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Damage in Multifamily Housing Walls With Vinyl Siding.

While I have had the opportunity to observe wall openings in many dozens of multifamily housing complexes with most other common types of siding, I have been present at wall openings at only seven multifamily housing complexes that each include numerous buildings with vinyl siding. In every one of those, most of the buildings had some damage to wall components behind the vinyl siding. The damage did not appear to be related to water intrusion into the walls from the outside. This article describes the damage that was observed and the conditions that existed. It also postulates how and why damage occurs in walls with contact-applied vinyl siding, at least in some parts of the Pacific Northwest, and under what conditions.

Findings from Single-Family Wall Moisture Field Studies

In three major field studies, (7-9) three or four openings in exterior walls were made to investigate whether moisture problems were occurring inside the walls of both older existing and new well-insulated single-family homes in the Northwest. The test homes were screened to remove those few with siding that exhibited water intrusion or related damage such as rainwater leaks and damage around windows, doors, and other penetrations. In about 1,000 such wall openings in homes in Portland, Ore.; Spokane, Wash.; Seattle; the Washington coast; and Montana, there were no cases of elevated moisture contents during warm weather, and there were no cases of decay or other wall damage. Thus, damage without water intrusion from the exterior is rare. In my experience, the same is true for multifamily housing walls.

In one of the 20 new homes in Montana, a sheathing moisture content (MC) as high as 47% was observed in February, but that had dried to 18% in April and 7% in July. (10) In 50 new Washington homes, 10% had sheathing MC values measured in mid-winter greater than 40%, and one home had a sheathing moisture content of 52%. Yet, despite the existence of such high moisture contents during cold weather, there was no sheathing or any other wall component damage. (9) So why was there no sheathing damage?

Requirements for Wood Decay Growth

The answer lies in the requirements for growth of wood decay. For wood to decay, the wood must be abnormally (i.e., soaking) wet with a moisture content greater than 26% to 30%, depending on the species of wood. (11) Furthermore, the wood must be warm. The optimal temperature range for the growth of decay fungi is about 65[degrees]F to 95[degrees]F (18[degrees]C to 35[degrees]C). (12) There is little or no growth below 50[degrees]F (10[degrees]C). So, decay only occurs in warm weather, and that is not a widely known fact.

Wood such as sheathing typically increases in moisture content during the winter in cold climates as water vapor naturally migrates from the warm indoors to the cold outdoors and condenses on the cold sheathing. As the weather warms, most walls with breathable siding dry out such that the sheathing is no longer wet enough for decay to occur.

That was the case, for example, in the Northwest Wall Study homes. (9) So for decay to occur such as has been observed in structural sheathing in walls with vinyl siding, the wood must be both wet and warm. The question is what causes the sheathing to remain wet even into warm weather when it would normally dry out?

Vapor Migration

If the water that leads to damage to wall cavity components such as structural sheathing is not entering the vinyl-clad walls from rain or other water from the outside, as was verified to be the case, then water must be entering the walls from the indoor spaces. Any water that migrates from the indoors to the outdoors during cold weather can be termed vapor migration. It is the migration of water vapor through walls by a combination of vapor diffusion and air movement or leakage that carries water vapor in the air with it.

Vapor migration is occurring in all walls and is a normal mechanism for removal of moisture generated by occupants and their activities from housing. Because an interior vapor retarder is required in the walls of Northwest housing, the amount of vapor diffusion, with certain exceptions to be described, is relatively small so most of the vapor migration is by air leakage (exfiltration).

An early rule of thumb (the Five to One Rule) (13,14) developed in the 1950s or so for walls in cold climates stated that cladding and sheathing should be five times more permeable than the inner wall permeability to allow water vapor migrating by diffusion to the outside in winter to freely exit and not build up in the sheathing. Canada Mortgage and Housing Corporation (CMHC) in its Best Practice Guidefor Wood-Frame Envelopes gives a rule of thumb that the permeance of the materials to the cold side should be 10 to 20 times that of the interior vapor retarder. (15) So what happens if these rules are violated?

Most claddings are vapor permeable and air can leak through them, so they allow water vapor to pass through them to the outdoors. In those instances, normal vapor migration from indoors is not a problem. However, that moisture can build up to a greater degree in the sheathing during cold and even warm weather if any materials on the outside of the wall are vapor impermeable and insufficiently air leaky such that normal drying to the outside does not occur as readily. Therefore, they trap water vapor that is normally migrating in winter from the warm indoors to the colder outdoors inside the wall cavities. Because the walls cannot dry out as readily in warm weather in such cases, moisture contents in the sheathing and other components remain elevated into warm weather. That can lead to staining, mold, and decay. See some examples of this in the sidebar "Single-Family Housing-Damage in Walls With Impermeable Cladding or WRB (water resistant barrier)."

Multifamily Housing with Damage Behind Vinyl Siding

I've been present at wall openings at seven multi-family housing complexes in the Northeast (Oregon and Washington) that have a few dozen buildings with contact-applied vinyl siding. See "Multifamily Housing--Damage Behind Vinyl Siding" sidebar. In each, damage to wall components such as sheathing and WRBs was observed in numerous locations. Unfortunately, this problem is not widely known, since damage is typically not observable from the outside of the walls, and not much vinyl siding gets removed and wall cavities behind it inspected. I am not aware of any other field studies or observations where such damage has been observed.

The walls of the buildings in the seven examples were constructed with gypsum wallboard, an interior vapor retarder, fiberglass batt insulation, OSB or plywood sheathing, gypsum sheathing for fire protection in most cases, housewrap or building paper WRB, and contact-applied vinyl siding. In all cases, there were numerous locations where indoor air could leak into the wall cavities. There was no rain screen or external insulation (typical construction with vinyl siding in Oregon and Washington).

We must make a critical distinction about multifamily versus single-family housing walls. Moisture generated inside housing during cold weather in cold climates is typically removed by vapor migration and mechanical ventilation. Single-family housing loses moisture generated indoors by vapor diffusion and mainly indoor air leaking to the outdoors through four or more walls, while multifamily housing loses moisture generated indoors through many fewer walls (typically one or two). Thus, the moisture load in multifamily walls is much greater. Therefore, moisture problems caused by vapor migration that includes air leaking out through walls in cold climates are more likely to occur in multifamily housing.

Recall that air leakage in walls, rather than vapor diffusion, is typically the dominant mechanism of moisture movement into and through walls. Any moisture damage that occurs will be influenced by the location of that air leakage.

No wall cavity moisture problems were observed in the dozens of both older and new single-family homes with vinyl or metal siding in the three Northwest wall moisture field studies described earlier. (7-9) However, damage has been observed in the seven multifamily housing complexes described in "Multifamily Housing-Damage Behind Vinyl Siding."

In most cases, there was no suggestion of a moisture problem within the walls from exterior appearances. Where siding was removed during cold weather, in many locations water was observed to be dripping from the back side of the vinyl siding where it had condensed. In a few cases water was observed leaking from behind the siding upon close inspection, and in two cases the leaking water had frozen.

In many cases, when the siding was removed, excessive wetting and damage to wall components was observed, such as to structural and gypsum sheathing or WRBs. In almost all cases, there was some decay of structural sheathing such as OSB or plywood, or framing. Where gypsum sheathing was installed in the wall cavities, water staining of the outer layer of the gypsum sheathing was commonly observed, along with mold growth. In some cases, the fiberglass batt insulation outer surfaces were moldy.

The worst damage behind the siding was typically located on north-facing walls, while solar-heated south-facing walls typically but not always had less damage. Damage was sporadic, and not every wall section exhibited damage. Damage also appeared to be strongly related to locations with the greatest air leakage such as between floors or where interior walls intersected the exterior walls. In one development where data loggers monitored interior temperature and relative humidity conditions, damage correlated with elevated indoor dew-point temperatures in bedrooms.

The damage does not appear to be related to rain or other water intrusion into the wall cavities from the outside based on careful inspection of the conditions at the damage sites. Rather, it appears to be related to the fact that vinyl siding is an impermeable surface on the outside of walls that leads to elevated moisture contents in the structural sheathing as well as other exterior wall components under the right set of conditions. The abnormally elevated moisture contents result from condensation of water vapor migrating from indoors during cold weather along with lack of sufficient drying to the outside.

The maritime Portland and Seattle area winter weather is cool, cloudy, humid, and rainy for about nine months of the year. It is possible this damage problem is limited to such climates. However, it is also possible that similar damage occurs in other climates. The fact that damage also occurred in arid eastern Washington, where the rainfall is significantly lower but the weather is significantly colder, suggests the possibility that a wider range of climates than just those like western Oregon and Washington may be susceptible to this problem.

Does Vinyl Siding's Leakiness Prevent Damage Behind It?

Many building scientists and contractors believe vinyl siding is relatively air leaky. That is likely because of the results of Canadian laboratory and test hut studies of vinyl siding. (1,5) In Reference 1, the following statement is made: "... claddings like vinyl siding have multiple air paths as the joints are intentionally installed loose to allow for thermal expansion. Furthermore, every second lap of vinyl siding has small circular holes for drainage. While the actual vinyl siding is impermeable to air and water, the joints (both horizontal and vertical), edge details, and drain holes all contribute to making the cladding extremely leaky."

Later, it is stated: "The air leakiness of the vinyl installation is seen as beneficial, as it allows water trapped behind the siding to dry via diffusion and convection through the leakage areas. Otherwise, as vinyl is a highly impermeable material, walls clad with vinyl would become more subject to wetting by outward moving vapor drives and the drying capability of wall assemblies would be inhibited."

Yet the large number of site observations described here of damage in dozens of buildings and many dozens of wall openings, coupled with the modeling results, suggest that the siding is not always leaky enough to completely prevent damage to wall components behind the siding. My observations in the field were that often the J-channels at the ends were relatively tight, coupled with much longer sections installed on the walls than the 2 ft to 4 ft (0.6 m to 1.2 m) wide pieces used in the Canadian and U.S. tests (such that there was more stagnant or less horizontal flow behind the siding laps).

Moreover, because longer pieces were often used, there were fewer vertical joints between overlapping pieces, and those typically appeared to be rather airtight. The Canadian tests (1) further verified that vertical airflow was minimal. In addition, individual pieces of vinyl lap siding usually include two or more laps. For those with two laps, the top lap drip edge has no weep holes, while the bottom lap drip edge has some typically spaced at 18 in. (457 mm). In almost all cases, the weep holes are blocked by the overlapping portions of individual pieces of siding. So, there is minimal if any air leakage through the drain holes. Thus, overall in real buildings, it is possible that contact-applied vinyl siding is in fact not always as air leaky as has been suggested by the Canadian research. (1)

The fact that damage occurred in sporadic locations may simply be because of differences in air leakiness of the siding or the interior walls in different areas. In some locations, the siding may be air leaky enough to prevent damage, while in other locations the siding is airtight enough that damage results.

The laboratory and test hut test conditions that led to the conclusion that sheathing readily dries because the siding is very air leaky might have been quite different than the conditions observed in the field in Oregon and Washington. For example, in the ASHRAE Research Project 1091 and WSU test hut studies, (1,6) the vinyl siding was installed only on south walls, which are often driest.

Yet such siding on north walls with less solar drying would clearly be a worst-case scenario. (4) Furthermore, the interior wall surfaces were often simulated in tests with a vapor and air impermeable surface coupled with some wetting of the sheathing. Yet in the field the interior vapor barriers often were not very good air barriers, likely leading to more water vapor intrusion into the wall cavities from inside. So, it is likely that the effect of the very high indoor relative humidities or dew points and air leaky interior surfaces associated with damage in the field was not really duplicated in those laboratory and test hut tests.

Furthermore, the presence of sheathing decay in the field would require moisture content levels above fiber saturation, and levels as high as in the seventies were sometimes measured. However, most of the lab or test hut studies reported wetted sheathing moisture contents at or below fiber saturation, so less drying would be required for the much dryer sheathing test conditions. So, almost all those laboratory and test hut studies did not test drying of really wet sheathing with values well above fiber saturation that are known to routinely occur in cold winter climates such as Oregon and Washington. (9-10)

Overall, it is unclear if the tests really simulated the conditions that often exist in the field in Oregon and Washington housing. Finally, as far as I know, the results of the lab and test hut studies have never been validated with studies of the performance of walls in actual multifamily housing in the field. While tests of air leakage across vinyl siding were conducted in the lab and test hut studies, apparently, none have been conducted with contact-applied vinyl siding in actual housing.


Wall component damage in northwest U.S. walls without a vapor retarder on the wall exterior is rare. When it does exist, it typically is caused by rain or other water intrusion from the outside. Whereas, there are numerous examples cited here of wall component damage caused by the existence of a vapor retarder on the exterior of residential walls. The exterior vapor retarder traps moisture migrating from indoors in the sheathing and keeps it from normally drying such that excessive moisture results and damage occurs. There is a clear pattern to the damage, and it does not appear to be just a coincidence.

A substantial cause of sheathing and other damage shown in most of these examples is the existence of impermeable (non-breathable) vinyl siding on the exterior of multifamily housing walls that appears to be relatively airtight, at least in places where there is damage behind it. Otherwise the sheathing should have dried out. While some previous laboratory and test hut studies (1,5) have suggested that contact-applied vinyl siding is fairly air leaky so interior wall components such as sheathing readily dry out and remain damage free, it appears that the conditions in those tests may not have replicated the conditions in some locations in actual multifamily buildings with contact-applied vinyl siding.

This damage problem with vinyl siding is not well known to building scientists or those in the building trades, in part because walls with vinyl siding do not often get opened up. However, based on the large number of examples shown here, it is possibly a more prevalent problem in multifamily housing in Oregon and Washington and similar cold climates, and perhaps even in other cold climates, especially where indoor dew-point temperatures are relatively high and/or where significant air leakage from indoors exists.

Thus, contact-applied vinyl siding as designed and typically installed may be a problematic siding material in some cold winter climates where condensation in sheathing occurs. The same could be said for any contact-applied metal siding in multifamily housing walls without venting or exterior insulation, although wall cavities with such siding have not been observed by the author.


Given the damage observed, it is recommended that, as a precaution, new contact-applied vinyl siding, or possibly metal siding, only be installed in cold winter climates on multifamily housing with a fully vented (both top and bottom) rain screen (1) or with exterior insulating sheathing behind the siding. (6,9,13,16) Both those approaches have been shown to reduce the incidence of moisture problems in residential walls. Rain screens are widely used in Canada to keep walls dry, often by code, although until recently they have seldom been used in most of the U.S. Incorporating either approach should avoid the damage noted here. While that will increase installation costs, it is better to be safe than sorry.

The safest approach likely is to install the siding using a fully vented rain screen with a gap space of about 0.5 in. (13 mm). If installing exterior insulating sheathing, then in milder cold climates such as western Oregon or Washington (Zone 4C) about 1 in. (25 mm) should be a satisfactory minimum thickness, but greater thicknesses may be required in colder cold climates depending on the wall construction (walls in colder climates require more exterior insulation). (16-18) However, the use of exterior insulating sheathing to avoid wall damage with vinyl siding has not been field tested to my knowledge.

Other approaches may also help reduce the incidence of damage, such as improving air sealing on the inside of wall cavities (19) or improving indoor ventilation, but both also involve extra cost. Moreover, as housing gets more airtight, that might lead to increased indoor humidities, which appear to be part of the cause of the problem in multifamily housing.

It would appear prudent to conduct further field studies of contact-applied vinyl sided walls where the siding is removed and the sheathing is examined, especially in other cold climates, to see if the problem is more wide-spread. As part of such studies, it would also be worthwhile to measure air exchange rates across the siding both in locations where there is damage and where there is not damage to determine if in fact the system is as airtight as indicated by the laboratory and test hut studies.

Finally, based on the findings noted here, it is recommended that owners of existing multifamily housing with contact-applied vinyl siding consider conducting wall inspections wherein some siding is removed to examine the condition of WRBs and sheathings. Openings would best be conducted on north walls and especially where indoor humidities are found to be high, say above 60%.

Single-Family Housing

Damage in Walls With Impermeable Cladding or WRB.

1970s Midwest Single-Family Homes. About 6,000 wood-sided single-family homes were built in Wisconsin, Minnesota, and Michigan in the 1970s. The walls were manufactured in a factory and site-assembled. An improper 0.24 perm WRB was specified and installed, which was a vapor retarder on the wrong side of the wall. Extensive plywood sheathing decay was first found in one of the homes about 15 years after construction. After that, numerous field inspections of those homes found a large percentage had widespread but sporadic decay of the plywood sheathing.

During an inspection of 15 homes, all the siding was removed from 11 of them, and 14 had decayed plywood sheathing (12 were severe enough that plywood could be torn apart by hand). (20) Photo 1 shows decayed plywood and a deteriorated WRB in one of those homes. The wood siding was in good condition and did not indicate any of the damage behind it.

The damage was not caused by water intrusion from the outside of the walls. Rather, the moisture and resultant condensation that caused the sheathing damage came from indoors. Air leakage clearly played a role in where damage occurred. That the locations of decayed plywood were random was attributed to variability in locations of air leakage.

Computer simulation of the weekly variation of sheathing and siding moisture content over one full year was undertaken using NIST's MOIST model. The key variable was the permeability of the WRB. Three different values were assumed: 50 for a housewrap, 20 for building paper, and 0.65 for the actual average measured value of 44 samples removed from the field. The results are shown in Figure 1. (20)

The key result is that when a low permeability WRB is assumed compared to conventional breathable housewrap or building paper, the moisture content in the plywood is much higher and is well above the 28% fiber saturation level above which decay can occur. Most importantly, elevated moisture content (MC) levels continue further into warmer weather when decay can and does occur. With more permeable building paper or housewrap, the sheathing moisture content is not elevated above the fiber saturation level during warm weather, so decay cannot occur.

Clearly the low permeability WRB in the Midwest homes acted as a vapor retarder on the outside of the walls and was the culprit that caused the extensive and widespread plywood sheathing damage.

Pre-1970 Northwest Mobile Homes. Prior to about 1970, many mobile homes had corrugated metal siding with a zero perm water-resistive barrier (WRB) right behind the siding. Water vapor would migrate from indoors through fiberglass insulation, and during cold weather it would condense on the cold plastic up against the metal siding. In a project sponsored by the Bonneville Power Administration,21 metal siding was removed from several older mobile homes, and water was observed condensing on the plastic and draining down onto floor framing members. The members were badly decayed (Photo 2). The damage occurred because the impermeable siding/WRB combination did not allow the moisture migrating through the walls during cold weather to dry to the outdoors. One must wonder if similar problems might exist in new multifamily housing with metal siding without proper venting or exterior insulation.

These examples illustrate that damage occurs in walls with a vapor retarder or barrier on the outside of the wall cavity other than just vinyl siding.

Multifamily Housing

Damage Behind Vinyl Siding.

Hillsboro, Ore., Apartments. The walls were constructed with an interior poly vapor retarder and housewrap. There were no kitchen exhaust fans. The bathroom exhaust fans were controlled only by a simple on-off switch, and there was mold in many of the apartments. When siding and housewrap were removed, the exterior paper face of the gypsum sheathing was observed to be notably water stained on some but not all exterior walls and covered breezeways. The staining pattern indicated water was running down the sheathing in a haphazard fashion.

When the gypsum sheathing was removed, the OSB was found to be moldy and badly decayed in places (Photo 3). The extensive damage was not caused by rainwater intrusion from the outside, in part because there was a 2 ft to 4 ft (0.6 m to 1.2 m) wide roof overhang that protected the top portions of the siding. Furthermore, following careful inspection there was no evidence that water was leaking through the siding except in very isolated areas such as some poorly flashed joints between the bottom of the siding and concrete breezeway floors. Therefore, the water that caused the damage had to be coming from moisture in the indoor air that was moving by vapor migration into the wall cavities. While the damage appears mostly uniform in this photo, the damage did not occur uniformly on all walls.

Vancouver, Wash., Multifamily Housing. The walls were constructed with housewrap. Many of the windows had extreme condensation on the inside glass, indicating elevated indoor humidity levels. During sub-freezing weather, water was observed draining from behind some of the siding laps, including well away from penetrations such as windows, and when the siding was opened, water drained out from the back side of the siding. There was a mixture of ice and water on the back side of the siding. The outside face of the gypsum sheathing was badly water stained (Photo 4) and in some cases moldy. The staining was the worst in the area between floors, presumably because there is more air leakage there. In most cases the flashing membranes around windows were properly integrated with the WRB.

Except for those few cases where there was some leakage around windows, there was no evidence of water intrusion from the outside. Many of the wall opening locations were without damage, but some damage existed in some walls. The walls were only opened in a few relatively small locations to determine the type of structural sheathing (OSB), so it was not determined if the OSB was damaged in this case. None of the gypsum sheathing was removed in locations where it was badly stained and/or moldy.

Colville, Wash., Low-Income Housing. The walls of this building in eastern Washington (the coldest climate of all the cases observed) were constructed with a poly vapor barrier, housewrap, and gypsum but no structural sheathing. Water was observed in mid-winter to leak from between siding laps (not through weep holes) in covered breezeways and freeze. That prompted opening walls in those areas and elsewhere. In none of those areas was there evidence of water entering the walls from the outside.

When the north wall siding was later opened in April, water drained out from the back side of the siding that was dripping wet. The exterior face of the gypsum sheathing was moldy and badly water stained. The staining was most prominent where the back edges of the vinyl siding laps pressed up against the gypsum sheathing. Water that condensed on the back side of each siding lap would drain by gravity down to the joint area where the siding intersected the sheathing and wet it, staining it the most.

An I-joist behind the gypsum sheathing in the floor area between the second and third stories was decayed (Photo 5). There was no vapor barrier behind the I-joist between floors. Note that the fiberglass insulation exterior surface was moldy. In some of the walls staining alone was observed, while in some there wasn't even any staining. So, again the staining and damage was not uniform. The worst staining or deterioration occurred on the north walls as well as in the covered breezeways where there is the least solar drying.

Renton, Wash., Multifamily Housing. The walls were constructed with a building paper WRB and a vapor barrier primer rated at one perm on the gypsum wallboard. Numerous walls were opened, and some had wet building paper and damage (even on a south wall), while in many areas there was no wet building paper or damage. One of the worst cases occurred on a north wall where the back side of the siding and the building paper were observed to be dripping wet (Photo 6). When the gypsum sheathing was removed, the OSB was observed to be moldy and decayed (Photo 7). Note that the OSB was not uniformly damaged. The damage was worse on the inside of the OSB than on the outside.

Because the primer was felt to possibly not be a true one perm vapor retarder as advertised on the interior of the wall cavity based on several court cases where nominal one perm vapor barrier primers were found to not be a vapor retarder at all, the drywall was removed and a poly vapor retarder was installed with new plywood and gypsum sheathing and building paper. Similar damage returned to the plywood. Therefore, vapor diffusion from the interior was apparently not the cause of the moisture damage since the new damage occurred after the interior polyethylene was installed. That suggests that air leakage from indoors with resultant condensation was the likely cause since careful inspection indicated there was no evidence of rainwater leakage from the outside.

Portland, Ore., Multifamily Housing. This example was observed during summertime repair of damaged vinyl-clad walls with house-wrap. The vinyl siding was replaced with fiber cement lap siding. One west-facing wall had stained gypsum sheathing where the back edge of the vinyl siding laps pressed up against it. On another south-facing wall, severe decay was observed to framing between floors and plywood sheathing, along with gypsum sheathing staining (Photo 8).

Lynwood, Wash., Multifamily Housing. The walls had a vapor retarder primer on gypsum wallboard and building paper WRB. When the siding was opened in cold weather in a covered breezeway (no possibility of water intrusion from outside) the back side of some of the siding was observed to be dripping wet (Photo 9).

A large section of the building paper was wetted by the condensed water on the back of the siding. However, the wall section to the far right did not have condensed water on it. Not every portion of the walls was adversely affected. In another covered walkway between buildings, water was leaking out from between the siding laps and puddled on the concrete walkway.

Decayed OSB and moldy, stained gypsum sheathing was observed in numerous openings in non-breezeway locations. Such damage was observed sporadically in most north walls, many east and west walls, but few south walls. There was no evidence of water leakage from the outside causing the damage.

Beaverton, Ore., Low-Income Housing. These walls had a one perm interior vapor retarder in the form of asphalt-impregnated kraft paper backing on inset R-19 fiberglass batts and housewrap. Upon removal of drywall, the interior vapor retarders were found to have numerous voids and air leaks that would allow airflow into the wall cavities behind, along with other air leaks into the wall cavities. There was not an effective air barrier on the inside of the walls. Kitchen fans were run continuously at 20 cfm (9.4 L/s). Only some of the bathroom exhaust fans had timer controls that allowed them to run for an hour after showering. There were passive air inlets in the bedrooms, but air circulation below the bedroom doors was obstructed by carpets.

After OSB decay was observed in one wall opening during a summer inspection, the walls of the eight buildings were opened five times between December and May in 48 random locations (one location was opened in July). Some of the openings had vinyl siding that was dripping wet on its back side (even in two locations in May and July), some had wet OSB (greater than 40% moisture content in about one-quarter of the locations), about a third of the OSB was moldy, some of the framing was decayed, and about one-quarter had OSB decay. (See an example of the latter in Photo 10 with vertical board and batten-style contact-vinyl siding. Most cases were with the usual horizontal lap siding.) The OSB decay was typically worse on the back side, suggesting moisture was coming from the inside. In fact, close inspection indicated that the damage was not caused by water intrusion from the outside.

OSB decay occurred sporadically on the walls, but was most prominent on some but not all locations of the north-facing walls, with some decay observed in east and west wall locations. There was less decay in south-facing walls, although the worst case occurred there in one very shaded location with high indoor dew-point temperatures. Widespread damage occurred to the OSB of one north-facing wall of one of the buildings where all the siding and WRB was removed (Photo 11). In that same wall, some decayed OSB was replaced with plywood during a summer prior to the wall openings, and the plywood was soaking wet and showing signs of decay the next spring.

Much of the worst sheathing damage there was between floors where the sheathing was exposed to air leakage because the vapor retarders were either missing or poorly sealed, or in locations outside of interior partition walls (such as in closet areas) where there was no interior vapor barrier or air barrier such that there presumably was relatively large air leakage. It is likely that damage typically occurring sporadically on walls (Photo 12) was due to localized variations in the air leakiness of the siding. There were numerous air leakage paths through the walls due to things like framing errors, localized lack of a vapor barrier, wiring holes, and even plumbing holes in the framing.

Temperature and relative humidity sensing data loggers were installed in 19 rooms. Damage correlated with bedrooms with high indoor air dew-point temperatures. Many, but not all, of the bedrooms had dew-point temperatures in the high fifties ([degrees]F) [tens ([degrees]C)] and/or relative humidity percentages in the 60s and 70s. The variability in indoor dew-point temperatures explains, at least in part, the variability in walls with elevated moisture contents and resultant damage. There was no mold observed on indoor surfaces except for very small amounts on window frames in a few bedrooms.

WUFI modeling (22) was undertaken to compare the performance of OSB sheathing in a north wall with vinyl siding to OSB in the same wall but with fiber cement siding. The width of the air cavity between the siding and WRB was assumed to be 0.39 in. (10 mm) without contact (one-dimensional analysis). A parametric analysis was performed to examine the effect of the air leakage across the siding. The modeling showed that no moisture contents above fiber saturation occurred (hence no decay) in either case with venting above about 10 air changes per hour (ach) (air leakage across the siding).

Under the same conditions with 4 ach venting assumed, the OSB moisture content remained well above its fiber saturation level that allows decay with vapor impermeable vinyl siding and well below with vapor permeable fiber cement siding. That suggested impermeable contact-applied vinyl siding with relatively low air exchange is likely a factor in the damage observed.

Of course, the modeling does not provide any information about how leaky the siding actually is. It merely indicates that the simulated wall is sensitive to moisture accumulation when the ventilation rate across the siding is assumed to be relatively low. The field observations appear to be consistent with low cladding ventilation rates, at least in those areas where damage occurred.


(1.) Straube, J., R. VanStraaten, E. Burnett, C. Schumacher. 2004. "Development of Design Strategies for Rainscreen and Sheathing Membrane Performance in Wood Frame Walls." ASHRAE Research Project RP- 1091, Final Report.

(2.) Burnett, E., A. Reynolds. 1991. "Final Report--Ontario Wall Drying Study." University of Waterloo, Building Engineering Group Report for Canada Mortgage and Housing Corp.

(3.) Forest. T., I. Walker. 1990. "Drying of Walls--Prairie Region." Canadian Mortgage and Housing Corp. Report by Dept. of Mechanical Engineering, University of Alberta.

(4.) McCuaig. L. 1988. "Final Report on the Drying of Walls--Atlantic Canada 1987." Canada Mortgage and Housing Corp; also, 1988. CMHS/CHBA Joint Task Force on Moisture Problems in Atlantic Canada Final Report.

(5.) Onysko, D., M. Bomberg, J.F. Straube, C.J. Schumacher. 2002. "Heat, air, and moisture control in walls of Canadian houses: lessons learned from field experience and monitoring studies." Proceedings of Canada-Japan Building Envelope ExpertMeeting.

(6.) Tichy, R., C. Murray. 2007. "Developing Innovative Wall Systems that Improve Hygrothermal Performance of Residential Buildings: Final Report." Washington State University Energy Program for the U.S. Department of Energy.

(7.) Tsongas, G.A., et al. 1979. 'A Field Study of Moisture Damage in Walls Insulated Without a Vapor Barrier." ORNL/Sub-78/97726/1. Oak Ridge National Laboratory; also, 1980. ASHRAE SP 28. Proceedings of the 1979ASHRAE/DOE-ORNL Conference on the Thermal Performance of the Exterior Envelopes of Buildings.

(8.) Tsongas, G.A. 1985. "The Spokane wall insulation project: a field study of moisture damage in walls insulated without a vapor barrier." Proceedings of the ASHRAE/DOE/BTECC Conference on Thermal Performance of the Exterior Envelopes of Buildings III; also, 1985. DOE/BP-541. U.S. DOE/Bonneville Power Administration.

(9.) Tsongas, G.A. 1990. "The Northwest Wall Moisture Study: A Field Study of Excess Moisture in Walls and Moisture Problems and Damage in New Northwest Homes." DOE/BP-91489-1. U.S. DOE/ Bonneville Power Administration.

(10.) Tsongas, G.A., G. Nelson. 1991. "A field test for correlation of air leakage and high moisture content sites in tightly built walls." ASHRAE Transactions 97(1).

(11.) U.S. Forest Products Lab. 1987. Wood Handbook: Wood as an Engineering Material.

(12.) Baker, M.C. 1969. "Decay of Wood." Canadian Building Digest CBD111. National Research Council of Canada.

(13.) Joy, F.A. 1951. "Basic Concepts of Water Vapor Migration and Their Application to Frame Walls." Technical Paper NO89. Pennsylvania State College Engineering Experiment Station.

(14.) Wang, F.S. 1979. "Comparative Studies of Vapor Condensation Potentials in Wood Framed Walls." ASHRAE/U.S. DOE Conference.

(15.) CMHC. 1999. Best Practice Guide for Wood-Frame Envelopes.

(16.) Straube, J. 2011. "BSD-163: Controlling Cold-Weather Condensation Using Insulation." Building Science Corporation.

(17.) International Residential Code (IRC)-2015, Table 702.2.1.

(18.) DeKorne, C. 2017. "Avoiding wet walls." Journal of Light Construction 35(8):47-51.

(19.) TenWolde, A., C.G. Carll. 1992. "Effect of cavity ventilation on moisture in walls and roofs." Proceedings of the Exterior Envelopes of Buildings V, pp. 555-562.

(20.) Tsongas, G., J. Olson. 1995. "Tri State homes: a case study of extensive decay in the walls of Wisconsin manufactured homes." Proceedings, Thermal Performance of the Exterior Envelopes of Buildings VI Conference.

(21.) Tsongas, G. 1995. "Wall Wood Decay and Weatherizing Older Mobile Homes," RCDP Technical Update, DOE/BP-2545. U.S. DOE/ Bonneville Power Administration.

(22.) Karagiozis, A., H. Kunzel, A. Holm. 2001. "WUFI-ORNL/IBP--A North American hygrothermal model." Proceedings of DOE/ASHRAE Buildings VIII Conference.

George A. Tsongas, Ph.D., P.E., is a consulting engineer and building scientist and professor emeritus of mechanical engineering at Portland State University, Portland, Ore.

Caption: PHOTO 1 Decayed plywood sheathing and deteriorated WRB in a Midwest single-family home wall.

Caption: FIGURE 1 Plywood sheathing and siding moisture contents as a function of WRB permeability.

Caption: PHOTO 2 Decayed floor framing in older mobile home with metal siding.

Caption: PHOTO 3 Moldy and decayed OSB sheathing.

Caption: PHOTO 4 Water stained gypsum sheathing (most severe between floors).

Caption: PHOTO 5 Badly water stained, moldy gypsum sheathing and decayed I-joist.

Caption: PHOTO 6 Soaking wet building paper behind the vinyl siding.

Caption: PHOTO 7 Moldy, decayed OSB behind gypsum sheathing and vinyl siding.

Caption: PHOTO 8 Decayed floor framing and plywood sheathing with stained gypsum.

Caption: PHOTO 9 Back side of siding dripping wet.

Caption: PHOTO 10 Moldy OSB and housewrap with OSB decay (not affected by vent).

Caption: PHOTO 11 Darker OSB areas indicating damage (serious decay to right).

Caption: PHOTO 12 Sporadic OSB sheathing damage on somewhat sheltered south-facing wall.
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Title Annotation:Cautionary Case Studies
Author:Tsongas, George A.
Publication:ASHRAE Journal
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2017
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