Dave Holbrook

Best practise for re-siding a home calls for stripping off existing siding, then installing a good weather condition barrier. This can decidedly complicate a new siding job on an existing house. Instead, the industry has migrated towards what is widely marketed as an easy fix and a way to improve thermal performance and water resistance at the same time: installing fan-fold foam board over existing siding. Only if it'south not practical properly, this route may lead to more bug than it fixes.

Not Insulation
At virtually ¼ inch thick, fan-fold foam lath adds merely R-ane to R-ane.5 to the wall system, a negligible insulation improvement. Foil-faced products often hope higher R-values, but that is false advertisement. There may exist some radiant effect that can reduce cooling loads, just only when a foil-faced material is installed with a minimum ¾-inch air space in front of information technology—an option that requires strapping the wall before hanging the siding.

According to cream-lath manufacturers, the advantage of fan-fold is really that the lath provides a consistent surface over the old siding, eliminating cracks and crevices that lead to free energy-robbing drafts. All manufacturers recommend taping the foam to aid stop air leaks and foreclose water that gets through the siding from leaking behind the cream. Unfortunately, record is often disregarded by siders pushing to get the job done as apace as possible. And fifty-fifty if you do tape, there may be problems.

"The danger is that you are putting an impermeable layer over the wall," explains Steve Easley, a building-science expert who specializes in helping builders eliminate callbacks. "If the wall behind the cream e'er gets wet—from a siding leak or a roof leak, fifty-fifty a plumbing leak from inside, or from excessive interior moisture condensing in the wall cavity, or any the moisture source—that wall will be very boring to dry." Even when a perforated foam production is used, a solution that minimally improves the permeability of the foam, Easley argues that the potential for moisture problems volition far outweigh the minor energy improvement afforded by the foam.

Stripping off siding and retrofitting exterior insulation is a superior move, but it requires extra attention to interior walls and at openings.
Ted Cushman Stripping off siding and retrofitting exterior insulation is a superior motility, but it requires extra attention to interior walls and at openings.

Stop the Air
The important matter to sympathize, says Easley, is that heat and wet move speedily through walls on air currents. "Stopping drafts is the all-time fashion to ameliorate thermal performance and ensure the long-term immovability of a wall system," he says. "Far more important than a vapor barrier is how well you stop the air menstruum. Air carries far more moisture than vapor diffusion in any climate. And then when weighing your options, you actually should be thinking about the best way to terminate air leaks." The vapor bulwark becomes a problem when it prevents a wall from drying out. If a wall gets wet, the just way information technology tin can dry is by evaporation. "You want a permeable wall that can dry out out before mold, mildew, or rot takes over," Easley says.

Continuous Outside Insulation
How nigh thicker foam? These days, many new homes are congenital with rigid foam on the exterior sheathing—a key thermal improvement that breaks the direct line of conductive estrus catamenia through wall framing. Continuous outside insulation works well for new construction when the walls are designed to dry to the interior. This means they accept no interior vapor control layers, peculiarly no vinyl wallpaper or vapor-blocking paints.

Remodeling is a different animal. If you lot install foam on the outside of an one-time firm, and the interior walls are covered with vinyl wallpaper or have a build-up of interior paint from years' and years' worth of repainting, you'll take a wall organisation that won't e'er dry if information technology gets wet. That will likely lead to mold or rot—liability no one wants to incur.

Continuous exterior foam can be retrofit to existing homes, every bit shown in the photograph on page 22, but doing so without incurring risk requires careful attention to the interior walls. The thicker foam too complicates how you finish around windows and doors and is all-time done as part of whole-firm rehab that includes new windows and doors, too.

Y'all likewise take to install enough foam. Because fan-fold doesn't really practise anything for thermal performance, there remains a take a chance of condensation. The same applies to as well thin a layer of rigid foam in a cold climate: The capsule remains common cold, and then there is a potential for water to condense on the inside surface—a quick style to start a mold farm. (To learn more than, see "Avoiding Wet Walls" in The Journal of Light Construction, May 2022).

Where'due south the Water?
"Also recall about how yous'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? Practise you know it'due south 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 risky, argues Easley. It'south really the only mode to adequately control the water, and it will provide a expert opportunity to improve air-sealing, which will get a long way toward improved condolement—a gene that may assistance sell the larger scope of piece of work.

Every bit for the insulation, if the walls are completely uninsulated, either dense-pack cellulose or fiberglass blown into the cavities is a simple nonetheless effective choice, though this may exist outside the telescopic of the replacement-siding contractor'due south piece of work.

Substrate options for vinyl siding. Fan-fold foam can work over existing siding but is a poor choice and viable only if the seams are taped.
Illustrations by Tim Healey Substrate options for vinyl siding. Fan-fold cream can work over existing siding but is a poor selection and feasible only if the seams are taped.

Vinyl Siding Substrates
The Vinyl Siding Institute considers vinyl siding to be office of a "water-resistive barrier system," not a h2o-resistive barrier by itself. Vinyl siding stops only some of h2o that hits the wall. Properly installed vinyl siding is attached somewhat loosely to let it to expand and contract with temperature changes, and so it's ever subject to some wet penetration from wind-driven rain and similar sources of h2o. A good water-resistive material roofing the sheathing is vital for protecting the wall.

Past pattern, vinyl works well as a rainscreen. Considering the panels are hollow and have slotted nail hems, they allow sufficient airflow every bit well as an escape path for moisture to drain off. The spaces and materials behind the vinyl dry out readily—even without strapping equally a spacer. However, if y'all are installing vinyl or whatever other siding over thick rigid cream, strapping is needed as a nail base, fifty-fifty if it's not technically required every bit a drainage space.