Installing rigid extruded polystyrene foam - Pro Construction Guide

Installing rigid extruded polystyrene foam

Installing rigid extruded polystyrene (XPS) foamThere are many innovative ways to build an energy-efficient wall in new construction.

However,  considering that 130 million of the residential structures in the United States are already built, with a median construction age of 40 years old, according to the 2013 U.S. Census, we need effective solutions to increase the thermal performance of the existing building stock.

Attaching rigid continuous exterior insulation boards − extruded polystyrene insulation (XPS) foam − to the exterior walls is one such method and is an effective alternative to drilling and filling cavities.

Exterior insulation creates an uninterrupted layer, which is a good way to address thermal bridging through wood studs and rim joists. It provides exceptional moisture resistance and can be applied over existing cladding, sheathing materials or directly to the studs.

Another advantage is that by putting the insulation on the outside rather than in the cavity, the dew point (when vapor condenses into water) is moved towards the exterior of the wall potentially reducing the wall getting wet in the first place.

We recommend application of a weather resistive barrier under the exterior insulation, but it can also be applied over the exterior insulation.

Installation of 2-inch rigid extruded polystyrene (XPS) foam

Exterior insulation boards typically come in 2x8s or 4x8s and have a tongue and groove to help lock them together. Use ⅜-inch galvanized roofing nails or 1-inch crown, 16-gauge staples or plastic cap nails to attach the panels to the wall.

Space the fasteners at least 24 inches apart along the studs. Use a utility knife or handsaw to trim the boards to fit around windows and doors. Do not caulk the joints as the foam will expand and contract and caulk may actually hold water against the wall rather than letting it drain away.

If you want to seal the panels, there are tapes made to seal the joints that will provide additional protection against air and moisture infiltration. Check with the manufacturer of the exterior foam panels for a compatible tape for the application.

Flashing around windows is also recommended to create a water-resistive seal, helping to prevent water damage. After the foam is in place, battens are strongly suggested to help keep the assembly dry and allow any water that gets behind the cladding to escape. Siding is then attached directly to these battens.

One-inch-thick boards will add an R5 to your building and installing 2 inches will add R10. While the 2-inch exterior insulation is better from an energy stance, it may lead to complexities with exterior trim around windows and doors, and with attachment of cladding.

This may sound like an expensive wall, but it isn’t when you consider the extra insulation is far less expensive than the installation of solar panels needed to generate the equivalent energy otherwise lost through an R20 wall.

Potential issues

It may be necessary to install jamb extensions around the window to compensate for the foam thickness. Jamb extensions can be made from durable exterior trim materials and are necessary to cover the side of the exposed foam before the window/door trim and siding are attached.

Another potential issue is how to attach the cladding. With 1 inch of exterior insulation, longer siding nails can typically be used to attach through the cladding and into the framing. However, with 2-inch exterior insulation, nominal 1×4 wood battens should be placed over the exterior insulation and structural screws installed to penetrate the framing.

The siding is then only attached to the wood battens. The screws should be 6 inches long for the 2-inch exterior insulation and wood batten system, since nails from a typical siding (coil type) nail gun are not long enough.

If you are attaching the wood battens with any insulation thickness, a drainage and ventilation space is created behind the cladding called a rain screen. This technique has been proven to provide a greater level of durability to both cladding and the building.

Building codes

Codes, standards and energy-efficiency programs are quickly moving away from prescribing nominal insulation values and towards recognition of the total conductive heat flow both through insulation and framing elements. Therefore, we expect to see exterior insulation on a number of new construction projects, as well as on existing retrofits.

This chart is an example of the types of Full Wall calculations the codes are using.

 2x6 wall, 16” OC, OSB Sheathing, R19 cavity insulation2x4 wall, 16” OC, 1” XPS  (R5) sheathing, R13 cavity insulation2x6 wall, 24” OC, 2” XPS sheathing, R24 blown cavity insulation
Exterior air film0.170.170.17
Hollow vinyl siding0.620.620.62
Exterior insulation0510
Interior air film0.680.680.68
TOTAL Cavity R-value21.5420.5436.54
Framing R-value8.3711.2518.37
Total Wall - effective R-value (Cavity + Framing)14.6815.7927.05

These calculations were done using the Isothermal Planes (Series-Parallel) method as described in 2009 ASHRAE Handbook Fundamentals. Note that using this method, a 2×4 wall with R14 cavity insulation and 1 inch of extruded foam sheathing has a slightly higher R value than a 2×6 wall with a nominal insulation value of R20.

Note also that an advanced framed wall with 2-inch XPS sheathing and R24 cavity insulation shows a 75 percent improvement in resistance to heat flow compared to the 2×6, R20 nominal insulation wall. This new emphasis on total effective resistance is the second reason to recalibrate our thinking about appropriate insulation levels and strategies.

This initiative requires process changes beyond the simple, historical approach of stuffing more and better insulation into cavities. These changes require consideration of at least cladding and window attachment, structural integrity, wall dimensions and resulting possible house design changes, and even water and air barrier detailing.

Despite the complexity of including insulated sheathing or implementing advanced framing, the relative value of continuous insulation and the cost savings of fewer wood members will be difficult to ignore as the thermal performance of buildings is advanced.

This special feature is provided by Construction Instruction. Among the most-respected building scientists in North America, Construction Instruction teaches best practice  building methods. Use Construction Instruction’s mobile app to get construction videos, building science articles, technical data and best practices  anytime.


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