Siding, and the part of the job you cannot go back and fix

Most siding pages are a materials comparison and nothing else, as though the decision were about the colour of the outside. The material matters. What is behind it matters more, and it is the only part you cannot go back and fix.

Start with the part nobody photographs

Water gets into a wall at the openings, not through the middle of a wall. What stops it is a water-resistive barrier lapped in the right order and flashing integrated into it at every window head, every jamb, every material transition, every deck ledger and every place a roof meets a wall. The code sections are R703.1 general weather resistance, R703.2 water-resistive barrier, R703.4 flashing at the head and sides of window and door openings, R703.4.1 pan flashing.

Caulk is not flashing. Caulk is a maintenance item with a service life measured in years; flashing is a permanent piece of the assembly that works by gravity and lapping. A wall relying on sealant to keep water out is on a countdown.

The drainage gap

Stucco absolutely needs an air gap. Manufactured stone, wood shingles and fiber cement all want 3/8 inch. Vinyl and aluminum need nothing extra, because their own geometry leaves plenty of air behind them.

What happens when we open the wall and find rot

It is not bad luck; it is the second most likely finding on any house with a roof-wall intersection and no kickout. What it should be is a written change order agreed before we start — a price per square foot of sheathing against the area actually found, not a number that appears on the final invoice. And the work is: remove everything compromised back to sound material, find and fix the source, replace framing where the decay reached the studs, then rebuild the barrier and flashing before any cladding goes back on. A contractor who covers rotten sheathing with new siding has turned a repairable defect into a hidden one.

OSB, which is behind most walls built since the eighties, fails fast enough once wet that people in the trade call it vertical mulch. A 1925 house with uninsulated walls dried itself out through its own air leaks. A modern insulated wall does not get that mercy.

Now the materials

Vinyl

Cheapest to buy and to install, never needs painting, and by an accident of its own geometry it is one of the better claddings for keeping a wall dry — it drains and vents behind itself without anyone designing that in. It fails by cracking on impact, worst in cold weather when the PVC is brittle, and by buckling when it is nailed too tight. Vinyl moves half an inch or more across a twelve-foot length with temperature, which is why the installation manual asks for roughly a dime's thickness of clearance under the fastener head. Nailed home, it tears at the slot or waves in the sun.

One failure mode almost nobody warns about: reflected sunlight from a neighbour's low-e windows can melt it. Vinyl softens around 165°F and measured reflected-solar temperatures have exceeded 300°F. The industry position is that this is beyond the manufacturer's control, which in warranty language means not covered.

Insulated vinyl

Vinyl with a contoured foam backer. It is flatter, quieter and considerably more dent-resistant, and the standard requires a minimum of R-2.0. Sell it, and buy it, for those reasons. Do not buy it as a code-level continuous insulation solution — the code contemplates R-5 to R-10 for that, and R-2 is not in the same conversation.

Fiber cement

The one product on this list that returns more at resale than it costs, in this region. It is dimensionally stable, it does not feed insects, and it is non-combustible — flame spread index of zero, smoke developed under five, classified non-combustible under the relevant standard.

It is also the material most often installed wrong, and the defects are specific and checkable: less than the required clearances (two inches to a roof, six inches to grade, one inch from gutter end caps), fasteners closer than three-eighths of an inch to a board edge, overdriven nails, unsealed field cuts wicking water into the core, missing Z-flashing at horizontal joints and window transitions, and head lap under the required inch and a quarter to inch and a half. Paint blistering on fiber cement is not a paint problem; it is moisture arriving from behind.

Honest downside: it is heavy, it is expensive to install, and the finish is the part that ages. Substrate life and finish life are two different numbers, and repainting is generally on a ten-to-fifteen-year cycle.

Engineered wood

Treated wood strand, lighter and easier to cut than fiber cement, with a longer nailing range and a warmer look. Failures concentrate at cut ends and anywhere the factory treatment is breached — so sealing field cuts is not optional. The only head-to-head hail data published puts it ahead of both fiber cement and vinyl, and that test was paid for by the manufacturer, which you should weigh accordingly.

Steel and aluminium

Steel is the hardest of these to dent through and rusts at cut edges and fastener penetrations if those are not handled. Aluminium does not rot or burn but chalks — the factory finish oxidises to a powder that comes off on your hand — and dents permanently. On a house with sound aluminium siding the real question is repaint versus replace, and repaint is often the honest answer.

Brick, and why we will not side it

A solid multi-wythe masonry wall — which is most of what was built here before the war — has no studs, no sheathing and no cavity. There is nothing to fasten to, nothing to lap a barrier into, and the wall works by absorbing rain and drying back out rather than shedding it. Cladding it means drilling through the one thing controlling the rain and then trapping water in a wall that has no way to release it. What these houses need is tuckpointing, lintel work and drainage at the base. Anyone offering to side your solid brick house is telling you something about themselves.

Synthetic stucco

The failure history is documented and dated. Roughly ninety-five per cent of American applications were the face-sealed kind with no drainage path, the failures traced overwhelmingly to omitted or badly installed flashing, and the specifications changed around 1996 and 1997. So pre-1997 is the high-risk cohort. Drainable systems installed since are a different product and are not the problem. Building Science Corporation's position is that face-sealed systems are unfit for use anywhere receiving more than twenty inches of rain a year. St. Louis gets over forty.

Going over the existing siding

The vinyl industry manual permits it and conditions it heavily: loose boards nailed down and rotten ones replaced first, furring strips where needed to get a flat continuous plane, a continuous water-resistive barrier, and fasteners penetrating at least an inch and a quarter into nailable wood. Vinyl must never go directly to studs without sheathing.

When it is a bad idea: any suspicion of sheathing rot, because you are burying the evidence. Over a wall whose windows were never properly flashed, because you are locking the defect in. Over solid masonry, because there is nothing to fasten to. Over asbestos-cement it can be the right answer, deliberately, because removal is what triggers the regulatory burden — but it has to be a disclosed decision, and it leaves the material there for the next owner.

Insulating while the siding is off

This is the upgrade worth considering and the one most often done badly. The mechanism matters: adding insulation to the outside of the sheathing makes it warmer and safer; adding air-permeable insulation to the stud cavity alone makes the sheathing colder and increases condensation risk. A thin layer of foam that is not enough to keep the sheathing above the dew point can be worse than no foam at all.

For Zone 4A the building-science guidance is that no interior vapour control is required on a normal framed wall with fibrous cavity insulation — and that interior Class I vapour retarders, meaning polyethylene sheet, foil facings and vinyl wallpaper, can cause premature enclosure failure through inward drive. St. Louis gets summer vapour drive inward and winter drive outward, so a wall sealed tight on the inside traps water from one direction or the other.

Sources: buildingscience.com · buildingscience.com · basc.pnnl.gov · ci.independence.mn.us · polymericexteriors.org · inspectorproinsurance.com · icc-es.org · greenbuildingadvisor.com · buildingscience.com · jlconline.com · nachi.org