What's the difference between new-construction and replacement window installation?

New-construction windows have a nailing fin and are installed into an open wall before siding goes on, allowing full access to flash the opening properly. Replacement windows fit into an existing frame with siding already in place, which limits how much of the original flashing can be redone. New construction gives an installer the best possible chance to get water management right from the start.

How do I check that a window contractor is actually qualified for a new-construction job in this area?

Ask for proof of licensing and insurance, and ask specifically how they handle sill pan flashing and shingle-lap sequencing — vague answers are a red flag. It also helps to ask how many new-construction jobs they've done in Whatcom County specifically, since local wet-climate experience shows up in the details.

Do certain window brands hold up better in Whatcom County's marine climate?

Most major window manufacturers offer products that perform well here as long as the material and hardware are rated for coastal exposure. The bigger factor than brand is usually installation quality — even a premium window will leak if the sill pan and flashing aren't done correctly.

What's a nailing fin and why does it matter for a new build?

The nailing fin is the flange around a new-construction window that gets fastened to the wall sheathing and integrated into the weather-resistive barrier and flashing system. It's what allows the shingle-lap flashing sequence that keeps water moving outward instead of into the wall.

How does Bellingham's moss season affect new windows specifically?

Moss and algae need sustained moisture and shade to establish, and poorly flashed sills or trim create exactly those conditions. Correctly flashed and sloped sills shed water fast enough that growth is much less likely to take hold around the window itself over the following wet seasons.

New-Construction Windows in Sehome | Bellingham Roofing Co.

New-Construction Windows Built for Sehome's Weather

Sehome sits close enough to Bellingham Bay that salt-laden air, wind-driven rain, and long stretches of gray, damp weather are just part of owning a home here. When you're framing a new build or a major addition in this neighborhood, the windows you choose and how they're installed will decide whether that house stays dry, efficient, and low-maintenance for the next 20-plus years — or starts showing rot, fogged glass, and drafts within the first five. New-construction window installation is different from a retrofit or replacement job. It happens before the siding goes on, while the wall assembly is still open, which means there's exactly one chance to get the flashing, the water management, and the window-to-wall integration right.

We install new-construction windows on homes throughout Whatcom County, and Sehome jobs get the same attention to moisture detailing that we'd give a house right on the waterfront. Being a few blocks or a mile inland from the Bay doesn't spare a structure from the marine layer, the wind that funnels through the valley, or the moss and algae that grow on anything that stays damp too long.

Why Sehome's Climate Changes the Job

Bellingham's marine climate means new construction here has to be built to handle sustained moisture exposure, not just the occasional storm. A few conditions specific to this area matter for window selection and installation:

Salt air corrosion: Even a few miles from saltwater, airborne salt accelerates corrosion on lower-grade hardware, fasteners, and cladding finishes over time.
Driving rain: Wind off the Bay pushes rain sideways against west- and south-facing walls, putting real pressure on flashing details and sill pans that a calm-weather installation wouldn't have to withstand.
Long moss and algae season: Whatcom County's wet months stretch long, and any spot on a house that stays shaded and damp — under a window sill, behind poorly lapped flashing — becomes a place where moss, algae, and eventually rot get a foothold.
Temperature swings: Cool, damp winters and mild summers mean window frames and seals go through constant expansion and contraction. Poor-quality seals or sloppy shimming show up as air leaks and condensation faster here than in a drier climate.

None of this means Sehome needs exotic materials. It means ordinary materials need to be installed correctly, every time, with water management treated as the priority it is.

What a Correct New-Construction Window Install Involves

Rough Opening Preparation

Before a single window goes in, the rough opening has to be square, correctly sized, and properly wrapped with a weather-resistive barrier. We check that the opening allows for the manufacturer's specified clearances and that the sill is sloped or otherwise set up to shed water outward, not let it pool.

Sill Pan Flashing

This is the step that separates a durable install from a future leak. A sill pan — whether a manufactured flexible-membrane pan or a properly built-up flashing system — creates a waterproof pocket under the window that channels any water that gets past the window itself back outside, instead of into the wall cavity. Given how much wind-driven rain this area sees, we treat sill pan flashing as non-negotiable, not an upgrade.

Window-to-Wall Integration (Shingle-Lap Sequencing)

Flashing tape and building paper have to be layered like shingles — each piece overlapping the one below it — so water is always directed down and out, never trapped behind a lap that runs the wrong way. Jamb flashing goes on next, then head flashing last, tucked behind the water-resistive barrier above the window so runoff from the wall above sheds past the window, not into it.

Fastening and Shimming

Windows get shimmed plumb, level, and square at every fastening point specified by the manufacturer — not just at the corners. Under-shimming leads to frame flex, seal failure, and operational problems (windows that stick or won't latch) well before their expected service life is up.

Insulation and Air Sealing

The gap between the window frame and the rough opening gets filled with a low-expansion foam or backer rod and sealant rated for window installation — never standard high-expansion foam, which can bow frames and interfere with proper operation.

Final Sealant and Trim

Exterior trim and sealant joints close out the installation, using sealants rated for the exposure the wall will actually see. On west- and south-facing elevations that catch the brunt of driving rain, we're more conservative about sealant choice and joint design than on a sheltered north wall.

Choosing Window Materials for This Climate

There's no single "best" window material for every house — the right choice depends on the home's design, budget, and exposure. Here's how the common options hold up under Sehome's conditions:

Material	Moisture Performance	Maintenance	Notes for This Area
Vinyl	Good; won't rot or corrode	Low	Budget-friendly, reliable seal life; a solid default for most new builds here
Fiberglass	Very good; dimensionally stable in temperature swings	Low	Handles our wet-to-dry seasonal cycling with minimal expansion/contraction stress
Aluminum-clad wood	Good exterior protection, but any cladding breach exposes wood beneath	Moderate	Attractive interior wood look; requires disciplined flashing and sealant maintenance in salt air
Bare wood	Poor without diligent upkeep	High	We're honest with clients about the maintenance burden here — repainting and sealant checks become a recurring job in this climate
Aluminum (non-thermally broken)	Prone to condensation and corrosion near salt air	Moderate to high	We generally steer new builds away from this option for exterior walls in Sehome

We don't push one brand or material on every job. We'll walk through the trade-offs for your specific design, orientation, and budget and let you make an informed call.

Our Process for a Sehome New-Construction Job

Site walk and plan review: We go over the architectural plans, window schedule, and rough openings before framing is even finished, so we can flag any spec or clearance issues early.
Material selection: We help you compare window lines and materials against your budget, the home's exposure, and your long-term maintenance preferences.
Coordinated timing: New-construction windows go in at a specific point in the build sequence — after the weather-resistive barrier is up, before siding. We coordinate directly with your builder or GC so the crews aren't working against each other.
Installation: Sill pan, shingle-lap flashing, shimming, fastening, insulation, and sealant — done to manufacturer spec and adjusted for the wall's actual sun and rain exposure.
Walkthrough and documentation: Every window gets checked for operation, seal, and square before we sign off, and we document the flashing details for your records and warranty file.

Common Mistakes We See — and Prevent

Skipping or improvising sill pan flashing to save time, which is the single biggest cause of hidden wall rot behind new windows.
Reversed flashing laps that funnel water into the wall instead of away from it.
High-expansion foam used around the frame, bowing it out of square and causing operational failures within a year or two.
Under-shimming at load points, leading to frame racking and premature seal failure.
Sealant choice that ignores which walls actually take direct, wind-driven rain.

These aren't rare mistakes — they're common ones on jobs where window installation is treated as a quick task squeezed between framing and siding crews, instead of its own trade with its own sequence.

Why It Matters to Hire a Crew That Already Works Sehome

A window installer who mostly works drier inland climates can do fine work there and still get water management wrong on a Bellingham job, because the margin for error is smaller here. Crews familiar with Whatcom County new construction know which elevations take the worst of the driving rain in a given neighborhood's typical layout, how long the moss season really runs, and why a sill pan detail that's "optional" in Eastern Washington is standard practice on the coast. That local pattern recognition is hard to get from a manual — it comes from having installed and later inspected windows through a few full wet seasons in this exact climate.

We also coordinate regularly with builders and GCs working in and around Bellingham, which keeps the install sequence tight — windows go in at the right point in the build, flashed correctly the first time, so your siding and trim crews aren't covering up a problem that surfaces two winters later.

A Quick Owner's Checklist

If you're a homeowner overseeing a new build or major addition in Sehome, here's what to ask your window installer before work starts:

Will you install a dedicated sill pan at every window opening, not just a bead of sealant?
What flashing tape and sequence will you use, and does it follow a proper shingle-lap order?
What foam or sealant will you use around the frame, and is it rated for window installation specifically?
Will window installation be scheduled and sequenced with the framing and siding crews, or squeezed in loosely?
Can you document the flashing details for my records, in case a warranty question comes up later?

A contractor who answers these without hesitation has done this enough times to know why each step matters.

Get a Free Estimate

If you're planning a new build or addition in Sehome and want windows installed correctly the first time, we're happy to take a look at your plans and talk through material and installation options. Fill out the form below for a free, no-pressure estimate.