Installing a window without a cavity

By - , Build 90

Want a quick guide to installing a window in a wall where a low level of weathertightness risk permits direct fixing of the cladding to the wall framing? Follow these easy steps for a snug-fitting window.

Wall wrap installed to outside of studs over window opening. Do not cut until window is ready to be installed.
Make diagonal cuts to the wall wrap and turn into the window opening. Staple in place.
Use flexible flashing tape to secure the wrap and reinforce corners. This helps provide both an air seal around the window and moisture protection to the framing left exposed after the wall wrap has been trimmed into the opening. Tape must extend at least 100 mm out from the corner in each direction.
Fit non-flexible flashing tape across the full width of the opening to provide back-up moisture protection to the horizontal timber surface. Turn tape 50 mm down the face of the wall wrap. Completely enclosing the sides and top of the trimmed opening with flashing tape may be required for some proprietary cladding systems and adds an additional layer of protection to the framing.
A partial (150 mm long) sill tray at each side of the opening (and under any mullion position) intercepts and drains any water that may leak through window mitre joints or get into the space around an aluminium window frame. Stop-ends prevent water migrating off the end of the flashing behind the cladding, directing water safely to the outside. Extending the sill tray (into the recess formed when a 20 mm jamb-packer is stopped short of the sill) allows it to pick up leaks from head and jambs. (Note: Cut ends of weatherboards must always be primed.)
For plank or weatherboard-type claddings, the cladding should be installed up to the top of the opening before the window is installed. Ensure adequate window flange covers to the cladding are maintained. (E2/AS1 requires 10 mm, BRANZ considers that greater cover improves the robustness of the detail by giving greater tolerances to work with, particularly where larger windows are being installed.)
With window in place, fix head flashing over first layer of wall wrap and lap top edge with selfadhesive tape or second layer of wrap (see BUILD Aug./Sept. 2003, page 14).
Fit cladding around the head flashing.
Scribers must be accurately cut to weatherboard and sealed to side of window. Seal the saw cut in the weatherboard at ends of head flashing. (Note: Screw fixings were used for our demonstration window but are not recommended for actual installations.)
A good air seal is required around any penetrations to prevent air leaks carrying water into the frame and insulation. First install a polyethylene backing (PEF) rod to stop sealant penetrating into the window trim cavity, then use low-expansion foam or other sealant to form the air seal.

For timber-framed buildings the Acceptable Solution E2/AS1, while not mandatory, is a deemed-to-comply solution. Alternative Solutions can be used as long as they show to the consenting authority that they meet the weathertightness performance requirements of Clause E2 of the New Zealand Building Code.

Alternative Solutions that have greater tolerance (and more peace of mind) have been promoted by BRANZ to encourage designers to detail more robustly. Currently there is no Verification Method for installing windows in a direct-fixed cladding situation, but installation is similar to a cavity-based situation.

Wall wrap installed to outside of studs over window opening. Do not cut until window is ready to be installed.
Make diagonal cuts to the wall wrap and turn into the window opening. Staple in place.
Use flexible flashing tape to secure the wrap and reinforce corners. This helps provide both an air seal around the window and moisture protection to the framing left exposed after the wall wrap has been trimmed into the opening. Tape must extend at least 100 mm out from the corner in each direction.
Fit non-flexible flashing tape across the full width of the opening to provide back-up moisture protection to the horizontal timber surface. Turn tape 50 mm down the face of the wall wrap. Completely enclosing the sides and top of the trimmed opening with flashing tape may be required for some proprietary cladding systems and adds an additional layer of protection to the framing.
A partial (150 mm long) sill tray at each side of the opening (and under any mullion position) intercepts and drains any water that may leak through window mitre joints or get into the space around an aluminium window frame. Stop-ends prevent water migrating off the end of the flashing behind the cladding, directing water safely to the outside. Extending the sill tray (into the recess formed when a 20 mm jamb-packer is stopped short of the sill) allows it to pick up leaks from head and jambs. (Note: Cut ends of weatherboards must always be primed.)
For plank or weatherboard-type claddings, the cladding should be installed up to the top of the opening before the window is installed. Ensure adequate window flange covers to the cladding are maintained. (E2/AS1 requires 10 mm, BRANZ considers that greater cover improves the robustness of the detail by giving greater tolerances to work with, particularly where larger windows are being installed.)
With window in place, fix head flashing over first layer of wall wrap and lap top edge with selfadhesive tape or second layer of wrap (see BUILD Aug./Sept. 2003, page 14).
Fit cladding around the head flashing.
Scribers must be accurately cut to weatherboard and sealed to side of window. Seal the saw cut in the weatherboard at ends of head flashing. (Note: Screw fixings were used for our demonstration window but are not recommended for actual installations.)
A good air seal is required around any penetrations to prevent air leaks carrying water into the frame and insulation. First install a polyethylene backing (PEF) rod to stop sealant penetrating into the window trim cavity, then use low-expansion foam or other sealant to form the air seal.

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

Wall wrap installed to outside of studs over window opening. Do not cut until window is ready to be installed.
Make diagonal cuts to the wall wrap and turn into the window opening. Staple in place.
Use flexible flashing tape to secure the wrap and reinforce corners. This helps provide both an air seal around the window and moisture protection to the framing left exposed after the wall wrap has been trimmed into the opening. Tape must extend at least 100 mm out from the corner in each direction.
Fit non-flexible flashing tape across the full width of the opening to provide back-up moisture protection to the horizontal timber surface. Turn tape 50 mm down the face of the wall wrap. Completely enclosing the sides and top of the trimmed opening with flashing tape may be required for some proprietary cladding systems and adds an additional layer of protection to the framing.
A partial (150 mm long) sill tray at each side of the opening (and under any mullion position) intercepts and drains any water that may leak through window mitre joints or get into the space around an aluminium window frame. Stop-ends prevent water migrating off the end of the flashing behind the cladding, directing water safely to the outside. Extending the sill tray (into the recess formed when a 20 mm jamb-packer is stopped short of the sill) allows it to pick up leaks from head and jambs. (Note: Cut ends of weatherboards must always be primed.)
For plank or weatherboard-type claddings, the cladding should be installed up to the top of the opening before the window is installed. Ensure adequate window flange covers to the cladding are maintained. (E2/AS1 requires 10 mm, BRANZ considers that greater cover improves the robustness of the detail by giving greater tolerances to work with, particularly where larger windows are being installed.)
With window in place, fix head flashing over first layer of wall wrap and lap top edge with selfadhesive tape or second layer of wrap (see BUILD Aug./Sept. 2003, page 14).
Fit cladding around the head flashing.
Scribers must be accurately cut to weatherboard and sealed to side of window. Seal the saw cut in the weatherboard at ends of head flashing. (Note: Screw fixings were used for our demonstration window but are not recommended for actual installations.)
A good air seal is required around any penetrations to prevent air leaks carrying water into the frame and insulation. First install a polyethylene backing (PEF) rod to stop sealant penetrating into the window trim cavity, then use low-expansion foam or other sealant to form the air seal.

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