Facilities (Learning Spaces)

Managing Moisture

Koliganek Huskies school


Schools in rural Alaska face some of the harshest conditions on earth — below-zero temperatures, wind, snow and driving rain.

And these villages, home for many Alaska natives, are so remote that there are no roads to the communities. Getting construction materials to these far-removed villages is a huge undertaking. The communities generally have between 200 and 900 residents and schools serve the entire K-12 population.

Designing these facilities takes a unique approach suited for their unique settings. Construction of a K-12 school is traditionally the largest, most expensive and most important building in the village. The schools must last 30 to 50 years in a hostile weather environment.

A strong building envelope is essential to long-term performance of the school building. Key considerations in the design process include understanding thermal and moisture management; accommodating a challenging construction environment; air tightness; continuity in vapor retarders, insulation materials and moisture barriers; and the durability of the exterior.

Thermal and moisture management

Just recently, my freezer demonstrated the challenges that cold-weather envelopes face. Like many Alaskans, I spend some portion of the summer catching, cleaning, processing and storing fish. My preferred method is to vacuum seal and freeze my fillets. After a seemingly wetter than average summer in southcentral Alaska, I was disappointed to find my outdoor freezer lid frozen shut and ice cascading from the lid to the bottom of my freezer encasing my bounty.

An afternoon with the freezer off, coolers handy, fans turning and gloves soaked, I was able to remedy the issue. But my freezer incident highlighted two of the basic issues that building envelopes must recognize in the Alaska heating climate: 1) air transport of vapor, and 2) moisture management.

Air transport of vapor: In general, water vapor moves from warm to cold. In the case of my freezer, the water moved from the outdoor summer air into where my fish packs are located by sneaking through in gaps in the seals. In the case of a school, it goes in reverse, from the warm interior to the below-freezing exterior or into the wall assembly all winter long.

cut away of building envelope construction

Moisture management of rain and condensation: The discussion is also relevant to seasonal changes in rainfall and humidity. In Alaska, this summer’s higher than average humidly caused issues with my freezer that I have not experienced in years with vapor and moisture seeping inside. For schools, we must design the building envelope to anticipate seasonal variations in rainfall and humidity.

diagram of school wall construction

Stay Warm (and Dry). Schools in rural Alaska face some of the harshest conditions on earth. Designing these facilities takes a unique approach suited for their unique settings. Key to success for a strong building envelope is the selection of materials that will endure in rural Alaska’s harsh environment. The structural frames in the schools we design are either a mix of glulam wood beams and steel or all steel. The key design goal is to limit any penetrations of structure from cold to warm space. The best way to ensure that you will have long-lasting schools is effectively control the vapor and moisture in the thermal envelope.

Key to success for a strong building envelope is the selection of materials that will endure in rural Alaska’s harsh environment. Structural Insulated Panels (SIPs) have proven invaluable on multiple rural school projects because they allow a tight envelope to be constructed quickly and easily. The SIPs consist of wood sheathing (OSB or plywood) sandwiched onto a core layer of Expanded Polystyrene Insulation (EPS). The thickness of the roof and wall panels are constructed with and based on dimensional lumber sizes and provide options for R-value or thermal resistance based on 2x4, 2x6, 2x8 and 2x12 standard thickness. Typically, the SIPs are used on the walls, roof and building soffits.

Accommodating realities in construction

Seasonal barge schedules set the timetable for rural Alaska construction. The villages are not connected the state’s road system. Everything is delivered by boat or airplane. The high cost and space limitations of air freight mean that a vast majority of materials and equipment arrive via barge — and those barges only travel about half the year. It is simply the reality of construction in remote Alaska.

If the design and bidding of a project is on schedule and the project site is ready, construction materials can arrive on the first barge of the season, generally in May. This allows for four or five months of site, foundation and framing construction to begin to accept the SIPs. The roof generally goes on in August/September, which is traditionally one of the wettest times of the year. In recent years, Western Alaska averages about 3¼ inches of rain in August and 2¾ inches in September.

Knowing that the roof will likely see heavy rains during construction, the school roof assembly is built with a vapor permeable self-adhered membrane and a 3½-inch ventilation space above that sits atop the SIPs. Above this is another layer of plywood, fully adhered non-permeable waterproof membrane and metal roofing. The vent space offers two key functions: 1) it allows the roof panels to dry from anticipated wetting that occurs during construction, and 2) it allows for future moisture to vent and dry out, ensuring a long functional roof life.

Air tightness

My freezer experience proves that vapor transport via thermally driven diffusion and air transport doesn’t take a day off. Just a small leak in the freezer door seal allowed nearly a gallon of water to slowly enter, condense and freeze over the course of two months.

It is the same principle with a school. The exterior envelope must be tight. Every wall penetration is an opportunity for air leakage and moisture damage. Limiting the penetrations from interior to exterior within the thermal and vapor barriers improves the tightness of the envelope.

Heating climates require the vapor retarder (polyethylene plastic sheet) to be on the warm side of the wall assembly. In typical construction, every outlet or light switch on an exterior wall is a potential leak. To limit this potential, the wall system assembly has the vapor retarder installed over the interior face of the SIPs without penetrations. Of course, a school needs lots of outlets, data wires and plumbing supply lines. Our designs place 2½-inch metal furring inside the SIPs to house all those elements within the warm space of the building. Gypsum wall board is the interior finish.

Because of the separation, all outlets, pipes and penetrations are no longer potential liabilities. It also is easier to update the school for future use without damaging the vapor retarder or requiring an expensive modification to the envelope.

Thermal envelope continuity

The structural frames in the schools we design are either a mix of glulam wood beams and steel or all steel. The key design goal is to limit any penetrations of structure from cold to warm space.

school construction site

In the subarctic region where most of these schools are going up, moisture in the air will condense on building elements (like moisture on a cold glass) and steel, especially with its conductive factors, will transfer that cold all the way into the building. As the cold moves into the school, it collects moisture from the heated air and releases it as condensation on other interior building finishes resulting in mold, mildew and possible decay. To reduce this, the best practice is to completely wrap or contain all structures within the thermal envelope. In some instances, such as support cantilevers, it is impossible to have them within the envelope. To limit condensation issues, cantilevers and other similar elements are built with wood or other material with a limited ability to conduct temperature.

Exterior durability

When the school is complete, the majority of the important building envelope elements are hidden behind the exterior finish. Especially in remote Alaska, the finish isn’t simply icing on a cake, it plays a crucial role in the overall functionality of the building and the envelope.

All of these remote Alaska schools are the center of the community and most are in use 12 to 15 hours a day. Even after the doors are locked, children are still playing at and around the school. The constant activity means that the wall, window and door surfaces are within arm’s reach or stone’s throw and need protection. Finishes need to be affordable, easy to install, easy to maintain and tough.

For the exterior walls, a heavier gauge (generally 22 gauge) large rib metal siding installed with flutes vertically aligned works well. It is more difficult to dent, allows the exterior to dry and is reasonably priced. The gauge can be lessened and the same profile metal used at adjacent less accessible areas, resulting in a consistent look but with some cost savings. High quality vinyl windows with double or triple panes are the standard choice for these schools. Protecting the windows is essential. The best practice is covering the exterior windows with a high-quality polycarbonate sheet. The polycarbonate is nearly as clear as glass and with proper detailing is easily replaceable without removing the windows.

Moisture outside, comfort inside

Controlling vapor and moisture in the thermal envelope means efficient, long-lasting schools. Keeping it out of my freezer means I get to enjoy salmon all winter and have a place to keep next season’s catch.

This article originally appeared in the December 2016 issue of School Planning & Management.

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