FHB Logo Facebook LinkedIn Email Pinterest Twitter X Instagram Tiktok YouTube Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Restoration
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In
How-To

Can exterior foam be added to a wall that has an interior vapor barrier?

By Martin Holladay Issue 245 - Aug/Sept 2014

Q:

I need to remove and replace a flanged window on my house in southern Kentucky. The 2×4 walls are sheathed with OSB and have a plastic vapor barrier on the interior, with fiberglass batts in the cavities. Since I have to pull apart that section of the house to replace the window, I thought it might be a good chance to boost my insulation by adding a 1/2-in. layer of rigid foam over the sheathing before reinstalling the siding and then continuing that approach on the other sides of the house as time allows. My fear is that adding foam to the outside and having the vapor barrier on the inside might trap moisture in the wall cavity. Is that a concern?

Don Gilbert, via GreenBuildingAdvisor.com, None

A:

Yes, you can put rigid foam on the exterior of your walls. Fortunately, in your mild climate zone, you don’t have to worry about possible moisture accumulation in your walls as a result of using such thin foam. Further north, you would certainly need thicker foam to keep your wall sheathing above the dew point during the winter. The main disadvantage of choosing such thin foam in your climate zone is poor energy performance; you would get much better performance if you chose foam that’s 1-1/2 in. or 2 in. thick.

Although it would be better if the poly weren’t there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven’t been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits aren’t as risky as some people fear, and he believes that these homes likely will be fine.

That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold until the causes of the problems are identified and fixed. If you plan to install exterior foam during a siding-replacement job, keep an eye out for evidence of moisture problems when stripping the old siding. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

If there is any sheathing rot, determine the cause—it’s commonly a flashing problem, but condensation of interior moisture is not impossible—and correct the problem.

Adding a rainscreen gap between the new rigid foam and the siding will go a long way toward avoiding future moisture problems in your walls. Of course, it’s important to be meticulous with your details when you install your new water-resistive barrier (WRB) and window flashing.

Once your retrofit work is complete, it’s also important to keep interior relative humidity within reasonable levels (below 35% or 40%) during the winter. Never use a humidifier.

It’s OK to put foam over your wall sheathing, even if you have an interior vapor barrier, but keep these four things in mind:

  1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. (Read more on this topic at FineHomebuilding.com/ extras.)
  2. As the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.
  3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.
  4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

Fine Homebuilding Recommended Products

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

Staple Gun

Although powered staplers are very popular, you still need a manual stapler on the job site. This staple gun from DEWALT® is reliable and easy to fire, reducing fatigue over prolonged use.
Buy at Amazon

Disposable Suit

This suit provides full body protection to help you stay clean from head to toe when installing insulation. We recommend wearing sweats underneath for additional coverage.
Buy at Amazon

Great Stuff Foam Cleaner

It’s important to clean and maintain a professional foam-dispensing gun, and this product fits the bill. It has a spray applicator for general cleaning, plus it screws onto the dispensing gun so you can clean the inside.
Buy at Amazon

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

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters
×

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

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

New Feature

Fine Homebuilding Forums

Ask questions, offer advice, and share your work

View Comments

  1. jhastings | Mar 20, 2018 09:58pm | #1

    I would like to add 2 layers of rigid foam Owens Corning foamular 250 which I understand compresses very little, but should I be concerned that my LP siding could slump? Has anyone experienced slippage between the 2 layers of rigid foam?....thanks,

  2. Paul_V | Jun 26, 2019 10:46am | #2

    Wouldn't putting a rain screen between the foam and the sheathing somewhat defeat the purpose of the foam by letting air circulate in the gap?

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Up Next

Video Shorts

Featured Story

Fast, Accurate Wall Framing

A rear addition provides a small-scale example of how to frame efficiently.

Featured Video

Video: Build a Fireplace, Brick by Brick

Watch mason Mike Mehaffey construct a traditional-style fireplace that burns well and meets current building codes.

Related Stories

  • Install Denim Insulation Like a Pro
  • Insulation for Homes in the Wildland Urban Interface
  • A Closer Look at Smart Water-Leak Detection Systems
  • FHB Podcast Segment: Sometimes Spray Foam Makes Sense, Sometimes It Doesn't

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

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Video

View All Videos
  • Install Denim Insulation Like a Pro
  • Insulation for Homes in the Wildland Urban Interface
  • FHB Podcast Segment: Sometimes Spray Foam Makes Sense, Sometimes It Doesn't
  • FHB Podcast Segment: Designing and Building an Efficient Slab-On-Grade Home
View All

Water and Moisture Control

View All Water and Moisture Control Articles
  • A Closer Look at Smart Water-Leak Detection Systems
  • Simple Self-Adhered Protection
  • Prevent WRB Tears at Corners
View All Water and Moisture Control Articles

Fine Homebuilding Magazine

  • Issue 332 - July 2025
    • Custom Built-ins With Job-Site Tools
    • Fight House Fires Through Design
    • Making the Move to Multifamily
  • Issue 331 - June 2025
    • A More Resilient Roof
    • Tool Test: You Need a Drywall Sander
    • Ducted vs. Ductless Heat Pumps
  • Issue 330 - April/May 2025
    • Deck Details for Durability
    • FAQs on HPWHs
    • 10 Tips for a Long-Lasting Paint Job
  • Issue 329 - Feb/Mar 2025
    • Smart Foundation for a Small Addition
    • A Kominka Comes West
    • Making Small Kitchens Work
  • Issue 328 - Dec/Jan 2025
    • How a Pro Replaces Columns
    • Passive House 3.0
    • Tool Test: Compact Line Lasers

Fine Home Building

Newsletter Sign-up

  • Fine Homebuilding

    Home building tips, offers, and expert advice in your inbox.

  • Green Building Advisor

    Building science and energy efficiency advice, plus special offers, in your inbox.

  • Old House Journal

    Repair, renovation, and restoration tips, plus special offers, in your inbox.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters

Follow

  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X
  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences
  • About
  • Contact
  • Advertise
  • Careers
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2025 Active Interest Media. All rights reserved.

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

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

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

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

Online Learning

  • Courses
  • Project Guides
  • Reader Projects
  • Podcast

More

  • FHB Ambassadors
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

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

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • X
  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok

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 70%

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