Wind damaged vinyl siding (C) Daniel FriedmanThe Need For and 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

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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 2015, 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

Photograph of leaks down a vinyl-sided building. Photograph of leaks down a vinyl-sided building.

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.

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.

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 FriedmanAt 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 BlissAs 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 FriedmanOur photo (left) 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.

In the two photographs 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.

Leak stains below vinyl siding (C) D Friedman 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 just below, courtesy J. Wiley & Sons, Best Practices Guide to Residential Construction.

Table of building wrap product performance (C) J Wiley, Steven Bliss

[Click to enlarge this or any table or image found at]

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:

  • Permeance to water vapor. The leading nonperforated products (Tyvek®, R-Wrap®, and Amowrap®) are significantly more permeable to water vapor, ranging from 48 to 59 perms, than nearly all the perforated materials.
  • Water resistance. All sheathing wraps adequately shed water on vertical surfaces. Pooled water, however, will leak through most perforated plastic housewraps over time, while the nonperforated materials will contain liquid water indefinitely. No. 15 asphalt felt retains water moderately well, but it allows some penetration over time.
  • Damage from extractives. Some researchers have noted that extractives leaching out of redwood and cedar siding can cause plastic housewrap to lose its water repellency and to deteriorate. Back-priming the wood siding or leaving an air space behind the siding will help prevent this. Stucco will also degrade plastic housewrap and is rarely installed over it.
  • Recommendations. Given their high permeance ratings and excellent resistance to liquid water, the nonperforated housewraps are a good choice for most building applications. Traditional asphalt felt is also a good option. Many contractors find plastic sheathing wraps more convenient than asphalt felt in that they weigh only 1 to 2 pounds per square, and they are more flexible in the cold and more tear-resistant. Also they are available in 9- to 10-foot-wide rolls versus 3-foot rolls for asphalt felt.

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?

TO: Editor, ASHI Reporter

RE: "Sizing up vinyl siding," by Mark Cramer, on p. 17 of the Jan 94 ASHI Reporter., (U.S. Building Codes) with annotations from letter, MC to DJ 1/11/94 and further annotations from Alan Carson (Canadian Building Code)
FROM: Daniel Friedman, ASHI Technical Committee

Mark's article on the need for a moisture barrier beneath vinyl siding was of considerable interest. It was particularly helpful that he included a "building code citation Section 1702.3." The reference to an authoritative source is the quickest way to distinguish between personal opinion and authority. In conversation, Mark has provided some additional information about vapor barriers and vinyl siding:

1. The "code" citation was from the Standard Building Code, published by SBCCI. Similar language is in the Uniform Building Code (UBC) Sec. 1708.(a) "All weather-exposed surfaces shall have a weather-resistive barrier to protect the interior wall covering." The code goes on to permit building paper ("red rosin paper") and felt. It makes no mention of newer building "wraps" such as "Tyvek(TM)" which are an excellent wind and water barrier, but permit water vapor to pass (from inside the house wall to outside).

2. According to these codes, the weather-resistive barrier can be omitted

  1. when exterior covering is of approved weatherproof panels
  2. in back-plastered construction
  3. when there is no human occupancy
  4. over water-repellent panel sheathing (UBC) ... including [weather-resistant exterior grade] plywood (SBC)
  5. under approved paper backed metal or wire fabric lath
  6. behind lath and portland cement plaster applied to the underside of roof and eave projections.

Much new construction uses exterior-grade plywood sheathing. I interpret 2.d (which is Section 1708.(a)4. in the UBC) to mean that the barrier is not needed in these cases.

3. The same paragraph requires "... exterior openings exposed to weather be flashed in such a manner as to make them weatherproof." Mark and I agree that problems associated with water leaks into siding are often at window and door penetrations, and at abutments of roofs to building sidewalls. We had trouble understanding how the presence of felt in these areas, also having to be cut around the opening, would in typical construction application, increase resistance to water entry (for example at the corners of windows). Many new windows come with pre-fab metal or plastic flashing which surrounds the corners.

4. Watch out: Since our readers may refer to Mark's article in supporting their field observations, it's important to distinguish between "weather-resistive" barrier and "vapor barrier." The latter, of which plastic and foil are examples, should not be placed on the outside of walls, as it threatens to entrap condensation (from the conditioned interior) inside the walls.

5. Mark offered other citations of authority for this matter, including the

Aluminum Association of Florida's "AAF Standards for the construction of aluminum and allied products," Std. 112 Siding, Soffit, & Fascia, which says:

"An approved membrane to protect against water intrusion shall be provided when installed over any non-waterproof surface, or the cladding shall be designed and constructed with an approved continuous edge, interlock, overlap, or seam to prevent water intrusion."

This authority also requires that siding shall be installed to the requirements of the SBC, AA incorporated/AAMA Aluminum Siding Application Manual and/or the Rigid Vinyl Siding Application Manual of VSI, in addition to the manufacturer's recommended procedures.

Here's where the topic got amusing:

First, the AAF's actual siding manual provided no details on flashing except to say "flash as required." And regarding the requirement of a moisture barrier, the text goes on to say that some manufacturers require it and some do not.

I'd like to know of any code jurisdictions where you know that the requirement for a moisture barrier is being monitored or enforced by local officials.

*** ANS: (Letter MC to DJF 1/11/94) Rev to 1988 SBC, (1990?) which was code in effect when subject house was constructed. Exceptions in this revision included water repellent panel sheathing but did not mention plywood or other products, as does the 1991 version.

Two questions seem to me to bear on this matter. First: the material behind siding is not normally visible. However from server al means one might know or suspect that the moisture barrier was omitted in some areas. Second: whether present or absent, the material does not excuse the inspector from looking for evidence of water damage to building components.

*** Comment: (ibid) The issue of felt under siding can be looked at as one of common sense. The felt is a dry-in layer just like the felt under the shingles on a roof. When a leak occurs, the felt serves to direct the moisture down the wall to the bottom where it can run off. As to the contention that leakage may still occur at penetrations such as windows, this is no different than a penetration in the roof, except that the sealing done at a window penetration would generally be better than what is done on the roof.

*** Comment: (Ibid) Vinyl siding is considered a water shedding material rather than a water-tight material. The tremendous expansion and contraction with temperature changes has a tendency to open things up when at the contracted state. Wind can blow rain between the laps at the ends of individual pieces of siding and at the ends of pieces at J-moldings. You would be surprised where wind can blow rain when velocities get up around 75 mph or so!

*** One source of water entry that just occurred to me is the laps at ends of pieces of siding when you have a sidewall flashed roof above with no gutter. (Common here [Florida].) Water shooting off the roof at an angle close to horizontal could easily get behind the siding at the laps.

*** Installation of flashings around windows and doors is typically not done correctly. In talking with people in the business I have found that there is a specific head flashing that manufacturers recommend [but] that is never used. Other installation defects are common, such as installing corner trim pieces with the upper piece beneath [lapped inside] the lower piece. In the real world you have to accommodate the [full range of training level of the] person who is installing the product. These are more reasons for the installation of a moisture barrier. You have to look at it as the underwear for the siding. NOt absolutely necessary at all times, but very handy under certain conditions.

*** I don't agree with the logic that because you have an opening in the felt you are going to have a leak anyway. If the felt is installed reasonably well, the chances of leaking are reduced. By reasonably well I mean install strips around and lapped into the opening, install the windows, caulking the flanges to the strips, and then install the felt over the wall, lapping it over the strips at he windows. This arrangement will be water tight.

[Comment DJF: the above caulking details during construction would be very rare - I've never seen it during any new construction, nor seen it specified in siding installation manuals. The detail that most prevents leaks in at windows is proper installation details of the J-flashing above and around the window, caulking at the sill, and assuring that siding below the sill does not protrude nor otherwise permit water entry.]

*** Regarding code enforcement, I supervised the construction of a 5000 sq. ft. custom home this year, with an acre of brick veneer on the front of the house. Portions of walls were wood frame. When the brick was 3/4 the way up the walls, I stopped by and noticed that there was no moisture barrier under the brick. I insisted that one be installed. The local building inspector backed me up. (Rare occasion.) There is enforcement of this requirement in most municipalities around here.

[Comment DJF: this points up the fundamental reality and problem that inspection of every construction detail (moisture barrier behind brick) is rarely achieved since the builders are simply not able to stop construction at every phase to wait for the off-site inspector to come on-site. Inexperienced or certain other crews are often pressured by time and cost to take shortcuts or leave out materials, the importance of which they may not really believe.]

*** I agree with not quoting code. I rarely do in a home inspection for the same reasons you provide, and others. But when you are telling a seller they have a multi-thousand dollar problem, your opinion is often questioned. That's an occasion when I will quote codes.

ASHI inspectors should be alert for evidence of damage or other problems associated with water penetration of siding. I'd like to know where you're finding damage, and what you can see as the apparent path of water entry - that is, what's the real source of the problem? Otherwise we have a "standard" that is more confusing than helpful.

It's always important for us to know the authoritative source for our assertions, just as it's important for us to identify when something is our opinion. Mr. Cramer is a licensed building code official and as such can state that something is a "violation." As a non-code official myself, I have less trouble when I express the opinion that something is a good or bad construction practice. That wording gives me considerable latitude to include many non "code" sources as well as the experienced of ASHI inspectors who study in-service field conditions.

Thanks again to Mark for the discussion.

Dan Friedman


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