Built Right Detail 11: Double-Stud & Deep Walls

A builder’s net zero envelope course by Xerox Builders · October 2026

Double-stud and other deep walls reach high R-values with ordinary framing and cellulose, but their outer sheathing runs cold and damp. This Detail covers how they are built, dense-pack cellulose, what research in our region shows, and how to make the wall airtight and able to dry.

Why deep walls

A deep wall gets a high R-value from ordinary framing. Framers already know how to build it, it is filled with cellulose (low cost and low embodied carbon), and offset studs leave very few thermal bridges through the full depth. The trade-off is that all of the insulation sits inside the sheathing, so the sheathing runs cold and damp through a coastal winter and the wall has less margin for air leaks and rain leaks.

Three ways to build thick

Dense-pack cellulose

Deep cavities are usually filled with dense-pack cellulose blown to about 56–72 kg/m³ (3.5–4.5 lb/ft³), or to the maker’s stated density. At roughly R-3.2 to R-3.5 per inch, a 300 mm wall reaches about R-40. In new walls the installer blows through breathable netting stapled to the framing; very deep cavities are divided so every corner can be reached. Density matters: low-density fill settles and leaves cold voids at the top of the wall. Count bags against the cavity volume and check by feel for soft spots.

Why the sheathing runs cold

RDH monitored two Passive House buildings: a deep, double-stud-type wall in Victoria and an exterior-insulated wall in Portland. On the north walls, with about 3 °C outdoors, the Victoria sheathing averaged about 4.5 °C against about 7.5 °C in Portland; at 80% outdoor humidity, sheathing humidity ran about 88% against about 76%. Mould risk was very low in both, with the Victoria north wall the highest. The authors found the exterior-insulated sheathing warmer and drier, and stressed that water-control details still decide whether any wall survives.

A Building Science Corporation study in Massachusetts found the north sheathing of a 300 mm cellulose wall above 20% moisture content in winter. It dried each summer and showed little damage when the walls were opened after three winters. But in the winter before the ventilation system was started, indoor humidity sat at 40–50% and all the test walls moved into the high-risk range, with condensation in the cellulose walls.

Where the moisture comes from

1. Air leakage of warm, moist indoor air that condenses on the cold sheathing. This carries by far the most moisture.
2. Vapour diffusion through drywall and paint, slowly.
3. Rain leaks at windows, doors and penetrations.

High indoor humidity makes all three worse, which is why working ventilation is part of the wall design.

Build it to dry outward

• Inside: an airtight vapour control layer, either a sealed membrane or a smart vapour retarder that becomes more open when humid, allowing some inward drying in summer. The Building America team recommended vapour control tighter than ordinary latex paint for cellulose walls, but not polyethylene in their climate, because it blocks inward drying. Your consultant should choose the layer for our coastal climate. In BC, Part 9 still requires a vapour barrier on the warm side; the 2018 BC Building Code limits its permeance to 60 ng/(Pa·s·m²) (Article 9.25.4.2), so confirm in the current edition that a smart retarder meets the requirement or is accepted by your building official.
• Outside: a vapour-open sheathing membrane and a drained, vented rainscreen cavity so the sheathing can dry.
• Sheathing: many designers prefer plywood in deep walls because it handles wetting better than OSB.
• Air barrier: decide which layer it is (interior membrane, or sheathing on the inner frame) and how it passes floors, plates and openings.

Design and detailing

Deep openings need plywood bucks, a sill pan and a decision on where the window sits (Detail 08). Rim joists and floor lines need a drawn path for both the air barrier and the two frames. Floor area is lost to the thicker wall, so check setbacks and floor-area rules early. And size the HRV and make sure it runs, because a well-ventilated house keeps winter humidity down.

What usually goes wrong

• Leaks at the hard spots. Rim joists, top plates, electrical boxes and window bucks are where moist air reaches the cold sheathing.
• Low or uneven density. Cellulose blown too loose settles and leaves a cold, empty band under the top plate.
• Wet framing closed in. Sheathing that soaked up rain during framing is sealed in before it dries.
• A house run humid. The HRV is switched off or undersized, and winter humidity climbs.

Site checklist

• Wall assembly reviewed by an envelope consultant
• Air barrier layer named and drawn through floors and openings
• Interior vapour control layer sealed at every lap
• Vapour-open sheathing membrane and rainscreen cavity installed
• Sheathing dry before it is closed in
• Cellulose density checked and bag count recorded
• No voids at top plates, corners or around boxes
• Window bucks flashed and air sealed
• Blower door test passed before drywall
• HRV commissioned and running

Test yourself

Try to answer each question, then click it to check.

Next: Detail 12, Decks, Balconies & Ledgers. For the physics of cold sheathing and vapour, see our Building Science Deep Dive.