Best practice calls for stripping off existing siding, then installing a good weather barrier.

Fan-fold foam board is widely marketed as an easy fix for improving the thermal performance and water resistance of walls when re-siding. But if not applied properly, this option could lead to moisture problems, according to leading building-science experts.

At about ¼-inch-thick, fan-fold foam board adds only R-1 to R-1.5 to the wall system, a negligible insulation improvement. Foil-faced products promise higher R-values, but only when installed with a minimum ¾-inch air space in front of them, an option that requires siding over strapping. According to foam board manufacturers, the advantage of fan-fold is really that the board provides a consistent surface over the old siding, eliminating cracks and crevices that lead to energy-robbing drafts. All manufacturers recommend taping the foam to help stop air leaks and prevent water that gets through the siding from leaking behind the foam.

The Journal of Light Construction

Certainly, the foam board provides a smooth working surface for installing new siding, but whether there is a significant improvement in thermal performance is hotly debated. “The danger is that you are putting an impermeable layer over the wall,” explains Steve Easley, a building consultant who specializes in helping builders eliminate call-backs. “If that wall ever gets wet from a siding leak or a roof leak, a plumbing leak, excessive interior moisture condensing in the wall cavity, or whatever the moisture source, that wall will be very slow to dry.” Even with a perforated foam product, a solution that minimally improves the permeability of the foam, Easley argues that the potential for moisture problems will far outweigh the minor energy improvement afforded by the foam.

Stop the Air The important thing to understand, says Easley, is that heat and moisture move rapidly through walls on air currents. “Stopping drafts is the best way to improve thermal performance and ensure the long-term durability of a wall system,” he says. “Far more important than a vapor barrier is how well you stop the air flow. Air carries far more moisture than vapor diffusion in any climate. So when weighing your options, you really should be thinking about the best way to stop air leaks.” Where the vapor barrier becomes a problem is when it prevents a wall from drying out. If a wall gets wet, the only way it can dry is by evaporation. “You want a permeable wall that can dry out before mold, mildew, or rot takes over,” Easley says.

Joe Lstiburek, a leading building researcher and principal of Building Science Corp. recommended foam sheathing in some of the high-performance wall systems his company designed for Building America's Houses That Work program ( But, it's important to understand that these walls were designed for new construction. Specifically, the walls that use foam on the outside are designed to dry to the inside. Remodeling is a very different animal. If you slap up foam on the outside and the interior walls are covered with vinyl wallpaper or years and years worth of repainting, you'll have a wall system that won't ever dry if it gets wet.

Where's the Water? “Think about how you're changing the water flow when installing new siding,” Easley says. “J-channel, in particular, rechannels the water. Do you know where it's going? Do you know it's draining away from the wall? The only way to know for sure is if you strip off the existing siding, wrap the walls with a permeable housewrap and flash the windows.”

Although many homeowners will be reluctant to pay for the tear-off, anything less is irresponsible, argues Easley. As for the insulation, if the walls are completely uninsulated, either dense-pack cellulose or fiberglass blown into the cavities is usually the best option. But reliably sealing the air leaks is the surest bet of all.