Wind damaged vinyl siding (C) Daniel FriedmanNeed For & Role of Vapor Barriers Beneath Vinyl & Other Building Siding

  • HOUSEWRAP AIR & VAPOR BARRIERS - CONTENTS: How to Select & Install house wraps, vapor barriers, house wrap behind vinyl siding and other building siding. Leaks into vinyl-sided building. Select & Use House Wrap. Code Requirements for Building Wrap. Sheathing Wrap Performance Measures. Water Resistance of Housewraps. Air Infiltration of Housewraps. Performance Table for Housewraps. Can the Vapor Barrier be Omitted? Guide to choosing & using synthetic, paper, felt, plastic & draining housewraps: house wrap requirements, Tyvek, Rwrap, Typar, building paper, roofing felt, rosin paper: the acceptability of various materials for use as house wrap or rain barrier under wall siding
  • POST a QUESTION or READ FAQs about moisture & vapor barrier requirements underneath building wall siding

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

Vinyl siding & other applications & requirements for house wrap:

The requirement for water or rain barrier on building walls: this article discusses of the need for external vapor barriers beneath vinyl, siding - building code requirements, interpretation, and home inspection concerns.

We include photographs and sketches of vinyl siding installation procedures and of common defects observed in vinyl exterior building siding, such as buckling, splitting, cracks, odors, and questions about the need for a vapor barrier behind vinyl siding and over building sheathing.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Examples of Leaks into a Vinyl-sided Building Demonstrate Need for a Water Barrier on Building Exterior Walls

Photograph of gutter leak running down vinyl siding.

Our page top photo shows significant leak stains on and into a building exterior wall where wind has blown vinyl siding off of this 1960's townhouse in Bellmawr, New Jersey.

[Click to enlarge any image]

Our siding leak photo (left) shows a typical source of concentrated water running down, onto, and into vinyl siding on a 1990's home in Pawling, New York.

Article Contents

The purpose of building sheathing paper (such as Typar™, Tyvek™ or "Housewrap") is to prevent drafts and the entry of wind-driven rain into the wall cavity. Certain types of siding consisting of large sheets or panels will perform this function, eliminating the need for sheathing paper.

This requirement applies to siding such as that commonly used on mobile homes but does not apply to siding installed in strips which is intended to simulate the appearance of a lapped wood siding.


Such material does not act as a substitute for sheathing paper since it incorporates provision for venting the wall cavity and has many joints. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction (see our BOOK REVIEW of that book), with additional code citations and explanation were provided by building experts Al Carson, Mark Cramer & Daniel Friedman.

- Daniel Friedman, Mark Cramer , Alan Carson

About these photographs of leaks on and perhaps into a vinyl-sided house wall

The page top photograph shows that a lot of water, more than you might guess, may be running over the surface of building exterior siding, vinyl, aluminum, wood, anything.

Photograph of leaks down a vinyl-sided building.

Many siding materials are not themselves a waterproof external building skin. Where high volumes of water run down siding, leaks into the building windows or walls are possible, leading to costly hidden damage such as from rot, mold, or insects. If we add the effects of wind pushing rain against a building these effects are increased still further.

Photograph of leaks down a vinyl-sided building.

The pair of photographs shown just above make clear that a lot of water was running on the exterior of this particular building, in this case due to an improper roof flashing and gutter design that combined a large volume of roof runoff falling into a gutter that leaks against the building.

Vinyl siding troubles in Portland Maine (C) D Friedman

At this unfortunate and new vinyl-sided home where workmanship was not the best, the leaky gutter and wet wall finally send water into a low area beneath a deck where water, trapped against the building wall, is likely to eventually find its way into the building basement.

As you can see from our horrible vinyl siding trouble at left (Portland, Maine), leaks may occur in vinyl siding for a wide range of reasons, including structural movement, bending, and breaking.

The building at above left asks more of the vinyl siding than the product can be expected to deliver.

Wet walls, wet basement, and hidden damage are the risks here. Installation of a water barrier behind this siding, combined with proper window and door flashing, would significantly reduce the risk of future building damage.

Typar house wrap example (C) Daniel Friedman

Shown in our photograph at above left, housewrap covered the building sheathing and framing around this first floor entry, and was lapped over the foundation wall for about twelve inches - more material was needed, lapped with its upper edge underneath the upper course of housewrap so that the finish siding could extend closer to the ground.

Guide to Selecting & Using Sheathing Wrap - House Wrap - Building Exterior Moisture Barriers

Felt paper house wrap (C) J Wiley, Steven Bliss

As described in Best Practices Guide to Residential Construction:

The primary goal of a sheathing wrap (or housewrap) is to protect a building’s structural components from water. At the same time, the sheathing wrap must be permeable enough to allow drying to the building’s exterior if the framing or sheathing should get wet.

While the permeance and water- resistance ratings of sheathing wraps vary significantly, how they are installed is far more important than the specific product used. The key is to always lap the sheathing wrap to shed water and to properly integrate the wrap with flashings so water is directed on top of the layer below.

All sheathing wraps fall into three basic types: asphalt felt, Grade D building paper, and synthetic housewrap. Grade D building paper is used primarily under stucco in the western United States and is essentially a lighter- weight version of asphalt felt.

Comparing one material to another is difficult since there is no single standard for all products, and even where manufacturers follow the same test standard, test conditions may vary dramatically from one company to the next.

Building Code Requirements for Building Sheathing Wrap

Typar house wrap example (C) Daniel Friedman

Our photo shows an example of Typar housewrap being applied during new construction. Continuing from from Best Practices Guide to Residential Construction:

The 2003 International Building Code calls for a “waterresistive barrier behind the exterior veneer” consisting of flashings and a “weather-resistant sheathing paper” lapped at least 2 inches horizontally and 6 inches vertically.

It specifies asphalt-saturated felt that weighs at least 14 pounds per square and complies with ASTM D226, which means that most unrated No. 15 felt paper sold at lumberyards (which weigh closer to 7 pounds per square) do not comply.

Nearly all the plastic housewraps have been submitted to the model code authorities and accepted as substitutes for ASTM rated No. 15 building paper. If building in an area that follows the Model Energy Code (MEC), builders must either install a “vapor-permeable housewrap” on the exterior or seal all the penetrations in the building by using some combination of polyethylene, caulks, and gaskets on the interior.

Performance Measures for Building Sheathing Wrap

What is the definition of permeance? Permeance ratings measure the rate at which water vapor passes through a material. One perm equals one grain of water vapor passing through one square foot of material per hour per inch of vapor-pressure difference.

Under ASTM standard E96, manufacturers can use either test A (dry cup) or test B (wet cup), which yield somewhat different results. Grade D building paper uses yet another standard for measuring permeance, which is roughly equivalent to a permeance rating of about 5, similar to asphalt felt materials. Plastic housewraps range in permeance from around 5 to over 50.

In general, a sheathing wrap should have a permeance of at least 5 to enable wall assemblies to dry out should they get wet. Since common sheathing products like plywood and oriented-strand board (OSB) have permeance ratings of less than one, the sheathing is more likely to interfere with drying than the sheathing wrap.

Research note on housewrap tests: According to U. Mass building scientist Paul Fiske who conducted his own independent tests of the permeance of several housewrap products:

Tyvek, Rwrap and 15-pound felt showed no signs of liquid water leakage throughout the 2-day test period. However, the blotter paper on the Tyvek and Rwrap samples felt slightly damp to the touch by the end of the period, suggesting that there had been some transfer of water vapor by diffusion. Blotter paper disks on the 15-pound felt paper samples remained bone dry to the touch.

Typar samples all leaked within the first 2-hours of the test. Two of the Typar samples leaked immediately – as soon as the blotter paper was placed onto the wrap. Blotter paper indicators for Barricade, Pink Wrap, and AmoWrap (all perforated wraps) became saturated before the layers of the test setups could be secured with an elastic band. ... Felt paper leaked after 30 minutes in the column test, but still makes my list as a strong candidate.

Fiske, who questions the viability of housewraps as weather barriers in general, prefers to rely on building flashing and good design [which in our opinion fails to address water penetration of building siding in the general field of the material during windy wet weather]. Fiske also likes felt paper better than plastic type housewraps because if water does penetrate behind it, the permeance of felt means that water can also escape.

Guide to Water Resistance of Building Sheathing Wraps, Building Papers, Housewraps

Vinyl Siding Stains & Brown Leak Stains on Foundation Walls Below Vinyl Siding

Vinyl siding with stains or moisture indicators (C) Daniel Friedman

Many siding materials are not themselves a waterproof external building skin. Where high volumes of water run down siding, leaks into the building windows or walls are possible, leading to costly hidden damage such as from rot, mold, or insects.

If we add the effects of wind pushing rain against a building these effects are increased still further.

Check out these companion articles: VINYL SIDING INSTALLATION and VINYL SIDING INSPECTION & REPAIR for illustrations of troubleshooting leaks, buckling, blowing off, loose or other vinyl siding troubles.

Leak stains below vinyl siding (C) D Friedman

In the two photographs above and just below you can see stains on the building foundation wall below a vinyl-sided home - these stain patterns are a common source of questions about where leaks are occurring and what will be their effect on the building.

Vinyl siding leaks down foundation wall (C) Daniel Friedman

Brown leak stains appearing on the foundation wall below vinyl building siding may be traced to a single event (a plumbing leak for example) or to recurrent events (wind-blown rain), either of which can send water down the building wall behind the siding itself. A sheathing wrap can't do much to stop the effects of a plumbing leak into the building wall. That water is moving from inside the building out.

But if the sheathing wrap is properly installed it can stop or significantly reduce the movement of wind-blown rain through the siding and down the building sheathing.

Housewrap Permeability

Permeability of housewraps by water: Several different tests are used to measure the ability of building papers to stop liquid water. Grade D building papers must have a 10-minute rating under ASTM D779, commonly called the “boat test,” in which a piece of building paper is folded in the shape of a boat and floated in a dish of water until it soaks through and wets a powder on top. Some Grade D papers are rated as high as 60 minutes.

In general, products with very high vapor permeability, such as DuPont’s Tyvek®, do poorly in the boat test since water vapor can pass through and wet the indicator powder. However, Tyvek® and other non perforated plastic housewraps perform well in the alternate “hydro-head” test in which the material is placed under a 22-inch column of water and must not leak for five hours. More importantly, non perforated plastic housewraps generally do a very good job of shedding liquid water in the field.

Guide to Air Infiltration Specifications of Building Sheathing Wraps, Building Papers, Housewraps

Air leak resistance of housewraps: Many sheathing wrap suppliers tout their products’ ability to block air infiltration, often citing proprietary test results. Some follow ASTM E283, in which an 8-foot-square wall section is tested before and after installation of the sheathing wrap. However, since the manufacturer is free to specify the type of wall assembly, one test is not comparable to another, and none simulates real job-site conditions with seams and holes in the sheathing wrap.

If a house already has a reasonably tight wall assembly, there is little evidence that a layer of housewrap will significantly tighten the building. In general, air-sealing efforts are better spent on the building’s interior, using caulks and gaskets or a continuous polyethylene air/vapor barrier.

See MOISTURE PROBLEMS: CAUSE & CURE for additional detail about air and moisture movement through buildings.

Table of Performance Characteristics of Building Housewraps & Sheathing-Wrap Materials

Installed carefully, any of the sheathing wraps can perform well and keep water out of walls. The three main choices are traditional asphalt felt, Grade D building paper, and the newer plastic housewraps. The optimal product will depend upon the siding choice, building details, and climate.

With any sheathing wrap material, however, the key to good performance is to carefully lap the material to shed water. This job has been made easier by the introduction of a number of peel-and-stick membranes for use around windows, doors, and other trouble spots. General performance characteristics of sheathing wraps are summarized the table below

Table of Permeability Ratings for Sheathing Wrap Products & for Building Sheathing Products

Material type Uses Advantages Disadvantages Perm
Comments & Recommendations
Asphalt felt, saturated felt, "tar paper" Sheathing wrap under siding & roofing Moderate resistance to liquid water. Absorbs & stores water. High permeability when wet Deteriorates if exposed to sunlight; prone to tearing; 5

Use min 14 lb. material, good option for all types of siding. Use with rain screen. Do not rely on for air-barrier



Grade D building paper, red rosin paper Sheating wrap under stucco Resists water from 20-60 minutes as per rating Deteriorates if saturated 5

Use two layers of rated material under stucco



Foil facing Insulation face, radiant barriers Waterproof

Traps water if any leaks into the building cavity

Easily torn

Close to zero

Used with or as part of insulating products, Not used as a housewrap without additional steps and in hot humid climates.



Housewrap: Tyvek & similar products Sheathing wrap under siding Resist water penetration, permit water vapor passage Water vapor permeability & perm ratings vary widely by product.

6.5-29 (Kaellner 2006)

48-59 (Bliss 2006), 77 (Gibson 1994)

Plastic housewrap, perforated Sheathing wrap under siding Good permeability Low resistance to liquid water 9 - 48 Use in relatively dry areas
Plastic housewrap, not perforated Sheathing wrap under siding Most have high permeability and high resistance to liquid water, good air barrier if taped Can trap liquid water fdrom wall leaks 9 - 59

Good option for all types of siding. Use with rain screen. Can also serve as air-infiltration barrier if taped at all seams & edges.


Draining housewraps Sheathing wrap under siding, drainage plane. Provides drainage plane and air space for rain screen Effectiveness varies by construction details. Variable

Good option for all types of siding in areas subject to wind-blown rain.


Plywood, 1/2" CDX Wall & roof sheathing Structural material; absorbs & stores water depending on coating; May function as a buffer by absorbing and releasing moisture over time. 1-30

Not intended as sole vapor barrer in most construction.

Permeability varies depending on ambient relative humidity.

See also PLYWOOD Roof, Wall, Floor Decks & Sheathing

OSB, 1/2" Wall & roof sheathing Structural material; absorbs & stores water depending on coating Lower perm than 1/2" CDX plywood, less variability 1-7

Not intended as sole vapor barrer in most construction.

Permeability varies depending on ambient relative humidity.




Adapted and expanded from Table 1-1, Sheathing Wrap Performance, [image file] found in Bliss, Steven. Best Practices Guide to Residential Construction: materials, finishes, and details. John Wiley & Sons, 2006.

Permeability of plywood vs OSB discussed by

  • Ahuja, Deepak, B. Matthew Smith, and M. Arch. "Potential Pitfalls of a Green Building Material: A Case Study of Cellulose Insulation." In Forensic Engineering 2012@ sGateway to a Safer Tomorrow, pp. 239-249. ASCE, 2013.
  • Gibson, Scott. "Air and Vapor Barriers." Fine Homebuilding 4, no. 94 (1994): 48-53.
  • International Building Code (2006),
    1404.2 Water-resistive barrier. A minimum of one layer of No. 15 asphalt felt, complying with ASTM D 226 for Type 1 felt, shall be attached to the studs or sheathing, with flashing as described in Section 1405.3, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer.

    2510.6 Water-resistive barriers. Waterresistive barriers shall be installed as required in Section 1404.2 and, where applied over wood-based sheathing, shall include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper. Exception: Where the water-resistive barrier that is applied over wood-based sheathing has a water resistance equal to or greater than that of 60 minute Grade D paper and is separated from the stucco by an intervening, substantially non-water-absorbing layer or drainage space.

    R703.1 General. Exterior walls shall provide the building with a weather-resistant exterior wall envelope. The exterior wall envelope shall include flashing as described in Section R703.8. The exterior wall envelope shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing a waterresistive barrier behind the exterior veneer as required by Section R703.2 and a means of draining water that enters the assembly to the exterior. Protection against condensation in the exterior wall assembly shall be provided in accordance with Chapter 11 of this code.

    R703.2 Water-Resistive Barrier. One layer of No. 15 asphalt felt, free from holes and breaks, complying with ASTM D 226 for Type 1 felt or other approved waterresistive barrier shall be applied over studs or sheathing of all exterior walls. Such felt or material shall be applied horizontally, with the upper layer lapped over the lower layer not less than 2 inches (51 mm). Where joints occur, felt shall be lapped not less than 6 inches (152 mm). Such felt or other approved material shall be continuous to the top of walls and terminated at penetrations and building appendages in such a manner to meet the requirements of the exterior wall envelope as described in Section R703.1.
    Exception: Such water-resistive barrier is permitted to be omitted in the following situations:
    1. In detached accessory buildings.
    2. Under exterior wall finish materials as permitted in Table R703.4.
    3. Under paperbacked stucco lath, when the paper backing is an approved weather-resistive sheathing paper.

    R703.6.3 Water-resistive barriers. Waterresistive barriers shall be installed as required in Section 1404.2 and, where applied over wood-based sheathing, shall include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper. Exception: Where the water-resistive barrier that is applied over wood-based sheathing has a water resistance equal to or greater than that of 60 minute Grade D paper and is separated from the stucco by an intervening, substantially non-waterabsorbing layer or drainage space.
  • Lstiburek (2015) - see REFERENCES
  • Kaellner, Bradford R., "Attachment# 14 Impact Of Title 24 Residential Leakage Reduction Credit On Water-resistive Barriers In California Homes", Architectural Energy Corporation, Boulder CO, USA, for California Energy Commission, Public Interest Energy Research Program, (2006).

The table above gives perm ratings for common building sheathing wrap products.

Table of WRB Performance Data: Air & Water Leakage & Perm Ratings for 17 Housewrap Products

In our separate articl eHOUSEWRAP PRODUCT CHOICES, the table at HOUSEWRAP DATA TABLE lists 27 housewrap products, giving manufacturer, air retarder rating, air leakage rates, water vaper permeance or perm rating, structural integrity, UV exposure rating, thickness, and flame/smoke classification for these products (listed in order of lowest to highest Perm rating: Vorted / Weather Trek, Rufco-wrap, FirstWrap, Weathermate Plus, Dri-Shield, Weathermate / Air-Gard, Prime Wrap, Barricade Building Wrap, GreenGuard RainDrop, Typar, PinkWrap, GreenGuard Classic Wrap, ProWrap, Tyvek ComemrcialWrap, GreenGuard Ultra Wrap, WrapShield,Tyvek StuccoWrap / DrainWrap, Tyvek HomeWrap, R-Wrap, Two-Ply Super Jumbo Tex 60 Minute, Super Jumbo 60 Minute, Fortify, Two-Ply Jumbo Tex, Two-Ply Jumbo Tex HD 30 Minute, Jumbo Tex HD 30 Minute, Jumbo Tex.

Asphalt Felt Used for Housewrap

Typar house wrap example (C) Daniel Friedman

The old standby, asphalt felt (photo at left, construction in 1978), has a perm rating of around 5 and moderately good water resistance, making it suitable for use as a sheathing wrap. However, unlike plastic housewraps, asphalt felt will absorb water when wet.

Once wet, its permeability jumps from around 5 to as high as 60. In the event of water leaking into the wall, asphalt felt may help store some of the water, and its high permeability when wet will promote drying to the exterior. Housewrap, in contrast, tends to trap any liquid water that gets behind it.

Some contractors find felt easier to install and weave into flashings because of its rigidity and narrow roll width. Felt, however, tends to get brittle and deteriorate under long-term exposure to UV radiation and is more prone to tear during installation than plastic housewraps. For situations where prolonged exposure is expected, plastic house- wraps are better suited. Otherwise, asphalt building felt remains a valid choice for modern homes.

Although traditional 15-pound rag felt weighed 15 pounds per 100 square feet, the material sold today as No. 15 felt is made of recycled cardboard and sawdust and actually weighs only 7 to 8 pounds per square.

Most of the lightweight building paper sold has no ASTM rating. ASTM-rated No. 15 felt is either a minimum of 7.6 pounds per square (ASTM D4689) or 11.5 pounds per square (ASTM D226). Similarly, the unrated variety of No. 30 felt typically weighs only 15 to 20 pounds per square versus 26 to 27 pounds for rated Type 2 felt (ASTM D226).

Grade D Building Paper Used for Housewrap

Grade D building paper is an asphalt-impregnated kraft-type paper, similar to the backing on fiberglass insulation. Unlike asphalt felt, it is made from new wood pulp, rather than recycled material.

Its most common use is under stucco in the western United States. The vapor permeance of Grade D paper is similar to asphalt felt. Its liquid water resistance ratings range from 20 to 60 minutes, as measured by using the boat test (see “Water Resistance,” previous page).

Because Grade D paper tends to deteriorate under prolonged wetting, the trend in three-coat stucco is to use two layers of 30-minute paper. Because the paper tends to wrinkle, the two layers tend to form a small air space, creating a rain-screen effect.

Plastic Housewrap Products

There are a wide range of plastic housewraps on the market. Most are non woven fabrics made from either polyethylene or polypropylene. Some have perforations to let water vapor pass through and the others are designed to let water vapor diffuse through the fabric itself. Because there is no single testing standard for plastic housewrap performance, it is difficult to make apples-to-apples comparisons. However, published performance data and limited field studies suggest the following:

Draining Housewraps

In the last few years, manufacturers have responded to the need for an air space and drainage plane with a variety of housewrap products that are either wrinkled or corrugated to provide an integrated air space. These include products intended primarily for stucco, such as DuPont’s StuccoWrap®, and others developed for siding, such as Raindrop Housewrap, which is a plastic drainage mat from Pactiv, Inc. (see “Resources,” page 47 of Best Practices Guide to Residential Construction).

The air space created by these products is minimal, ranging from 0.02 inch thick for StuccoWrap to 0.008 for RainDrop®. Although these materials may allow for some drainage, it is unlikely that they will provide any measurable airflow to promote drying.

A more promising approach is a 1/4-inch nylon matrix, called HomeSlicker®, which has vertical drainage channels and installs between the sheathing wrap and siding. The material is rigid and thick enough to resist compression by the siding but thin enough that windows, doors, and trim can be installed without furring.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Sizing up Vinyl Siding - The Requirement for a Moisture Barrier Beneath Vinyl Siding - When Can the Vapor Barrier be Omitted?

This discussion is now found at VAPOR BARRIERS, VINYL SIDING

House Wrap, Air & Water Barrier Articles


Continue reading at VAPOR CONDENSATION & BUILDING SHEATHING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Suggested citation for this web page

VAPOR BARRIERS & HOUSEWRAP at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about moisture & vapor barrier requirements underneath building wall siding.

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

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman