Roof Insulation: Types & Costs

Roof Insulation: Types & Costs

Key roof build-ups (how insulation is placed)

• Warm Roof (above deck/over rafters) — Continuous rigid boards above structure; keeps deck and structure warm.
• Cold Roof (between/below rafters) — Insulation within/under rafters; ventilated void above.
• Inverted Roof (flat roofs) — Waterproofing under the insulation; XPS on top with ballast/pavers.
• Hybrid Roof — Combination (e.g., between rafters + a thin continuous layer over/below to cut thermal bridges).
• Green Roof build-up — Insulation as part of a vegetated roof system (extensive/intensive).

Main insulation materials

• Mineral Wool (glass/rock) — Batts/rolls for between rafters; non-combustible, sound-absorbing.
• PIR/PUR Rigid Boards — High thermal performance per thickness; ideal for warm roofs.
• XPS Rigid Boards — Moisture-resistant, compressive strength; used in inverted roofs/terraces.
• EPS Rigid Boards — Cost-effective; various densities for pitched/flat roofs.
• Cellulose (blown-in) — Recycled fiber blown into cavities; good for retrofits.
• Spray Foam (open- or closed-cell) — Air-sealing + insulation in one; requires expert design/ventilation strategy.
• Reflective Foil Layers — Radiant barriers/foils as part of systems; not stand-alone insulation.
• VIP (Vacuum Panels) — Very thin, high R; niche, detail-sensitive.

Where each shines

• Warm Roof + PIR/EPS: New build or full re-roof; best thermal continuity (few bridges).
• Cold Roof + Mineral Wool/Cellulose: Loft conversions/retrofit where above-deck access is limited.
• Inverted Roof + XPS: Flat roofs with pavers/ballast, terraces, podiums.
• Hybrid: Upgrades where you must control condensation and cut bridging without full strip.
• Green Roof: Noise attenuation, summer heat buffering, stormwater benefits.

Pros & cons (at a glance)

Warm Roof (rigid boards over deck)

• + Minimal thermal bridges; protects structure from condensation; ideal for low-slope roofs.
• − Higher upfront %, roof height details to solve (eaves, parapets).

Cold Roof (between rafters)

• + Easier/cheaper in retrofits; materials widely available.
• − Thermal bridges at rafters; needs reliable ventilation to avoid condensation.

Inverted Roof (XPS above waterproofing)

• + Membrane protected; easy overlay/refurb; ideal under paving.
• − Heavier; needs ballast/drainage layer; careful edge/stone retention.

Hybrid

• + Balances cost and performance; improves dew-point safety.
• − More layers/details; coordination effort.

Material notes

• Mineral Wool: + Non-combustible, acoustic; − needs perfect vapor/air control.
• PIR/PUR: + High R per mm; − fire/smoke specs vary—use rated boards & overlays.
• XPS: + Water/pressure resistant; − lower R/mm than PIR, combustibility consideration.
• EPS: + Best value; − thicker for same U-value vs PIR.
• Cellulose: + Fills gaps well, eco; − moisture management crucial.
• Spray Foam: + Air seal + insulation; − hard to reverse; ventilation/condensation design is critical.

Cost guide (percentages only; baseline = Cold Roof with Mineral Wool between rafters = 0%)

(Combined materials + typical installation for comparable U-value; ranges vary by thickness, access, roof complexity.)

• Warm Roof (PIR rigid over deck): +20–45% vs baseline
• Warm Roof (EPS rigid over deck): +10–30% vs baseline
• Hybrid (between rafters + thin PIR over/below): +15–35% vs baseline
• Inverted Roof (XPS + ballast/pavers): +25–55% vs baseline
• Cellulose blown-in (cold roof cavities): −5–+15% vs baseline
• Spray Foam (pitched roof interior): +25–60% vs baseline
• VIP inserts (localized thin build-ups): +150–300% vs baseline (niche)

Lifecycle/maintenance over ~15–20 yrs

• Warm Roof (PIR/EPS): −5–15% vs baseline (fewer condensation/repair issues)
• Inverted XPS (under pavers): −5–10% vs baseline (membrane protected)
• Spray Foam: 0–10% vs baseline (low routine care, but hard to modify)
• Cellulose/Mineral Wool: 0–5% vs baseline (check ventilation & vapor control)

Energy impact (annual heating/cooling) for same U-value but better air-tightness/bridging control

• Warm/Hybrid vs Cold: typically −5–15% energy use vs baseline (due to reduced thermal bridges + improved airtightness)

Risks & how to avoid them

• Condensation (all roofs): Wrong vapor control or missed ventilation void.
  • Fix: Hygrothermal/dew-point check; correct vapor control layer (VCL) placement; maintain vent paths in cold roofs.
• Thermal bridges at rafters/parapets:
  • Fix: Add continuous over/under-rafter layer (hybrid/warm), insulate upstands/edges.
• Air leakage:
  • Fix: Continuous airtight layer; tape penetrations; service cavity under VCL.
• Wet insulation during install (MW/cellulose):
  • Fix: Weather protection; dry substrate; moisture checks before close-up.
• Fire performance/spec:
  • Fix: Use rated systems (BROOF(t4) / FM approvals as applicable), correct overlays and separations.
• Load & drainage on inverted/green roofs:
  • Fix: Structural check; proper falls (1–2%), water outlets, filter/drain layers.

Quick choice guide

• Best overall performance (flat/pitched re-roof): Warm Roof (PIR/EPS)
• Budget retrofit without stripping roof: Cold Roof (Mineral Wool/Cellulose)
• Terraces/podiums with pavers: Inverted Roof (XPS)
• Thin build-up at tricky details: Hybrid or localized VIP (niche)
• Eco & cavity fill retrofits: Cellulose blown-in (with robust vapor strategy)

Short FAQ

• Do I still need ventilation?
  • Cold roofs: yes (eaves/ridge vents). Warm/inverted: ventilation generally not required above the deck, but follow system design.
• UFH in roof/loft rooms—any limits?
  • Insulation must meet U-value and airtightness targets; avoid compressing batts; respect service cavities.
• Can I add insulation over old felt?
  • Often yes during re-roof (warm roof overlay), if deck is sound and details are updated (VCL, upstands, height transitions).