be a foam sill sealer between the bottom plate and the concrete foundation wall. But voids in the wood or concrete, or tears in the sill sealer, can create gaps where air and insects can penetrate. While you’re sealing the rim board, cover the bottom plate and top off the concrete as well. There’s no need to picture-frame the joist spaces before filling them, but don’t spray a lift more than 2 in. thick.

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Scraping Overspray With a Curry Comb | Construction Pro Tips
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Scrape overspray with a curry comb

Run a straightedge along the outside of the studs and scrape down any high spots with a curry comb. Remove large chunks of foam with a handsaw. Curry combs are designed to groom horses and are available online or at farm supply stores for less than $10.

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Laying fiberglass over spray foam | Construction Pro Tips
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Combine foam with fiberglass

Fiberglass insulation can be used in conjunction with spray foam. The process is called “flash and batt.” Just picture-frame the wall cavities and lay down one lift, and then fill the rest of the cavity with fiberglass insulation. This is a popular way to save money but still take advantage of the outstanding air-sealing properties of foam.

For 2×4 stud walls, the fiberglass insulation will have to be split to fit. In this case, we sprayed just under 2 in. of foam (R-10.8), and split 3-1/2-in. R-13 batts in half (R-6.5) to achieve an R-17.3 insulation value. Never cram oversize batts into a wall cavity because fiberglass insulation loses R-value when it’s compressed. You could save money by spraying 1 in. of foam and adding more fiberglass, but don’t spray less than 1 in.—the foam will no longer serve as an air barrier. This particular foam creates its own vapor barrier if sprayed 2 in. thick.