I have been told all new windows need trickle vents as build. regs. require
This seems to break the thermal envelope. Are trickle vents mandatory?
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It depends on what ventilation system/rates you are intending to have e.g. if using MVHR you don’t need them.
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As far as I am concerned trickle vents are a failed concept. The idea was that as British homes become less draughty you need to introduce deliberate ventilation to shift humidity. The problem is that when the wind isn’t blowing there is negligible air movement but when it is howling a gale outside you get excessive air movement. The draughtiness of a house was limited by building regulations to 10 air changes an hour, i.e. the air you heat in your house gets fully replaced every 6 minutes. Under such regulations trickle vents compare favourably with leaving all the windows open.
The building industry pressurised the government into scrapping any real improvement in the latest building regs as improvement would be “too onerous”. (Compare that with Passivhaus regulations of not greater than 0.6 ach.) If your house complied with building regs, only, then you don’t really need trickle vents. A few years ago I had my house tested and it had 7 ach. I have since made improvements. Despite various window replacements over the years I have never owned a trickle vent and don’t intend to do so. There are currently 5 people making humidity. I do not get condensation or mould in the house, partly due to an effective ventilation system and partly due to even heating in all rooms preventing cold patches.
I would go further than @john.d and say that if using MVHR you should avoid them. In fact any ventilation system that introduces fresh air is disadvantaged by having trickle vents.
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We had patio doors and got them replaced when we had a change in wind direction with heavy near horizontal rain and water came through the trickle vents. You can selectively control the opening of windows as and when you need it.
Even if the water doesn’t get into the room it can still run down inside the frame, wicking away warmth and causing problems as it goes.
Sadly the architectural profession doesn’t seem to have caught up with the concept of thermal envelopes. I had virtually the same argument with ours who wanted to install trickle vents even though we already have a MVHR installed!!
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I think that if you are replacing the odd window you can ask for like for like, so if your house doesn’t have trickle vents already it can stay like that. Alternatively you can follow current regulations, as below.
If you are replacing the majority of windows or building a new extension then you need to comply with building regs. However, building regs do not mandate trickle vents, just adequate ventilation. If you can show that your ventilation meets the requirements then you don’t need additional ventilation, be that trickle vents, air bricks or something else.