FHB Logo Facebook LinkedIn Email Pinterest Twitter Instagram Plus Icon Close Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Main Menu
Subscribe

In every issue you'll find...

  • Expert insights on techniques and principles
  • Unbiased tool reviews
  • Step-by-step details to master the job
  • Field-tested advice and know-how
Subscribe Now!
Subscribe
  • How-To
  • Design
  • Tools & Materials
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
  • Join
  • Log In
Main Menu Subscribe
How it Works

How it Works: Vapor Drive

To understand the importance of permeable and impermeable products as they relate to the construction of your home, you need to understand how vapor drive works

By Rob Yagid Issue 204
Article Image
A hidden nightmare. If unimpeded in certain climates, water vapor can travel through a wall assembly toward the cold side of a house. When that vapor comes in contact with a cold surface, it can condense into a liquid and become a threat to the health and durability of the structure.

When considering building materials, you hear a lot about their permeance. This issue’s article on spray-foam insulation, “Spray Foam: What Do You Really Know?”, is no exception. To understand the importance of permeable and impermeable products as they relate to the construction of your home, you need to understand vapor drive. Here’s how it works.

Seasonal factors play a part

Water vapor constantly diffuses through building materials from the warm, humid side of a house toward the cold, dry side. The cause of this movement is heat and moisture. So in the summer, when a lot of homes are air-conditioned, vapor is driven toward the interior. In the winter, vapor drive is toward the exterior. Vapor drive is the least significant cause of moisture problems in a house, but it’s still really important. When water vapor passes through an assembly and comes in contact with a surface that has a temperature below the dew point (the temperature at which water vapor condenses), then it becomes a wood-rotting, mold-feeding liquid.

There are three ways to reduce condensation within an assembly. By placing a vapor retarder on the warm side of the house, the rate of vapor infiltration is decreased. Using relatively vapor-permeable materials on the cold side of the house helps to allow any vapor in the assembly to dry. Finally, the temperature of building materials can be kept above the dew point by adding a layer of exterior rigid-foam insulation or by filling the cavity with closed-cell spray foam. Closed-cell foam performs well because it’s semi-impermeable, so vapor is always contained to the warm side of the wall.

Climate dictates solutions

Until 2007, the International Residential Code (IRC) treated the country like a single cold climate and offered only one solution to vapor drive. The code required that a vapor retarder be installed on the interior (warm) side of assemblies. However, most of the country has both heating and cooling seasons, so sometimes the cold side is the inside of the wall or roof instead of the outside. The old codes created a risky situation that could cause problems in the summer, especially if the local code official insisted that a vapor retarder meant a polyethylene sheet. When you fill a wall with a highly vapor-permeable insulation (fiberglass batts) and cover one side of it with a virtually nonpermeable vapor retarder (poly), the vapor retarder will be on the wrong side of the assembly for part of the year and inhibit the wall’s ability to dry.

The IRC now breaks the country into eight climate zones and recognizes three classes of vapor retarders that have different levels of permeance (see “What’s the Difference?” FHB #202). Generally, the IRC demands that a class-I or -II vapor retarder be installed on the interior side of homes in climate zones 5 and above, and in marine 4. However, if you’re building in a humid climate in zone 4, 5, or 6, and you air-condition your house in the summer, you may be concerned about having a vapor retarder in the “wrong” position for part of the year. If this is the case, just be sure to use a class-II vapor retarder on the interior of the wall. You also can use closed-cell spray foam in the cavity or a layer of exterior rigid foam, with a class-III vapor retarder on the interior.

When building in hot, humid climates (zones 1 to 3), you shouldn’t have a vapor retarder on the interior side of the wall. This allows any water vapor that makes its way into the wall, which can be tempered by closed-cell foam or exterior rigid foam, to dry to the interior.

Drawings by: Don Mannes

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Sign Up

Get home building tips, offers, and expert advice in your inbox

×
X
X

New Feature

Fine Homebuilding Forums

Ask questions, offer advice, and share your work

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

More How it Works

View All
  • how it works wood beams

    How it Works: Simple Wood Beams

  • electrical breaker panel

    Electrical Breaker Panels

  • Ice Dams

  • Drain-Waste-Vent Systems

View All

Up Next

Featured Story

FHB Podcast 334: Becoming a Builder, Leaky Apartments and Complexities of a Custom Home

Podcast 334: Becoming a Builder, Leaky Apartments, and Complexities of a Custom Home

Rob, Kiley, and Patrick hear from listeners about going all electric and not messing up historic houses before taking listener questions about becoming a high-performance builder, fixing a leaky apartment, and planning a new home.

Featured Video

Micro-Adjust Deck-Baluster Spacing for an Eye-Deceiving Layout

No math, no measuring—just a simple jig made from an elastic band is all you need to lay out a good-looking deck railing.

Related Stories

  • More New Products at the Virtual International Builders’ Show

  • Protecting Foundations From Frost

  • Wet-Room Drainage

  • Make a Copper Flashing Boot

Discussion Forum

Recent Posts and Replies

  • |
  • |
  • |
  • |
  • |
  • |
View More Create Post

Highlights

  • Fine Homebuilding All Access
  • Fine Homebuilding Podcast
  • Tool Tech

    Plus, get an extra 20% off with code GIFT20

  • "I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.

    Get home building tips, offers, and expert advice in your inbox

    Sign Up See all newsletters

Video

View All
  • carpenter installing ICF foundation forms

    Benefits of Building With ICFs

  • 4 types of wall assemblies

    2nd Annual Wall-Assembly Contest Kickoff

  • FHB House ep5

    Double-Stud Walls and Insulated Window Bucks

  • Podcast 291: Overdriven Nails, Indoor-Air Quality, and What is This Attic Insulation?

View All

Water and Moisture Control

View All
  • Installing ZIP System Sheathing: Penetrations

    Installing ZIP System Sheathing: Sealing Penetrations

  • Installing ZIP System Sheathing: Sheathing to Foundation Connection

    Installing ZIP System Sheathing: Sheathing-to-Foundation Connection

  • Installing ZIP System Sheathing: Taping Seams and Corners

    Installing ZIP System Sheathing: Taping Seams and Corners

  • Why We Need Building Sensors

View All

BOOKS, DVDs, & MERCH

Shop the Store
  • Code Check Building 4th Edition

    Buy Now
  • Fine Homebuilding Magazine Slipcase

    Buy Now
  • 2020 Fine Homebuilding Archive

    Buy Now
  • Musings of an Energy Nerd

    Buy Now
  • Shop the Store

Fine Homebuilding Magazine

  • Issue 297 - Feb/Mar 2021

    • Foundations for Success
    • Staircase Renovation
    • Ditch the Hose
  • Issue 296 - Dec 2020/Jan 2021

    • Insulating Glass Keeps Getting Better
    • Simple Way to Make Old Walls Straight and Plumb
    • Making Sense of Minisplits
  • Issue 295 - Kitchens & Baths

    • Condo Kitchen Reimagined
    • Row-House Remodel
    • Rise of the IAQ Monitor
  • Issue 294 - Oct/Nov 2020

    • Schoolhouse Reimagined
    • Tool Test: Folding Sawhorses
    • A Better Way to Build Tall Walls
  • Issue 293 - Aug/Sept 2020

    • A Practical Guide to Fall Protection
    • Installing Frameless Cabinets
    • Make Any Tablesaw Safer

Fine Homebuilding

Follow

Newsletter

Get home building tips, offers, and expert advice in your inbox

Sign Up See all newsletters

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Manage Preferences

Taunton Network

  • Green Building Advisor
  • Fine Woodworking
  • Fine Gardening
  • Threads
  • About
  • Contact
  • Advertise
  • Privacy Policy
  • Careers
  • Copyright
  • Terms of Use
  • Accessibility
  • CA Privacy Rights

© 2021 The Taunton Press, Inc. All rights reserved.

    Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Reader Projects
  • Magazine
  • Members
  • FHB House
  • Podcasts

  • FHB Podcast
  • ProTalk
  • Account

  • Log In
  • Join

    Podcasts

  • FHB Podcast
  • ProTalk
  • Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

    Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

    All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

    Shop the Store

  • Books
  • DVDs
  • Taunton Workshops

    More

  • FHB Ambassadors
  • Reader Projects
  • Podcast
  • Customer Support

    Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Sign Up See all newsletters

Follow

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 52%

Subscribe

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in