Passive House windows and doors in Seattle: what makes them different and what to ask

A Passive House window is held to an installed U-0.15, about half what Washington code allows. What that takes, why doors are harder, and what to ask.

A Passive House window is not a window that happens to have three panes of glass. It is an assembly held to a measured number. In the Passive House Institute's cool-temperate climate zone, the installed whole-window U-value has to come in at 0.85 W/(m²K), which is U-0.15 in the units printed on an American spec sheet. Washington's energy code lets a window through at U-0.30. So the target is roughly half the heat loss of a window that is completely legal to install today, and the glass is the easy part of getting there.

Doors are held to the same order of number, plus two requirements that catch people out: a threshold profile no taller than 20 mm, about three quarters of an inch, and a tested airtightness class rather than a claimed one.

The number is a comfort spec, not an energy target

PHI does not derive the window U-value from an energy budget. It derives it from comfort. The rule is that the interior surface of a window may not sit more than 4.2 K, about 7.5°F, below the room's operative temperature, and that air movement in the occupied space stays under 0.1 m/s. Those two together describe the cold draft that falls off a cool pane and runs across the floor. Perimeter radiators under windows exist to cancel exactly that draft. Get the surface temperature right and you no longer need them, which is why a well-built Passive House can put a sofa against the glass in February.

The second criterion is about damp, and it matters more here than it does in a cold dry climate. PHI limits water activity on interior surfaces to 0.80, the level below which mold does not get established, and turns that into a required temperature factor at the coldest point of the frame: 0.70 in the cool-temperate zone. It is a condensation spec. Black spotting along the bottom of the glass and on the sill is a cold line in the frame and the glass edge, not a housekeeping failure, and a warm frame edge is what actually fixes it.

One practical note on the zone: PHI publishes these limits per climate region and issues certificates against a specific one. A certificate written for a warmer region is held to looser numbers. Ask which region a product's certificate covers rather than assuming the seal means one thing everywhere.

Four parts, and the glass is the easy one

For the cool-temperate zone the reference glazing sits at 0.70 W/(m²K), roughly U-0.12. The window as a component has to reach 0.80, about U-0.14. Installed, 0.85, about U-0.15.

The glazing. Triple, two low-e coatings, argon or krypton fill. This is close to a commodity now and it is rarely where a project fails.

The frame. This is where the number is won or lost. The frame is almost always the worst-performing part of a window, and it is the part a homeowner never asks about. Aluminum with a thermal break generally does not get there on its own. The systems that do are multi-chamber PVC with the right reinforcement, insulated fiberglass, and timber with an insulated outer skin. PHI certifies the frame, glazing and spacer together as one system, because the three of them only make sense as a set.

The spacer. The band around the perimeter of the sealed glass unit. An aluminum spacer conducts heat straight around the edge of the glass and produces the cold line where condensation shows up first. At this level a warm-edge spacer is mandatory, and it is one of the few specifications you can ask about and get a one-word answer to.

The hardware and the seals. This is the reason European tilt-turn hardware dominates the category. A tilt-turn sash, or a multipoint casement, pulls the sash inward against two or three continuous gaskets and compresses them. A single-hung or a sliding sash cannot compress anything: the sash slides past a brush or a fin, and it has to keep sliding. That is a fact about the motion, not about brand quality, which means the sash type is an airtightness decision before it is a styling one.

Then there is solar gain, which is a separate specification from U-value and gets ignored more often. Every pane and every low-e coating cuts the solar energy coming through, so a triple with two coatings passes well under half the solar heat of a clear double unit. In a house this tight, that is a lever in both directions. South glass generally wants gain, paired with an overhang that shades it in July and lets it through in January. West glass usually wants the opposite, because west sun arrives late in the day at a low angle that no overhang catches. Most suppliers will quote one glazing package for the whole house. Ask for it by elevation.

Doors are the harder half

Windows get the attention. Doors are where the blower door test usually goes wrong.

PHI certifies entrance doors as their own category and tests them under climate load: conditioned air on one face, cold on the other, and for timber doors a hot humid exterior condition as well. The reason is that a door slab with 70°F on one side and 32°F on the other wants to cup. A cupped slab stops touching its seals at the corners. A door that measured tight in a showroom in September can leak in January, which is why the test matters more than the datasheet.

The threshold is the genuinely hard detail. PHI caps the profile at 20 mm, and that piece has to be low enough to walk over, thermally broken so it is not a frozen strip across the entry, sealed against wind-driven rain, and able to drain. Most air leakage at an entry door is the threshold and the two bottom corners.

Sliding doors are their own problem. A conventional slider physically cannot compress a seal, for the same reason a sliding window cannot. A lift-and-slide drops the panel down onto compression seals when you turn the handle and lifts it clear of them to move. If you want a wide glass opening in an airtight house, that mechanism is the thing to ask for by name. PHI treats sliding doors as a separate certification category with its own airtightness testing, which tells you how much of a weak point they historically were.

One more that gets missed on remodels and additions: if there is an attached garage, the garage sits outside the thermal envelope. The door between the house and the garage is an exterior door and needs exterior door detailing, insulation and seals. It is frequently specified as an interior door, and at that point it is a large uninsulated hole in a wall you paid to insulate.

The installed number is the only one that counts

PHI publishes the component U-value and the installed U-value as two separate figures on purpose. The gap between them, 0.05 W/(m²K), is the allowance for the thermal bridge created by the installation itself, and it is charged against the window. A certified window installed badly is not a certified assembly.

Three things close that gap.

The window sits in the plane of the insulation rather than at the face of the framing. On a wall with exterior continuous insulation, that means the frame is carried outboard of the structure on brackets or a plywood buck, which is a structural detail that has to be drawn before the wall is built.

The frame is over-insulated. The exterior insulation laps over the outer face of the frame so that the coldest part of the window is covered rather than exposed. This is also why the window sizes have to be settled early: the rough opening for a Passive House install is not the rough opening for a nail-fin window.

The perimeter joint is sealed twice, and asymmetrically. Airtight and vapor-tight on the inside, weather-tight and vapor-open on the outside. Tighter inside than out, so that anything that does get into the joint can dry outward. Reversed, which happens, you have built a moisture trap around every opening in a climate that rains for seven months.

The mistake that costs the most money is simpler than any of this: buying by the center-of-glass number. A datasheet quoting U-0.10 is almost always describing the glass alone. The whole-window figure is worse because the frame and the glass edge drag it up, and the installed figure is worse again. U-value also changes with unit size, since a large fixed pane is mostly glass and a small operable one is mostly frame. Ask for the whole-window number at your actual sizes.

Washington's code already pays for a lot of this

None of these targets are exotic in this state. They are the top rows of a table our own energy code prints.

The residential energy credit table gives a house credit for vertical fenestration at U-0.22, more for U-0.25 in combination with envelope upgrades, and up to 2.5 credits for the package built around U-0.18 windows. In the air leakage category, the largest available credit goes to a house that tests at 0.6 air changes per hour at 50 pascals with a heat recovery ventilator at 80 percent sensible efficiency. That 0.6 figure is the Passive House airtightness standard, sitting in Washington code as the top of the scale.

That changes how the window premium should be read. Better windows are not only an out-of-pocket upgrade. They are also credits you are not buying somewhere else, and they usually cost less than buying the same credits through mechanical equipment.

Lead time is the other budget, and it is measured in months rather than weeks from approved shop drawings, longer again for imported units that have to cross an ocean and clear customs. On a DADU or an addition the window order is often the item that sets the framing date, not the other way round, and a wrong unit means starting that clock again.

What to ask before anything is ordered

• The whole-window U-factor at my actual unit sizes, and the installed figure if you have it. Not center-of-glass.
• Which climate region the certificate was issued against.
• Spacer type at the glass edge.
• Solar heat gain coefficient by elevation, not one package for the whole house.
• The threshold detail for every exterior door, drawn, showing the thermal break and how it drains.
• Whether sliding units are lift-and-slide or conventional.
• Who owns the air seal at the perimeter, which tapes go on inside and outside, and whether that trade returns after the blower door test to fix what the test finds.
• Lead time from approved shop drawings rather than from deposit, and what happens to the schedule if a unit arrives wrong.

Where this actually gets decided

The window and door schedule is not a selection you make once the plans are done. Because the frames sit in the plane of the insulation, the wall assembly and the window install detail are the same drawing, and the rough openings depend on which system you chose. Deciding the windows late means redrawing the walls, and that is the point at which people quietly drop back to a code-minimum unit and lose the number they were building for.

If you are weighing this for a new build, a DADU, or a deep remodel, the useful first conversation is about which elevations get which glass, how the walls are built, and what the entries have to do. What Passive House actually means for a Seattle home covers the rest of the envelope those choices sit inside.

Love Construction builds Passive House projects in Seattle, Bellevue, Kirkland, Issaquah, Newcastle and SeaTac. Bring the window and door schedule to the first design meeting rather than the last, and ask for every number in writing at your actual unit sizes.