Expanding foam is a polyurethane foam sealant used for filling gaps and cracks in buildings, boats, and other structures. It comes in pressurized cans and expands rapidly when released, allowing it to fill cracks and voids. While very useful, there are some places where expanding foam should not be used. In this article, we will explore the properties of expanding foam and when it is not suitable.
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What is Expanding Foam?
Expanding foam, also known as polyurethane foam sealant, is a polyurethane-based material that comes as a two-part liquid compound. Inside the expanding foam can are the two components that make up the foam – polymeric isocyanate and polyol resin. These remain separated until the can is pressurized, which forces the components to blend together.
When released from the can, the liquid begins to expand rapidly due to a chemical reaction between the two components. Carbon dioxide is generated as a byproduct of this reaction, which causes the foam to inflate up to 30-60 times its original liquid volume within seconds.
The expansive properties allow the foam to easily fill cracks, gaps, and holes as a sealant and insulation material. As it cures, the foam becomes a rigid plastic with a closed cell structure. This gives it insulating properties, strength, and water resistance.
Common uses for expanding foam include:
– Filling gaps around window frames, door frames, pipes, and wires
– Insulating between stud wall cavities
– Sealing air leaks in roofs, walls, attics
– Waterproofing foundations and below-grade spaces
– Bonding and gap filling in masonry and concrete
– Securing insulation batts and panels in place
Advantages of Expanding Foam
Expanding foam offers several benefits that make it a versatile DIY and construction sealant:
– Expands to Fill Voids – The high expansion ratio allows the foam to mold into hard to reach cracks and completely fill gaps. Just a small amount can expand to seal large voids.
– Adheres Well – It bonds tenaciously to most building materials like wood, masonry, insulation, pipes, wires, and metal.
– Insulates – The rigid foam has good insulative properties for both temperature control and sound damping.
– Stops Air Leaks – By filling cracks and gaps, expanding foam blocks drafts, moisture, pests, dust, and noise.
– Lightweight – At full cure, expanding foam is a low density foam plastic that adds little weight.
– Quick Cure – Many formulas fully expand and cure in just 5-20 minutes.
– High Strength – It forms a rigid plastic foam capable of supporting weight and pressure when cured.
– Water Resistant – Closed cell foam resists water absorption and degradation.
– DIY Friendly – Available in convenient pressurized cans for simple point-and-shoot application.
Types of Expanding Foam
There are a few different versions and grades of expanding foam available:
– Gaps & Cracks Filler – General purpose foam for non load-bearing filling applications.
– Window & Door Foam – Low expansion formula designed for sealing around windows and doors without distortion.
– Big Gap Filler – High yield formula for extra large gaps up to 4 inches wide.
– Fireblock Foam – Fire rated foam for sealing penetrations in fire separations.
– Minimal Expanding Foam – Controls expansion for small gaps. Less post-trim required.
– Removable Expanding Foam – Temporary foam that can be removed after serving its purpose.
– Low Pressure Foam – Dispenses as a thicker bead instead of a spray for more control.
– Fast Cure Foam – Becomes tack free in just 5 minutes.
How to Use Expanding Foam
Using expanding foam properly is important for getting good results:
– Wear gloves and eye protection when dispensing. Uncured foam can irritate skin and eyes.
– Shake the can vigorously for 30 seconds before use and periodically during use. This helps mix the components.
– Ensure surfaces are clean before applying foam. Oils and dirt can prevent adhesion.
– Moisten surfaces lightly with water before use for better curing.
– Hold the straw tip 1-2 inches from the surface when spraying. This allows room for expansion.
– Dispense foam in layers, allowing each layer to partially cure before adding more.
– Stop filling when foam is 50-70% larger than the gap to allow for drying shrinkage.
– Allow 4 hours for foam to fully cure and harden before trimming or painting.
– Uncured foam can be cleaned immediately with acetone or polyurethane cleaner. Cured foam must be cut or sanded away.
Limitations of Expanding Foam
While very useful in many applications, expanding foam does have some limitations:
– Permanent – Once cured, expanding foam can only be removed mechanically. It cannot be remelted or redissolved.
– Messy – Excess uncured foam requires immediate cleanup. It sticks aggressively to most surfaces.
– Expanding Pressure – Foam can warp or distort surfaces if expansion is confined.
– Overexpansion – Gaps larger than 2 inches may cause excessive outward pressure and improper curing.
– UV and Temperature Sensitivity – Cured foam will degrade over time when exposed to sunlight and extremes of hot or cold.
– Solvent Sensitivity – Uncured foam may be dissolved or weakened by contact with solvents like gasoline, paint thinner and acetone spills.
– Flammability – While fireblock foams are rated for fire separation gaps, most foams are combustible and give off toxic fumes when burned.
These limitations mean expanding foam may not be the right solution in every situation. Understanding where not to use expanding foam is important.
Where Should You Avoid Using Expanding Foam?
Now that we’ve looked at what expanding foam is and how it works, let’s focus on places you should avoid using expanding foam sealant:
1. Structural Support Applications
While cured foam is rigid and strong in compression, it has very little tensile strength or shear strength. Expanding foam should not be used for any load bearing or structural support purposes. This includes:
– Framing joints – Do not use in place of framing fasteners and connectors.
– Floor joists – Not designed to support gravity, point, or dynamic loads.
– Door and window headers – Cannot transfer structural loads or span openings.
– Drywall backing – Will not provide drywall support or take the place of studs.
– Stair stringers – Lacks the tension, flexural, and shear strength for stairs.
2. Confined Spaces
Expanding foam generates internal pressure as it expands. In a tightly confined space, this can result in distortion – or even damage – to surfaces as the foam exerts excessive force while curing. Do not use expanding foam:
– In gaps less than 1/4″ thick – May cause permanent warping or buckling.
– Underneath flooring – Can push up floor boards, vinyl, or tiles.
– Inside wall cavities – Will bulge and distort drywall or paneling.
– Behind trim – Can warp interior finish boards away from the wall.
– Around pipes and conduits – Will distort conduits and stress pipe joints.
3. Fire Separation Penetrations
Most expanding foams are flammable and will not provide an adequate fire rated seal. Use only fireblock rated foam tested specifically for sealing penetrations in fire separations like:
– Pipe, conduit, and wire penetrations in fire rated walls.
– Ductwork and electrical boxes passing through ceiling or floor assemblies.
– Openings around chimneys and flues.
– Top plate penetrations in firewalls.
4. Below Grade Applications
Since expanding foam is porous when cured, it should not be used where it will be exposed to ground moisture. Avoid using expanding foam:
– To fill footer or foundation cracks.
– As a substitute for foundation waterproofing.
– Below slabs as a vapor barrier.
– In crawl spaces in contact with the ground.
Instead use solid hydraulic cement or specialized waterproofing materials.
5. High Temperature Surfaces
While expanding foam is reasonably fire resistant, it should not be used where it will be exposed to sustained high temperatures above 240°F, such as:
– Engine exhaust manifolds or mufflers.
– Furnace flues and chimney breast walls.
– Direct contact with steam lines or pipes.
– Inside incandescent light fixtures.
– Attics where roof temps exceed 150°F.
High temps can soften, melt or accelerate aging of the cured foam over time.
6. Cladding Attachment
Because rigid foam insulation has such low shear strength, it should not be used to attach facing materials like:
– Brick, stone, stucco, or cultured stone veneers.
– Siding, shakes, or exterior trim boards.
– Tile backer boards.
– Bathroom accessories like towel bars or grab handles.
The weight of the facing can shear the foam away from the substrate. Use mechanical fasteners instead.
7. Mirror and Picture Hanging
While the adhesive properties of expanding foam may seem ideal for hanging mirrors and picture frames, the flexibility and softness of cured foam allows too much movement. Items can detached from the wall over time due to:
– Vibration from opening and closing doors.
– Gravity pulling down on the mass of the object.
– Humidity induced expansion and contraction.
Use traditional mounting hardware designed for mirrors and framed art instead.
8. Overhead Applications
Expanding foam’s low shear strength makes it unsuitable for overhead applications where gravity applies constant shear force trying to pull or detach the foam:
– Filling ceiling joist cavities.
– bonding ceiling tiles or panels.
– Mounting lighting fixtures or ceiling fans.
– Filling roof deck seams.
Liquid foams can also run and drip before curing, creating a mess. Use alternatives like fiberglass batts, construction adhesives, or solid blocking.
9. Movement Joints
Expanding foam is also problematic when used in moving joints and dynamic gaps subject to regular expansion and contraction:
– Deck ledger board gaps.
– Seams between dissimilar materials.
– Intersections between walls, floors, and roofs.
– Changes in material thickness.
The rigid foam will restrict free movement and can crack from the stresses. Use backer rod and sealant instead.
10. Automotive Uses
Avoid using expanding foam as a quick fix substitute for proper auto body materials and repair techniques:
– Filling dented body panels – Will prevent proper hammer and dolly repair.
– Sound deadening panels – Foam absorbs moisture and degrades over time.
– Sealing windshields – Elastomeric auto sealant gives a better seal.
– Mounting accessories – Plastic foams fail from engine heat and vibration.
– Insulating convertible tops – Better aftermarket insulators are available.
Best Practices When Using Expanding Foam
When used in appropriate applications, expanding foam can provide excellent air sealing and insulation. Here are some best practices for success:
– Read manufacturer instructions – Different brands have varying properties, dry times, expansion rates, and limitations.
– Use gloves and eye protection – Uncured foam residue is difficult to remove from skin and can irritate eyes.
– Do test sprays first – To gauge spray pattern, expansion rate, drying time, and finished appearance before doing the final application.
– Allow for expansion – Fill cavities only 50-70% full to allow room for rising foam.
– Dispense in layers – Adding thinner beads gives better density and curing results compared to one thick shot.
– Ensure adequate ventilation – Fresh air allows moisture to evaporate so foam cures properly.
– Confirm compatibility – Check that foam bonds well to application surfaces and that any chemicals present won’t inhibit curing.
– Check for movement – Look for any potential for movement that could compromise the foam seal over time.
– Cover promptly – Protect cured foam from UV exposure by covering it with solid materials. Uncovered foam will deteriorate.
– Trim carefully – Use sharp knives or foam planer tools to trim away excess material. Jagged edges tear more easily.
– Seal it in – Paint or otherwise seal cured foam to prevent air and moisture reaching the porous foam cells.
By carefully following usage guidelines and best practices, expanding spray foam can be used effectively as an air barrier sealant and building insulator. Pay close attention to manufacturer instructions, allow for proper expansion, provide good ventilation, and trim and cover the foam neatly. Most importantly, avoid unsuitable applications where the foam’s properties and performance limitations make it a poor choice.
Frequently Asked Questions
Here are answers to some common questions about where and where not to use expanding foam:
Is expanding foam waterproof?
Once fully cured, expanding foam is water resistant but not fully waterproof. It can withstand incidental moisture but should not be used as a primary water barrier. The foam’s porous cell structure allows water under pressure to permeate over time.
Can I use expanding foam as car soundproofing?
Expanding foam would not be an effective or durable soundproofing solution inside a vehicle. The combination of heat, humidity, and vibration found in car interiors causes expanding foam to degrade rapidly. Specialized auto sound damping materials are better suited for dampening road noise in vehicles.
Is expanding foam a good DIY fix for drafty windows?
Expanding foam can be very effective for sealing small gaps around window frames as long as the proper expanding window and door formula is used. It will fill cracks and create an air tight seal. Allow at least 1 hour to cure before disturbing the window.
Can I use expanding foam instead of drywall screws?
No, expanding foam has very little structural strength and cannot replace drywall screws, nails, or other mechanical fasteners. Use it only as a supplemental sealant in drywall applications. The foam does not create a secure attachment on its own.
Is expanding foam better than fiberglass for insulating basement rim joists?
In most cases, fiberglass batt insulation with an exterior air barrier cover will provide better thermal performance and moisture resistance compared to expanding foam when insulating basement rim joists. The foam’s thin coverage can leave cold spots.
Conclusion
Expanding spray foam is a versatile product for filling gaps, sealing cracks, and enhancing insulation when used properly. However, its limitations mean that caution should be exercised in certain applications such as structural support, fire separations, high temperatures, and below grade. Follow manufacturer instructions closely, allow room for expansion, and cover or seal the cured foam for best results. With attention to detail, expanding foam can serve as a handy air barrier sealant and insulator in many common situations faced by DIYers and building professionals.