Why would you want to use attic ventilation fans in the UK? Surely they’re just for hot countries where they want to try and get the heat out of the roof space?
Well in my opinion in hot countries they don’t work at removing heat, but that argument is for another day.
Here in the UK the main reason for considering an attic ventilation fan is likely to be for the purpose of trying to reduce the chances of condensation forming in your roof void, either on the underside of the roof cover, roofing felt or worse still within the loft insulation.
Before I answer that question, we need to understand what condensation is and why it forms anywhere in or home?
Condensation forms where the moisture in the air (water vapour) comes into contact with an object that has a surface temperature that is below the dew point. And dew point is the temperature at which the water vapour ceases being a gas and forms a liquid. (You can read more about condensation here).
As part of living our lives we generate water vapour by showering, cooking, drying clothes etc. All this water vapour floats around our homes until it either condenses to form condensation or it’s taken outside by way of ventilation.
So how I hear you ask does it get into the loft where I don’t cook or do any of those things mentioned?
You need to think about the construction of your home. The ceiling between the living accommodation and the loft area is typically made of plasterboard which isn’t a true vapour barrier but it does a pretty good job of stopping water vapour.
However through this plasterboard we cut holes to get wires for lights through and we cut out a big rectangle to form a roof hatch. Then over time we start to see cracks form where the plasterboard meets the walls and so on.
These cracks and openings allow the water vapour to get into the loft area. The issue is then exacerbated by the fact that most homes have whats called “cold roof” construction, where the insulation is laid directly on top of the living accommodation ceilings. Which is great when it comes to keeping heat in the living area but it also means the area above the insulation tends to be cold.
So any water vapour that manages to get into the loft and past the insulation will condense when it comes into contact with any cold surfaces, like the underside of the roof and roof timbers.
There are several ways:
Installing attic ventilation fans would in theory help, but only if you also provide additional ventilation to match what the fan removes. In other words if your fan extracts 4 litres/sec. second you need to make sure that you provide additional ventilation that will let in at least 4 litres/sec. to replace it.
If you don’t do this all you’ll do is put undue stress on your attic ventilation fan and the fan will try to draw air up from the living area below which is full of water vapour, and bringing more water vapour into the loft is the last thing you want.
Attic ventilation fans can make a difference but you need to make sure that in conjunction with fitting the fan you make sure you’ve created a vapour barrier to the ceiling of the living area below, so that no more water vapour can get into the loft.
So, I'd have to say, why not just create a vapour barrier and not bother with the attic fan!