Tyvek HouseWrap - Product Attributes
Weather-tightness
The product resists liquid water penetration and wind-blown snow and will protect the sheathing and frame from external moisture.
The period prior to the installation of the brickwork should be kept to a minimum. The membranes should not be used as a temporary waterproof covering during this time.
Risk of condensation
The product has a design resistance to water vapour transmission of less than or equal to 0.6 MNsg–1 and is defined as a ‘breather membrane’ in accordance with BS 5250 : 2002. Therefore, it will contribute towards minimising the risk of interstitial condensation in suitably designed walls.
The risk of condensation occurring within the wall of a timber-frame building will depend upon the properties and vapour resistance of other materials used in the construction, the internal and external conditions and the effectiveness of the internal vapour control layer.
Convective water transfer into the wall construction can be reduced by installing a continuous airtight sheet such as Tyvek SD2 behind the internal lining. This may also contribute to a successful pressure test by achieving the required design air permeability of 10 m3h–1m–2 in accordance with Approved Document L (See section 12.3).
Strength
The product will resist the loads associated with construction and installation into timber-frame constructions.
The product is not adversely affected by water and will retain its properties when wet.
Properties in relation to fire
The product will have similar properties to polyolefin membranes in relation to fire, tending to burn and shrink away from the heat source. The product is unclassifiable in terms of the Building Regulations and this should be considered when assessing the overall fire risk.
Cavity barriers should be used to satisfy the requirements of the national Buildings Regulations.
Maintenance
As the product is confined within a wall construction it has suitable durability (see section 10), maintenance is not required.
Durability
The product will be virtually unaffected by the normal conditions found in a timber-frame wall and will have a life comparable with other similar elements of construction, eg vapour control layers.
Tyvek HouseWrap - Product Installation
Tyvek Housewrap must be installed in accordance with BBA Certificate No. 90/2548 instructions and the recommendations given in NHBC Standards, Section 6.2, where appropriate.
Procedure
Starting 300 mm from a corner, wrap the Tyvek membrane completely around the external face of the sheathing board.
Make sure that the membrane overlaps the lowest timber member by 100 mm.
Normal practice is to unroll the membrane horizontally, but vertical application is also acceptable.
Fix the membrane with galvanised nails or stainless steel staples as follows:
Horizontal centres = max 600 mm
Vertical centres = max 300 mm Extend the Tyvek membrane to cover window and door openings and staple or nail around the openings at 150 mm centres.
Any vertical laps formed in the membrane should be a minimum of 150 mm.
Horizontal laps between sheets and any laps onto cavity trays or flashings should be a minimum of 100 mm.
Sealing the laps with Tyvek Tape is optional.
Once the Tyvek membrane installation is complete cut an ‘X’ from corner to corner at window and door openings, fold inwards and staple/nail to reveals.
Make good to the corners with Tyvek Tape. If windows and doors are already in place turn the membrane up against the frame to prevent water ingress.
Stud positions for wall ties can be easily marked by using an indelible marker pen.
Procedure for Lapping and jointing
The membrane should be fixed in such a way as to shed water away from the sheathing, and below the lowest timber. Upper layers should be lapped over lower layers.
Laps should be at least 100 mm and vertical laps 150 mm.
Vertical laps should be staggered wherever possible.
To assist in achieving the design air permeability the lap joints and penetrations through the underlay can be sealed with Tyvek Acrylic Tape or Tyvek Butyl Tape.
Fixing
The membrane must be secured at regular intervals with nails and staples to prevent damage by wind
Nails should be of galvanized or sherardized mild steel, austenitic stainless steel, phosphor bronze or silicon bronze and staples should be of austenitic stainless steel.
Marking stud positions
It is essential that the positions of studs are marked to enable wall tie fixing.
Lowest timbers
It is essential that the lowest timbers in the wall are protected by the breather membrane.