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Cavity Walls Using Exterior Insulating Sheathing for Commercial Construction

January 8, 2014

Plasti-FabWelcome back to our blog series on commercial construction and Plasti-Fab EPS insulation solutions. Before the holidays we covered grade beams, frost walls, under slab insulation, and foundation insulation solutions for commercial building applications. Moving up the building, we begin to discuss exterior insulating sheathing solutions, starting this week with the application known as Cavity wall construction using exterior insulating sheathing.

Rain Screens (Cavity) Walls-1What is a Rain Screen (Cavity) Wall?

Cavity wall construction is a modification of cavity wall construction that was developed to prevent rain penetration through the interior masonry wythe (A vertical section of bricks or other masonry that is one unit thick) by providing a drainage cavity within the wall assembly. The cavity between the interior and exterior wythes of the wall construction is an ideal place for insulation, however filling the cavity with insulation takes away its value as a drainage area for moisture, hence the development of rain screen cavity wall design..

Architects and designers seeking better moisture management and energy efficiency for the exterior walls of their projects are turning to rain screen wall designs incorporating insulating sheathing as a solution for both. Rain screen cavity walls use exterior sheathing insulation attached over the weather-resistant barrier which has been applied over the interior wythe of the wall assembly. A drainage cavity is created between the face of the insulating sheathing and the back of the exterior cladding.  A rigid insulating sheathing board helps to protect the weather-resistant barrier and the drainage cavity allows any moisture that may pass through the exterior cladding to easily drain away from the wall assembly. By allowing air to flow behind the exterior cladding pressure equalization occurs in the drainage cavity which accelerates evaporation of any residual moisture.  The surface of the insulating sheathing provides an additional drainage plane.

Plasti-Fab’s PlastiSpan insulation is recommended for rain screen cavity walls. It is a rigid insulation that provides continuous insulation over the wall surface. As well, it provides a capillary breaking layer that resists penetration of moisture.

Exterior insulating sheathing should be viewed as a building envelope insulation, whose primary function is not to provide barrier protection against water penetration (which is provided by the weather-resistant barrier), but instead is designed to limit the amount of water that could potentially come into contact with the building envelope’s primary moisture barrier, reducing the chance of water finding a way into the wall assembly.

Why should you design and use rain screen cavity walls in commercial construction? 

A rain screen system allows architects, designers and builders numerous benefits in moisture-management and energy-efficiency, including:

  •  Insulating sheathing keeps moisture away from the interior of the wall assembly
  • Water behind cladding can escape by means of evaporation, reducing the potential for mold
  • Reduces energy costs by limiting heat transfer through the wall
  • Promotes water drainage by channeling moisture to the outside of the wall assembly
  • Air space between exterior cladding and drainage plane helps drainage and facilitates drying

How-to install PlastiSpan Insulation to achieve a Rain Screen (Cavity) Wall: 

Preparation
Provide through-wall flashings at the base of walls, at headers, over windows and at other locations where water could gather in the cavity. Provide weep holes or vents as required.

1) A vertical section of bricks or other masonry that is one unit thick, known as a wythe, is built up first. Cut insulation board to fit around projections, into returns, etc. to provide a completely insulated wall.

2) Connect inner and outer wythes with non-corroding metal cladding ties spaced as specified by exterior cladding supplier so that insulation boards can be fitted between them.

3) Provide fire stops where required at floor levels, if construction system does not provide for them.

4) Use adhesive to caulk around cladding ties and to fill any other breaks in the insulation.

5) Lay outer wythe, known as the final exterior cladding, usually brick, with care so that the drainage cavity is not filled with mortar. Ensure that drain and vent openings are clear.

For more information on rain screens and their application, visit www.plastifab.com or contact us today 1-88-THINK-EPS®

Stay tuned for next week’s blog as we cover another form of exterior sheathing insulation for your commercial structure.

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