Using various design and construction techniques designed to eliminate moisture problems

by Sally Bouorm | December 1, 2011 3:03 pm

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By Jason Smith

Recently, MZO Group, an architecture firm based in Boston, Mass., was tasked with a challenging problem—design and build a residential indoor, heated pool in the often cold, snowy Massachusetts climate, a scenario that could lead to significant moisture problems.

It was challenging request, made more complicated by the fact the pool house had to be isolated from the rest of the client’s newly constructed home along a shared 10.6-m (35-ft) common wall. The design goal was to isolate the pool house from both the outdoors and the interior of the home as much as possible.

“I aimed to build a structure that you could turn upside down and it still wouldn’t leak,” says architect Eric Gjerde, MZO Group. To achieve this, a ‘belt and suspenders’ approach, with redundant safeguards against moisture problems, was used.

Building this unique structure required a well-planned design from architectural, heating, ventilating and cooling equipment (HVAC) and building science perspectives. Throughout the project, Jensen Development Corp., the homebuilder, and J&J Mechanical, the HVAC contractor, worked closely with the architects to ensure all systems would work well together.

Design considerations

Designing a heated indoor pool in a wood frame structure for a cold climate presents a unique set of challenges, as the potential for high humidity air to condense on a cold surface and cause mould or rot is high. When designing the pool house, there were several design requirements to consider, including:

• a continuous air barrier system;
• a continuous vapour retarder;
• highly insulated wall assemblies; and
• elimination of thermal bridging.

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To help meet these requirements, closed-cell spray polyurethane foam (SPF) was selected, because it is the only insulant that can serve as an air, moisture and thermal barrier at once. SPF is the ‘belt’ in the belt-and-suspenders design, meeting the first three of the aforementioned design requirements. To meet the final requirement, 25 mm (1 in.) of polyisocyanurate board was used to eliminate thermal bridging from the studs.

“Closed-cell SPF is a great choice because it has a high R-value, or thermal resistance, per inch, serves as an air barrier and moisture retarder and increases the structural performance of traditional light frame wood construction,” Gjerde says.

The ‘suspenders’ comprise a redundant self-adhered waterproofing membrane and moisture retarder between the polyisocyanurate board and interior gypsum. A mould- and moisture-resistant gypsum board was used and painted with a moisture-resistant epoxy coating. The exterior was sheathed in plywood, a less moisture-sensitive alternative to oriented strand board (OSB). The sheathing was then covered in a weather-resistive barrier and finished with cedar shingles.

Air barrier and vapour retarder

Due to the potential for high humidity from the pool interior, both an air barrier and vapour retarder were critical to the project. Moisture vapour is transported via two different mechanisms into wall assemblies: air movement and diffusion.

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Air movement is typically the larger source of moisture problems. At 21 C (70 F) and 40 per cent relative humidity, a 25-mm (1-in.) hole will allow 90 times more water into a wall via air leakage than a 1.2- x 2.4-m (4- x 8-ft) gypsum board will via diffusion. Thus, an air barrier not only improves energy efficiency, it also serves as the first line of defence against potential moisture problems.

The vapour retarder is the second line of defence, protecting against moisture problems due to the moisture diffusion mechanism. With any design, the vapour retarder should be on the ‘warm’ side of the insulation, as it was in this project; this prevents moisture diffusion to the cold side of the assembly.

When designing an assembly, each potential condensing surface should either be warmer than the dew point or sufficiently protected from encountering moisture via diffusion or air infiltration. While multiple vapour retarders are typically discouraged, these three layers are locked into a solid, airtight assembly, preventing a moisture-trap scenario.

Insulation

Extremely humid air brings with it a high risk of condensation on walls and windows. To eliminate condensation in this project, the structure was super-insulated with R-values approximately twice the code requirements. Each cavity was filled to its entire depth to avoid any air space within the system. Not only does this eliminate the condensation potential, it also significantly reduces the heating bill for an otherwise energy-intensive building.

As the relative humidity of the indoor air rises, the difference in temperature between the air and wall surface at which condensation occurs narrows. With conditions of 25.5 C (78 F) and 80 per cent humidity, a surface temperature of 21.6 C (71 F) will cause condensation. Therefore, it was critical to strengthen the thermal weak spots in the wall. In this case, to eliminate thermal bridging at the studs, foil-faced 25-mm (1-in.) polyisocyanurate board was used on the interior of the studs adding a continuous insulation of R-value 6.5. For the glazing, high-performance, highly insulated windows were installed.

Mechanical matters

The pool house’s HVAC unit was a stand-alone system, disconnected from the main house. Supply registers were placed above the windows to keep warm air flowing over the surface, preventing condensation. Because of the airtightness of the structure, controlled mechanical ventilation (at rates defined by American Society of Heating, Refrigerating and Air Conditioning Engineers [ASHRAE] Standards) was provided via a heat-recovery ventilator (HRV). Lastly, the design team set out to control humidity by installing a separate dehumidification system decoupled from the heating and cooling systems. The target relative humidity range was 50 to 60 per cent.

Occupant behaviour

In addition to the careful design and construction of the project, the builder also coached the homeowner on how to keep relative humidity down and lower the run time and energy usage of the dehumidification system. First, to minimize evaporation, the clients were counseled to use the pool cover whenever the pool is not in use. They were also advised to set the pool water temperature one to two degrees ( lower than the pool house air temperature and keep it lower than 26.6 C (80 F) to slow the evaporation rate.

Conclusion

By managing the moisture hazard on multiple levels, MZO Group designed an enclosure that will ensure the client’s indoor pool will last a long time. By providing an effective air barrier, along with a moisture vapour retarder to the warm side of the insulation, the probability that moisture will enter the wall assembly (where it could condense and cause mould or rot) is minimized. The proper amount of insulation and high-quality windows ensure condensation does not occur on the interior surfaces.

By designing the indoor space as though it were outdoors, bringing in the mechanical contractor early in the design process and ensuring high-quality, detail-oriented work by subtrades, the project team was able to design and build an outstanding structure that will withstand the challenges of a difficult environment.

 

 

 

Jason Smith is the market development specialist in the Americas region for the Spray Polyurethane Foam (SPF) team within the Polyurethanes Division of Huntsman. Jason has eight years of experience in the chemical industry, including both technical and commercial roles. He can be reached via e-mail at jason_smith@huntsman.com[4].

Source URL: https://www.poolspamarketing.com/trade/using-various-design-and-construction-techniques-designed-to-eliminate-moisture-problems/

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