Caulk and block method for thermal shell details (C) Daniel Friedman Vapor Barrier Details for Sealing at the Band Joist

  • VAPOR BARRIERS & AIR SEALING BAND JOISTS - CONTENTS: Vapor barriers and condensation in buildings - leaks at the rim joist or band joist. Construction & sealing details to provide a continuous vapor barrier in buildings at the rim joist or band joist. Caulk & rigid insulation details protect the integrity of the building's thermal shell
  • Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about vapor barrier installation details on buildings

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Band joist - rim joist vapor barrier sealing details: this article describes how to install vapor barriers and how to avoid problems with building indoor condensation: we provide details for caulking, sealing, & insulating at the building band joist or rim joist will protect the integrity of the building's thermal shell.

Multiple approaches to building vapor barrier sealing details are provided to address this air and moisture leak problem: the caulk and block method to seal the rim joist is effective but time consuming; offsetting the band joist to permit exterior rigid foam insulation is described; details are provided for sealing building overhangs such as found on a raised-ranch home; other building air leak sealing methods developed by researchers at the University of Toronto are described.

Green links show where you are. © Copyright 2015, All Rights Reserved.

Vapor Barriers & Building Condensation - Sealing the Band Joist at Building Floors - part III

Vapor and condensation table (C) Daniel FriedmanSketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

This discussion of vapor barriers and condensation in buildings in this article series begins at part I, VAPOR BARRIERS & CONDENSATION in buildings, (when and why condensation occurs inside buildings, explains the problems caused by excessive indoor condensation, explains how moisture enters building wall and ceiling cavities, and summarizes the best approaches to prevention of indoor moisture and condensation problems), continues with part II at VAPOR CONDENSATION & BUILDING SHEATHING (detailed questions and answers about various building wall sheathing and insulating materials and their impact on building condensation problems) followed by VAPOR BARRIERS & AIR SEALING at BAND JOISTS. Readers should also see VAPOR BARRIERS & HOUSEWRAP.

"Dealing with Band Joists. Don't let them sabotage your thermal shell" - links to the original article in PDF form follow the expanded/updated online version of this article just below.

Traditional Methods for Sealing the Building Rim Joist or Band Joist

Part 1 of this article on building details to seal air and vapor leaks at the band joist or rim joist describes two traditional, effective, but labor-intensive methods to stop air, moisture, and heat leaks around the perimeter of building floors and overhangs.

  • Figure 1 (page top sketch and shown in more detail here), describes the caulk and block method of band joist insulation and sealing: compressible pre-cut foam blocks or custom-cut rigid foam insulation blocks are placed over fiberglass-insulation between the floor joists at the building perimeter, sealed on all sides with caulk. The author suggests a few construction tricks that can reduce the labor associated with this building air, heat, and moisture vapor leak sealing method.
  • Figure 2 (at left just above) shows a physically easier method for sealing the building floor perimeter by offsetting the band joist inwards and routing of the poly vapor barrier inside of the building floor frame. This method will require a different than usual construction sequence, applying part of the vapor barrier around the floor structure early in the building framing sequence.

Better Methods for Sealing the Building Band Joist

Floor overhang sealing and insulating details (C) Daniel Friedman

  • Figure 3(left) demonstrates how floor overhangs such as on a raised ranch may be sealed using caulking and blocking combined with insulation of the floor overhang soffit space. Notice that in addition to insulating the floor overhang soffit, rigid foam insulation blocks are cut and caulked between each joist pair to seal the building interior against air, heat, and moisture leaks into the soffit space.

    Using this method for sealing floor overhangs, the poly vapor barrier is run down the wall and under the subfloor to extend into the interior past the overhanging section of the floor.

    If the plans call for gluing wood parquet flooring directly to the subflooring, the poly is omitted in that section, relying on the glue coating to also serve as a vapor barrier.

    The author, Mr. Bliss, points out that building heat, air, and moisture sealing is concerned with leakage of air and moisture into building cavities, not diffusion of moisture through building materials. That is because moisture diffusion through materials has been shown to be miniscule in import compared with air and moisture leaks at openings.

Insulating and sealing details for cape cod design homes (C) Daniel Friedman

  • Figure 4 (left) shows details of air, moisture, and heat loss sealing details at the walls, attic knee wall floor, knee walls, and ceiling of a typical cape design building. The author points out that in a Cape Cod design, convective coupling of eaves and ceiling/floor joists can "spell trouble".

    Air leaks through floor framing into the space behind the knee walls of a cape, ultimately into the outdoors, as well as air leaks upwards through the knee walls into the attic space above are all encouraged by natural convective currents in the building, especially if the floors and walls are not adequately sealed and fully insulated.

    "The solution is the same - blocking between the joists to continue the air/vapor barrier on the warm side of the insulation. [See detail in the illustration at left.] This is often overlooked, resulting in air passage between the cold eaves and the floor/ceiling cavity. Since this cavity is between two heated spaces, it is a heated space also. If it is convectively coupled to the outdoors, you'll be heating the outdoors."

Building sealing without poly vapor barrier (C) Daniel Friedman

  • Figure 5 (left) demonstrates building sealing methods where the poly vapor barrier is omitted. The author cites a radical Canadian splinter group who argues that the polyethylene air/vapor barrier should be omitted entirely, claiming that the vapor barrier will oxidize, become brittle and fail over time, that under peak wind loading or pressurization testing the poly will tear and leak around staples [this is doubtful if if the poly is adequately secured and protected from movement by its covering building materials], and finally "... to do it the right way with the right materials is just too expensive for the tract builder to consider."

    The Canadian alternative (described in the sketch at left and proposed by Joseph W. Lstiburek, P.E. in Toronto) focuses on controlling air leakage rather than vapor diffusion by using compressible foam gaskets in five locations around the floor perimeter combined with installation of 2 1/2" rigid foam insulating blocks, finishing by painting drywall with two coats of vapor retarder paint.

    OPINION: Our field experience [DJF] demonstrates that poly vapor barriers remain intact for decades, certainly 20 to 30 years based on our own direct experience in construction using poly vapor barriers and subsequent invasive inspection of the same buildings decades later.

  • Provided that the vapor barrier polyethylene is protected by having been covered by other building materials, it has a good life. We prefer to use 6-mil rather than 4-mil polyethylene, however, because we've found that the thicker material is less likely to be damaged during construction and because production control does not maintain full material thickness. --DJF

    OPINION-DF: While we fully agree that building scientists, including Dr. Lstiburek, have shown compelling evidence that air and moisture leaks in buildings are far more significant in building impact and energy losses than vapor diffusion through building materials, our field work as both a builder and a building inspector suggest that Dr. Lstiburek's non-poly building sealing methods involve a larger number of components and materials (foam gaskets, foam blocks, vapor retarder paint in two coats) that must be accurately placed than the use of polyurethane vapor barriers and caulk - materials familiar to most builders. This approach was reported in May 1984 in Solar Age Magazine - we look forward to seeing the results of subsequent research and field experience.

For details about use of acoustical sealants or tapes to seal polyethylene vapor barriers, see ACOUSTICAL SEALANT CHOICES.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article in PDF format

More Reading

Green link shows where you are in this article series.


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about vapor barrier installation details on buildings

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References