Ventilation Strips

All attic and skillion insulated roofs require passive ventilation if condensation problems are to be avoided. Don't get the blame for a leaky roof when it is the house condensation that is to blame.

Insist on air movement by using TVS slotted aluminium strips at soffits, ridges and barges to provide adequate the necessary ventilation.

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Because different roofs require different balanced amounts of ventilation at the eaves and ridge a range of TVS is offered to suit every situation and which are simple to use when fixing flashings. Thomson Eave Ventilation Strips (TEVS) are made in two standard widths with 3mm slots either 50mm or 25mm wide and 2400mm long. Although other special sizes can be made to order.

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The broad parameters for design are between 1/150 and 1/300 of the flat area of an insulated roof. Low pitched roofs and those in sheltered areas require more ventilation.The TEVS 50 has a NFA (net free area) of 17,500mm2 per lineal metre). TEVS 25 has a NFA of 9,500mm2 per lineal metre.

The Aluminium TEVS can be powder coated or have a permeable backing for marine environments. The TEVS fits into the standard groove of a fascia board while the two different sizes provide the designer with the option to balance the ventilation for the site. Aluminium soffit vents can also be used on sloping barges and apex ridges.

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The drawing above highlights a common mistake made by insulation installers who often block off any air movement. A 20mm space is needed between the roof cladding or the purlin to provide ventilation between the soffit and the ridge or gable vents. While there are proprietary products made for this purpose, safety mesh stapled to the bottom or top chord is equally effective.

The principle of passive ventilation is well known. If there is a temperature or a pressure differential, then under the laws of equilibrium the temperatures and pressures will try to equalise. Air movement takes place and the hot air carries with it the moisture (water vapour is 33% lighter than dry air).

A designated space is required to vent it outside the building, however for the stack effect to work a balance of intake (lower) vents and outlet (higher) vents is required. If the design is a monopitch this requires equal air inlet and outlet areas whereas a duo-pitch gable would require soffit vents on both sides totalling the area of the ridge outlet vent.

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While a duo-pitch gable requires soffit vents on both sides it can have either a ridge vent or gable end vents. For skillion construction it is best to vent the barge on either side while a mono-pitch can either have soffit and ridge vents or barge vents on both sides up the slope.

It is important to retain a ventilation path and where possible allow a 20mm air space either up and down or across the roof.

There are four very different situations attic, skillion, monpitch and duopitch and a different design type is required for each.
New building and retrofits also need a slightly different approach.

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