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

Discussion Forum

Discussion Forum

Heard of Performance based vs prescriptive based insulation standards for a foam roof job?

Kimball_house | Posted in Energy, Heating & Insulation on September 19, 2010 05:04am

I received 4 bids for a spray foam insulation job in our 3rd floor renovation (a bedroom and bathroom and attic area).  Closed cell foam is to be applied to the gable roof surfaces and vertical walls.  I think code is R30 roof and R19 for the walls.  Some of the rafter joists are 2 x 6 so I thought foam would be a much better solution than a vented roof assembly with fiberglass.

One bid came in with 2.5″ of CC foam on the roof surfaces.  The other bids specify 4 inches of CC foam. on the roof.  The rationale for the bid with shallower fill was that one can use a “performance based standard” rather than a “prescriptive based standard”.  As CC foam is ~R7 / inch a 4″ fill satisfies the R30 code requirement.  The “performance based standard” is based on the claim that foam at 2.5″ provides vapor barrier, complete sealing, and sufficient insulation to make the roof perform at R30.    The claim is that in the real world the 2.5″ foam would make the roof perform as good as a roof insulated to R30 with fiberglass batts (which is probably not R30 in practice, I suppose).  The shallower fill is ~20% less expensive than some other bids.

I’m not familiar with performance based standards.  How would one really document this, by measuring heat flux through real world roofs using thermal imaging?  Can anyone explain that the shallow fill foam solution is a sound idea?  It seems logical that thicker foam will have greater performance.  At some point more foam becomes a poor use of resources.  I’m inclined to have 4 inches put in. 

Curiously, the bids vary in cost by 100%, even though the work is clearly specified.  No one is recommending additional work that would in any way explain the difference.  I don’t think I’ve ever seen bids that vary this much. 

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

Replies

  1. davidmeiland | Sep 19, 2010 05:46pm | #1

    The 2-1/2" guy

    is trying to give you a justification to go along with the lower price he gave you, which got your attention. While I wouldn't buy what he's selling, he is pointing out one thing that you already know, which is that in a real-world situation, his foam might outperform much thicker fiberglass batts in a vented roof, where wind-washing could reduce the value of the FG to a very low level. But, his 2-1/2" will max out at R17 or thereabouts, and I wouldn't be be able to sell that to the inspector here as R30 no matter what. At 4" you are probably close to R30 but not quite there. Why not have them fill the 5-1/2" rafters full and do similar in other areas? It'll never be as cheap as it is right now to get a well-insulated job.

  2. 50_Pascals | Sep 19, 2010 11:39pm | #2

    It's B.S.

    "Performance based" codes typically apply to new construction or substantial renovation.

    With that said, 2 1/2" wouldn't even pass in a "performance" code.  Because a performance code is a holistic and takes into account the heat sources and all the other building components.  In some locales, you may be able to meet performance codes with R-17 as part of an entire envelope package, but it is incorrectly applied here.

    And while it may work better than garbage, it will not work better than 4" of foam.

    -Rob

  3. Clewless1 | Sep 20, 2010 09:42am | #3

    Codes and particularly energy codes often offer different methods of compliance. Prescriptive, component performance, and whole building performance. Prescriptive simply spells out e.g. R-values for this and that (walls, ceilings, windows, etc.). You simply install the prescribed value ... done and easy ... but often more stringent than another option. Component performance allows you to put e.g. less in the walls and more in the ceiling if the resulting heat loss will be equal or less ... requiring a fairly simple calculation. To say a certain R-value is 'required' is a bit of a misnomer under this option; technically no specific minimum R-value would be required. Whole building allows tradeoffs between systems ... e.g. walls and furnace efficiency.

    I've been working w/ energy codes for 30 years, now. Your contractors suggestion that his performance based standard is plausible. However, we've seen in the energy industry many claims for 'equivalent' R-values through a variety of strategies ... radiant barriers, thermal mass, etc. While these may work under some circumstances, they often fall short of the claims of "equivalent R-value" and are often backed by unsubstantiated or shaky tests.

    Your contractor is saying that the install will substantially reduce e.g. air leakage ... and therefore increase the 'effective' R-value of the construction. A little right and a little misleading. Right ... substantiall reduce air leakage. But other insulations [properly installed] would not necessarily promote high air leakage. It is improper to combine the affect of [reduced] air leakage with the R-value. He is making claims unsubstantiated for your house. While I respect that his product will provide a lot of additional energy benefit, it will NOT be a higher R-value for his 2.5" of insulation. That could be proven by a standard hot box test used to determine R-values of any insulation.

    If you specified a specific R-value or if the prescriptive part of the code applies to this install, he needs to install a full thickness to achieve the specified/required R-value. He's playing games with the concepts here and he shouldn't be doing that. Shame on him. It is generally considered pretty sleazy to make this kind of a claim. Given 2 bids for the same material, he shouldn't be claiming equivalent performance. Make him drop the hype and correct his bid if he wants to compete.

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Video Shorts

Categories

  • Business
  • Code Questions
  • Construction Techniques
  • Energy, Heating & Insulation
  • General Discussion
  • Help/Work Wanted
  • Photo Gallery
  • Reader Classified
  • Tools for Home Building

Discussion Forum

Recent Posts and Replies

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

Up Next

Video Shorts

Featured Story

FHB Podcast Segment: Finding the Right Fixer-Upper

Get expert guidance on finding a fixer-upper that's worth the effort.

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

  • Guest Suite With a Garden House
  • Podcast Episode 688: Obstructed Ridge Vent, Buying Fixer-Uppers, and Flashing Ledgers
  • FHB Podcast Segment: Finding the Right Fixer-Upper
  • Keeping It Cottage-Sized

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

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 2024
    • 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.

  • 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 70%

Subscribe

Enjoy unlimited access to Fine Homebuilding. Join Now

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