Executive Summary
For walls, roofs and attics, the choice between spray foam, fiberglass batts and blown cellulose shapes comfort, bills and moisture risk for decades. R-value is only one piece: air-tightness, detailing and carbon footprint matter too.
- Closed-cell spray foam delivers high R per inch and air-sealing, but at higher cost and embodied carbon.
- Fiberglass is cheap and familiar but relies heavily on installation quality and air-sealing by other means.
- Cellulose offers good performance, low drafts in dense-pack assemblies and relatively low embodied carbon when installed correctly.
At Energy Solutions we benchmark assemblies, not just products, to show how insulation choice behaves over a full building life-cycle.
Contents
- 1. Basics: conduction, air leaks and assemblies
- 2. Performance: R-values, air-tightness & moisture
- 3. Where each insulation makes sense
- 4. Costs, carbon & programme design
- 5. Case studies: real retrofit projects
- 6. Devil's advocate: insulation pitfalls
- 7. Outlook to 2030
- 8. FAQ for designers & homeowners
1. Basics: conduction, air leaks and assemblies
Insulation slows heat flow, but real-world performance is dominated by thermal bridges and air leaks. A moderate-R material in an airtight, well-detailed assembly often outperforms a higher-R product installed with gaps and voids, especially in windy or very cold climates.
Three mechanisms matter in practice:
- Conduction through framing, sheathing and insulation.
- Convection within cavities and through cracks, which can short-circuit even high-R materials.
- Radiation across air spaces and from hot to cold surfaces.
Good envelopes treat insulation, air-barriers and vapour control as one coordinated system rather than a single product choice in isolation.
2. Performance: R-values, air-tightness & moisture
The table below uses stylised centre-of-cavity R-values for 100 mm thickness in typical 2026 products. Actual assemblies include framing, sheathing, service cavities and cladding, all of which reduce effective R-value compared with brochure figures.
| Insulation type (illustrative) | R-value per 100 mm | Air-tightness contribution | Moisture & detailing notes |
|---|---|---|---|
| Fiberglass batt | ~R-3.5–3.8 | Low (needs separate air barrier) | Gaps around wiring and framing common if not carefully fitted. |
| Dense-pack cellulose | ~R-3.7–3.9 | Medium (fills cavities, reduces convective loops) | Requires controls on moisture entry; often paired with smart vapour retarders. |
| Closed-cell spray foam | ~R-6.0–6.5 | High (insulation + air barrier in one) | Must manage drying potential; difficult to remove or modify later. |
Designers increasingly combine products—for example, exterior mineral wool boards plus interior cellulose—to balance performance, drying and carbon. Spray foam is then used more sparingly at difficult junctions or in areas with very limited depth.
Illustrative installed cost index by insulation type
3. Where each insulation makes sense
No single product is "best" everywhere. Instead, each tends to shine in particular assemblies and project types:
- Fiberglass batts: cost-sensitive new builds where framing is simple, quality control is good and a separate air-barrier layer is already in the design.
- Dense-pack cellulose: retrofits of existing wall and roof cavities, and high-performance walls that target low embodied carbon and robust moisture control.
- Closed-cell spray foam: complex junctions, limited-depth rim joists, and assemblies where combining insulation and air-barrier in one layer offsets higher material cost.
Climate also matters. In cold, heating-dominated regions with high energy prices, the value of extra R-value and airtightness is amplified. In milder climates, simpler, lower-carbon assemblies can deliver most of the comfort and bill gains at lower cost.
4. Costs, carbon & programme design
On a typical project, fiberglass batts are the lowest upfront cost, cellulose slightly higher, and spray foam the clear premium option per unit area. When lifetime energy savings and carbon are considered, many high-performance projects favour cellulose or hybrid assemblies over all-foam solutions.
Policy programmes are starting to move from paying for R-value alone toward rewarding measured performance: blower-door results, thermal-bridge detailing and whole-building heating demand. This shift tends to favour teams that understand building physics and are comfortable combining products intelligently.
Indicative embodied carbon index by insulation type
5. Case studies: real retrofit projects
Case Study A: 1960s bungalow retrofit – Minnesota, USA
- Scope: Attic and wall cavity insulation upgrade.
- Approach: Dense-pack cellulose in walls, blown fiberglass in attic to R-60.
- Results: Heating bills down 42%; blower-door test improved from 12 ACH50 to 4.5 ACH50.
- Lesson: Cellulose in walls + fiberglass in attic is a cost-effective, low-carbon combo for cold-climate retrofits.
Case Study B: Commercial office – London, UK
- Scope: External wall insulation (EWI) on 1970s concrete frame.
- Approach: 120 mm mineral wool boards with render finish.
- Results: U-value improved from 1.5 to 0.25 W/m˛K; heating demand cut 55%.
- Lesson: External insulation avoids thermal bridges and interior disruption, ideal for occupied buildings.
Case Study C: Industrial cold store – Sydney, Australia
- Scope: Roof and wall insulation for temperature-controlled warehouse.
- Approach: Closed-cell spray foam (100 mm) for air-tightness and vapour control.
- Results: Energy use for refrigeration down 28%; no condensation issues after 3 years.
- Lesson: Spray foam excels where air-tightness and vapour control are critical, despite higher cost.
6. Devil's advocate: insulation pitfalls
Installation quality: Even the best insulation fails if poorly installed. Gaps, compression and missing air-sealing are common in fiberglass and cellulose jobs.
Moisture traps: Spray foam can trap moisture if assemblies don't allow drying. Failures are expensive to fix and may require full removal.
Thermal bridges: Insulating cavities alone won't fix steel studs, concrete slabs or window frames that bypass the insulation layer.
Embodied carbon: Spray foam has 2–3× the embodied carbon of cellulose. Over-specifying foam increases upfront carbon for marginal energy gains.
Bottom line: Focus on whole-assembly performance, not just R-value. Air-sealing, thermal-bridge details and moisture management matter as much as insulation choice.
7. Outlook to 2030
2026–2027: Low-GWP blowing agents become standard for spray foam, reducing climate impact. Cellulose gains market share in high-performance retrofits.
2028–2030: Building codes tighten toward near-zero energy; hybrid assemblies (exterior + cavity insulation) become mainstream. Embodied carbon limits may enter codes in EU and some US states.
Wildcards: Aerogel-enhanced blankets could offer high R-value in thin profiles for space-constrained retrofits. Bio-based foams (mycelium, hemp) may reach commercial scale.
Projected insulation market share by type (illustrative)
Methodology Note
R-values, costs and carbon figures are illustrative composites based on Energy Solutions analysis of manufacturer data, retrofit programme results and building-science literature (2024–26). Actual values vary by product, installation quality and climate.
8. FAQ: picking the right insulation mix
Is spray foam always the "best" insulation?
No. It has high R per inch and air-sealing, but also higher cost and embodied carbon. In many walls and attics, careful air-sealing plus cellulose or fiberglass can deliver similar bills at lower environmental impact.
Where does cellulose shine?
Dense-pack cellulose performs well in retrofits of existing cavities and in high-performance walls that prioritise low-carbon materials and good moisture control.
What should homeowners ask installers?
Focus on whole assemblies: air-barrier strategy, vapour control, thermal bridges and blower-door targets—not just advertised R-values.
Can I combine different insulation types?
Yes, and it's often the best approach. For example, exterior mineral wool + interior cellulose balances thermal performance, drying potential and cost.
How important is air-sealing vs R-value?
Very. A well-air-sealed assembly with moderate R-value often outperforms a leaky assembly with high R-value, especially in windy or cold climates.
Is fiberglass still a good choice?
Yes, for cost-sensitive projects with good installation quality and a separate air-barrier layer. It's the most affordable option and performs well when detailed correctly.
What about moisture and mould risk?
All insulation types can fail if moisture isn't managed. Cellulose and fiberglass need vapour retarders in cold climates; spray foam must allow drying in at least one direction.
How do I verify installation quality?
Request a blower-door test (ACH50) and thermal imaging after installation. These reveal gaps, thermal bridges and air leaks that visual inspection misses.