ADU BUILDING SCIENCE

How to Build Dry, Durable, Energy-Efficient Backyard Homes in the Pacific Northwest

What Homeowners Usually Think

“An ADU is just a smaller house. If it’s built to code, it should perform the same.”

What Actually Happens

ADUs are more vulnerable to moisture problems than full-size homes because they are:

  • smaller

  • tighter

  • more humid (showers + cooking in a compact space)

  • closer to the ground

  • often built with low-slope roofs

  • surrounded by shading and vegetation

  • built over crawlspaces, slabs, or mixed assemblies

Without careful moisture management, ADUs easily suffer from mold, condensation, poor indoor air quality, and short material lifespan — especially in the Pacific Northwest.

Building an ADU “to code” is not enough.
Building it to building-science standards is what keeps it healthy and durable.

Why ADUs Are Higher Risk in the PNW

1. Small Volume = Big Moisture Load

A shower in a small ADU produces far more humidity per cubic foot than in a full-size home.
If ventilation is undersized, condensation shows up on windows, walls, and roof assemblies.

2. Lots of Corners, Low Rooflines, and Modern Shapes

Many ADUs use:

  • shed roofs

  • low slopes

  • parapets

  • boxed overhangs

  • large window walls

These increase the risk of trapping moisture and create complex flashing conditions.

3. Built Close to Trees & Fences

Shade slows drying.
Vegetation increases localized humidity.
Wind-driven rain splashes onto siding and windows.

4. Crawlspace or Slab Challenges

ADUs often sit in the wettest corner of a lot.
Without proper drainage, ground moisture loads the interior.

5. High Window Ratio

ADUs rely heavily on natural light.
More windows = more potential water intrusion points and more condensation risk.

6. Multi-Use Loads

Many ADUs combine:

  • bedroom

  • kitchen

  • laundry

  • bathroom

All in a compact footprint.
Heat, moisture, and air quality challenges multiply.

Best Practices for ADU Building Science in the PNW

1. A True Air-Tightness Strategy

ADUs must be air-sealed more carefully than larger homes because airflow has fewer places to “dilute” moisture.

We seal:

  • top/bottom plates

  • electrical penetrations

  • bath fans

  • attic access

  • rim joists

  • window/door interfaces

  • mechanical chases

Tight + ventilated beats leaky + moldy.

2. Ventilation Designed for Small Spaces

Every ADU should have:

  • a continuous or demand-controlled bath fan

  • a dedicated kitchen exhaust

  • balanced or supplemental fresh-air intake

  • humidity-controlled operation

Simply following code CFM numbers is not enough.
Moisture loads in ADUs are disproportionately high.

3. Roof Assemblies: No Vented Flat or Low-Slope Attics

ADUs often have shed or low-slope roofs that cannot rely on traditional venting.

We use:

  • non-vented warm roof assemblies

  • rigid insulation above the sheathing

  • continuous air barrier below

  • correct flashing at all terminations

  • deep overhangs to shed water

A correct roof assembly is critical — most ADU failures start here.

4. Drainage and Site Water Management

ADUs sit in backyards that often slope toward the structure.

Best practice includes:

  • grading the pad

    • 6” over 10 ft away from the ADU

  • extending downspouts

  • foundation drain integration

  • dimple mat or drainage board

  • slab or crawlspace moisture control

A dry site = a healthy ADU.

5. Walls Built for Slow Drying

ADUs are small buildings with high exterior exposure.

We use:

  • full rainscreen assemblies

  • high-performance WRBs

  • careful window flashing

  • correct insulation for the wall’s drying profile

  • smart vapor retarders when needed

ADUs shouldn’t rely on “builder-grade” wraps or insulation.

6. High-Performance Windows + Proper Flashing

Larger window ratios mean more risk.
Correct flashing integration is non-negotiable.

We install:

  • full sill pans

  • flexible flashing

  • head flashing with slope

  • WRB integrated correctly

  • rainscreen behind cladding

This keeps the walls dry for the lifetime of the ADU.

7. Durable, Moisture-Resistant Interior Materials

Because ADUs run humid, we select materials that can tolerate moisture swings:

  • moisture-tolerant drywall finishes

  • high-quality bath ventilation

  • mildew-resistant paints

  • water-resistant flooring

  • proper air sealing around plumbing and mechanical penetrations

How Emerald State Contracting Builds Better ADUs

1. Building-Science-Driven Design

We start with the moisture loads, ventilation strategy, and roof/wall assemblies — before talking about finishes.

2. Site Drainage Assessment

We evaluate soil, slope, downspouts, groundwater, and rain patterns to design a dry building pad.

3. High-Performance Envelope Construction

Our ADUs include:

  • full rainscreen siding assemblies

  • robust flashing at every opening

  • air-tight, heat-efficient wall and roof assemblies

  • conditioned crawlspace or insulated slab

  • high-performance windows

4. Mechanical Systems Sized for Small Spaces

We design systems that manage humidity, not just temperature.

5. Long-Term Durability

Our goal is to build ADUs that stay dry, clean, quiet, and efficient — and that don’t trap moisture inside walls, roofs, or floors.

Why This Matters

A poorly built ADU will show problems fast:

  • window condensation

  • musty smell

  • mold behind drywall

  • roof deck rot

  • high humidity

  • peeling paint

  • swollen trim

  • poor comfort

A well-built ADU feels like a luxury guest suite — dry, warm, comfortable, and healthy.

When to Call a Professional

You should consult a building-science-trained contractor if:

  • You’re planning to build an ADU

  • Your lot slopes or holds water

  • You want a low-slope or modern roof

  • You want large windows or high glass ratios

  • You want the ADU to be energy-efficient

  • You want a space that stays dry and mold-free long term