Drained cavity window installation

By - , Build 130

Recent changes to E2/AS1 have slightly altered the construction requirements for window installation in drained cavity construction. What do designers, builders and window installers need to know?

Steps 7, 12 and 13: Install window and finish sill.
Steps 1–2: Install wall underlay and flexible flashing tape.
Steps 3–4: Install support bar and cavity battens.
Steps 5–6: Fix cladding to sill level and top of window.
Step 8–12: Window head.

Amendment 5 in 2011 to Acceptable Solution E2/AS1 External moisture included changes to both direct-fix and drained cavity construction window installation requirements. The changes to direct-fix window installation were described in Build 128 February/March 2012, pages 16–17.

Sill support bar

The most significant change for drained cavity window installation requires the fitting of a sill support bar to all doors and windows over 600 mm wide.

The sill support bar must allow any water that gets past the external cladding to drain away, and it must maintain at least 1,000 mm² clear opening per metre length of window between the drainage cavity and the window or door trim cavity to allow air passage. To do this, sill support bars incorporate drainage and ventilation openings.

Sill support bars must end within 100 mm of the trimming stud.

Sill support bars must also comply with BRANZ Evaluation Method EM6, Verification Method E2/VM1 and Acceptable Solution B2/AS1.

Additionally, manufacturers must provide information about the support bar loading limits.

Jamb flange gap

For fibre-cement sheet or ply claddings, E2/AS1 now requires a 5 mm gap for sealant to be left between the jamb flange and cladding.

Other cavity construction changes

There are several other changes:

  • In extra high (EH) wind zone situations, flashing upstand dimensions must be 25 mm more than the dimensions stated in E2/AS1 section 4.5.1 or Table 7, and all flashings must have a hook or a hem.
  • In very high (VH) and extra high (EH) wind zones, sealant must be inserted between the head flashing and the window head flange as shown in E2/AS1 Figure 71(c).
  • The minimum cover to the cladding for window sill flanges has been reduced to 8 mm, although the minimum jamb flange cover remains at 10 mm.
  • Factory-fitted soakers are required behind the sill/jamb mitred frame joints of aluminium windows and doors.

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More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Steps 7, 12 and 13: Install window and finish sill.
Steps 1–2: Install wall underlay and flexible flashing tape.
Steps 3–4: Install support bar and cavity battens.
Steps 5–6: Fix cladding to sill level and top of window.
Step 8–12: Window head.

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