Caulk is the most used and most misused material in home maintenance: the right sealant in the right joint quietly works for a decade, the wrong one fails in a season, and a bead in a joint that should never have been sealed traps the very water it advertised against. The types of caulk sort into a short taxonomy, and the craft is matching product to joint, plus knowing the joints that stay open on purpose. This reference covers both.
The taxonomy
Acrylic latex (“painter’s caulk”): water cleanup, easy tooling, paintable, cheap, and modestly flexible, the interior workhorse for trim gaps, baseboards, and crack-filling before paint, and an exterior failure waiting to happen anywhere real movement or weather lives; siliconized acrylics stretch its range slightly. Silicone: the flexibility and water champion, adhering to glass, tile, and metal through decades of wet cycles, which makes it the tub, shower, sink, and glazing sealant, and the unpaintable one, paint beading off it, and future sealants refusing to stick where its residue remains, the property that earns its “silicone is forever” reputation on both sides of the ledger. Kitchen-and-bath silicones add mildewcides for the wet-room biology. Polyurethane: the exterior heavyweight, high movement capability, tenacious adhesion, abrasion resistance, paintable after cure, the traditional pick for siding joints, masonry, and driveways, tempered by tougher tooling and cleanup. Hybrids (MS polymer / silyl-modified): the modern exterior standard-bearers, near-polyurethane performance with easier tooling, paintability, and adhesion across mixed materials, worth their premium at every joint that matters. The specialists round out the shelf: butyl for gutters and metal laps, fire-rated sealants at code-required penetrations, concrete-crack products for slabs, and the removable winter-seal caulks for old-window weatherization.
Matching product to joint
The joint’s movement, moisture, and paint future pick the tube. Interior painted trim and drywall gaps: acrylic latex, tooled and painted. Wet rooms at tub, tile, and counters: silicone, with the old bead fully stripped first, since nothing bonds over silicone residue including silicone. Exterior siding-to-trim, trim-to-masonry, and penetration collars: hybrid or polyurethane, paintable and rated for the joint’s swing. Glass and metal glazing details: silicone’s home turf. And the sizing rule that outlasts every product choice: joints wider than about a quarter inch, and all deep joints, get backer rod first, the foam cord that sets the sealant’s depth to roughly half its width and creates the two-sided adhesion flexible joints require, caulk bonded on three sides cracking as the joint moves no matter the label’s promises.
The joints that stay open
The exterior’s drainage joints are designed gaps, and sealing them converts breathing assemblies into water traps: the bottom edges of siding courses and panels, which weep the water the rain-screen design sheds behind itself; weep holes in brick veneer and at window and storm-window sills, the wall’s and unit’s drains; the underside gaps at drip caps and head flashings, whose lap-and-drain geometry the flashing references cover, caulk across a flashing lap being the classic confession of a misdiagnosed leak; and the garage-slab and stucco screed lines that drain their assemblies. The working rule: horizontal-ish tops of joints shed water and may be sealed; bottoms of laps drain water and stay open, and when in doubt, the assembly’s designer left the gap for a reason worth learning before filling.
Application: the ten minutes that decide the decade
Failed caulk is usually failed prep: old sealant cut and stripped fully (razor and mineral spirits, or the dedicated removers, and absolute silicone-residue honesty), surfaces dry and dust-free, backer rod set in the wide joints, tape for crisp lines where finish matters. The bead gets pushed, not dragged, filling the joint rather than bridging it, tooled promptly with a finger or tool to compress adhesion and shape the concave profile, tape pulled while wet, and cure honored before paint or water per the tube. Temperature windows matter outdoors, most products wanting mild dry days, and the annual audit closes the loop: a walk of the exterior’s beads each spring, cracked, pulled, or missing joints recut and resealed while they are maintenance instead of moisture archaeology.
The bottom line
Acrylic inside where paint follows, silicone where water and glass live, hybrids and polyurethane outside where joints move, backer rod in anything wide, and no caulk at all in the gaps the wall drains through. Strip the old bead, prep like it matters, tool it wet, and walk the beads every spring, because sealant is cheap exactly once per joint.