Ventilation – Preventing back draughts

Back draught occurs through extraction ventilator grilles and is caused by the wind blowing into the holes of the ventilation grille and into the building it serves. The holes are designed to allow moist air extracted from damp rooms such as kitchens and bathrooms via a fan and extracted outside. Unfortunately the very same holes that allow the moist air out, can also let air in and this can lead to draughts, commonly known as “back draughts” inside the house. This can create a draught within the room but can also be noisy, allowing wind noise to be heard via the ducting.

Remember – back draught is a phenomenon where wind is forced into an extractor outlet , not into a vent designed to allow air in (see note below) 

 

One of the simplest ways to reduce the effects of back draughts is to plan the positioning of the extractor outlets and position them at the leeward side of the house or building. In the UK this will mean positioning extractor outlets (ie hob extracts, tumble dryer outlets, kitchen, bathroom and WC outlets) on the East wall. If that’s not possible then South or North facing wall will be fine so long as it’s not positioned on a West wall, which is the direction of the prevailing wind. Of course, it’s not always possible to avoid using a West wall and we’ll be looking at some of the products we have that can be used to help prevent or at least reduce the back draught.

One key criteria when deciding on location of extractor outlets is to remember that the outlet is designed to allow the extraction of moist air from the house . To do this as efficiently as possible care must be taken to reach a balance between efficient extraction and avoidance of back draughts. The best route for an extractor is as short as possible with as few bends in the duct as possible. If this can be achieved and positioned extracting directly to an East wall then this would be ideal, however, it would be counter-intuitive to run a 20 metre duct from one side of a building to the other with 3 x 90 degree bends just to avoid ventilating out a West facing wall.

Thankfully we have many products that look good on listed buildings and period properties to allow, subject to consent, extraction to be on any wall. Gone are the days of running a duct within the ceiling voids the entire length of a house to put nasty plastic grilles out of sight at the back of the house. The Conservation Vent is aptly named because, if the Victorians had invented tumble dryers and bathroom extractors, it’s just the sort of vent they would have used and, thankfully, the sort of vent grille that gets approval on many of the countries listed and period buildings. It’s a stunning vent that looks on any wall – even at the front!

The conservation vent is one of the best products we have to reduce back draughts, lets take a look at some other ideas and products…..

   

Flaps, positioned withing the ducting can be used to allow the extracted air to flow when the fan is running but shut off when the fan isn’t in use. It is cheap to buy but care is required installing to ensure it is positioned as above and fixed in position because it requires gravity to close when the fan isnt running and this is only achieved when installed with the flap completely horizontal. The main disadvantage with flaps is that they can be heard closing and opening, especially if they are positioned near the room they extract from.

Mesh installed complete with the grille on the external wall will reduce back draught and driving rain. We sell mesh complete with many grilles and as “belt and braces” for those installing conservation vents in especially windy areas and in situations where the wind can be directed upwards. Again, this is cheap to install but consideration needs to be given to the fact that it reduces the air flow in “both” directions so will reduce the amount of air being extracted as well as the back draught. 

   Louvre air bricks are not a new invention, designed by Victorians, they are a common feature in period rail stations. Originally, it is thought they were predominately used to prevent careless smokers discarding cigarette butts in the vents and causing a fire hazard. Indeed they are still useful to this effect but are also extremely good at preventing back draughts whilst allowing a decent free area for extraction and ventilation. They are available in various sizes but because of the amount of iron required for the structure, the smallest, size 9×3 has very narrow slits and therefore very little ventilation, the larger size 9×6 is much preferred. Unlike the hit and miss air brick (see note at the bottom) , the cast iron louvre air brick was one of the Victorians finest ventilation products! 

It is a common misconception that louvre air bricks or indeed any air bricks can be combined with mesh. Fitted behind the louvres or air bricks to reduce draught further and also prevent flies and insect ingress can lead to problems in the future and can lead to increased occurrence of dry rot. This is because the mesh is hidden away – gathering dust and debris, slowly reducing the ventilation each day until it eventually becomes blocked. To overcome this problem we have designed air bricks specifically for use with mesh were the mesh can easily be seen from the front of the grille and can be removed for regular cleaning, whilst leaving the frame of the air brick mortared in the wall – ingenious!  These air bricks are called Flyscreen air bricks and are available in 9×3, 9×6 and 9×9 sizes…

 

Please note ….

1. An old design from the past is the hit and miss cast iron air brick. An air brick with a sliding closure that can be shut off when the wind picks up or room ventilation isn’t required.  We still make these for property restorers to replace like for like broken ones but we don’t recommend fitting them where they aren’t already installed. The idea, in principal, sounds good but unfortunately they’re not a great idea, not one of the Victorians best inventions. As they are made from cast iron , they’re prone to rusting and seizing in either the open or closed position. They are often used for ventilation to rooms where , in 21st century it would be illegal to shut off the ventilation as the air is required for open fire (gas boilers, oil fired boilers , log burners etc).
2. This article is regarding back draughts through extraction vent grilles, ie vents that are designed to expel air, not allow air in. It is of course not applicable to ventilation that is natural and allows air (wind) through the vent. many examples of these vents are installed in period properties and the majority are used to ventilate underneath ground floor wooden floor boards to prevent dry rot occurring.  In such circumstances it may be beneficial to site these into the prevailing wind and have them without mesh.