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• Vinyl siding defects analysis
• Need for moisture barrier behind vinyl siding

This article discusses common defects observed in vinyl exterior buiding siding, such as buckling, splitting, cracks, odors, and questions about the need for a vapor barrier behind vinyl siding and over building sheathing. Included are comments from several recognized building inspection and construction authorities. Also see ALGAE, FUNGUS, LICHENS, MOSS for algae and stains on vinyl building siding and on vinyl exterior trim.

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Vinyl Siding Concerns, Defects, Vapor Barriers, Inspection, and Diagnosis

Review and technical comments about "Sizing Up Vinyl Siding"

"Sizing up vinyl siding," by Mark Cramer, on p. 17 of the Jan 94 ASHI Reporter, with annotations from letter, MC to DJF 01/11/1994 and later annotations and updates about the requirement for and function of house wrap or other barriers below vinyl siding on buildings.

Mark Cramer'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.

If our photo (at left) makes you think of your siding, see STAIN DIAGNOSIS on Building Exteriors for diagnosis of green, black, and other stains and growths on vinyl siding.

In conversation, Mark has provided some additional information:

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 t he corners of windows). Many new windows come with pre-fab metal or plastic flashing which surrounds the corners.
   
   
4. Since our members 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 several 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 wight 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 DJ Friedman: 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 sqft 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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