Bulk insulation
Bulk insulation resists heat flow by trapping still air in fibre (glasswool, polyester, wool) or foam. R-value derives from thickness. Standard ceiling/wall insulation.
Ask Chalkline about this →Bulk insulation is the family of insulation products that resist heat flow by trapping still air within fibres or closed-cell foam, producing a measurable R-value proportional to the material’s thickness and conductivity. It is the dominant residential insulation type in Australia, used in ceilings, walls, and (less commonly) floors. Unlike reflective insulation, bulk insulation works on direct contact with adjacent surfaces, doesn’t require an airspace, and contributes its R-value as installed (subject to compression and moisture). Verified per AS/NZS 4859.1:2018 (2026-05-16).
Types of bulk insulation:
| Type | Composition | Typical R-value/m | Use |
|---|---|---|---|
| Glasswool batts | Spun glass fibre + binder | 22-24 W/m K → R3.5-R4.0 at standard thickness | Ceilings, walls; most common; cheap |
| Polyester batts | Polyethylene terephthalate (PET) fibres | 25-28 W/m K → R2.5-R3.5 | Walls, ceilings; pet-friendly, fewer fibres airborne |
| Wool batts (sheep’s wool) | Natural wool fibre | 28-32 W/m K → R2.5-R3.0 | Premium residential; moisture-tolerant |
| Rock wool / mineral wool | Stone fibre | 30-34 W/m K → R3.0-R3.5 | Fire-rated walls, acoustic, hot environments |
| Cellulose loose-fill | Recycled paper + flame retardant | 35-45 W/m K → R3.0-R3.5 at 150 mm | Retrofit ceilings, blown-in |
| Rigid PIR boards | Polyisocyanurate | 22-26 W/m K → R4.0-R4.5/m | Walls, under-slab, exposed framing; high-performance |
| Rigid XPS boards | Extruded polystyrene | 28-32 W/m K → R3.5-R4.0/m | Under-slab, plinth, foundation walls |
| Rigid EPS boards | Expanded polystyrene | 30-35 W/m K → R3.0-R3.5/m | Cheaper rigid option; SIP panels |
Standard residential thicknesses and R-values:
| Element | Typical thickness | Typical R-value |
|---|---|---|
| Ceiling batt | 175-300 mm | R5.0-R7.0 |
| External wall batt | 90-150 mm (90 mm framing) | R2.5-R4.0 |
| Internal acoustic wall batt | 70-90 mm | R1.5-R2.5 (acoustic-rated; AS 1191 STC) |
| Suspended floor batt | 90-150 mm | R2.5-R3.0 |
| Under-slab rigid board | 50-100 mm | R2.0-R4.5 |
NCC 2022 minimum R-values (climate zones):
| Zone | Climate | Ceiling R | Wall R | Floor R |
|---|---|---|---|---|
| 1 | Tropical | R4.1 | R2.8 | varies |
| 2 | Warm temperate | R4.1 | R2.8 | R1.5 |
| 3 | Hot dry | R4.1 | R2.8 | varies |
| 4 | Cool/Cool temperate | R5.1 | R2.8 | R2.0 |
| 5 | Sub-tropical | R4.1 | R2.8 | R1.5 |
| 6 | Mild temperate (Sydney/Melbourne typical) | R5.1 | R2.8 | R2.0 |
| 7 | Cool temperate (Canberra) | R6.0 | R2.8 | R2.5 |
| 8 | Alpine | R6.0 | R3.5 | R3.5 |
(These are the minimum elemental R-values under DTS. The 7-star whole-house assessment under Specification 42 typically requires higher R-values to balance against window losses and air leakage.)
R-value loss factors:
| Cause | Effect |
|---|---|
| Compression (batt squashed into smaller cavity) | R-value falls proportional to compression; 50% compression → ~30% R-value loss |
| Gaps between batts | A 5% area gap reduces effective R-value by 50% (gaps shortcut the thermal path) |
| Settling (loose-fill cellulose) | Settles 15-25% over years; less performance over time |
| Moisture (water absorbed by fibre) | Wet glasswool loses 90% R-value; polyester less affected |
| Mechanical compression (services, cables routed through) | Local R-value reduction |
A “best practice” install achieves the rated R-value uniformly; a poor install achieves 60-70% of the rated R-value due to compression and gaps.
Bulk vs reflective vs composite:
| Approach | Mechanism | Best for |
|---|---|---|
| Bulk (this) | Still air in fibre/foam | All conductive heat flow; bedrooms, walls |
| Reflective | Low-emittance foil + airspace | Radiant heat (hot roofs in summer) |
| Composite (foil-faced batt) | Combines bulk + reflective | High-performance hybrid; common in modern builds |
For most residential applications, bulk does 90% of the work. Reflective is an addition for hot-summer climates.
Common defects:
- Batts cut to fit awkward cavities and stuffed: compression and tearing; reduces R-value.
- Batts not extending to the wall plate: gap at the top reduces ceiling R-value at the junction.
- Downlights cut into ceiling batts without IC-rated separation: fire risk + thermal short.
- Wet batts left in walls (e.g. rain ingress during construction): wet glasswool stays wet; mould; R-value loss.
- Wrong batt orientation in walls (long edge facing wrong): batts don’t fit cavity properly.
- No batts in eaves/soffit cavity: air leakage path; entire ceiling R-value compromised by the connected airspace.
Builder takeaway:
- Choose product type for the climate (rock wool for fire-rating, polyester for pet/asthma sensitivity, glasswool for cost).
- Match R-value to NCC minimum for the climate zone + Specification 42 calculation.
- Install carefully: no compression, no gaps, no moisture.
- For high-performance builds (8-star, Passive House), use rigid PIR or composite systems to push past glasswool’s practical R-value ceiling.
Also known as: mass insulation; conductive insulation; thermal batts; fibre insulation; mineral wool (specific subtype); thermal insulation (loose term).
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.