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

Insulating a floor over a crawlspace

By Marc Rosenbaum Issue 102
  • X
  • facebook
  • linkedin
  • pinterest
  • email
  • add to favorites Log in or Sign up to save your favorite articles

Q:

My 67-year-old farmhouse has pine flooring with no subfloor on 2×6 joists over an open crawlspace. The wood all appears to be in excellent condition, kept dry no doubt by cold winter winds. What would be the best way to insulate under the floors? Would a perforated radiant barrier work well in this situation? Is fiberglass my only option, and if so, how can I keep the rodents from nesting in it?

Isabelle Burns, Brenham, TX

A:

Marc Rosenbaum of Energysmiths in Meriden, New Hampshire, replies: Your situation seems to call first for a good air barrier to prevent those winter winds from blowing through the pine flooring. A perforated radiant barrier is probably not the best choice because its insulating value depends on the foil surface remaining shiny. Over time, the foil surface facing up would become covered with dirt sifting through the floor boards. Plus, aluminum surfaces tend to oxidize, reducing the shininess that is critical to maintaining radiant- barrier performance.

However, there are a couple of better ways to insulate your floors. First, you can fill the floor-joist cavity with unfaced, friction-fit fiberglass batts and then staple housewrap to the underside of the floor joists. The fiberglass provides the insulation (and in your cooling climate, R-19 is more than adequate), and the housewrap provides the air barrier. Seal the housewrap at the sill and at all joints with an approved tape, such as 3M 8086 Contractor’s Tape. It helps to make all of your housewrap joints fall on joists so that you can put adequate pressure on the tape to ensure good long-term adhesion. For the best job, seal all penetrations through the floor, such as wiring and plumbing holes, with caulk or expanding foam before you insulate. The bad news is that the housewrap won’t do much to keep those pesky rodents out of the fiberglass insulation.

The second, less rodent-friendly method involves insulating and air-sealing the floor-joist cavity with rigid-foam insulation board. Installing the foam against the flooring will maximize the airtightness of the installation. If the framing is irregular or twisted, cut the foam board slightly smaller than the cavity width and foam it in place with a low-expansion urethane foam. If your framing is straight and fairly regular, cut the foam slightly larger that the width of the joist bay and wedge it into place.

In your situation, I think that a vapor barrier is unnecessary. But if the only foam board you have available has an aluminum-foil skin on one side, install it with the foil facing down to put the vapor barrier in the correct location for your cooling climate.

I would try for an R-value of 10 to 12 for the crawlspace in your house, which would be 1 in. to 1-1/2 in. of polyisocyanurate foam; 2 in. to 2-1/2 in. of extruded polystyrene foam; or 2-1/2 in. to 3 in. of expanded polystyrene foam. In some areas, local code may not allow foam insulation because of fire hazard. However, because the crawlspace is not habitable, I think you’ll be okay with this strategy.

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.

Respirator Mask

A full-face respirator prevents fog up and is more comfortable than constantly replacing particle makes that get fouled or sweaty and is ideal for the pro installer.
Buy at Amazon

Caulking Gun

It's important for any insulator to have a good caulking gun for air sealing or adhesives, and this one features a revolving frame that makes orienting the tip a breeze.
Buy at Amazon

Insulation Knife

If you like insulating with mineral wool, this knife makes cuts that are clean, neat, and easy — it's much better than a utility knife which can be slow and messy.
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

  • X
  • facebook
  • linkedin
  • pinterest
  • email
  • add to favorites Log in or Sign up to save your favorite articles

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

FHB Podcast Segment: Repairing an Old Home While Maintaining Its Integrity

Learn about different approaches to making updates and repairs to older homes with historic charm.

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

  • A Practical Approach to Exterior Insulation
  • FHB Podcast Segment: Designing and Building an Efficient Slab-On-Grade Home
  • Makeshift Foam Tip for Tight Spots
  • Insulation Shield for Wafer Lights

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
  • FHB Podcast Segment: Designing and Building an Efficient Slab-On-Grade Home
  • FHB Podcast Segment: Insulating and Air-Sealing Behind a Masonry Wall in a Remodel
  • FHB Podcast Segment: Insulating the Roof of a Small Balloon-Framed House
  • FHB Podcast Segment: Old Home Insulation Troubles - How to Find the Right Contractor
View All

Insulation

View All Insulation Articles
  • FHB Podcast Segment: Designing and Building an Efficient Slab-On-Grade Home
  • Ranch Redux
  • Save Batt Bags for Reuse
View All Insulation Articles

Fine Homebuilding Magazine

  • 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 2024
    • How a Pro Replaces Columns
    • Passive House 3.0
    • Tool Test: Compact Line Lasers
  • Issue 327 - November 2024
    • Repairing Damaged Walls and Ceilings
    • Plumbing Protection
    • Talking Shop

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.

  • Home Group
  • Antique Trader
  • Arts & Crafts Homes
  • Bank Note Reporter
  • Cabin Life
  • Cuisine at Home
  • Fine Gardening
  • Fine Woodworking
  • Green Building Advisor
  • Garden Gate
  • Horticulture
  • Keep Craft Alive
  • Log Home Living
  • Military Trader/Vehicles
  • Numismatic News
  • Numismaster
  • Old Cars Weekly
  • Old House Journal
  • Period Homes
  • Popular Woodworking
  • Script
  • ShopNotes
  • Sports Collectors Digest
  • Threads
  • Timber Home Living
  • Traditional Building
  • Woodsmith
  • World Coin News
  • Writer's Digest
Active Interest Media logo
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 81%

Subscribe

You have 1 free article remaining.

Get complete site access, including thousands of videos, how-to tips, tool reviews, and design features.

Start your FREE trial

Already a member? Log in

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

Privacy Policy Update

We use cookies, pixels, script and other tracking technologies to analyze and improve our service, to improve and personalize content, and for advertising to you. We also share information about your use of our site with third-party social media, advertising and analytics partners. You can view our Privacy Policy here and our Terms of Use here.

Cookies

Analytics

These cookies help us track site metrics to improve our sites and provide a better user experience.

Advertising/Social Media

These cookies are used to serve advertisements aligned with your interests.

Essential

These cookies are required to provide basic functions like page navigation and access to secure areas of the website.

Delete My Data

Delete all cookies and associated data