Chapter 28 Key Takeaways: Drywall, Plaster, and Interior Walls
Drywall Types — Choose the Right Product
| Type | Thickness | Application |
|---|---|---|
| Standard | 1/2 inch | Interior walls and ceilings |
| Standard | 5/8 inch | Ceilings (sag resistance), higher rigidity |
| Moisture-resistant (green board) | 1/2 inch | High-humidity areas, NOT shower tile substrate |
| Type X fire-rated | 5/8 inch | Garage-to-living walls, stairwells — required by code |
| Cement board | 1/2 inch | Tile substrate in wet areas |
| Sound-dampening | 5/8 inch | Where acoustic performance is a priority |
The Three-Coat Finishing System
New drywall is finished in three coats of joint compound: 1. Tape coat — embeds paper tape over joints and fills screw dimples 2. Fill coat — feathered wider, builds over the tape 3. Finish coat — lightest coat, widest feather, eliminates ridges
Each coat must dry fully (24+ hours in normal conditions) before the next. Never rush this.
Level 4 finish is standard for living spaces with flat/eggshell paint. Level 5 finish (full skim coat) is required for gloss paint or rooms with raking light. Always prime new drywall before painting — skipping primer causes seam shadowing.
Drywall Repair — Match Method to Hole Size
| Hole Size | Method | Notes |
|---|---|---|
| Nail/screw pop | Drive new screws 2" above and below, cover heads | Don't just hammer back in |
| Under 1" | Lightweight spackle | Don't use toothpaste or soap |
| 1"–6" | Self-adhesive mesh patch OR California patch | California patch gives better raking-light result |
| Over 6" | Cut to studs, new drywall piece, tape all edges | Standard joint technique |
Texture matching is the hardest part of drywall repair. Practice on cardboard. Apply texture after priming the patch, not before.
Plaster Walls — What You Have and What to Do
The thump test: Tap plaster — hollow sound or movement under palm pressure means failed keys (delamination). Safe to walk on; unsafe if on a ceiling.
Repair approaches by scale: - Small cracks: latex caulk (at movement joints) or patching plaster (stable areas) - Loose-but-intact plaster: plaster washers fastened through into lath — genuinely effective - 10–30% failure: washers plus skim coating - Over 30% failure: evaluate replacement with new drywall
Three-coat plaster (pre-1950s construction) is thicker, denser, and better at sound isolation than drywall. Preserve it where structurally sound.
Soundproofing — Honest Performance Expectations
What works: - Mass (more drywall) adds 3–5 STC points per added layer when direct-screwed - Resilient channels (decoupling) add 8–12 STC points over baseline when correctly installed - Double stud walls achieve STC 55–60+
What doesn't work for transmission reduction: - Acoustic foam panels (absorbs room echo, not transmission) - Spray foam in wall cavities
The critical failure mode: Short-circuited resilient channels (screws accidentally going into studs through the channel) eliminate most of the channel system's benefit.
Flanking transmission through floors, ceilings, and HVAC ducts limits any wall assembly's real-world performance.
Paint Prep — The 90% That Isn't Painting
Required before painting any wall: 1. Clean surfaces (TSP substitute in kitchens/grimy areas) 2. Repair all holes, nail pops, and cracks 3. Caulk all transitions (wall-ceiling, trim-to-wall) 4. Sand all repairs smooth with a sanding block (not bare sandpaper) 5. Prime: PVA primer on new drywall; stain-blocking primer over water stains or smoke 6. Check for lead paint in pre-1978 homes before sanding
Paint Sheen Selection
| Sheen | Where to Use |
|---|---|
| Flat/Matte | Ceilings, low-traffic adult bedrooms |
| Eggshell/Satin | Living rooms, dining rooms, hallways — general living spaces |
| Semi-gloss | Kitchens, bathrooms, all trim and doors |
| High-gloss | Trim, cabinetry — requires Level 5 prep |
The Lead Paint Rule
Homes built before 1978 may have lead paint. Intact lead paint is not a hazard. Disturbed lead paint (sanding, scraping) is. Test before sanding in pre-1978 homes. Follow EPA lead-safe renovation guidelines when lead is present.