Polyisocyanurate (polyiso) insulation is widely used in roofs, walls, and basements for its high thermal performance (typically R-6.0–R-6.5 per inch). But a persistent question lingers: Does its R-value degrade over time? The answer involves chemistry, temperature, and installation practices.
The Short Answer
Polyiso does not experience significant long-term R-value loss under normal conditions. However, two factors can cause initial, minimal reduction:
Thermal drift: A slight drop in R-value occurs as blowing agents (gases trapped in cells during manufacturing) diffuse out. This stabilizes after 1–2 years.
Temperature dependence: Polyiso’s R-value temporarily decreases when the mean temperature through the material falls below 40°F (4°C) .
Key Factors Affecting Polyiso’s R-Value
1. Temperature Sensitivity
- Below 40°F (4°C), polyiso’s R-value declines. For example:
- At 25°F (−4°C), R-6.0 polyiso may perform closer to R-5.5.
- At 0°F (−18°C), it may drop to R-5.0.
- Above 40°F, it delivers near-labeled R-value.
- Why? The insulation’s hydrocarbon blowing agents condense at low temperatures, reducing their thermal resistance .
2. Edge Sealing & Moisture Exposure
- Cut edges exposed to moisture (e.g., basement slabs without vapor barriers) can absorb water, lowering R-value. Foil facers mitigate this.
- Unfaced polyiso is vulnerable in damp conditions. Always seal edges in below-grade applications .
3. Aging (“Thermal Drift”)
- After installation, polyiso loses 2–5% R-value as blowing agents escape, stabilizing within 24 months.
- Long-term performance: Aged R-value remains 90–95% of the labeled value, comparable to XPS and EPS .
Polyiso vs. Other Insulations: R-Value Stability
Table: Comparing Insulation Performance Over Time
| Material | R-Value Per Inch | Cold-Temp Performance | Aging Impact (After 20 Years) |
|---|---|---|---|
| Polyiso | R-6.0–R-6.5 | Declines below 40°F | ~5% loss (stabilizes) |
| XPS | R-5.0 | Stable | 10–20% loss |
| EPS | R-3.6–R-4.4 | Stable | <5% loss |
- Polyiso vs. XPS: XPS maintains R-value better in cold but loses more long-term due to global warming potential (GWP) gases diffusing out .
- Polyiso vs. EPS: EPS is temperature-stable but offers lower R-value. Polyiso outperforms it in warm climates or interior applications.
Best Practices to Maximize Polyiso’s R-Value
- Multi-Layer Installation:
- Stagger joints between layers to reduce thermal bridging and air leaks. This also boosts durability .
- Use in Above-40°F Applications:
- Ideal for interior walls, roofs, and conditioned basements. Avoid below-grade exterior use in cold climates.
- Seal Edges & Use Vapor Barriers:
- Protect cut edges with foil tape or sealants. Install over a vapor barrier in basements .
- Pair with Cover Boards:
- High-density (HD) polyiso cover boards add compressive strength and maintain R-value in roofing systems .
The Verdict
Polyiso’s R-value does not meaningfully degrade over decades if installed correctly. Its initial thermal drift is minor (≤5%), and it stabilizes after 2 years. While cold temperatures reduce its short-term efficiency, it remains a top choice for:
- Roof replacements (using tapered or HD cover boards) .
- Above-grade walls and conditioned basements.
- Projects prioritizing sustainability (polyiso uses pentane, a low-GWP blowing agent).
For cold climates (<40°F), pair polyiso with complementary insulations (e.g., EPS) or increase thickness by 10–20% to offset temperature effects.
💡 Pro Tip: Always specify aged R-value (not initial) when calculating energy code compliance. Manufacturers provide this data upon request.
