Why Vapor-Permeable Air Barriers Are Essential for New Hampshire Homes
- John Barker
- Jun 15
- 3 min read
Living in New Hampshire’s cold, humid climate (Climate Zones 5 or 6) means your home’s walls must handle harsh winters and humid summers while staying energy-efficient and moisture-free. According to BuildingScience.com, using vapor-permeable air barriers on both sides of your wall assembly is key to preventing mold, rot, and energy loss. This article explains why this approach, paired with required continuous exterior insulation, is ideal for New Hampshire homeowners, offering a durable, code-compliant solution.
Understanding Moisture in New Hampshire Homes
In New Hampshire, moisture moves through walls differently by season:
Winter: Warm, humid indoor air (from showers, cooking, or breathing) tries to escape to the cold, dry outdoors, risking condensation inside walls if not managed properly.
Summer: Humid outdoor air can move inward, especially in air-conditioned homes, potentially trapping moisture if walls can’t “breathe.”
Moisture trapped in walls can lead to mold, wood rot, and costly repairs. Our #1 source for building science literature, BuildingScience.com emphasizes that walls must dry out if they get wet—from rain, leaks, or indoor humidity—rather than just blocking moisture entirely, as no wall is perfectly waterproof.
The Role of Vapor-Permeable Air Barriers
An air barrier stops air leaks, which carry far more moisture than vapor diffusion (think of air leaks as a highway for moisture, diffusion as a slow trail). A vapor-permeable air barrier does this while allowing water vapor to pass through, letting walls dry out. In New Hampshire, placing these barriers on both sides of the wall ensures moisture can escape inward or outward, depending on the season.
Why Both Sides?
Exterior Air Barrier: A vapor-permeable housewrap (like Tyvek, with >10 perms) stops wind-driven air and rain from entering while letting moisture in the wall escape outward. This is crucial in New Hampshire, where rain and snow can wet walls, especially behind brick or siding.
Interior Air Barrier: Sealed drywall with latex paint stops humid indoor air from leaking into walls during winter, while its permeability (~10 perms) allows drying inward during summer, preventing moisture buildup in air-conditioned homes.
Using impermeable materials (like plastic sheeting or vinyl wallpaper) on either side can trap moisture, leading to damage. BuildingScience.com warns against “double vapor barriers” and recommends breathable materials for cold climates like New Hampshire’s.
Continuous Exterior Insulation: A Must for Energy and Moisture Control
The 2021 International Energy Conservation Code (IECC) requires continuous exterior insulation (R-5 in Zone 5, R-10 in Zone 6) for New Hampshire homes to boost energy efficiency by reducing heat loss through studs. This insulation also keeps the wall’s sheathing warmer in winter, preventing condensation where humid indoor air meets cold surfaces.
For best results, choose vapor-permeable insulation like mineral wool (e.g., Rockwool, ~30 perms). It allows drying to the exterior, complementing the housewrap, unlike low-permeance foams (e.g., XPS) that can limit drying. This setup meets IECC standards while enhancing wall durability.
A Recommended Wall Assembly for New Hampshire
Here we have outlined a practical, code-compliant wall design for New Hampshire homes, based on BuildingScience.com’s principles:
Exterior Cladding: Wood, vinyl siding, or brick with a 1/4-inch air gap to promote drying.
Exterior Air Barrier: Vapor-permeable housewrap (e.g., Tyvek, >10 perms), taped for air and water tightness.
Continuous Insulation: Mineral wool (R-5 or R-10), highly permeable for drying.
Sheathing: Plywood or OSB (~10–15 perms).
Cavity Insulation: Unfaced fiberglass or cellulose (R-20 or R-21).
Interior Air Barrier: Sealed drywall, caulked at seams, to stop air leaks.
Interior Finish: Latex paint (~10 perms) for breathability.
This assembly:
Meets IECC energy and air barrier requirements.
Stops air leaks that carry moisture.
Allows drying to both sides, preventing mold.
Handles New Hampshire’s seasonal moisture flows.
Email us at AtlasRemodelingNH@gmail.com or text the owner John Barker at (603)-688-6096 to book an estimate/assessment for the purpose of upgrading your home for the sake of health and durability! Sources: BuildingScience.com (BSD-106, RR-0004, BSI-073, Info-311); 2021 IECC and IRC.
