Window trickle vents

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?


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!!

:man_facepalming: :man_shrugging:    

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.

I wonder if the building industry couldn’t cope with the truth about non breathable products being used in buildings mean’t to breathe. That amount of pressure might be too much!

Currently I’m about to purchase a window with trickle vents for the bedroom where we intend to replace the gypsum plaster and silk painted walls, to revert back to the original Victorian house ‘breathable’lime.
I have realised that the damp in said room is because of penetration through pointing and saturation in the gypsum from the external walls and non breathable paint trapping moisture.

Reading your comments, which make perfect sense to me , I’m back at fighting against trickle vents.
My friend also mentioned that because I will have the MV in the bathroom, I could leave the doors open and count that as my ventilation for the bedroom too.
What do you think - the bathroom is about 2m x 3m and the bedroom is 2 meters opposite .

I’m going to write to my MP …

*Additionally, I’m actually starting to wonder if gypsum is better saved for internal stud walls only.

Regarding the bathroom + bedroom ventilation, I think that would be pushing things a bit. At 2m separation there is plenty of possibility (i.e. probability) that air will be drawn from the rest of the house too, unless you start playing with ducts between otherwise airtight rooms.

Gypsum is a good and relatively cheap product when used in the right situations. Having the humidity more or less the same both sides or avoiding ever exposing it to the dew point are required, so, yes, internal walls should be fine. Likewise, if the wall is water proofed, e.g. by tiling, gypsum would be fine, until the waterproofing fails, but you know that story already!

When I had my extension built and replaced other windows there was no suggestion of having trickle vents, so I didn’t need to fight my corner. When a building inspector came he raised an eyebrow until I pointed out the MVHR and he accepted my solution immediately.

I’m in the process of ordering new windows - without trickle vents.
I’m of the understanding via my window fitter that he won’t be able to certify my windows and the windows could potentially be flagged up for inspection.

Having monitored the situation in the specific room we’re about to retrofit, we want to achieve ventilation in other ways , recognising how trickle vents are an air leak and a source of heat loss .

In addition to this, it made me think how this blanket approach to ventilation is sending the wrong messages about managing water ingress and resulting damp and mould in building fabric.

I have written to my MP letting him know that I anticipate a problem and my concerns . It’s a long email and I think I’ve probably waffled a lot :grimacing: trying to help him get of picture of the circumstances .

I’m sharing it here. Feedback and comments are most welcome.

Dear Mike,
first of all I wish to thankyou for the continued work and commitment to helping people - in health, climate change and the environment.
You are most appreciated. :pray:t3:

I am contacting you again re. a problem I am anticipating about the phased retrofit choices I am making for my home. Resonating loudly alongside this is the greater relevance to Awaab’s law and the case for ensuring healthy homes for all through building practices and materials used.

My concern is about trickle vents and moisture management in the home.
My issue - I do not want trickle vents in my new windows. I have good reason for this as well as concerns about the emerging blanket effect message of one size fits all in our individual homes.

Using the facts about housing and managing moisture, the law about trickle vents on windows seems to be a knee jerk response to managing moisture in our homes - trickle vents will not eradicate the main causes of mould created in building fabric away from windows.
In fact, enforcing trickle vents on all windows will create other problems for householders trying their best to reduce their carbon footprint via heat loss and managing moisture build up via other more effective ventilation methods.
I am not against ventilation - I am against being told how to achieve it and how it will mislead the public into thinking trickle vents will solve damp and mould problems in walls.

Here I will try my best to explain why I do not think trickle vents are necessary in the windows I am about to purchase.

We are retrofitting the front bedroom of our Victorian home which has these moisture problems:-

  • penetrating damp via failed pointing and the external window reveals

*condensation build up on the two cold uninsulated external solid walls of the bedroom.

*constant air leakage from the warped pvc window frames, which won’t close properly and have failed double glazing units, this providing a constant draught and heat loss; It is an unmanaged source of ventilation, which hasn’t prevented the condensation buildup on the walls and windows in this room. Indeed in wet weather - the ambient humidity from outside comes into the room through the window adding to the moisture content and condensation buildup in the room.

*In addition to this are the extreme temperature changes throughout the seasons, which, at times have made this room unbearable for my older daughter who is studying for her nursing masters. We know we need to improve the insulating values of these walls.

As you may recall -via the Carbon Coop - I have taken a lot of time to learn about retrofit alingside achieving a healthy and sustainable living environment.
Such is my passion about this, that if I could afford to leave the security of my current job, I would take time to help and educate others who struggle in these areas.
Householders currently do not have enough correct information available to them to help them make the best choices for their homes when making improvements.

*My house being of Victorian solid wall construction was built with brick and lime cement and internal lime plastered walls - all materials which breathe and do not trap water molecules - unlike gypsum plaster that absorbs moisture but doesn’t release it; This is very relevant to the problems in our bedroom, owing to a modernisation that removed the original lime parge and plaster in favour of dot and dab gypsum board and skim.
This action did two things - reduced the insulation values of the internal wall coverings, as well as losing the hydroscopic moisture managing qualities of the existing lime.
In addition to this, the external lime cement was repointed with concrete and we lost more of the breathable moisture managing qualities of the original materials. The concrete cement effectively traps moisture and blows the bricks in freezing conditions.

Of great interest to me is the performance of my next door neighbour’s home - which still has all the original lime wall coverings , external lime cement pointing and no trickle vents on any of her upvc double glazing. Her upstairs rooms on the same front elevation are temperate throughout the seasons - without a spot of damp! Even more amazing is the cupboard situated against the external cold wall , ram jammed full of items in storage with no signs of damp or mould; versus my cupboard (also against the external wall) riddled with moisture and mould - it is unusable.

I feel like this is a lot of information- but the bottom line is that I know installing trickle vents on my new windows is not the answer to the damp and mould problems in our room.
The moisture problem is engrained in the overall fabric, which we are addressing. We are removing the gypsum and going back to an original lime parge on the walls.
About ventilation, moisture and heat loss, we wish to manage this by opening windows and doors when necessary, having carefully sited and managed storage to allow airflow around the affected walls. Alongside this we will be carefully monitoring and checking this room for any signs of damp or mould.
We are not be going to all this trouble and expense of replacing windows and wall coverings to create further problems. Indeed we know that in some situations trickle vents, an a strategically placed air-brick or additional mechanical ventilation might be necessary.
My final argument is about heat loss and reducing our carbon footprint- trickle vents will be a constant unmanageable source of heat loss. Furthermore, they negatively affect all the improved values of an energy efficient window.

Moreover, we are just one lower income family trying our best to reduce our carbon footprints and expenses by creating a healthier, more temperate and sustainable, future proofed living environment.
Everyone needs this - the planet needs it.
Awash Ishnaq and his family needed it.
Devastatingly , it is too late for them, but I hope my little attempt to stand up and add to this conversation will result in better decision making further down the line.

I’m not sure how much support my reaching out to you will add in my argument against trickle vents in certain situations, but hopefully by keeping you up to date with my progress in my specific case , it will provide you with an understanding to support others too.

If your MP looks into this he could justifiably reply that trickle vents are not compulsory. It is, I suspect, the glazing industry that promotes this view as it is in their interests to have a uniform production process and one system suits all approach. Anything that requires a different approach, even if it involves less being done to the window during manufacture, adds complication and expense.

Building regulations specify the requirements for ventilation and mention several options, however,

A disclaimer signed by the homeowner stating that they do not wish to have background ventilators or that they will be installed in future is not a suitable way of complying with the Building Regulations. Work must comply with the Building Regulations and competent person schemes must monitor their registrants and take action against any registrant who is found to have carried out non-compliant work.

(Source: Approved Document F, Volume 1: Dwellings - frequently asked questions - GOV.UK)

In other words you need the ventilation in place before changing the window.

Could I argue the case using my mechanical ventilation in the bathroom as a tick in this box ? The bathroom is the room opposite the small bedroom .

It is worth a try.

If the bathroom has extract only there is an argument that a vent is needed for replacement air, try to avoid that argument if you can but have backup locations ready to demonstrate optimal airflow.

Edit: have you fitted that MVHR that has been sitting in its box for about 2 years? :flushed:

I have never had any building regs certifications for any window or door I have had changed. I didn’t even know that was possible until reading up on it 2 days ago.

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I think we need to get it fitted asap!

In addition to the ventilation I found an article about the case for lime plaster for preventing mould. Indeed as I talk here , I am watching the Call the Midwife episode which is about a tennant living in an apartment riddled with mould. The housing department try to blame the tennant for the mould . While gathering evidence as the tennants child lies seriously sick in hospital, it is stated to the mother that the mould is NOT her fault - it is a problem seen in dry wall and poor building practices.
To hear this was a problem so long ago and is still a problem to this day beggars belief.
I’m asking myself if building regulations have ever really regulated effectively about the installation of dry wall materials upon which mould can grow .
Moreover, should gypsum be used in the external facing walls of solid wall construction buildings, because from everything I read , the indications are it should not and nor should non breathable paints.,to%20treat%20fungus%20and%20mould.

Cavity walls came in before lime plaster went out, so original structures should be ok. It is repairs and “improvements” that cause the problems.

I have never seen an episode of “Call the midwife”.

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I can see how this evenings episode contained a topic of great public interest following Awaab Ishnaq’s tragic death.
It will certainly bring about more conversation and hopefully education about tackling damp and mould.

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Showing that this has been going on so long without progress being made should provoke public outrage. Unfortunately politicians don’t react to less.