Building Science Part 4 Moisture

Generally, proximity to water and water views increase the value of your properties. So why is managing moisture in your building investment just as important? It is a money saver. Dry building envelopes are more thermally efficient and more energy-efficient. And, when dry, building envelopes are more durable and require less maintenance and upkeep. A dry building envelope also means a more comfortable and healthy occupant experience (OX). Allowing too much moisture into the building envelope can result in a host of problems affecting the sustainability of the building and the health of its occupants. One of the biggest concerns is the damage caused by sustained exposure of a building envelope to moisture.

Moisture exposure can lead to condensation, staining, mold and mildew. Wood and wood-based materials in the building can decay if exposed to moisture for too long. Masonry and concrete are also prone to moisture damage during freeze-thaw cycles, especially in colder climates. Other materials like standard drywall under extreme conditions can hydrate and ultimately turn to mush. Of course, metals will corrode and, in addition, there’s damage caused by hygric expansion of materials. Wet cement, for example, will expand, then dry and contract, which can cause gaps and cracks in structures over time. Finally, moisture can cause aesthetic problems, such as the buckling of wood, efflorescence of brick and mortar, and paint failure. But, that’s not all. There’s also moisture damage that’s not as visible.

Moisture Flow: Why It Matters and How to Control It

Hidden within building cavities, wet insulation loses R-value and makes the building less energy-efficient. Over time, many building components deteriorate and fail from being repeatedly saturated. Mold can form on many wet surfaces, especially when there’s moisture trapped in wall, floor and ceiling assemblies. For mold to form and grow, there must be a food source, moderate temperatures, oxygen and sufficient moisture present. Any carbon-based substance can be a mold food source, especially one that is cellulose- or wood-based. Once moisture is exposed to the food source, long-dormant mold spores can gain a foothold. As long as the food and moisture sources are supplied, the mold colonies will continue to grow and prosper.

Hidden Moisture Damage

What makes Mold Growth and Corrosion possible?

We can evaluate the risk for mold growth as well as the other enemy, corrosion, by observing surface relative humidity averaged over time within a specific temperature range. Conditions that are ripe for mold and corrosion are: a surface relative humidity greater than 80 percent and temperatures ranging between 41° F and 104° F, on a 30-day running average.

Know How Moisture Flows

There are four mechanisms that cause the transmission of moisture within a building. Here are some tips for you and your team on preventing each from spreading moisture throughout your investment.

Gravity

Gravity moves rainwater down the building’s exterior surfaces. If there are holes through those assemblies, water will pass through them. To prevent this, first demand flashings to keep rainwater out and wherever there are joints in the building envelope, overlap the components with no reversed laps.

Capillary

Capillary suction is caused by surface tension of water, and water is drawn in through tiny pores in various materials, often so small they are invisible to the eye. To prevent this sponge-like flow it is important to break the continuity of materials from the exterior through to the interior so moisture doesn’t have a clear path. Create breaks in the components, small cavities that prevent moisture from migrating through all the layers of materials. These spaces should drain and vent to the exterior so drying can occur.

Airborne Moisture Flow

Another way moisture is carried into a building is by air flow. Air movement will bring a great deal of moisture into a building unless it is prevented by good construction practices. An open 1-inch hole in in an assembly to permit air flow and airborne moisture flow can add up to 30 quarts of water to a cavity or an interior over a typical heating season. This creates a dramatic case for making assemblies airtight and preventing moisture from condensing on cold surfaces.

Water Vapor Diffusion

Water vapor will move or diffuse through building materials when areas of high vapor pressure and low vapor pressure exist on opposite sides of that material. The movement is from the high vapor pressure side to the low-pressure side. The best way to prevent water vapor diffusion is with a good vapor retarder.

Construction Moisture

Construction moisture is unavoidable, so there’s a risk all your projects face right from the start. For instance, concrete mixes require large amounts of water, and concrete assemblies take years to dry out — not weeks or months. Unfortunately, that’s one moisture source that we have to live with unless substantially reducing with the use of mass-timber construction.

Ground Moisture

It’s important to do your best to protect your building from ground moisture, such as rainwater. Use vapor barriers under concrete slabs to help keep ground moisture from migrating upward. Demand proper footing drainage at the perimeter of the building. Rainwater that hits the building should be directed away from the building and runoff should go into a drainage system or the water must be directed far enough away from the building that it doesn't accumulate at the foundation.

Mechanical Equipment

Mechanical equipment rooms can be problematic, as they’re usually located at the foundation level and frequently generate moisture. It’s important to isolate and air seal these rooms because of the strong stack effect usually associated with them.

Internal Sources

And lastly, there are many internal sources for moisture, including: commercial kitchens, lavatories, fountains, plant life, maintenance (washing floors, pressure washing of interior spaces, etc.) and people going about their daily activities. Think of all of these internal sources pressed together in a four-story office building. They might seem small in comparison to the external sources, but combined, they’re a force to be reckoned with.

Moisture Sources

The wall assembly is the first place to fortify a building’s defense against moisture, with good wall material choices and, most importantly, adequate insulation, air barriers and vapor retarders. Brick, concrete block and stucco have a fairly large capacity to store moisture within them. We consider them storage reservoir systems. This difference, reservoir or non-reservoir, literally defines how your building envelope must be designed.

The most common commercial wall assembly is the steel stud cavity. This is the best recommendation for long life and optimum performance. Here are some guidelines on designing a steel stud cavity assembly: First, always use a water-resistive barrier (WRB). It’s the first line of defense against rainwater intrusion. Water will enter somewhere, somehow, so use a ventilation and drainage space between the masonry façade and the WRB. You want to maximize condensation control several ways — first, by using insulating sheathings. Use exterior air/wind barriers, since air can transport considerable moisture into assemblies, if not blocked. Use interior air barriers to help control wintertime moisture from migrating and condensing on cold surfaces. And, use a smart vapor retarder to not only control the wintertime moisture but also allow assemblies to breathe during other seasons.

Commercial Wall Assemblies

Climate Consideration

The weather patterns from season to season depend on the climate of the building’s location. When you are designing a building envelope, you have to consider the climate of the location. Factors to take into account are average cold temperatures and precipitation. Once you find your location, you will have two important data points for predicting what weather patterns your investment can be expected to face down the road.

Today’s building codes consider the impact of temperature
and moisture together. Generally speaking, building
envelopes in cold and extreme cold climate zones focus on
heating systems, while building envelopes in hot-dry and
hot-humid climates focus on air conditioning systems. For
areas of “mixed” experience both hot and cold climates, it
is wise to investigate the microclimate and local experience
when planning a building.

There are many methods of moisture control, each
adaptable to specific climates. Be sure to have your team
pay special attention to:

Rainwater Management
Air/Wind Barriers
Vapor Retarder requirements for the Climate

It’s always a good idea to check the local building code, as
it may vary across all the asset locations for your portfolio.

Solar Heat

Solar heat drives moisture into building materials. This is
especially true in the case of claddings that absorb and
retain large amounts of moisture. The sun comes out, creates a high vapor pressure at the outside, and this drives the stored moisture inward, into building assemblies. Since it’s nearly impossible to prevent moisture from entering 100 percent of the time, using a smart vapor retarder permits water to escape from wall assemblies as water vapor, which permits them to dry.

More in this Series:

Leveraging Occupant Experience to Increase Value

Improving Occupant Experience with Thermal Control

Keeping the Good Air In and the Bad Air Out

Protect Your Investment with Moisture Control

Don't Hope for Good Acoustics

Building Sustainable Investments

An Occupant Experience Focus Leads to Healthier, More Productive Spaces

Following the above guidelines will help you lead your teams toward a better building envelope with good moisture management performance. Good moisture management creates a healthier, more comfortable environment for occupants and durable value for your investment.

Want complimentary building science expertise and insight? Solving issues before they become problems is just one area in which Saint-Gobain Commercial Solution’s building scientists deliver value. By helping our strategic owner and developer partners discover the pitfalls and unintended consequences of projects, they can avoid the need for costly change orders or post-occupancy renovations and deliver a more profitable result.

We are passionate about great occupant experiences and helping clients understand what they are really going to get for their investment. We take great pride in providing our clients more confidence in the construction decisions that they are making. It is just one way we achieve our vision of making the world a better home.

For Your Success

When designing a building envelope one of the best tools for predicting its moisture management performance is hygrothermal analysis. Hygrothermal analysis done by experienced building scientists can help predict the impact of transient heat and moisture on building envelopes over time. It may be used on construction projects you are planning and on existing buildings with moisture problems. It involves specialized software that helps visualize surface condensation and mold growth potential; the wetting and drying potential of the building envelope; and moisture content of building components. This analysis can help you evaluate potential pre-construction moisture risks and analyze and resolve moisture problems post-construction.

Hygrothermal analysis takes into consideration both the geographic location and the building orientation. Reports can be generated to compare any two materials in a building system for surface condensation and moisture content, combining the data to show potential mold growth risk. The hygrothermal analysis reporting described here is available to strategic partners of Saint-Gobain Commercial Solutions.

Predicting Moisture Risk

Moisture Control in Roofing & Fenestration

When it comes to roofing, windows, curtain walls and doors, proper installation and flashing is critical for protection from moisture. Airtight systems work best at reducing or eliminating moisture infiltration. With low-sloped roofs, a good drainage system will prevent roof damage from standing water. Isolate parapets from all hidden wall cavities and roof-ceiling cavities to reduce airborne moisture flow. We’ve seen how this flow can cause ice damming and the expensive damage this can create over time. Finally, with curtain walls, you will want to make sure to thermally isolate hidden metal surfaces to reduce the potential for surface condensation.