Quick answer
A bouncy or sagging floor is almost always a structural issue with the subfloor or joists—not the insulation. Insulation affects temperature and sound, but it doesn’t carry load. Start by inspecting from above and below: tighten or replace a loose subfloor, then evaluate joist span, size, and condition. Fixes range from re-screwing and gluing the subfloor, to sistering joists, adding blocking/bridging, or installing a mid-span beam/post. Severe rot, insect damage, or engineered joists/trusses warrant a professional.
What’s going on and how to check
Bounce comes from deflection (flexing), while sag shows as a dip. Both result from:
- Loose subfloor-to-joist connections
- Overspanned or undersized joists
- Notched/drilled/damaged joists
- Inadequate support or settlement at beams/posts
- Moisture or insect damage
Insulation can hide problems from view, but it doesn’t cause bounce. If insulation is wet or moldy, you likely have moisture damage to wood.
Tools you’ll find helpful
- 6–8 ft level or laser line level
- Tape measure and framing square
- Impact driver, drill, and countersink
- Construction adhesive (polyurethane) and wood glue
- Subfloor screws (2–2 1/2 in) and structural screws (Simpson SDS/LedgerLOK/GRK) for framing
- Pry bar, clamps, wood shims
- Reciprocating saw or circular saw (for subfloor repair)
- Moisture meter and flashlight/headlamp
- Adjustable jack posts, bottle jack, and temporary shoring lumber (as needed)
- PPE: eye/ear protection, gloves, dust mask/respirator
Step-by-step diagnosis
- Walk test and mark problem zones: Note where the floor feels springy or dips. Drop a marble; watch if it rolls to a low spot.
- Check flatness: Lay a long level or a straightedge and measure gaps to spot sagging joists. Use a laser to sight across a room.
- Access from below (basement/crawlspace):
- Look for cracked, notched, or bored joists, especially near mid-span or over openings.
- Check joist size, spacing, and span. Confirm hangers and bearing points are secure.
- Identify any moisture stains, rot, or termite damage.
- Inspect beams and posts for settlement or loose connections.
- Assess subfloor: From above, squeaks and bounce near seams often mean loose sheathing. From below, look for gaps between joist tops and subfloor.
- Measure spans and compare to guidelines.
Target deflection (residential): L/360 (hardwood/carpet), L/720 preferred for brittle tile/stone.
Common spans @ 16" o.c. (typical SPF):
- 2x8: ~11–12 ft
- 2x10: ~14–16 ft
- 2x12: ~17–18 ft
Subfloor thickness: 3/4" T&G plywood or OSB for 16" o.c.; consider 1-1/8" for 24" o.c.
Screw pattern (re-secure): 6" o.c. along panel edges, 8" o.c. in the field.
Common fixes and how to do them
1) Loose subfloor (squeaks/bounce mostly near seams)
- From above (best if flooring is off): Apply construction adhesive to joist tops, lay/relaminate subfloor, and screw it down to the pattern above.
- From below (finished floors): Drive 2–2 1/2 in screws up through joists into subfloor where you see gaps. Use adhesive injection where possible. Specialty kits (squeak repair) let you screw from above through carpet and snap the heads.
- Consider adding 3/8–1/2 in plywood underlayment (glue and screw) over an existing thin or damaged subfloor to stiffen the feel.
Time/cost: 2–4 hours per room, $50–$200 in screws/adhesive/plywood (excluding new finish flooring).
2) Overspanned/undersized joists (general bounce across the room)
- Sistering: Glue and screw a new joist or LVL alongside the existing for at least the center two-thirds of the span; full‑length, bearing‑to‑bearing is best. Clamp tight to eliminate gaps and use structural screws/bolts and adhesive.
- Add mid-span support: Install a beam with posts (adjustable steel columns or framed posts) on proper footings to cut the span in half. Jack slowly to correct sag before locking in the beam.
- Add blocking/bridging: Solid blocking or metal X‑bridging every 6–8 ft improves load sharing and reduces vibration. It won’t fix sag alone but helps.
Time/cost: Sistering 1–2 hours per joist, $60–$300 per joist. New beam/posts with footings: 1–2 days, $500–$3,000+ depending on length and concrete work.
3) Damaged/notched/drilled joists
- Sister over and beyond the damaged area (ideally 4+ ft past) with adhesive and structural fasteners. For severe damage, replace the joist or engage an engineer.
- Never notch the middle third of a joist depth; keep holes small and in the middle third vertically. Avoid cutting engineered I‑joists or trusses without manufacturer specs.
4) Beam/post settlement
- Add temporary shoring, then jack the beam up slowly (no more than 1/8–1/4 in per day) to avoid cracking finishes. Install new posts on proper footings and secure with rated connectors.
5) Moisture/rot/insects
- Fix water sources (leaks, poor ventilation). Replace compromised wood. Treat for termites if present. Replace wet insulation and ensure it doesn’t trap moisture against subfloor.
About insulation
Insulation neither causes nor cures bounce. Remove it temporarily to inspect joists and subfloor. If it’s wet or sagging, track down the moisture problem and replace it after structural repairs.
Safety and best practices
- Check for plumbing/electrical before drilling or driving long screws from below.
- Wear PPE; older homes may have lead paint or asbestos in finishes—test before demolition.
- Structural changes may require permits. Use rated connectors (Simpson, etc.) and structural screws rather than generic deck screws for framing.
- Jacking: Go slow and shore properly to avoid sudden load shifts.
Common mistakes
- Relying on bridging alone to fix sag.
- Sistering short “patch” pieces that don’t reach bearing.
- Skipping adhesive between members or between subfloor and joists.
- Over‑tightening jacks quickly, causing drywall/tile cracks.
- Using nails only for subfloor reattachment (screws hold better).
When to call a pro
- Sag over ~1 inch, widespread deflection, or unknown cause.
- Engineered joists/trusses or steel beams involved.
- Significant rot, termite damage, or foundation settlement.
- Adding beams/posts and footings that need engineering/permits.
Rough cost guide
- Re‑screw/glue subfloor: $50–$200 DIY materials.
- Add blocking/bridging: $5–$10 per bay.
- Sistering solid lumber: $60–$150 each; LVL: $150–$300 each plus hardware.
- Beam with posts/footings: $500–$3,000+.
- Structural engineer consult: $300–$800.
Tip: Start with the simplest fix—resecure the subfloor—and work up to joist reinforcement or new support. A systematic inspection saves money and ensures you solve the actual cause, not just the symptom.