The Washington Energy Code in Plain English

If someone at the permit counter just told you your plans have to meet the energy code, and you walked out with a handout full of words like fenestration and R406 credits — this page is for you.

No code sections. No assumptions about what you already know. Read it start to finish and you’ll understand what the county is asking for and why. It takes about ten minutes.

The whole thing in one paragraph

Washington requires new homes to hit a minimum level of energy efficiency. You prove you hit it on paper, before you build, as part of your permit application. Most people prove it with a point system: efficient choices earn credits, your house needs a certain number of credits based on its size, and you pick whichever efficient choices you were going to make anyway until the total is high enough. That’s it. Everything below is detail.

Does this even apply to me?

Yes, if you’re doing any of these:

• Building a new house, duplex, or townhouse
• Adding on to an existing house, if the addition is over 150 square feet
• A remodel large enough that your building department is treating it as new construction

It applies no matter who’s building it. Licensed general contractor, framing crew, or you and your brother-in-law on weekends — the code applies to the building, not the builder. Owner-builders don’t get an exemption from this.

Probably not, if:

• Your addition is under 150 square feet — though some requirements still follow the work you’re doing. See Are small additions exempt?
• You’re replacing like-for-like — a window swap, a new furnace in the same spot — though equipment efficiency rules may still apply

When in doubt, ask your building department before you draw anything. Five minutes on the phone beats a correction notice six weeks later.

The words you’re going to run into

These are the terms that show up on plan review checklists and correction notices. Each one links from other pages on this site, so you can always jump straight back here.

The building itself

Building envelope

The shell that separates heated space from unheated space. Walls, roof, floor, windows, doors, and the insulation and air sealing in all of them.

Picture your house as a cooler. The envelope is the cooler’s walls and lid. Everything the energy code cares about is either part of that shell or part of the equipment that heats and cools the air inside it. You’ll also see this called the thermal envelope — same thing.

The part people get wrong: the envelope follows the heated space, not the walls of the building. An unheated attached garage is outside the envelope, so the wall between the house and the garage is an exterior wall as far as the code is concerned, and it needs to be insulated like one.

Fenestration

Windows and glass doors. That’s the whole definition.

It’s an old architectural word that survived into the code because it’s a tidy way to say “every opening in the envelope that you can see through.” A sliding glass door is fenestration. A skylight is fenestration. A solid front door is not.

It matters for two reasons. Glass loses far more heat than an insulated wall, so the code limits how inefficient your windows can be. And your total glass area helps determine how many credits your house needs — a small house with a lot of glass gets bumped into a higher requirement. See How many credits do I need?

Conditioned floor area

The heated square footage — the space inside the envelope.

The test is not whether a space is finished. It’s whether the space sits inside the thermal envelope and is heated, either directly by a register or indirectly by being open to heated space. An unfinished basement with insulated foundation walls and the furnace sitting in it is conditioned floor area, drywall or no drywall. An unheated detached garage or a vented crawlspace outside the envelope is not.

This number sets your credit requirement, so pin it down before you assume which tier you’re in. If a space is a judgment call — a partially heated bonus room over the garage, a basement you’re leaving unfinished for now — ask your AHJ rather than guessing. Being wrong here moves your whole credit target.

Continuous insulation, or “ci”

A layer of insulation that runs unbroken across the framing, instead of sitting between the studs.

You’ll see wall requirements written like R-20+5ci or R-13+10ci. Those are two routes to the same performance: R-20 batts in the stud cavities plus a continuous R-5 layer — usually rigid foam sheathing — over the outside of the studs, or less in the cavity with a thicker continuous layer doing more of the work.

The plus sign is doing real work there. It’s cavity insulation and continuous insulation, not a choice between them. Reading “R-13+10ci” as “R-13 or R-10” is a fast way to fail plan review.

Why does the code offer more than one combination? Because wood conducts heat. Every stud is a small gap in your insulation, and a wall full of studs leaks more heat than the batt rating suggests. A continuous layer covers the studs too, so less insulation does more work. This is called thermal bridging, and it’s the reason the two options aren’t the same R-number.

Ducts in conditioned space

Running your heating ducts inside the insulated shell rather than through the attic or crawlspace.

Ducts in a cold attic lose heat on the way to the room, and every leak dumps air you paid to heat into a space nobody lives in. Keeping the ductwork inside the envelope earns credits, and it’s usually a design decision rather than a cost — you have to plan for it early or you can’t do it at all.

The numbers

R-value

How well a material resists heat flow. Higher is better.

Used for insulation. R-60 in the ceiling holds heat back better than R-49. When your plans call out a ceiling minimum, that’s the least the code will accept up there — and if you’re wondering why that number keeps climbing every code cycle, it’s because attic insulation is the cheapest performance you can buy, so the code leans on it hardest.

U-factor

How much heat passes through an assembly. Lower is better.

Used for windows, where it’s the standard measure. U-factor is roughly the inverse of R-value, which is why the two run in opposite directions and trip everyone up at first. A window rated U-0.28 lets less heat through than one rated U-0.32.

The number that counts comes off the NFRC label — the sticker on the glass from the National Fenestration Rating Council. Not the manufacturer’s marketing number, not the number for a different size of the same window. Plan reviewers check this, and so do inspectors — they will look at the actual stickers on the actual windows in the actual house and compare them to what you put on the plans. Leave the labels on until after the inspection.

ACH50

How leaky the house is. Lower is tighter.

Back to the cooler for a second. R-value is how thick the cooler walls are. ACH50 is the gap around the lid — and a cooler with thick walls and a bad seal still won’t hold ice overnight. Insulation and air sealing are two different problems, and the code scores them separately.

ACH50 stands for air changes per hour at 50 pascals. A fan pressurizes the house to a set pressure and the meter reads how much air the building swaps with the outdoors every hour to hold it there. A house at 3.0 ACH50 is meaningfully tighter than one at 5.0.

The code sets a ceiling you have to stay under, and beating that ceiling by a good margin earns credits. Check Air sealing & blower door testing for the current thresholds and what earns what.

Blower door

The test that produces your ACH50 number.

A technician seals a calibrated fan into an exterior doorway, runs it, and measures. The question it’s really asking is: how well did you turn this paper bag into a plastic bag?

It happens near the end of construction, and it’s a pass/fail item on your permit — which is what makes it the most stressful number in the whole code. You’ve already built the house by the time you find out.

The fix is sequencing, not heroics. Air sealing is cheap and easy while the framing is open and nearly impossible once it’s covered. If you fail, there are options: Failed blower door rescue.

Climate zone

Washington has two zones, and knowing yours matters — though not the way you’d expect.

CZ4 Marine is western Washington, west of the Cascade crest. CZ5 is eastern Washington. Your county’s assignment is set in Table R301.1, and it’s legally operative — reviewers reference it.

Here’s the surprise: for the prescriptive envelope, it doesn’t change your numbers. Washington’s Table R402.1.3 covers "Climate Zone 5 and Marine 4" in one column — the whole state, one set of values. If someone told you eastern Washington needs more insulation, they’re describing a rule that holds across the model code’s full range of zones, not across the particular pair Washington contains.

The one geographic variation Washington’s energy code does make is altitude. Above 4,000 feet, or in a windborne debris region, vertical fenestration may be U-0.32 instead of U-0.30 — a relaxation, not a tightening.

Full detail and a ZIP lookup: Climate zones: CZ4 vs CZ5

The equipment

HRV / ERV

A ventilation unit that recovers heat from the air it exhausts.

Heat Recovery Ventilator, or Energy Recovery Ventilator.

The cooler analogy finally turns on you here. Seal the lid perfectly and the air inside goes stale — nobody wants to live in a ziploc bag. An old leaky house ventilated itself by accident through all the gaps you’re now being asked to close. Once the house is genuinely tight, fresh air stops being a happy accident and has to become a system.

But a plain exhaust fan throws away air you paid to heat. An HRV pulls the heat out of the outgoing air and hands it to the incoming air, so you get the fresh air without buying the heat twice. An ERV does the same and moves some moisture along with it.

That’s why the code groups air sealing and ventilation in the same credit category — it won’t let you do the first without the second.

HPWH — heat pump water heater

A water heater that moves heat instead of making it.

A conventional electric water heater turns electricity directly into heat. A heat pump water heater pulls warmth out of the surrounding air and pumps it into the tank, which takes far less electricity for the same hot water. It’s one of the larger credit opportunities in the code, which is why it shows up in so many compliance packages.

You’ll see them rated by NEEA tier — a Northwest efficiency standard, where a higher tier means a more efficient unit and more credits.

The paperwork and the process

Energy credits (R406)

The point system. This is the part most people are actually dealing with.

Every efficient choice is worth a set number of credits. Your house needs a total based on its size — a small house needs fewer, a large house needs more. You pick efficient options until you reach the number, and you submit the tally with your permit.

The elegant part: you choose which efficient choices. If solar doesn’t fit your budget but you were already installing a heat pump, take the heat pump credits. The code sets the target, not the route.

Two tables feed your total: fuel normalization and the seven credit categories. The full breakdown is at How many credits do I need?

Fuel normalization

Credits based on what heats your house.

A separate, smaller table where you pick exactly one option — your primary heating system’s fuel type — and it contributes to your total. A standard electric heat pump earns the most. A gas furnace earns none.

This is usually your single biggest decision, because it can move your total by several credits before you’ve chosen anything else. Worth settling before you design the rest of the package: Choosing your heating system

Prescriptive path

Follow the recipe. Meet each listed minimum — this much wall insulation, this much ceiling, windows no worse than this — and you comply. No modeling, no calculations. It’s the simplest route and the one most single-family projects take, combined with the credit system.

Performance path / ERI

Model the whole house and prove it beats a reference house.

Instead of meeting each individual requirement, an energy modeler simulates your actual design and demonstrates it performs at or above a target. More work and it costs money, but it buys flexibility — useful when a design feature you won’t give up would fail the prescriptive checklist. Detail: ERI / performance path

UA trade-off

Rob Peter to pay Paul, with math.

The envelope gets evaluated as a whole rather than component by component. Fall short on windows, make it up with extra ceiling insulation, and if the total heat loss still meets the target, you comply. Handy when one element can’t hit its individual number. Detail: The UA trade-off path

Glazing schedule

A table on your plans listing every window and door: size, location, U-factor, and total area.

A missing or incomplete glazing schedule is one of the most common reasons a set gets kicked back, and one of the easiest to avoid. Build the table before you submit. See WSEC-R permit correction notices

C3 spreadsheet

The official calculator from WSU Extension. An Excel workbook that does the credit math.

It’s thorough and it’s the tool the state built. It’s also a dense spreadsheet that assumes you already know the code, it’s a rough ride on a Mac, and it’s where most first-timers get stuck. Common C3 problems · Running C3 on a Mac

wsec.ai runs the same R406 calculation in a guided format that explains each choice as you make it.

AHJ — Authority Having Jurisdiction

Whoever actually issues your permit.

Sometimes the county, sometimes the city, depending on where your parcel sits. They’re the ones who interpret the code for your project, and where the code leaves room for judgment, their reading is the one that counts. Find yours: Washington counties

Certificate of occupancy

The document that says you’re allowed to live in it.

Issued after final inspection. Energy code compliance — including a passing blower door test — has to be satisfied before you get one.

So how do you actually prove it?

Three routes. Almost everyone takes the first.

1. Prescriptive + credits. Meet the listed minimums for insulation and windows, then earn enough R406 credits to hit your target. Standard for single-family.
2. UA trade-off. Same idea, but the envelope is judged as a whole so a weak spot can be offset by a strong one.
3. Performance path / ERI. Model the house and prove it beats the target. Most flexible, most effort.

You do not use REScheck. It’s the common national tool and it does not support Washington, because Washington wrote its own code. Submitting a REScheck report gets your plans returned. Why REScheck doesn’t work here

What you actually hand the county

Requirements vary by jurisdiction, so confirm with yours — but a typical residential energy submittal includes:

• The completed R406 credit worksheet, showing which options you selected and that the total meets your requirement
• A glazing schedule on the plans — every window and door with its U-factor
• Insulation values called out on the plans for walls, ceiling, floor, and slab edge
• Heating and water heating equipment identified with make, model, and efficiency rating
• A note on the plans committing to the blower door test and the threshold you’re targeting

Your building department may have its own checklist. Ask for it at pre-application — it’s the cheapest correction notice you’ll ever avoid.

Where people actually get stuck

The five that generate the most correction notices:

1. The credit worksheet is incomplete — options selected but the total doesn’t reach the requirement, or the fuel normalization row is blank.
2. Missing glazing schedule — plans show windows but never list their U-factors.
3. Windows don’t match — the U-factor on the plans doesn’t match the NFRC label on what actually got ordered.
4. Failed blower door — air sealing treated as a finish-stage task instead of a framing-stage one.
5. Assuming an addition is exempt — over 150 square feet and the code is in play.

Every one of these is cheaper to fix on paper than in the field. Full detail: WSEC-R permit correction notices

Frequently Asked Questions

I’m building it myself. Does this still apply?

Yes. The energy code applies to the building, not the builder. Owner-builders are exempt from contractor registration under RCW 18.27.090, but that’s a separate matter — your permit still requires energy code compliance documentation.

Do I have to install solar?

No. Solar is one option among many in the credit system, and plenty of compliant houses have none. Take credits wherever they fit your project.

Can I just copy what worked on my last house?

Careful. If it was a different size tier or a different climate zone, the requirement changed. And if it was under an older code cycle, the whole table may have changed. Re-run it.

Who decides if my project complies?

Your AHJ — the city or county issuing the permit. Where the code allows interpretation, theirs is the one that governs.

This is a lot. Is there a shortcut?

The credit math is the part worth automating. The concepts on this page are worth understanding either way, because they’re what your plan reviewer and your inspector will be talking about.