Built Right Detail 15: Roof-to-Wall Transitions

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

The top of the wall is where the wall and ceiling air barriers meet, and where a missed joint sends warm, moist air straight into the attic. This Detail shows how to choose the path over the top, pre-strip the top plate before the trusses go up, and protect the insulation and venting at the eaves.

Why the top of the wall matters

At the top of an exterior wall, two parts of the air barrier have to meet: the wall air barrier and the ceiling or roof air barrier. Between them sit the top plates, the truss heels and the blocking, and none of those are airtight on their own. In winter, warm, moist indoor air pushes outward at the top of the house, so a gap here sends moisture straight into the cold attic, where it can frost on the underside of the roof sheathing and later drip back onto the ceiling. Thin insulation at the same spot makes cold interior corners that are prone to condensation and mould. The physics are covered in our Building Science Deep Dive; this Detail is about the build.

The key point is timing. Once the trusses are standing, the top plate is buried under them and the best moment to seal it has passed. This joint has to be designed and drawn before framing starts.

Pick the path first

Trace the air barrier over the top of the wall on the building section, using the pencil test from Detail 01. Most houses use one of three paths:

Path B is very common, because many builders now use an exterior air barrier on the walls but still build a vented attic with an interior ceiling air barrier.

Pre-stripping the top plate

BC Housing’s airtightness guide describes pre-stripping for parapets, balconies and canopies: installing air barrier material behind framing that would otherwise block access later. The same idea works at the top plate under trusses. The sequence is:

1. Sheathe the wall and install the wall membrane (or tape the sheathing).
2. Before the trusses arrive, lay a strip of compatible membrane over the top plates.
3. Lap it down the outside face, over the wall membrane, so it sheds water. Leave a lip hanging to the inside, wide enough to seal to later.
4. Set the trusses on the strip.
5. After the trusses, seal the ceiling air barrier to the lip with tape or sealant.

The strip runs under every truss in one continuous piece. With airtight drywall at the ceiling, the lip is replaced by a gasket or sealant bead between the drywall and the top plate, and the joint between the plate and the sheathing is sealed instead.

Room for insulation at the edge

With a standard truss, the top chord comes down right over the wall, leaving only a few centimetres for insulation above the top plate, which is exactly where the air barrier joint is. A raised-heel (energy-heel) truss lifts the top chord above the wall so the ceiling insulation can run full depth to the outside edge of the wall. Sheathing or blocking at the heel holds the insulation in place and can double as the wind block.

Venting and wind washing at the eaves

In a vented attic, the BC Building Code (Section 9.19, Roof Spaces) calls for at least 63 mm of clear space between the top of the insulation and the underside of the roof sheathing. At the junction of a sloped roof and an exterior wall, that space may be reduced to 25 mm where a preformed baffle is installed that extends at least 50 mm above the insulation. The code also sets a minimum unobstructed vent area (1/300 of the insulated ceiling area for most roofs) and requires vents spread on opposite sides, with at least 25% of the required area at the top of the space and 25% at the bottom. These figures are from the BCBC 2018 text; confirm the edition that applies to your permit. In practice that means:

• A baffle in every rafter or truss bay at the eave, holding a clear path from the soffit vent up past the insulation.
• A wind block at the heel, so soffit air cannot blow through the edge of the insulation (wind washing).
• No insulation choking the soffit vents.

Gables, partitions and soffits

At a gable end, the attic wall above the ceiling is outside the house, so the ceiling air barrier must be sealed to the gable top plate just like at the eaves. Interior partitions under the attic need their top plates sealed, along with every wire and pipe hole through them. Dropped soffits, kitchen bulkheads, stairwells and knee walls that open into the attic must be capped with a rigid material and sealed before the ceiling insulation is blown in. Detail 13 covers the rest of the attic air barrier.

Flat roofs and parapets

On a low-slope roof, an air barrier membrane on the roof deck, under the insulation, is usually the simplest choice (path C). It turns up the inside of the parapet, over the top, and joins the wall air barrier on the outside. The corner where the deck meets the wall is almost impossible to reach once the parapet is framed, so pre-strip it first. Where the roof and wall membranes are not compatible, use a transition membrane that works with both. Detail 14 covers vented and flat roofs in more depth.

Site checklist

• Air barrier path over the top of the wall drawn on the section
• Top plate pre-strip installed before trusses, lapped over the wall membrane
• Inside lip of the strip sealed to the ceiling air barrier all round
• Raised-heel trusses or other full-depth insulation detail at the eaves
• Baffle in every bay at the eave, with clear vent space above the insulation
• Wind block at every heel; soffit vents not blocked
• Gable top plates and partition top plates sealed
• Bulkheads, stairwells and knee walls capped and sealed
• Top plates smoke-tested at the blower door test and photographed

Test yourself

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

Next: Detail 16, Pipes, Ducts & Wires Through the Envelope. See also our Roofing Systems & Ice Dam Prevention Field Guide.