How to Inject Insulation

Select one of four types of injectable foam: cementitious, phenolic, polyisocyanurate or polyurethane, all related to plastic. Use a water-soluble version, tripolymer, that provides excellent resistance to fire. All foam types are injected with pressure sprayers that distribute the material from containers into a space with a flexible hose and nozzle; all use a two-container method to mix insulation material with a foaming agent into the sprayer.

Rent spray equipment or use a prepared kit, which comes with the liquid material in disposable containers with the hose and nozzle. Follow the directions provided with the kit precisely. Wear a face mask, work gloves and protective goggles with either type of dispenser. Spray gently by depressing the trigger nozzle only about a third to halfway until you learn how to control the output.

Locate cavities between studs. Use a stud finder, locate nailing points on siding or identify a known stud location such as at the end of the wall and measuring 16-inch spaces. Remove a row of siding to expose the wall sheathing and drill 2-inch holes with a hole saw into every cavity, near the top of the wall. Drill through both siding and sheathing as an alternative, saving the cutouts for replacement later. Turn on the foam sprayer tanks. Insert the spray hose into the cavity, drop it down inside the wall and fill the cavity with foam. Work slowly and carefully. Move from one wall cavity to the next until all are filled.

Trim any excess material that expands out of the hole or through any cracks or gaps in the sheathing with a utility knife; spray foam cuts easily once it has cured, which typically takes 24 hours.

Replace the wood discs in the fill holes. Secure the discs with construction adhesive and fill the gaps around the holes with strong wood filler. Use polystyrene plugs or slices cut from large dowel rods as alternatives, but finish the surface with wood filler or a stucco compound to smooth the repair surface. Replace any siding that was removed. Or replace the siding cut-outs, if you cut through the siding as well as sheathing, and then repaint.

Inject foam into cavities between roof rafters from inside the attic. Spray a thin layer of foam on the underside of the roof decking between rafters or truss top chords. Use this technique in an unvented attic in extremely hot climates; do not use this if other insulation has already been applied to the exterior of the roof and inside attic joists.