A Guide to Selecting Underlay

Roof and wall underlays come in many forms. Here’s a guide to the considerations typical of a medium density project:

What’s the cladding? Direct or cavity fix? Absorbent (timber, brick, fibre cement) or non-absorbent (metal or plastic) cladding? Steel or timber framing? What properties are stipulated by the cladding manufacturer?
Compatibility and reduced risk: A growing concern and requirement across the industry — is documentation available to prove compatibility of the underlay with the sealants, tapes, cladding and frame? Is the underlay part of a wider system or solution that could be used? How long is the the warranty?
Roof underlay: What is the purlin span? Is underlay support needed? Can the underlay be used on pitches under 10 degrees?
Fire: Is the AS 1530 Part 2 Flammability Index of the underlay no greater than 5 in order to meet the NZBC Acceptable Solution C/AS2? As best practice, Thermakraft suggests fire retardant products be used across the entire build of a medium density build, even if minimum standards allow otherwise.
Wind zone: Does the underlay meet the Wind Zone requirements?
Air barrier: You may need an air barrier in specific areas only, but could it be more efficient to use an underlay with air barrier properties across the build?
Exposure rating: Consider the longer build times on Medium Density projects; how many days can the underlay be exposed before its no longer warranted?
Optimisation: Wide width rolls and cut-to-length could reduce labour and waste resulting in significant savings.
Supply: Delays in supply may compromise the integrity of other products. Is the underlay readily available?
For the client: Are they keen on the marketing potential of custom branding? Medium density projects present a big billboard opportunity.

What underlays meet all these considerations?

Thermakraft, manufacturing underlays in New Zealand, for New Zealand conditions and building code requirements, suggests the following compatible and approved solutions for a Medium Density build:

Watergate Plus Wall Wrap
Thermaflash Flashing Tape
OneSeal Pipe Penetration Seals
Covertek 405 or Covertek 407 Roof Underlay

If the build goes higher or rigid air barriers are required, Thermakraft recommends James Hardie RAB system with Watergate Plus flexible underlay if required, Thermaflash flashing Tape, Premium Joining Tape and OneSeal pipe and cable penetration seals.

Learn more: Book an 'Underlay — What's the difference?' presentation with Thermakraft

Thermakraft Premium Joining Tape

Kingspan Thermakraft

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Premium Joining Tape, BRANZ approved to join James Hardie RAB sheets and a wide range of flexible underlays and rigid sheathing products.

BRANZ Appraised as part of the James Hardie RAB system
Join the laps of wall and roof underlays, damp-proof membranes and vapour control products such as foil underlays and rigid sheathing products
Suitable to use with air barrier underlays in unlined spaces
Suitable to be used in conjunction with thermal breaks on steel framed buildings

NZBC:

B2
E2/AS1
F2

BRANZ Appraised

Thermakraft Thermaflash

Kingspan Thermakraft

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Thermaflash high adhesion synthetic window flashing tape is a high performance self-adhering synthetic tape.

Thermaflash window flashing tape can be used as a flexible flashing system around window and door joinery openings on timber and steel framed buildings within the following scope:

Installed into and around joinery openings over the wall underlay and exposed frame to cover both the face and edge of the opening as a secondary weather resistant barrier
Installed at joinery heads to seal flashing upstands to the wall underlay
To assist the overall weathertightness performance of window and door joinery installations
To be used in conjunction with air seals and joinery flashing systems. Installer must check for air seal product compatibility with Thermaflash
Situated in NZS3604 Wind Zones up to, and including, ‘Extra High’
Suitable for residential builds (E2/AS1 timber framed and E2/AS4 steel framed) and medium rise buildings (E2/VM2)

Thermaflash window flashing tape has been appraised by BRANZ as a flexible system for use around window and door joinery for buildings with the following scope:

Constructed with timber and steel framing in accordance with scope limitations of NZBC Acceptable Solution E2/AS1 and NASH Building Envelope Solutions, Paragraph 1.1
With a risk score of 0-20, calculated in accordance with NZBC Acceptable Solution E2/AS1 or NASH Building Envelope Solutions, Table 2
With wall cladding systems complying with NZBC Acceptable Solution E2/AS1 or NASH Building Envelope Solutions, or a valid BRANZ Appraisal that specifies a flexible flashing system
With flexible wall underlays compatible with the flashing tape and complying with the NZBC

Clause B2 DURABILITY: Performance B2.3.1 (b), 15 years and B2.3.2. Thermaflash Flashing Tape meets these requirements
Clause E2 EXTERNAL MOISTURE: Performance E2.3.2. Thermaflash Flashing Tape contributes to meeting this requirement
Clause F2 HAZARDOUS BUILDING MATERIALS: Performance F2.3.1. Thermaflash Flashing Tape meets this requirement and will not present a health hazard to people

BRANZ Appraised

Thermakraft CoverTek 405 — Roof & Wall Underlay

Kingspan Thermakraft

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Thermakraft CoverTek 405 is a fire retardant self supporting synthetic roofing underlay for use under all claddings.

Thermakraft CoverTek 405 Fire Retardant Self-Supporting Roof Underlay has been appraised for use as a self-supporting roof underlay on buildings within the following scope:

the scope limitations of NZBC Acceptable Solutions E2/AS1, Paragraph 1.1 with regards to building height and floor plan area; and,
with masonry tile roof cladding; and,
with metal tile and profiled metal roof cladding; and,
situated in NZS 3604 Wind Zones up to, and including Extra High

Clause B2 Durability: Performance B2.3.1 (a), not less than 50 years, B2.3.1 (b), 15 years and B2.3.2. Thermakraft CoverTek 405 meets these requirements.

Clause E2 External Moisture: Performance E2.3.2. When used as part of the cladding system, Thermakraft CoverTek 405 will contribute to meeting this requirement.

Clause F2 Hazardous Building Materials: Performance F2.3.1. Thermakraft CoverTek 405 meets this requirement and will not present a health hazard to people.

Thermakraft CoverTek 405 Roof Underlay has an AS 1530 Part 2 flammability Index of not greater than 5 and therefore meets the requirements of NZBC Acceptable Solutions C/AS2 to C/AS6, Paragraph 4.17.8 b), for the surface finish requirements of suspended flexible fabric used as an underlay to exterior cladding that is exposed to view in occupied spaces. It may therefore be used with no restrictions in all buildings.

BRANZ Appraised

CodeMark

Thermakraft Watergate Plus

Kingspan Thermakraft

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Thermakraft Watergate Plus is a fire retardant absorbent breathable wall underlay.

Thermakraft Watergate Plus can be used as a wall underlay on timber and steel framed buildings within the following scope:

The scope limitations of NZBC Acceptable Solution E2/AS1, Paragraph 1.1, with regards to building height and floor plan area; and,
With absorbent wall claddings directly fixed to timber and steel framing; and,
With non-absorbent wall claddings directly fixed to timber and steel framing; and,
With absorbent and non-absorbent wall claddings installed over an 18mm minimum drained cavity;
With masonry veneer in accordance with NZS 3604, NZBC Acceptable Solution E2/AS1 for timber framed buildings, or specific engineering design for steel framed buildings; and,
Situated in NZS3604 Building Wind Zones up to, and including ‘Very High’ when used as a standalone flexible underlay, and “Extra High” when used as a flexible underlay over a rigid wall underlay.
Can be used as a non-rigid backing material for Stucco Plaster in accordance with the requirements of NZBC Acceptable Solution E2/AS1 Paragraph 9.3.5.1.
The Underlay must be supported with 75mm galvanized mesh, or Thermakraft Stud Strap at 150mm centres run across cavity battens to limit deflection to a maximum of 5mm.
May also be used as a slip layer over rigid backing for Stucco Plaster in accordance with the requirements of NZBC E2/AS1 Paragraph 9.3.3(b).
Refer BRANZ Appraisal No 695 (2017) for full details.
May be used with LOSP treated timber.
Is suitable as an air barrier in unlined wall spaces.
Is suitable to be used in conjunction with thermal breaks on steel framed buildings.
Is suitable for direct fixing behind metal cladding

Meets the Performance Requirements of NZBC Clauses B2, Durability (B2.3.1 (a) 50 years, B2.3.1 (b) 15 years and B2.3.2), E2 External Moisture (E2.3.2), and F2 Hazardous Building Materials F2.3.1, providing

It is not damaged
Installed in accordance with instruction guide
Is not left exposed for more than 90 days
Installed by or under guidance of Licensed Building Practitioners
Is compatible with cladding system used

BRANZ Appraised

CodeMark

Thermakraft CoverTek 407 — Roof & Wall Underlay

Kingspan Thermakraft

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Thermakraft CoverTek 407 is a fire retardant self supporting synthetic building underlay for the use under all claddings.

Thermakraft CoverTek 407 Fire Retardant Self-Supporting Roof Underlay has been appraised for use as a self-supporting roof underlay on buildings within the following scope:

The scope limitations of NZBC Acceptable Solutions E2/AS1, Paragraph 1.1 with regards to building height and floor plan area; and,
With masonry tile roof cladding; and,
With metal tile and profiled metal roof cladding; and,
Situated in NZS 3604 Wind Zones up to, and including Extra High

Clause B2 Durability: Performance B2.3.1 (a), not less than 50 years, B2.3.1 (b), 15 years and B2.3.2. Thermakraft CoverTek 407 meets these requirements.

Clause E2 External Moisture: Performance E2.3.2. When used as part of the cladding system, Thermakraft CoverTek 407 will contribute to meeting this requirement.

Clause F2 Hazardous Building Materials: Performance F2.3.1. Thermakraft CoverTek 407 meets this requirement and will not present a health hazard to people.

Thermakraft CoverTek 407 Roof Underlay has an AS 1530 Part 2 flammability Index of not greater than 5 and therefore meets the requirements of NZBC Acceptable Solutions C/AS2 to C/AS6, Paragraph 4.17.8 b), for the surface finish requirements of suspended flexible fabric used as an underlay to exterior cladding that is exposed to view in occupied spaces. It may therefore be used with no restrictions in all buildings.

BRANZ Appraised

CodeMark

Thermakraft OneSeal Multi-fit Pipe and Cable Penetration Seals

Kingspan Thermakraft

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OneSeal Multi-fit Pipe and Cable Penetration Seals are a range of pipe and service penetration seals consisting of a soft, flexible EPDM membrane with a self-adhesvive flange.

OneSeal Multi-fit Pipe and Cable Penetration Seals have been appraised for use as pipe and service penetration seals for use where the penetration penetrates the wall underlay on timber and steel framed buildings within the following scope:

the scope limitations of NZBC Acceptable Solutions E2/AS1, Paragraph 1.1 with regards to building height and floor plan area; and,
with stand-alone flexible wall underlays covered by a valid BRANZ Appraisal; or,
with flexible wall underlays covered by a valid BRANZ Appraisal for use over rigid wall underlays; or,
with proprietary plywood or fibre cement based rigid wall underlays cover by a valid BRANZ Appraisal; and,
situated in NZS 3604 Wind Zones up to, and including Very High where stand-alone flexible wall underlays are used; or,
situated in NZS 3604 Wind Zones up to, and including Extra High where flexible wall underlays are used over rigid wall underlays, or proprietary rigid wall underlays are used.

OneSeal Multi-fit Piep and Cable Penetration Seals have also been appraised for use on buildings subject to specific weathertightness design. Building designers are responsible for the building design and for the incorporation of Multi-Fit Penetration Seals into their design in accordance with the declared properties and the instructions of Thermakraft Ltd

Clause B2 Durability: Performance B2.3.1 (b), 15 years and B2.3.2. Multi-Fit Penetration Seals meet these requirements.
Clause E2 External Moisture: Performance E2.3.2. When used as part of the wall cladding system, Multi-Fit Penetration Seals will contribute to meeting this requirement.
Clause F2 Hazardous Building Materials: Performance F2.3.1. Multi-Fit Penetration Seals meet this requirement and will not present a health hazard to people.

BRANZ Appraised

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