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23 March 2018

Natural Ventilation: Was the Old Way So Bad?

State House Window 1

Over this extended summer season I became more conscious than usual of natural ventilation as the primary means of controlling the cooling of a house’s interior. With the prevalence of heat pumps in our homes, it is so easy to switch the unit into air conditioning mode to make up for any deficiencies in the arrangement and type of opening windows.

Most modern houses have top hung awning windows whose geometric form makes for economic and easily installed units, but are not the best from a natural ventilation point of view. This is especially true for flexibility of options to better suit the changing weather, specifically the wind direction and strength.

The support of opening sashes, especially with the advent of double glazed units (and the approximate doubling of weight), is best provided by a symmetrical arrangement of the hinges across the top rail of the sash. Ignoring dynamic loads such as wind, this means that gravity is the only significant static load to be catered for. With a side-hung casement sash the gravity load is still needed to be supported, but the turning moment (trying to rotate within the plane of the glass), becomes the dominant static force to be considered in the design of the frames. This is especially important for the heavier double glazing. Also with awning sashes, if the hold-open stays fail, the sash usually just drops to the closed position, whereas casements will flap in the wind and can be damaged.

From a cost and structural design position, awning sashes are simpler and easier to manufacture than casements, but is that the primary design brief for opening window sashes? In my opinion it is more important that they act as effective and adaptable natural ventilation units over their lifetime, rather than as a means of reducing the one-off initial capital cost of the windows when delivered to site.

The best demonstration of the value of casement sashes is provided by the windows of the original State House design. Apart from single windows, which can only open in one direction, most windows have a pair of casements opening in opposite directions. A common variation has a fixed pane between, and top-hung fan-lights are often included above if ceiling height allows.

The simple vertical sliding double-hung windows of the old villas are very flexible in the options they offer for controlling the natural ventilation of the home which is perhaps why various modern versions are available in the market. Having the options of a top opening only, bottom opening only, both opening at the same time, and also fully closed so that the sashes also act as fixed glazing, allows for substantial modification of the internal environment by simple and quick means. The ability to easily vary the degree of opening at the top and bottom of the window provides fine-tuning of the natural ventilation. Of course there is the significant problem of dealing with unexpected rain. This is substantially dealt with by a hood or wide eaves.

The lowering of ceiling height with the arrival of the state house necessitated the redesign of the windows and how opening sashes could be incorporated. The horizontally side-hung casement sash was the result. Because domestic mechanical ventilation systems were not available in the 1930s, (and I’m sure would not have come to mind), retention of the flexibility of the double-hung arrangement would have been uppermost in the designer’s considerations. The opposing opening direction of a pair of casement sashes was a logical solution, and also retained aesthetic symmetry. Although paired sashes are the most common, there is no problem adding more to the window unit.

Houses have windows on all walls which are each continually subject to differing daily wind pressures; high on the windward side pushing air inwards, and low on the lee side where the partial vacuum sucks air out of the house. The adjustable casement-stay allows for positive control of the degree of opening, even to the extent of 100% on a calm day. Awning sashes do not have this flexibility. The other fundamental advantage over the awning style is that the opposing opening direction allows for easy control of the direction of air flow. Having the opening facing the breeze brings air into the house, and with only the other open air is drawn from the interior. During inclement weather both are closed to give a sealed window. At the whole house level, this inward/outward air flow can be adjusted to suit the windward and leeward air pressures and the variations of the seasons. Awning sashes do not provide this fine level of flexibility.

Yes, top-hung awning windows give rain protection from unexpected rain. The incorporation of fanlights in the state house, when open, covered the V-gap at the casement head as also does the closeness to the soffit above. This soffit protection is also offered in the modern house. With the common availability of a wide range of mechanical automatic window controls, unexpected rain shower and sudden excessive wind events are easily catered for.

The recent promotion of recessing windows will reduce the ability of awning windows to provide natural ventilation as the jamb and sill facings will restrict the free movement of air, especially when in ‘trickle-vent mode’ (see October 2017's blog 'Recessed Windows: Is a New Aesthetic Needed). This would be partly mitigated by opening the sashes to a greater extent, but that increases the forced-entry potential. Also is there adjustable fixing hardware capable of positively and securely holding the open awning sash in place? With recessed casements the opening is away from the jamb facing so there is no added restriction, and the effect on the small proportion of opening area at the top and bottom V-gap is minimal.

In my opinion it is now an appropriate time, especially with the rethinking of aluminium frame profiles for recessed windows, to go back to fundamentals and fully reconsider the design of the natural ventilation aspects of our windows for life-time use, not just the manufacturing convenience.

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