New Construction: the Three Predictable Failure Points.

The call surprises people every time: a house less than two years old, in a clean new subdivision in Bend or Hillsboro or south Eugene, with mice in the pantry. Surely a new house, built to a modern energy code and wrapped tight enough to need mechanical ventilation, should be rodent-proof. It is not, and the reasons are completely predictable. Energy-tight is not the same as rodent-tight — air sealing stops drafts, not animals — and modern construction has three details that fail with such regularity we can almost guarantee them on a walk-through. The good news is that on a new house all three are easy to reach and easy to fix.

Air-tight is not rodent-tight. A new house can pass a blower-door test and still have a half-inch hole a mouse walks through.

Failure point 1: the weep screed and the siding base

Modern stucco and many siding systems are designed to drain — water that gets behind the cladding has to weep out at the bottom. On stucco that exit is the weep screed, a metal flashing at the base of the wall with a continuous slot along it; on lap and panel siding it is the gap left under the starter course and behind the trim. These openings are intentional and necessary, but they often lead into the wall cavity, and where the builder left the bottom of the cavity open, a mouse that finds the weep slot is inside the wall. The fix is not to block the drainage — that causes rot — but to ensure the bottom of the stud bay is sealed off from the weep path, and to screen any oversized base gaps behind the siding.

Failure point 2: utility penetrations the trades left open

This is the big one, and it is a coordination failure more than a design flaw. A modern house is pierced by an astonishing number of services — the HVAC line set and condensate, the gas line, the electrical service and low-voltage entries, the plumbing supply, the radon and sump lines, the dryer and bath fan terminations, the hose bibs. Each trade drills its hole, runs its line, and moves on; sealing the annular gap around the pipe is nobody's specific job, and on a tight construction schedule it is the step that gets skipped. The result is a brand-new house with a dozen finger-width gaps where lines pass the rim joist and the exterior wall — and the rim joist in new construction, behind that line set or the laundry box, is exactly where we find fresh mouse sign.

What the fix looks like

1. Walk the exterior and find every point where a pipe, wire, or duct passes through the wall or the foundation.
2. At each one, check the annular gap around the line. Pack it with copper mesh and seal it with an appropriate sealant — fire-rated where it passes a rated assembly.
3. Inside, check the same penetrations at the rim joist, behind the water heater, the laundry box, and the HVAC closet, where the gaps are usually larger and never finished.

Failure point 3: the garage door corners

The third failure is the same one old houses have, because the garage door is not an energy-code assembly and gets no air-sealing attention at all. New garage doors leave the same two triangular gaps at the bottom corners covered in our threshold article, and new builds frequently ship with no corner seals and a connecting door to the house that has a generous gap underneath and no sweep. On a new home the garage is also where the water heater, the HVAC, and a knee-wall into the house often live, so a rodent in the garage is a short walk from conditioned space. Corner seals and a gasket on the connecting door close it for a few dollars.

Why new houses fool people

The reason new construction catches owners off guard is that it looks finished. There is no visible decay, no obvious gap, no century-old foundation crumbling at the corner — just clean siding and a fresh slab. But rodent exclusion was never part of the inspection that signed off the house. The energy code cares about air leakage measured at pressure; it does not care whether a half-inch hole around the gas line is screened, because air sealing and rodent proofing are different goals achieved with different materials. A house can be genuinely tight and still have three open doors a mouse can find in a night.

When to call

For a new house this is often a satisfying DIY afternoon, because everything is accessible, undamaged, and easy to reach — you are sealing clean penetrations, not chasing rot. Walk the perimeter, seal every line that passes the wall, screen the weep-path gaps without blocking drainage, and put corner seals on the garage. If your new home is on a crawl space or has a complex roofline, or if you would rather have someone confirm you have found every penetration the trades left, an exclusion-focused operator can do the full walk and close it in one visit. Either way, the lesson is the same: do not assume new means sealed. New means accessible — which is the best time there is to rodent-proof a house properly, before any animal has moved in.