Fiberglass vs Cellulose Insulation
Fiberglass vs Cellulose Insulation
Fiberglass and cellulose both work in blown attic applications, but cellulose is usually discussed for dense coverage while fiberglass depends heavily on depth control and wind washing protection.
Attic retrofit
Both can top up existing attic insulation, but the settled depth and ventilation path need to be checked first.
Blown-in behavior
Cellulose can settle; blown fiberglass can lose performance if air movement crosses the layer.
Example thickness
At R-3.5 per inch, R-38 needs about 10.9 inches; at R-3.2 per inch, it needs about 11.9 inches.
How to use this page
The insulation comparison page is best used to narrow options, not to make an automatic design decision. Start with a realistic thickness, compare lambda, R-value, and installation limits, then check shortlisted materials against current data sheets.
The result must be read together with the application. A material that works well on an external wall is not automatically the best choice below a screed, on a flat roof, or in timber construction. Moisture, load, fire behaviour, and fixing method can outweigh a small lambda difference.
When two materials look thermally similar, look at system availability, tolerances, and whether the insulation layer can be made continuous. A slightly weaker material installed cleanly can perform better than a theoretically stronger option with gaps or thermal bridges.
After comparing materials, move to the U-value calculator and test the complete assembly. The comparison table shows material differences, but the full build-up reveals the effect of render, cladding, air spaces, and surface resistances.
If two results are close, do not choose from one number alone. Check moisture behaviour, fire performance, available board formats, system requirements, and whether the layer can be installed continuously without open joints.
For renovation work, also check substrate condition, condensation risk, and details around windows, ring beams, and balconies. A strong lambda value will not fix a layer interrupted at the most important junctions.
The final step should be comparison with the current manufacturer data sheet. The database helps narrow the direction, but the documentation for the exact product matters most for ordering and construction.
In practice, make a short decision table: thermal result, installation difficulty, moisture behaviour, fire behaviour, acoustics, and system availability. That reduces the risk of choosing a material only because one number looks best.
If the comparison is used in a supplier discussion, record the data-check date and assumed thickness. It becomes easier to separate a price or product change from a real change in thermal performance.
A useful comparison includes at least three variants: current state, economical option, and option with margin. That shows whether extra thickness still delivers a meaningful improvement.
If the comparison is tied to legal requirements, do not rely on an internet average. Use the local threshold, the correct calculation method, and data for the product that will actually be installed.
Settling and air leakage checks
For attic retrofits, material choice is only one part of the decision. Air sealing, baffles, safe clearances, and installed density can matter as much as nominal R per inch.
- Do not cover blocked soffit ventilation or recessed fixtures without checking required clearances.
- Measure current settled depth before calculating the added R-value.
- Use the attic calculator for an added-R estimate, then confirm product coverage charts.
- Use Fiberglass vs Cellulose Insulation as a screening page: set the target R-value, compare typical per-inch values, then check a declared product.
- If two options look close thermally, prioritize fit, moisture control, fire covering, access, and the full assembly before choosing.
- Keep manufacturer data, local code review, and installer documentation with the calculation so later changes can be checked.
The comparison narrows the choice, but the result still depends on declared lambda, thickness, moisture, and installation quality. Before making a decision, check the calculation method, definitions, and current material table. methodology, glossary, lambda table.