Humidity and building envelope failures

Specialized expertise is needed when designing and maintaining an enclosure for an indoor spa, pool or steam room to avoid the negative effects of condensation, such as mold growth, rotting wood, flaking paint or rust and damage to exterior building envelope elements

By Chander Thusu September 7, 2022
Courtesy: IRC Building Sciences Group, a Rimkus Co.


Learning Objectives

  • Learn about the obstacles or impediments a building science design engineer must overcome in designing high-humidity enclosures.
  • Learn the various parameters that must be adapted to for the successful design of a high-humidity enclosure.
  • Understand the primary and secondary lines of defense to mitigate collateral damage to a building caused by high moisture and thermal gradient.

Building Enclosure Insights

  • Building enclosures with high humidity in places such as swimming pools, spa pools and steam rooms come with additional challenges for building designers because humidity can have a major effect on the immediate and surrounding area.
  • Designers also must account for seasonal changes because what is required for the winter is very different from what is required during the summer months. Humidity, steam and heat each provide their own challenges despite being similar regarding temperature.

High-humidity building enclosures are often building occupancies where the relative humidity and temperature of indoor air are very high all year-round. This paper will focus on building enclosure types classified under the following categories:

  • Swimming pools.
  • Spa pools.
  • Steam rooms.

All three enclosures (swimming pools, spa pools and steam rooms) may have building envelope elements linked to exterior ambient conditions for all four seasons and therefore result in a challenge for the building science engineer to design for an efficient moisture management and building envelope system — especially for the ambient conditions prevalent during late fall, winter and early spring seasons.

However, if swimming and spa pools have no direct link to exterior ambient conditions, the design of these enclosures to mitigate condensation becomes comparatively easy. Condensation’s negative effects such as the formation of mold spores, bacteria, fungi, rotting of wood, flaking of paint and rust formation on the metal surfaces, is minimized by careful and efficient design.

In swimming pool and spa pool enclosures, the air temperature ranges between 81°F to 86°F and relative humidity ranges between 100% at the surface of pool water to 50% to 60% at upper levels in enclosed areas of spa and swimming pools. In steam rooms, relative humidity is around 99% and air temperature around 113°F.

In winter, any hot, moist air may escape from the pool enclosure into the wall cavity, either due to poor design and/or poor construction of the wall system and will condense in the incipient areas of the wall cavity. This will compromise the integrity of the wall system’s components due to secondary effects of condensed water such as freeze-thaw damage in the exterior masonry cavity walls. This phenomenon of condensation may also happen during spring or fall if the temperature any plane of the cavity wall is below dewpoint. The condensed water in these seasons will lead to formation of fungi and mold and rotting of some building materials susceptible to moisture.

On the other hand, steam rooms, which generally operate at 99% RH and 113°F temperature, will take a drop in temperature of less than (~1°F) to exhibit surface condensation. As a result, the hot, moist air prevalent in steam rooms will diffuse through the interior static air mass serving as an insulating surface in front of the walls and ceiling surfaces of the steam room and condense because the temperature of these walls and ceiling surfaces are below the dewpoint.

Building enclosures in high-humidity areas

Hot, moist air common in high-humidity enclosures is transported from the interior to exterior by two ways:

Convection/advection: Differential air pressure between the interior and exterior of the enclosure.
Vapor diffusion: Differential vapor pressure between the interior and exterior of the enclosure.

Vapor diffusion in the presence of an effective vapor retarder is a very slow process. Any condensation problem developed due to vapor diffusion through the series of building materials forming the wall or roof element is not a serious problem because any hot, moist air, which may diffuse through the vapor retarder, will be very minuscule in the quantity. The diffused hot, moist air will condense within some plane of the cross section of wall or the roof, where the temperature of the condensed surface will be below the dewpoint. Any condensed water in the inverted roofing system or the exterior insulated cavity wall system will eventually evaporate to the exterior without causing measurable damage to any component of the building envelope. If the hot, moist air bypasses the vapor retarder of the conventional roofing system, where the insulating layer is between the vapor retarder and the finished roofing membrane, reverse vapor diffusion in combination with the effective dehumidification system will minimize the moisture damage within the conventional roofing system. However, some of the hot, moist air may condense within the conventional roofing system and thereby compromise its integrity.

Most of the building envelope problems in high-humidity enclosures are due to advection. Advection can take place through penetration points such as a poorly-sealed fenestration system, fasteners securing the roofing insulation boards, electrical conduits, light fixtures, sprinkler heads, plumbing pipes and sheet metal ducts. These penetrations create discontinuity in the air/vapor barrier systems of the enclosure and thereby provide an easy path for transportation of moisture around the crack lines, which are discontinuity lines between the penetrating elements and adjoining air/vapor barrier. In absence of effective building science design for such enclosures, especially steam rooms, condensation of hot, moist air in the concealed spaces is always present, resulting in a massive damage to the concealed elements of the structure after one to two years of commissioning of such enclosures.

The condensed water also may facilitate the growth of mold spores where organics are evident, creating health issues for the users. This is especially true in steam rooms and standalone spa pools, which are often small enclosures compared to wider enclosures such as swimming pools.
The ambient conditions prevalent in all four seasons are poles asunder, resulting in a challenge for the building science engineer to design high-humidity enclosures for an efficient moisture management and building envelope system. This is especially true for the ambient conditions prevalent during late fall, winter and early spring seasons.
Transportation of hot, moist air by advection in swimming pools can be mitigated by good mechanical design incorporating an effective dehumidification system, maintaining slightly lower (negative) pressure within swimming pools compared to the exterior and blowing warm, dry air on the fenestration elements to raise the dewpoint of the fenestration components. Even by maintaining the negative air pressure in swimming pools, vapor diffusion may still be prevalent in these elements, which should be mitigated by incorporating an effective vapor retarder in the wall and roof assemblies forming part of the building envelope. These instances represent unique building science challenges when different climates interact within the building and across the building enclosure.

Building enclosure challenges

It is possible the consultants and designers may overlook the intricacies of these interior enclosures because they are not part of the traditional building envelope design. However, the potential issues that could arise in a building with high humidity and high temperature indoor environments can be avoided or mitigated during the design phase with proper considerations and the right approach.

However, it is not as simple as specifying typical building materials for the various control layers (air vapor, thermal and moisture) and providing generic details for such control layers will not eliminate the potential problems in the building envelope in high-humidity buildings without careful thinking on part of the building science design engineer and adopting proper building science principles to overcome the challenges in the design of high-humidity enclosures.

It is imperative to understand the fundamental principles of building science to analyze the incipient conditions in these enclosures, as depicted below in two separate case studies involving high humidity and high temperature conditions.

Swimming pool building enclosure

This project included a large facility with an indoor pool enclosed by exterior walls composed of a masonry cavity wall and fenestration system incorporated within the exterior wall system. The façade system showed extensive water staining and formation of icicles during winter months (Photograph 1).

Photograph 1: Hot, moist air is egressing from the pool enclosure due to discontinuities in transition membrane around the perimeter of the window/curtainwall. Courtesy: IRC Building Sciences Group, a Rimkus Co.

The composition of the masonry wall depicted in Photograph 1 is:
Masonry concrete block inner wythe.
Torch-grade, modified bitumen air/vapor barrier membrane.
Semi-rigid mineral wool insulation.
Architectural block exterior wythe.
Peel-and-stick transition membrane between the fenestration and façade.

The façade and fenestration system depicted above exhibited massive transport of hot, moist air from the pool enclosure to exterior, where the transition membrane was either missing or compromised. In selected locations, it was established most of the hot, moist air was egressing along the windowsill line, where the air/vapor barrier transition membrane was totally compromised.

A breach in the continuity of the air/vapor barrier between the roof and wall was evident as manifested in the formation of icicles along the underside of roof flashing membrane (Photograph 2). Hot, moist air egressed from the swimming pool along the junction between the parapet, curtainwall and inner wythe of the pool cavity wall, resulting in the condensation of hot, moist air and the eventual formation of icicles in sub-zero temperatures.

Photograph 2: Interface of roof, masonry wall and curtainwall as viewed from ground level. Moisture escaped from the swimming pool and facilitated the formation of icicles. Courtesy: IRC Building Sciences Group, a Rimkus Co.

Photograph 3: Electrical junction boxes and conduits installed to accommodate pool lighting over the plywood substrate over the tongue-and-groove wood deck, thereby causing a potential path for egress of hot, moist air from the pool to the underside of the thermoplastic polyolefin membrane. Courtesy: IRC Building Sciences Group, a Rimkus Co.

In closure, to summarize:

  • In swimming pools, the interior pressure should be slightly below the exterior to prevent transportation of hot air to the exterior and to protect building envelope elements enclosing the pool and surroundings from damaging effects of condensation.
  • Design of the walls, windows and roof membrane assembly should be such so the transition membrane necessary to prevent the egression of hot, moist air will stay functional during the entire life span of the enclosure.
  • It is imperative good design is complemented with good construction practices to ensure the pool, spa and steam room enclosures do not compromise the integrity of the surrounding structures.
  • The heating and moisture load should be thoroughly evaluated to choose efficient dehumidification and pool heating systems.
  • Any penetration in the wall, window or roof system should be adequately addressed in the design stage to minimize the future moisture egression problems from the swimming pool enclosure.
  • In steam rooms, it is imperative to incorporate multiple levels of moisture, air and vapor control layers to prevent the transport of hot, moist air outside the steam room enclosure.
  • Steam rooms should have an exhaust system installed at systematic locations to prevent moisture buildup outside the steam room enclosure.

Author Bio: Chander Thusu is an engineer with more than 32 years of experience. He serves as Building Sciences Practice Leader with IRC Building Sciences Group, A Rimkus Company.