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Damp toolbox part 4: Ventilation...practical solutions putting you in control


The headlines

The previous article described how reducing the amount of moisture getting into the air helps, and why ventilation is the best solution to stop damp air building up.

This article describes practical effective ways to ventilate a house properly, both to get rid of moisture building up in the air indoors, but crucially also, get rid of indoor air pollution that is now a serious issue in the UK.

There are many ways to do this, and to provide the many fresh changes of air homes need to be healthy:

It starts with the dead simple stuff, and then moves on to more substantial solutions:

  1. Simple things to do a little differently in your home day to day. This includes finding out what the Germans do!

  2. Some small physical changes such as installing trickle vents.

  3. Increasingly common products that ACTIVELY ventilate a home and completely eliminate damp and condensation issues.

How much you need to ventilate your home will vary depending on a very wide range of things. A home with a single occupant will need less than a home with many occupants. A home where laundry is dried outdoors will need less than a home where laundry is dried indoors.


In practice, it will need some experimentation, trying different methods and gradually increasing ventilation to resolve issues you may have.


The general guideline in building regulations in the UK is that the air in a home in the UK needs to be refreshed and replaced with fresh air at least 15 times a day to get rid of moisture and pollution created indoors.


Some trivia I can't resist sharing: In 1752 a London prison was persuaded by the English scientist Stephen Hales to open ventilation holes in the cell walls. A giant bellows was installed to force fresh air into the cells. The health of the inmates then dramatically improved.


This is a recent Facebook post from someone a little while after they read this article:

 

Quick link to sections below.

 

General simple tips on ventilation

If you have a professional to fit insulation, draught-proofing or new windows or doors: Ask them what they have done to check that ventilation will still be adequate.

Let air in the home get to cold spots or surfaces that are giving you problems:

  • Pull furniture away from cold walls to let the air in the room circulate to the wall. A couple of inches of space will allow air to circulate to the wall at the back.

  • Exterior walls are the most important in the above tips as they will be coolest.

  • If you have a cool room that suffers from mould or condensation leave the door open when the heating is off. This will help air circulate around the home and into the room. Keeping the door closed all the time will actually help mould survive and even thrive.

  • Similarly, if you have mould issues in a built-in wardrobe, leave the door to it open when you can to let air circulate into it.

  • Try to avoid opening your windows at peak times of traffic if you know you are in a high pollution area. You can also check your local outdoor air pollution levels.

 

Air vents or vent bricks....never block them!

Vent bricks are there for a reason and should never be blocked up. They provide essential ventilation to keep the structure of the house dry such as inside cavity walls.

I know someone who was spending over £600 a year running a dehumidifier to help with a musty damp smell issue in their cellar.

They then discovered that the vent bricks for the cellar had been blocked up by a previous owner. They have now unblocked the air bricks and now no longer need to use the dehumidifier.

Similarly, do not block indoor vents on chimney breasts.

If they are blocked up, do unblock them, otherwise you'll risk creating damp problems inside the chimney breast.


 

Trickle vents

Why have them

These were introduced in new build homes SPECIFICALLY in the early nineties to give a basic ventilation solution for indoor pollution and damp issues.

With a few exceptions, from June 2022 they are a mandatory fitment on all new and replacement windows.

They are usually fitted at head height to minimise the creation of low-level draughts.

We need more homes with them though....

Only around 10% of homes have trickle vents throughout the home. All the remaining homes rely on 'luck' for ventilation through random draughts and gaps in the building. In many cases this is completely inadequate.

Even when you have them they don't provide enough ventilation

This survey suggests that in the vast majority of cases (85%) the trickle vents were actually inadequate for the needs of the home. Studies have shown they are inadequate for good quality sleep. (See the geeky section at the end if interested)

Even though they provide barely enough ventilation they need to be used

The same survey also found that 70% of trickle vents in homes were shut by householders either to:

  • keep noise out or keep heat in, or

  • because the householders were not aware of what they did.

They reduce draughts elsewhere. When trickle vents are open, they make it harder for air to find its way into a home than through hidden draught sources. So, while you will have fresher air at face level from trickle vents, you should have fewer or weaker cold draughts coming in at foot level. Try to think of trickle vents as nice and healthy draughts.

Retrofit of trickle vents is pretty easy

With some exceptions, trickle vents can be retrofitted on almost any window by anyone reasonably confident in DIY. This includes wooden window frames.

They come in a range of colours and are inexpensive (under £5). These two YouTube videos show you exactly how to fit them, by a choice of methods:

  • Installing a trickle vent without removing the opening part of the window. I don't recommend walking on the roof like the guy does at the end though!

  • Installing a trickle vent by removing the opening part of the window.


Summary & key tips - if you have trickle vents, get what value you can out of them

  • Trickle vents should be considered the barest minimum needed in any home and should always be left open.

  • On their own, they do not provide enough ventilation to cope with intense moisture generating activities such as drying laundry indoors.

  • They only provide enough ventilation for bedrooms if a door is left open at night.

  • If you have a room with damp or condensation issues and a trickle vent, leave the door open when the heating is off. It will improve the flow of fresh air from the trickle vents through the house and the problem room.

  • Doors should have at least 10mm gap underneath them for trickle vents to work best in ventilating the whole home.

  • Retrofit them if you can.

  • Yes, you might feel cool air from them if your put your hand near them, but they will definitely help with damp problems. It is why they exist. Without them, some rooms or homes really are sealed boxes,

Through flow of ventilation, even very gently is by far the best way to keep a home healthy
 

Use or even upgrade extractor fans - bathrooms etc

They are essential for dealing damp and air pollution in wet rooms and kitchens.

However, they often aren't actually as good as they should be.


Bathrooms and wet rooms

This 2019 survey found that less than half the extractor fans in bathrooms and wet rooms were extracting enough air. In some cases, they were extracting substantially less than was needed. The reasons were because either the wrong fans had been specified, or the wrong speed selected.


Upgrading these fans is relatively easy and cheap to do. Most modern extractor fans far exceed the minimum extractor rate. You can also buy bathroom extractor fans with humidity sensors built into them. The fans will switch on and off automatically to control humidity in a wet room.


Extractors won't help with damp problems if they aren't used

I know that sounds an obvious thing to say! But, this survey found 40% of householders had switched their bathroom and WC extractors off at the wall.


Helping kitchen extractors help you

This survey found that 15% of homes never use their kitchen extractors. It also found that the amount of air they were extracting was inadequate because of dirty filters.


When using kitchen extractors, you can help them do their job by leaving the door into the kitchen open. This will allow the extractor fan to suck moist and polluted air out of the house as well as the kitchen.

 

Leave some windows partially open

A 2020 survey showed that 80% of households deal with their damp issues by opening windows.


If security is an issue, most windows have something known as a "night vent position". This allows the window to be latched and even locked in a 'cracked open' position.

The night vent position provides more ventilation than a trickle vent can. This position is ideal for providing enough ventilation for drying laundry in a room. Even wooden framed windows often have latches that allow you to do this such as the one in the photo.

Wooden frame latch
 

Do what ze Germans do!

This is one of the most effective simple and low cost ventilation methods. If you've never heard of this, it understandably might seem a bonkers thing to do! There are a couple of reasons why you might think that:

  • The UK media often use the expression "UK homes are among the leakiest in Europe". This is largely true, but the use of the word is very misleading. Using this word implies AIR leakage and has resulted in a common belief that any way for air to enter or a leave a home is bad. The most important way to stop "heat leakage" is through insulation. The media really need to be re-educated to use the words "UK homes are the most poorly insulated".

  • Air doesn't actually hold much heat energy. If the inside of your home is warm, most of the energy is stored in all of the walls, furniture, flooring etc etc. I've estimated the cost of using this method at the bottom of this section.

The Germans have a tradition they call Stosslüften (sometimes known as shock or surge ventilation). They open all the doors and windows for up to ten minutes twice a day.

They do this regardless of the weather! This Guardian newspaper article describes the German tradition as:

'The custom is something of a national obsession, with many Germans habitually opening windows twice a day, even in winter. Often the requirement is included as a legally binding clause in rental agreements, mainly to protect against mould and bad smells.'

When I say 'open' the windows, I mean WIDE open!.

This shock tactic is generally regarded as much better than the trickle vents or leaving a window open crack. It gives a quick change of air without having much effect on the temperature of the contents and interior surfaces of a home. BOOM...stale damp air out....fresh air in.


To minimise the heat loss this is best done at times when the house is coolest, so when the heating has been off for a little while.

Opening all internal doors is an important thing to do when you do this to ensure that ALL rooms get fresh air.

This is especially so for rooms that are either sources of moisture (such as bathrooms) AND rooms that have specific damp or mould issues.

The Energy Savings Trust recommend windows on opposite sides of the room are opened two or three times a day for 5 minutes. Again, not just opening the windows a little, but wide open!

So how much does the German method cost?

I've done some sums and I reckon each time you do a full ventilation the German way it costs under 10p in heating costs. By that I mean it will cost around 10p for your heating system to warm that air back up again (assuming you heat your home with gas).

 

Take control and experiment

If you haven't seen it take a look at my other article on how to reduce the amount of moisture you are generating in the home as by making reductions where you can and adequate ventilation I believe it is possible to substantially or completely solve all or most damp issues.



With all of the tools and tips, I'd encourage you to:

  • Have a go.

  • Ventilate plentifully, give fresh air a chance.

  • Give it a few days to see what happens to your condensation issues.

  • Give it a few weeks to see what happens to any mould issues.

  • Then gradually reduce the ventilation until you start to see condensation issues creeping back - you will then know you have gone too far and need to ventilate a little more.

Try stuff out, even for just a week or two.

 

More complicated but more effective solutions

The above conventional solutions are largely simple changes or low-cost investments.

It is increasingly clear that the above methods are insufficient for all homes.


In the next section, I describe two increasingly popular and effective solutions. They are automatic and ACTIVE ventilation solutions. and are increasingly being encouraged in building regulations.

They are very well proven but are an investment.


Positive input ventilation products

These are also known as 'PIV' systems. They are essentially the opposite of an extractor fan, bringing in fresh air rather than trying to extract stale air.

They have been sold in large numbers since the 1970s. Around 1 million homes in the UK now have them.


Despite the popular belief in the effectiveness of PIV systems they were REMOVED from building regulations in 2021. This is because while they are very successful at addresssing visible condensation and mould issues they create a risk of driving moist air into the sub structure of a home causing damage to the building such as timber rot.


If PIV is used it is ESSENTIAL to take good advice on its suitability. In particilar, it is essential to ensure that as fresh air is brought into a home by a PIV unit, stale and humid air in a home has an effective and reliable way of LEAVING the home.


How they work

  • They use a very low power and almost silent fan that gently brings fresh air into a home.

  • By doing this, damp polluted air is gently forced out of cracks and gaps that always exist in a home. This includes forcing it out through bathroom extractors, cooker hoods and trickle vents.

  • They are normally mounted in a loft (though there are alternatives). They take air from the loft, filter it, and distribute it through a single central vent, normally on the landing ceiling.


What people say about them

PIV systems are widely praised on social media for how quickly they eliminate damp and condensation issues. Here are some examples:

  • This is a six minute video of a UK householder giving a review of his PIV system.

  • Also this recent two minute TikTok video of a different real UK home experience.

  • Loft air is normally a few degrees warmer than the outside air, even in very cold winter weather. The air it brings in is obviously cooler than the air in the house. Some people do find the air they bring in is too cold, particularly in very cold weather. Where you are in the UK will affect this of course. To help with this concern there are versions of these products that have a low power electric heater built in. These heaters are just powerful enough to take any chill edge off the air coming in. They have smart controls to minimise energy consumption.


How PIV units cost to run

The annual costs of running one with no heater are around £32 (assuming a 27p electricity unit cost tariff cap from Oct 2023).

Units that use heaters do cost more to run. Trials by one of the manufacturers (Nuaire) showed that the energy costs varied depending on where a home is in the country. They found that for a three-bed semi, the running costs vary with the climate. They found the annual costs were:

  • London £55

  • Manchester £100

  • Glasgow £153 in Glasgow because of the colder climate.

That sounds a lot, but they are incredibly good at solving damp and indoor air quality problems.

I am sure you are thinking that this is a mad idea and PIV units will also increase the heating costs in a home bring cool air in. This is completely understandable. In practice though they don't increase heating costs substantially. The reason is that because they are putting positive pressure DOWN into a home. This downward air pressure pushes back and reduces cold draughts coming in from other parts of the home. Because draughts are reduced comfort and warmth, particularly downstairs can improve, especially if the draughts are at foot level.


PIV installation and costs

  • PIV units without heaters start at around £300 and ones with heaters around £400. This source suggests using a professional to install them roughly doubles these costs.

  • Installation is pretty easy DIY job. They need an electricity supply safely providing.

  • This video gives you a very practical guide to what these products actually look like and how to fit them.

 

Whole house heat recovery ventilation

These are the ideal energy efficient solution, although the most complicated.


They are called Mechanical Ventilation with Heat Recovery systems, or MVHR.


I fitted one many years ago and think it is awesome. We have no mould or condensation of any sort and have really low humidity indoors throughout the year, often below 40% RH.



How MVHR systems work

They have a fan unit, usually mounted in the loft that continually and silently sucks stale moist polluted air out of the kitchen and bathroom(s).


At the same time the fan unit also pumps fresh air from outside and into living rooms and bedrooms.

The air that is pumped in is pre-warmed as the loft unit transfers the heat from the outgoing air to the incoming air using something called a heat exchanger.

The result is that the fresh air being pumped into the home will be almost as warm as the air that is already in those rooms.

They use very little energy. This all happens very slowly, almost unnoticeably.

The loft units are a little smaller than a washing machine (and a lot lighter!).

This short video gives a good visual guide to how they work and what they do. You don't need sound to watch it. For a more detailed description of how they work this web page is also very good.



My own experience of MVHR

I fitted one of these systems in 2009.

Domestic installation of a MVHR heat recovery ventilation system
My own heat recovery unit and some of its ducting

It cost me around £1500 to install myself using insulated flexible ducting. The annual electricity cost to run mine it is around £40.

The only maintenance is replacing filters on it every 6 months or so. I do this myself with a pair of spare filters that I clean and recycle.


Because of this system we have also found:

  • We have no damp, mould or condensation issues of any sort, even in the shower which is used daily and not cleaned terribly often.

  • We have brilliant good sleep quality as we have fresh air vents into each bedroom.

  • Towels in the bathroom dry really quickly, even just hanging in the room.

  • If we dry laundry indoors it dries very quickly

  • Because the air in the house is at such a low humidity we don't need the house to be as warm as it would if it were more humid. So, ultimately this means MVHR systems actually help to reduce heating costs.

  • My Wife and I rarely get coughs or colds since we put the system in, as the better ventilation prevents the build up of viruses in the air indoors.

  • I have recently started monitoring levels of carbon dioxide in my home as build up of this gas (from humans breathing) reduces sleep quality and affects the ability to focus concentrate. I have been doing some experiments to understand how well the MVHR system gets rid of this unwanted gas, with some astounding results - it gets rid of it brilliantly!


Installation practicalities and costs with MVHR

How easy they are to fit will depend on the house. I was lucky, with a loft and room layout that made it pretty easy. It was a weekends DIY job.

I know with some lofts and houses there can be all sorts of practical challenges on where to run ducting.

In general, these systems should be designed and installed by professionals. This is to ensure the right amount of ventilation and minimise energy costs.

I felt confident enough to work it out myself and had a go. I had to do some experimentation with different flow rates into different rooms but the end result for us is amazing. It was fun doing it to!

Terminology: These systems are often known as Mechanical Ventilation Heat Recovery (MVHR) systems or sometimes Heat Recovery Ventilation systems (HRV). They are becoming increasingly used on new homes and homes being renovated.


Tips on buying one of these MVHR systems

If you are considering one of these systems these are my recommendations when talking to installers:

  • The basic air tightness of the home needs to be pretty reasonable for these systems to work efficiently. They don't need to be as airtight as passive houses, so do scrutinise any companies you talk to about this and ask them to satisfy you that your home is airtight enough. To do this a company should do an air tightness test on your home using something called a 'blower door test'.

  • There is no formal accreditation system for the design and installation of these systems. Understanding what the companies track record is, is really important. I personally would always want to speak to previous customers of the company to understand the experiences of others who they have installed systems for.

  • The ventilation needs of a home will vary for different households, so it is really important they fully understand and appreciate how you use the home, which rooms you spend most time in, typical number of occupants in the home.

  • The filters on the unit will need changing every few months. This only takes two minutes to do, but the location of the ventilation unit is important for this. In my case I didn't think this through originally, and my unit is at the far end of the loft which is a pain. With hindsight I'd have put it right next to the loft hatch. If it can be located in the confines of the house below this might be even more convenient.


 

The geeky stuff

Why are trickle vents inadequate for good sleep quality in a bedroom

If carbon dioxide levels are too high in a bedroom, sleep quality is poor. Studies have also shown that this also affects alertness and the ability to focus the following day.


Sleep studies (including this one) have also shown that:

  • Sleeping in a room with no ventilation and the door shut: This leads to carbon dioxide levels at twice the level recommended by the World Health Organisation.

  • Sleeping with trickle vents open but with a shut door: This is inadequate, leading to carbon dioxide levels 50% above recommended levels.

  • Sleeping with both trickle vents AND bedroom doors open IS just adequate.

  • Sleeping with a window open gives excellent results.


For PIV units - information on loft temperatures

The chart below shows how the temperature is several degrees warmer in the loft than outdoors, even in very cold weather. In this case the outdoor temperature (light blue line) fell to -2C but the minimum loft temperature (orange line) was 4C. (This was for a loft with 150mm loft insulation).


 

Quick links to the other main articles on this subject.

Other supporting articles on damp:

 

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I run this website as a hobby, because I care about this stuff, and do it for no commercial purpose. If you have valued what you've seen, please tell other people about it.


If you have any other suggestions for additions or changes to site content do please let me know. While I have made every effort to ensure that the information contained on this website is correct, I cannot take responsibility for errors or omissions.

All content on the site should be treated as information and not advice. You should take professional advice where appropriate to different site articles.


Thanks.


Mark Thompson


Get Energy Savvy - simple practical home energy efficiency information

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