Built up roofs (BUR) are a common choice for commercial and industrial buildings in the United States due to their durability and long service life. The R value of a BUR system does not come from the asphalt or felts alone; it primarily depends on the insulation layer beneath and how the assembly is designed. This article explains how R value applies to built up roofs, how to calculate it, and practical steps to optimize energy efficiency in BUR installations.
What Is A Built Up Roof And Why The R Value Matters
A built up roof combines multiple plies of bitumen with reinforcing fabrics, typically topped by gravel or a membrane. While the BUR layers provide weather resistance and durability, the insulating performance is largely governed by the underlying or added insulation. The R value measures thermal resistance; higher R values indicate better resistance to heat flow. For building owners, the R value of a BUR assembly influences heating and cooling costs, indoor comfort, and code compliance.
How R Value Is Calculated In BUR Assemblies
R value is additive. In a BUR system, the total R value equals the sum of the insulation R value plus any other components that contribute to thermal resistance. The bituminous layers and roof deck contribute minimal R value on their own, so the insulation layer is the critical factor. When estimating R value for a BUR assembly, consider:
- Insulation Type: Polyiso (polyisocyanurate), EPS (expanded polystyrene), or XPS (extruded polystyrene) each carries a different R per inch. Polyiso typically offers higher R per inch than other common options.
- Insulation Thickness: The total inches of insulation directly raise the assembly’s R value; increasing thickness yields greater thermal resistance.
- Additional Layers: Any radiant barriers, reflective foils, or edge insulation can modify the effective R value, particularly in certain climate zones.
- Installation Quality: Gaps, compression, or moisture can reduce actual performance versus labeled R values.
Typical R Values By Insulation Type Used Under BUR
Because BUR performance hinges on the insulation beneath, building professionals often specify R values based on climate zone and energy targets. General ranges include:
- Polyiso (Polyiso Board): Approximately R-6.5 to R-7 per inch. Common thicknesses range from 2 to 6 inches, yielding R-13 to R-42.
- EPS (Expanded Polystyrene): Roughly R-3.6 to R-4 per inch. Typical thicknesses can give R-12 to R-30.
- XPS (Extruded Polystyrene): About R-5 per inch. Used in precise assemblies to reach target R values with moderate thickness.
- Other Insulations (less common under BUR): Mineral wool and spray foams add their own R values per inch, with spray foams often contributing higher R per inch than boardstock materials.
Note: The U.S. energy codes (IECC) set minimum R values by climate zone for roof assemblies. In practice, many commercial BUR installations exceed minimums to improve energy performance, reduce peak loads, and support long-term operating costs.
Calculating Your BUR Assembly’s R Value
To estimate R value for a BUR system, follow this simplified approach:
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- Identify the insulation material and its thickness in inches.
- Look up the insulation’s labeled R per inch from the manufacturer.
- Multiply insulation R per inch by the insulation thickness to get the insulation’s R value.
- Add any significant radiant barrier or reflective components if present and rated for R value.
- Recognize that the bitumen and plies contribute a small additional R value, often negligible compared to insulation.
Example: A BUR roof with 4 inches of polyiso insulation (R-7 per inch) yields R = 4 × 7 = R-28. If a reflective mid-layer adds an estimated R of 0.5, the total might be around R-28.5, with the insulation driving most of the performance.
Design Considerations To Maximize R Value In BUR Installations
Maximizing R value for BUR systems involves selecting appropriate insulation, thickness, and accessory features. Consider these strategies:
- Climate Zone Targeting: Align insulation thickness with local IECC requirements and energy goals. Warmer climates may prioritize moisture management alongside moderate insulation, while colder regions demand higher R values.
- Insulation Type Selection: In many cases, higher R per inch materials like polyiso provide better performance in thinner assemblies, saving space and labor costs.
- Air and Vapor Management: Ensure proper air barrier placement and vapor control to maintain insulation effectiveness and prevent condensation, which can lower real-world R.
- Installation Quality: Use continuous insulation where possible and minimize gaps at perimeters and penetrations to prevent thermal bridging.
- Maintenance And Monitoring: Regular inspections for moisture intrusion or insulation degradation help preserve R value over time.
Code Implications And Practical Guidance
Energy codes like IECC set minimum roof insulation values by climate zone and often influence BUR design decisions. Some guidance for practitioners includes:
- Consult the local building code to determine the required roof insulation R value for the project’s climate zone.
- Document insulation type, thickness, and installation details to validate compliance and aid future retrofits.
- Consider long-term operational costs alongside upfront costs; a higher R value typically reduces heating and cooling expenses over the roof’s life.
Maintenance, Retrofits, And Life-Cycle Considerations
As buildings age, BUR systems may undergo retrofits to improve energy performance. Practical retrofits include upgrading insulation thickness, replacing aging BUR with higher-performance assemblies, or adding film or radiant barriers where compatible. Before retrofitting, assess:
- Current roof condition and moisture levels that could affect insulation performance.
- Existing membrane compatibility with new insulation or overlays.
- Cost-benefit analysis of insulation upgrades versus energy savings over time.
Key takeaway: For BUR systems, R value primarily comes from the insulation layer. Selecting the right insulation type and thickness, while ensuring proper installation and moisture control, yields the most reliable energy performance gains.