Drywall, also known as gypsum board or sheetrock, is a popular building material used for interior walls and ceilings. It consists of a core made of gypsum plaster pressed between two thick sheets of paper. Joint compound is a white powder that is mixed with water to create a plaster-like material for sealing joints between drywall panels and finishing the seams to create a seamless surface. The composition of both drywall and joint compound contributes to their usefulness in construction.
Composition of Drywall
Drywall is relatively simple in composition, containing just a few primary ingredients:
Gypsum
Gypsum, specifically calcium sulfate dihydrate, is the main component of the drywall core, comprising over 90% of its weight. Gypsum is a soft mineral that is mined and processed into a fine powder. When water is added, it forms crystals that interlock, creating a rigid structure. This allows the gypsum core to provide fire resistance and durability while remaining lightweight and easy to work with. Gypsum has sound-dampening qualities as well.
Paper Facings
The gypsum core of drywall is faced with heavy paper on both sides. The paper facings provide strength and protect the core. The front paper facing is typically pre-decorated with a colored, textured, or printed design for aesthetic purposes. The back paper facing is covered with a gray adhesive so drywall panels can be bonded together. Paper facings are made from recycled paper fibers and strengthen the core while allowing for easy installation and painting.
Additives
Various additives may be mixed into the gypsum core to provide additional properties. For example, fiberglass or plastic fibers reinforce the core, making it more resistant to cracks from shrinkage. Foaming agents introduce air pockets that lower density and improve sound absorption. Environmentally friendly drywall may contain recycled materials or bio-based binders. Chemical additives help control the set time and provide fire and moisture resistance.
Composition of Joint Compound
Joint compound has a similar base composition to drywall but also contains several other ingredients. The primary components of joint compound are:
Calcium Carbonate
Calcium carbonate, typically in the form of limestone, makes up 65-75% of joint compound. It is a finely ground white powder that provides bulk and body. Calcium carbonate enables the compound to be smoothly sanded down once dry for a seamless finish.
Calcium Sulfate Dihydrate
The same gypsum material found in the drywall core comprises 15-30% of joint compound. Powdered gypsum binds the ingredients together as the compound dries.
Mica or Talc
Mica or talc, making up 1-5% of joint compound, is included as a filler and texture modifier. It increases adhesion and workability while decreasing shrinkage. The small plate-like particles of mica also provide reinforcement.
Ethenol or Vinyl Acetate Polymer
A small amount of ethenol, polyvinyl acetate, or other binding polymer is added to improve the strength and adhesion of the joint compound. It allows the material to bond tightly to the drywall paper.
Attapulgite Clay
Attapulgite clay thickens the joint compound mixture and prevents separation of the solids and liquids. This improves the application properties of the compound.
Water
Water mixed into the dry joint compound powder activates the ingredients and allows application as a mud or paste. The water causes the gypsum to crystallize as the compound hardens.
Other Minor Additives
Other minor additives in joint compound may include perlite for texture, cellulose for thickness, sodium citrate to control setting time, preservatives like potassium sorbate to prevent mold, and titanium dioxide for whiteness.
Why These Compositions Work Well
The compositions of both drywall and joint compound contribute to their usefulness in construction and finishing of interior walls and ceilings:
Fire and Moisture Resistance
The gypsum core of drywall and the high concentration of calcium sulfate dihydrate in joint compound provide excellent fire resistance. The gypsum crystals contain water molecules that are released as steam when exposed to fire, delaying temperature rise. Joint compound and drywall also resist moisture damage.
High Bonding Strength
Joint compound adheres tightly to drywall thanks to the polymer binders and gypsum. The dried compound forms a hard, integrated layer with the paper facings. Strong bonds prevent cracks and separation at joints.
Smooth Sandability
Large amounts of calcium carbonate and mica create a soft, sandy texture and feel in joint compound. This enables the material to be sanded down smoothly once dry for seamless finishing. Good sandability is vital for achieving a flat, uniform surface.
Low Shrinkage
Additives like mica and clay limit shrinkage of joint compound as it dries. Minimal shrinkage prevents cracked seams, nail pops, and uneven finishes. The paper facings on drywall also provide dimensional stability.
Workability
When wet, both drywall and joint compound are easy to handle and shape. Joint compound has a malleable, plaster-like consistency for quick application over seams. Drywall can be cut and fastened in place with minimal effort.
Economical Materials
The primary raw materials in both products – gypsum and calcium carbonate – are abundant minerals mined from the earth. This keeps manufacturing costs relatively low. The recycled paper facings also reduce material expenses.
Lightweight
Despite good strength and fire resistance, drywall and joint compound set up into lightweight finished products. This reduces construction time and the need for heavy-duty building supports. The air pockets in some drywall core add-ins also decrease weight.
The synergistic compositions of drywall and joint compound allow the materials to be easily installed and finished into durable, fire-resistant, smooth wall and ceiling surfaces. Understanding what gives these products their beneficial properties aids builders and contractors in selecting quality drywall systems. With the right products and proper technique, drywall and joint compound pair up for cost-effective and long-lasting interior construction.
Drywall Core Materials
The core of a drywall panel, made mostly of gypsum, provides the base structure and properties. However, various other materials may be incorporated into the gypsum core to improve certain qualities:
Fiberglass Fibers
Fiberglass added to the core makes drywall more resistant to shrinkage cracks. The fine fibers bind the gypsum crystals together. They also reduce the weight slightly. Fiberglass mesh may also be embedded into specialized board types.
Paper Fibers
Recycled paper fibers help strengthen the gypsum core and reduce production costs. The short fibers become randomly dispersed through the core. Even small amounts significantly control cracking.
Foaming Agents
Soaps and detergents create air bubbles as the core slurry forms. This makes the board lighter in weight and improves sound dampening qualities. Foaming agents are only used in specific types of lightweight and sound-proof drywall.
Glass Fibers
Glass fibers increase flexural strength and prevent screw pull-through failure. They are commonly used in mold-resistant gypsum board as well as exterior products. The fibers reinforce the core to resist damage.
Microspheres
Tiny glass microspheres can be used to lower the density of the core further and improve sound absorption. The hollow microspheres basically act like microscopic bubbles dispersed through the gypsum matrix.
Perlite
Perlite is a volcanic mineral that is puffed with heat to form lightweight, low-density particles. It reduces the overall weight of the gypsum core while enhancing fire resistance. Perlite also gives the core better acoustic properties.
Starches
Adding binding starches improves the adhesion of the gypsum crystals and paper facings as well as increasing flexural strength. Starches derived from crops like corn are commonly used. This increases recyclability.
Boric Acid
Boric acid powder helps control fungus, mold growth, and termites in gypsum board. It allows the product to better resist moisture damage in damp environments. The acid is neutralized in the core slurry.
Silicon Dioxide
Silicon dioxide, or silica, in the form of quartz powder adds hardness to the core and improves fire resistance. It increases drywall’s resistance to surface indentation and damage. Silica also helps control shrinkage.
The choice of core additives depends on the specialized properties needed. With a basic understanding of core materials, builders can select the proper drywall products for their specific application and environment.
Paper Facings Role in Drywall Strength
Though the gypsum plaster core provides the bulk of drywall’s structural strength, the paper facings bonded to both sides also play some key roles:
Flexural Strength
The facings help resist tension forces on the back and compression on the front when drywall bends, increasing flexural strength. This prevents cracking when handled. Paper also increases tensile strength parallel to the sheet face.
Surface Hardness
The hard paper surfaces protect the softer gypsum core from damage on both sides. This prevents dents, gouges, and abrasion damage to the fragile plaster interior.
Fastener Holding
The relatively thick paper coverings reinforce areas around nails or screws driven through the drywall. This prevents fasteners from loosening or “popping” as the gypsum core shrinks and settles.
Impact Resistance
Face papers distribute impact forces across broad areas instead of allowing concentrated damage. Several layers of paper also absorb energy from impacts and increase crack resistance.
Dimensional Stability
Paper facings provide structural reinforcement to minimize expansion or contraction of the gypsum core with temperature and moisture changes. This maintains stable drywall dimensions.
Shear Strength
Overlapped seams or bonded edges result in a strong composite section with layers of paper on the outside. This creates exceptional shear strength and diaphragm stiffness.
Surface Quality
The smooth paper substrates provide an ideal surface for decorative finishes to be applied. The facings form a strong bonding surface for paints, wall coverings, and textures.
Handling Strength
The facings allow easy handling and installation of drywall panels without risk of damage. Workers can carry and fasten the panels without chunking or crumbling of the core.
While the gypsum core gives drywall its essential structural performance, the paper coverings are also engineered to provide many supplemental properties important for a durable and functional interior wall material.
Standard Drywall Sheet Sizes
Drywall panels are manufactured in a range of widths and lengths to suit different applications:
- 4′ x 8′ – The most common size for walls and ceilings. Easy to transport and lift. Can be cut to fit any room size.
- 4′ x 10′ – Longer sheets that reduce seams on walls over 8′ tall. Require two people to carry.
- 4′ x 12′ – Maximize coverage on high walls or long runs. Difficult for one person to install. May require lift equipment.
- 4′ x 14′ – Used for cathedral ceilings over 20′ tall. Require specialized installation methods.
- 4′ x 16′ – Extremely large sheets mainly used in commercial construction with specialized equipment. Challenging to transport.
- 54″ x 8′ – Wider sheets to reduce seams on large expanses. Heavier and harder to cut than 48″ width.
- 54″ x 10′ – Used to cover 9′ ceilings with fewer seams. Require two installers.
- 54″ x 12′ – Allow fast coverage of tall walls with just one vertical joint. Hard to handle and lift.
- 60″ x 8′ – Extra wide sheets for fewer seams on walls or ceilings. Difficult for one person to install.
- 2′ x 8′ – Smaller sheets used for patching jobs, soffits, small areas. Easy to carry and install alone.
- 2′ x 10′ – Ideal for repairing larger holes or damaged areas needing larger patches.
- 2′ x 12′ – Large patches for covering major ceiling water damage or long wall gaps.
- 1′ x 8′ – Precut strips for fast repairs of cracks or holes. Convenient to store scraps.
Knowing the common drywall sizes available allows matching the appropriate sheets to each application for efficient installation with a minimum of cutting waste.
Drywall Sheet Types for Specialized Uses
While regular drywall works for most indoor applications, specialized panels have been engineered with enhanced properties for specific environments:
Fire-Resistant Drywall
Fire-resistant or Type X panels contain glass fibers and mineral additives to further improve fire performance ratings beyond regular board. Provides extra protection of building structure.
Water-Resistant Drywall
Water-resistant panels feature an acrylic-coated paper facing that acts as a water barrier. Reduces moisture damage in bathrooms, laundry rooms, and other damp locations.
Mold-Resistant Drywall
Mold-resistant or “greenboard” contains additives to prevent mold growth. Ideal for moist areas. May include embedded fiberglass mesh for added protection.
Soundproof Drywall
Soundproof board has a dense, laminated, or damped gypsum core to block noise transmission through walls and ceilings separating rooms. Improves sound privacy.
Impact-Resistant Drywall
Impact-resistant panels feature a steel-reinforced facing, fiber mesh, or foam plastic core to withstand damage from repeated surface impacts. Used in high-traffic areas.
Exterior Drywall
Exterior board is coated with asphalt for water protection and surfaced with a strong, abrasion-resistant facing to withstand outdoor exposure. Suitable for protected exterior soffits.
Flexible Drywall
Flexible panels made from gypsum reinforced with cellulose fibers can bend slightly to fit curved walls. Requires careful handling to prevent cracks during shaping.
Lightweight Drywall
Lightweight, low-density board contains foaming agents or plastic microspheres to reduce weight by up to 20%. Easier to lift and fasten overhead.
Choosing the right specialty drywall for conditions like moisture, fire risk, sound transmission, or traffic impact improves performance while avoiding problems. An awareness of the options helps select the optimal materials.
Paper Joint Tape Purpose and Types
Joint tape is a thin paper material applied over the seams between drywall panels before finishing with joint compound. It serves several purposes:
Prevents Cracks
The paper tape reinforces the joint and helps minimize cracking from building movement and shrinkage of materials. This provides seam durability.
Creates Straight Edge
The thin edge of the tape provides a straight guide for smoothing joint compound to a uniform feathered finish. This maintains a professional appearance.
Controls Compound Absorption
Tape prevents the paper drywall facings from drawing water out of the wet joint compound too rapidly, allowing it to dry evenly.
Reinforces Joint
In multi-coat finishing, the embedded tape strengthens the hardened compound and ties the panels together rigidly.
Provides a Bonding Layer
The porous paper tape gives joint compound something to grip, improving adhesion between panels.
There are two main types of paper joint tape:
Paper Mesh Tape
- Woven open fiberglass mesh centered in a paper coating. Somewhat flexible.
Paper Tape
- Solid paper strip with perforated edges. Easier to use for beginners.
Paper tape is a basic component of quality finishing. The right type and proper application prevents cracks and builds durable, long-lasting drywall seams.
Drywall Primer Purpose and Types
Primer coat paints applied before decorating topcoats serve important purposes on drywall:
Seals Surface
Primer penetrates and seals the porous drywall paper covering. This prevents the topcoat from being absorbed unevenly, showing lap marks and texture differences.
Binds Surface Dust
Lightly sands and smooths the paper fibers, plaster dust, and residual joint compound. A primer coat encapsulates these particles to prevent blistering and bubbling of the topcoat.
Uniforms Absorption
The penetrating sealer equalizes the paint absorption characteristics across repairs, fastener dimples, and differences between joint compound and paper facings. This allows an even topcoat.
Improves Adhesion
Primers chemically bond to the drywall surface and provide an ideal intermediate layer to adhere the decorative topcoat. This prevents future peeling or flaking.
Hides Minor Flaws
The flat uniform primer helps conceal residual drywall defects and flaws before the final paint finish goes on. Two finish coats are still recommended over primer.
Common drywall primer types include:
PVA Primer
- Low-cost polyvinyl acetate primer suitable for low-moisture areas. Dries fast.
Drywall Primer
- All-purpose alkali-resistant primer formulated for new drywall. Medium price.
Stain-Blocking Primer
- Seals stains from water, smoke, markers, etc. before topcoat. More expensive.
Applying the right primer is a critical first step for quality, long-lasting paint finishes on drywall surfaces. It maximizes the performance of the final decorative topcoats.
Joint Compound vs. Spackle
While joint compound and spackle are both used to patch holes and seams in drywall, there are some key differences:
Composition
Joint compound contains limestone and mica for easy sanding, while spackle uses softer binders like polyvinyl acetate for a smoother finish.
Drying Time
Pre-mixed joint compound dries in 24 hours, while lightweight spackling paste can dry in as little as 15-30 minutes for fast repairs.
Properties When Dry
Joint compound sands smoothly but is harder and less flexible. Spackle sands easily but remains somewhat spongy and flexible when dry.
Shrinkage
Joint compound shrinks minimally for durable repairs, while spackle has more shrinkage as it dries quickly, increasing risk of cracks reappearing.
Uses
Joint compound is ideal for taping and finishing seams between drywall panels in new construction. Spackle works better for small drywall repairs.
Sanding
Joint compound requires more sanding for smooth results. However, lightweight spackle develops undesirable g