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Optimizing Insulation for Your New Hampshire Home: Why Continuous Exterior Insulation Matters

  • Writer: John Barker
    John Barker
  • Jun 28
  • 5 min read
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New Hampshire’s winters are unforgiving, with freezing temperatures and high heating demands that test the energy efficiency and comfort of any home. For homeowners and builders striving to create durable, high-performance residences, choosing the right insulation strategy is crucial. This guide explores why continuous exterior insulation—comprising at least one-third of a wall’s total R-value—is essential to combat thermal bridging and condensation in New Hampshire’s Climate Zones 5 and 6. Drawing on the International Energy Conservation Code (IECC) and expertise from Building Science Corporation’s resources, Atlas Remodeling offers practical insights to help you build or renovate a home that stays warm, dry, and cost-effective.


New Hampshire’s Climate and Residential Code Requirements

New Hampshire falls within IECC Climate Zones 5 and 6, where cold winters demand robust insulation to meet energy codes and ensure home comfort. The 2009 IECC, adopted statewide, sets minimum R-values for wood-frame residential buildings, as shown below:

Component

Climate Zone 5 R-Value

Climate Zone 6 R-Value

Ceiling

38

49

Wood Frame Wall

20 or 13 + 5

20 or 13 + 5

Mass Wall

13/17

15/19

Floor

30

30

Basement Wall

10/13

15/19

Slab R-Value and Depth

10, 2 ft

10, 4 ft

Crawl Space Wall

10/13

10/13

For wood-frame walls, the IECC allows either R-20 continuous insulation or R-13 cavity insulation paired with R-5 continuous exterior insulation. Building Science Corporation’s article, BSI-126: Dirty Harry Does Insulation, recommends that at least one-third of the total wall R-value be exterior to the sheathing to optimize performance. For an R-20 wall, this means at least R-6.7 in exterior insulation, ensuring energy efficiency and moisture control. This aligns with the New Hampshire Public Utilities Commission’s energy code guidelines PUC Energy Codes.


Nominal vs. Effective R-Value: Why It Matters

R-value measures a material’s resistance to heat flow—higher values mean better insulation. The distinction between nominal and effective R-values is key to understanding a home’s true thermal performance:

  • Nominal R-Value: The rated thermal resistance of the insulation material alone, as tested in a lab. For example, R-19 fiberglass batts in a 2x6 wall cavity have a nominal R-value of 19, assuming perfect installation.

  • Effective R-Value: The actual thermal performance of the entire wall assembly, accounting for heat loss through studs, sheathing, and framing. In a 2x6 wall with R-19 fiberglass batts, wood studs (R-1.1 per inch) can reduce the effective R-value to R-15 or lower due to thermal bridging, as noted in Building Science Corporation’s BSI-126: Dirty Harry Does Insulation BSC.


In New Hampshire’s cold climate, homes without continuous exterior insulation are prone to condensation within walls. Cold studs create spots where warm, humid indoor air condenses, especially on drywall. This moisture, likely present in homes lacking exterior insulation, can irreversibly degrade fiberglass insulation by causing clumping or compression, further reducing the effective R-value. Building Science Corporation’s Guide to Insulating Sheathing explains that this degradation compromises thermal performance permanently BSC. Continuous exterior insulation keeps sheathing and studs warmer, preventing condensation and maintaining the effective R-value closer to the nominal value.


Why Continuous Exterior Insulation Is Essential

Continuous exterior insulation, using rigid foam boards like extruded polystyrene (XPS), expanded polystyrene (EPS), or polyisocyanurate (polyiso), creates a seamless thermal barrier over a home’s exterior sheathing. Unlike cavity insulation, which leaves framing exposed to heat loss, exterior insulation—when at least one-third of the total R-value—minimizes thermal bridging, boosts energy efficiency, and prevents moisture issues.


For basement walls, Building Science Corporation’s Information Sheet: Basement Insulation recommends non-water-sensitive insulation (e.g., 2 inches of XPS for R-10) placed exterior to the concrete, with vapor semi-impermeable materials (>0.1 perm, like XPS or foil-faced polyiso) on the exterior side of the concrete. This setup allows inward drying while preventing condensation by keeping the concrete warmer BSC. For above-grade walls, exterior insulation with sealed joints provides similar benefits, maintaining warmer sheathing and framing to enhance durability, as supported by industry insights Rubcorp.


Thermal Bridging: The Hidden Threat to Your Home

Thermal bridging occurs when conductive materials like wood studs bypass insulation, creating pathways for heat loss. In a 2x6 wall with R-19 fiberglass batts (nominal R-19), studs—comprising 15-20% of the wall—can reduce the effective R-value to R-15 or less. Without exterior insulation, studs conduct cold to the interior drywall, forming cold spots where condensation can occur, risking mold and damage to finishes. In New Hampshire homes without exterior insulation, this condensation likely has occurred, permanently reducing fiberglass insulation’s effectiveness, as detailed in Building Science Corporation’s Guide to Insulating Sheathing BSC.


By placing at least one-third of the R-value exterior to the sheathing (e.g., R-7 for an R-20 wall), continuous insulation covers studs and sheathing, minimizing thermal bridges and maintaining a uniform wall temperature. This ensures the effective R-value closely matches the nominal R-value, improving energy efficiency and reducing moisture risks.


Performance Comparison: Walls With vs. Without Exterior Insulation

Consider a 2x6 residential wall targeting a nominal R-20 in Climate Zone 5. The table below compares cavity insulation alone versus a system with exterior insulation meeting the one-third R-value rule:

Metric

Cavity Insulation Only (R-19)

Cavity + Exterior Insulation (R-13 + R-7)

Nominal R-Value

19

20

Effective R-Value

~15-16 (or lower with condensation)

~18-19

Thermal Bridging

Significant (studs uninsulated)

Minimal (studs insulated)

Energy Efficiency

Moderate (20-30% heat loss)

High (5-10% heat loss)

Condensation Risk

High (cold studs, likely past condensation)

Low (warmer studs and sheathing)

Adding R-7 exterior insulation (e.g., 1.4 inches of XPS at R-5/inch) significantly boosts the effective R-value, as confirmed by Green Attic Insulation. This keeps sheathing above the dew point, preventing condensation and protecting finishes, unlike cavity-only walls where moisture likely has degraded fiberglass insulation, per Building Science Corporation’s Information Sheet: Basement Insulation BSC.


Why Even High R-Value Walls Need Exterior Insulation

A 2x6 wall with R-21 fiberglass batts (nominal R-21) may seem sufficient, but without exterior insulation, thermal bridging through studs reduces the effective R-value to around R-15. In New Hampshire’s cold climate, these cold studs likely have caused condensation, permanently compromising fiberglass insulation’s performance through moisture-induced clumping. Adding R-7 exterior insulation (one-third of R-21) insulates the studs, keeping sheathing above 45°F to prevent moisture issues, as recommended by Building Science Corporation’s Guide to Insulating Sheathing BSC. Materials like polyiso (R-6/inch) or XPS (R-5/inch) are durable and effective choices Rubcorp.


Conclusion

For New Hampshire homes, continuous exterior insulation—making up at least one-third of the wall’s R-value—is a must for energy efficiency and durability. By tackling thermal bridging and preventing condensation, this strategy aligns with IECC standards and Building Science Corporation’s expertise, ensuring your home stays warm, dry, and cost-effective. Atlas Remodeling is committed to helping you build or renovate with these advanced solutions. Text (603) 688-6096 or email us at atlasremodelingnh@gmail.com to book an assessment now! We provide a comprehensive analysis on what order you should upgrade/fix your house in to get the most out of your investment long term!


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