Air barriers must resist severe wind pressures. Use strong air barrier materials like structural wood paneling, drywall, or sheet metal to seal large air leaks, especially if your region has powerful winds. Attach these strong materials with mechanical fasteners and seal them with caulk or adhesive. If a technician needs to access a shaft or penetration in the future use a caulk that isn’t a strong adhesive, such as acrylic latex.
Use caulk by itself for sealing small cracks. Use liquid foam for cracks larger than 1/4 inch.
Air barrier materials should themselves be air barriers and also rated as fire blocks.
Three-quarter-inch plywood, OSB, and particle board are IRC-approved fire-blocking materials, and they’re strong enough to resist windstorms. Attach these structural sheets with screws or nails along with any sealant or adhesive that effectively air seals the joint.
Half-inch drywall constitutes a 15 minute thermal barrier, and is also an ignition barrier. When air sealing a fire-rated assembly in a commercial or multi-family building, choose five-eighths -inch drywall and a fire-rated caulking. Fasten drywall with screws and construction adhesive. Don’t use drywall in damp locations where it may get wet.
Steel and Aluminum Sheet Metal
Being non-combustible, sheet metal is used to seal around chimneys and other heat producing components. To seal around chimneys, cut the galvanized steel accurately, with less than a 1/8-inch gap, so that you can seal the gap with high temperature, non combustible caulk labeled ASTM E136.
Foam board may be a desirable product for air sealing; however it has less structural strength and fire resistance than the other materials discussed previously.
Cross-Linked Polyethylene House Wrap
House wrap and polyethylene sheeting are air barriers. These flexible materials aren’t rated as fire-blocks, and they are structurally weak.
Stuffing materials are used to insulate a cavity, to give the cavity a bottom, or to serve as supporting part of an air seal.
Backer rod and caulking are the most reliable and long-lasting air seals. Backer rod is closed-cell polyethylene foam that creates a bottom barrier in a gap before caulking. Backer rod doesn’t bond to the caulking, and so prevents three-sided adhesion that could tear the caulking bead apart with the materials’ expansion and contraction of temperature extremes.
Fiberglass batts reduce air convection in cavities where they’re installed. However, fiberglass batts are air permeable, even if compressed. Batts can support two-part foam sprayed over the opening of a cavity. Fiberglass batting is a good stuffing material for use with non-combustible caulk for penetrations through fire-rated assemblies because of its low combustibility.
Blown cellulose and fiberglass reduce air convection and air leakage through closed cavities. However neither material is an air barrier even when blown at high densities. Both are considered fire-blocks when installed in closed cavities because they block the passage of flames.
The adhesion and durability of caulking and adhesives depends on their formulation and on the surfaces to which they’re applied. Some caulks and adhesives are sensitive to dirt and only work well on particular surfaces, while others are versatile and dirt-tolerant. Remove debris and clean the joint to prepare the surfaces for caulking.
Water-Based Caulks
A wide variety of paintable caulks are sold under the description of acrylic latex and vinyl. These are the most commonly used caulks and the easiest to apply and clean up. Siliconized latex caulks are among the most adhesive and durable sealants in this group. Don’t apply water-based caulks to building exteriors when rain is forecast since they aren’t waterproof until cured, and they stain nearby materials if they are rained upon while curing. Don’t apply water-based caulks during freezing weather.
Silicone has great flexibility, but its adhesion varies among different surfaces. Silicone is easy to gun even in cold weather. Silicone isn’t as easy to clean up as water based caulks, though it’s easier than polyurethane or butyl. Silicone isn’t paintable, so choose an appropriate color. High-temperature silicone may be used with galvanized steel to air seal around chimneys if labeled ASTM E136, meaning that the caulk is non-combustible.
Polyurethane has the best adhesion and elasticity of any common caulk. It works very well for cracks between different materials like brick and wood. Polyurethane resists abrasion and is used to seal critical joints in concrete slabs and walls. It is also good for sealing the fastening fins of windows to walls. Polyurethane is almost as sticky and adhesive as a construction adhesive. Cleaning it up is difficult so neat workmanship is essential. Polyurethane caulk doesn’t gun easily, and should be room temperature or higher. Polyurethane caulk doesn’t hold paint.
This solvent-based or water-based adhesive is used to seal laps in polyethylene film and house wrap. Acoustical sealant is very sticky, adheres well to most construction materials, and remains flexible. Acoustical sealant is used to seal building assembles for sound deadening. Acoustical sealant is also used to seal the seams of polyethylene vapor barriers and ground moisture barriers.
Water Soluble Duct Mastic
Duct mastic is the best material for sealing ducts, including cavities used for return ducts. A messy but highly effective sealant, duct mastic can be applied with a thickness of 1/8-inch with a brush or rubber glove. Have a bucket of warm water handy to clean your gloved hands and a rag to dry the gloves. Spread the mastic and use fiberglass fabric web tape to reinforce cracks more than 1/8-inch in diameter. Thorough cleaning of dust and loose material isn’t necessary. Mastic bonds tenaciously to everything, including skin and clothing.
Stove cement is a material that can withstand temperatures up to 2000° F. It is used to seal wood stove chimneys and to cement wood stove door gaskets in place.
Some elastomeric caulks are designed specifically for use in fire-rated assemblies. They are labeled ASTM E136, meaning that the caulk is non-combustible. Use this type of sealant when sealing penetrations through fire-rated assemblies in multifamily buildings.
Fire-Rated Mortar
Used with other air-sealing materials to seal various sized holes and gaps in multifamily buildings with fire-rated masonry building assemblies. This mortar often covers a foam air sealant to create a non-combustible surface for an combustible air seal.
Construction adhesives are designed primarily to bond materials together. But they also create an air seal if applied continuously around the perimeter of a rigid material. They are often used with fasteners like screws or nails but can also be used by themselves. Some construction adhesives act like contact adhesives to bond lightweight materials without mechanical fasteners.
Use specially designed construction adhesives for polystyrene foam insulation because many general-purpose adhesives decompose the foam’s surface.
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SWS Detail: 4.9901.1 General Information on Spray Polyurethane Foam (SPF), 4.1401.1 Band/Rim Joists—Spray Polyurethane Foam (SPF) Installation, |
Liquid closed-cell polyurethane foam is a versatile air sealing material. Closed-cell foam is packaged in a one-part injectable variety and a two-part sprayable variety. It has a high R-value per inch and is ideal for insulating and air sealing small, poorly insulated, and leaky areas in a single application.
Installation is easy compared to other materials to accomplish the same air sealing tasks. However, cleanup is difficult enough that you probably don’t want to clean up multiple times on the same job. Instead identify all the spots needing foam application, make a list, and foam them one after another.
This gap filler has tenacious adhesion. One-part foam is best applied with a foam gun rather than the disposable cans. Cleanup is difficult if you’re careless. When squirted skillfully into gaps, one-part foam reduces air leakage, thermal bridging, and air convection. One-part foam isn’t effective or easy to apply to gaps over about one inch or to bottomless gaps. This product can leave small air leaks because it cracks when the materials around it move or shift.
Good for bridging gaps larger than one inch. Two-part foam is popular for use with rigid patching materials to seal large openings. Cut foam board to close-enough tolerances around obstacles and fill the edges with the two-part foam. Two-part foam should be sprayed to at least an inch of thickness when it serves as an adhesive for foamboard patches over large holes for strength.
Foam Construction Adhesive
Polyurethane foam dispensed from foam guns is an excellent adhesive for joining many kinds of building materials. It works well for joining foam sheets together into thick slabs for vertical access doors and attic hatches.
Caution: Two-part foam is hazardous to installers and building occupants. Installers must wear special personal protective equipment and ventilate spaces during installation to avoid lung, skin, and eye damage. SPF can harm occupants who breathe the toxic vapors during installation. SPF requires precise mixing of the two components at specific temperature ranges. Improperly mixed or installed spray foam can emit vapors for months or years resulting in long-term respiratory hazards.
