COLD DRAUGHTS - Here's why you don't want them
Why do I need to seal up my home? ….Will I be able to open the windows?
One of the first questions I get asked when discussing a refurbishment that includes retrofitting to lower the bills is often to do with blocking draughts and making an airtight home.
So, why is airtightness important?
Imagine it is a winter day… there is a frost, and the air is quite still. You decide to go out to enjoy the morning sunshine. You put on a woolly hat and gloves and a big oversized woollen jumper. Outside you love the feel of the crisp air and feel quite cosy all wrapped up. It’s such a lovely day, you think, oh, I’ll take a stroll and then after a while reaching the top of a small hill, you sit down and enjoy the view. Here though… it is quite windy. Soon you start to cool down and reach into your bag for your raincoat. It is a good wind barrier and thankfully stops the biting cold from reaching you.
Our houses are no different. Houses, particularly the old ones, were built to be leaky, to be ventilated naturally. Airbricks ventilate the timber joists that hold up your floorboards to keep them from getting damp or wet and rotting. The floorboards have gaps that let cold air into the room. The air that comes in at the eaves ventilates the rafters in your roof for the same reason as the ventilated suspended timber ground floor. Fireplaces are open to the sky and a great route for warm air to rise upwards and leave your home.
Old timber sash windows, metal or timber casement, and even lesser quality double-glazing tend to have gaps causing draughts. I remember placing one of my kid's artworks on a window sill of a double-glazed window in a house we once lived in. The art piece was made of paper and on a windy day, I noticed that I could hear rustling as some of the paper elements flapped in the ensuing draught. This was double-glazing, the type that would give you a better EPC rating due to your house having reduced heat loss. But was it performing well? Was it correct that there should be such a noticeable and visible draught? The window has reduced heat loss through being double glazed but is the minimising heat loss working if hot air can escape bypassing the double glazing only to be replaced by a cold draught?
Older properties were designed to be leaky, this has kept them generally dry, a testament to this working well is that many of you may be living in an older property that was built over 100 years ago.
We trust and have got used to our leaky properties, to the extent that I often get worried questions such as… What happens when you seal up all the draughts? Will we be able to open the windows? Will it be stuffy? Let me address these, one by one.
What happens when you seal up all the draughts?
When you make your home airtight you are preventing the cold air from reaching you via draughts, like the raincoat in the story above. You prevent cold draughts from cooling you near your front door, by your windows, through your floorboards, or around your loft hatch. The more extreme version of this includes tackling every possible source of air infiltration, this includes adding airtightness tape around electrical sockets and switches, sealing joist ends to the wall they connect to, even the addition of an airtightness layer in the form of oriented strand board (OSB) with all the nail holes and joins taped up as part of new roof construction as pictured above. This makes any insulation you put in more effective. It means you need to heat the air less to compensate for that cooling effect.
Will I be able to open the windows?
I remember asking that very question myself when I started on this journey. And the answer is…yes, you can open them anytime but from my own experience of our retrofitted house; you probably won’t need to as much as you might do now. This is because retrofitted properties that are made airtight, move from having uncontrolled ventilation to having controlled ventilation. Un-retrofitted homes may feel like the wind is blowing through them on a windy day causing far too much cooling in one go and at the other end of the scale have too little airflow on a hot, still day. As part of the retrofitting package, ventilation must be addressed and there are a number of ventilation systems that are go-to solutions for this.
Image from Muswell Hill Low Carbon House windows with the highest rating of airtightness Class 4
Will it be stuffy?
As for if will be stuffy? Yes, if you don’t add back in controlled ventilation. Until I took up retrofitting I didn’t realise that a stuffy room is a sign of too much CO2 in the room. CO2 builds up from the air we exhale; think of a sleepy afternoon lecture in a packed airless hall. With the number of people, the balance of CO2 in the air increases and that makes the room feel stuffy, it also causes drowsiness.
Controlled ventilation does amazing things:
*It regulates the air providing enough air exchanges to keep the air feeling fresh.
*It keeps the CO2 levels balanced and some types such as Mechanical Ventilation and Heat Recovery units (MVHR) can recover a good 90% of the heat from the air before it exits the building.
*Another ventilation system I like to use is a Demand Controlled Ventilation system. This reduces the amount of cold air coming in by having some moisture-sensitive pistons that open and close vents according to how much moisture is in the room. This simple solution reduces the amount of air needed for adequate ventilation and reduces the subsequent heat loss. When the humidity levels are low the piston shuts the vent. This works well as we raise the amount of moisture in a room simply by being in it. So you enter a room and raise the moisture and CO2 levels; the additional moisture triggers the pistons to open the vent and a continually active centralised fan (in another part of the house) draws the air across the room, by default reducing and keeping the levels of CO2 balanced.
Do I have to have trickle vents?
Building Regulations Part F insist on us providing adequate ventilation. It is often assumed that trickle vents in windows when sized properly provide an adequate amount of air to meet the regulations. This is true but this can be replaced by the sophisticated ventilation systems described above in order to reduce heat loss.
What are the best windows?
The best windows from a sustainable perspective are made of timber, due to this being a grown material, and they should have the highest rating of airtightness (Class 4).
Does how they are fitted matter?
Yes, how they are fitted is super important. To maintain airtightness around a window, they need to be sealed on both sides with an airtight seal. These come in a tape that can be plastered on to internally and a second airtight sealing foam tape applied in the gap between the frame and the wall. The foam tape is made of memory foam that expands into place and should there be any settling of the construction, the tape keeps expanding so that no cracks can appear over time. Some window suppliers work specifically with the retrofit industry and these companies in particular have the necessary knowledge to fit the windows in this long-lasting airtight way.
Windows that get fitted without this knowledge may be at risk of having draughts and cold bridges around the window.
Image from Muswell Hill Low Carbon House: airtight sealing foam tape
What are the results?
Does airtightness make a difference? Well, each house varies, however, the average heat loss through air loss via infiltration is 15% in an un-retrofitted house. If you insulate and reduce the heat loss component through the walls, roof, floors and windows, the proportion of heat loss goes up to a rule-of-thumb average of 40% for air leakage. This is a significant amount. It is a hugely important measure and to my own surprise can be as important as insulating.
What levels of airtightness are there?
The ultimate measured standard for energy efficiency PassiveHaus Standard requires the airtightness to be a max 0.6h-1 for new build and 1.0h-1 for retrofit when certified to an Enerphit standard. There are other standards out there; for example, the AECB Retrofit Standard level 2 requires an airtightness level of max 2.0h-1.
How does this compare to Building Control required standards, I hear you ask….
Building Control Part L (Conservation of Fuel and Power) including the recent 2023 amendments has a max 8.0h-1 for new builds (recently reduced from 10h-1) and this equates to no more air leakage than 8m3/hr compared to the ultimate of 0.6m3/hr for Passivehaus certified buildings. As I write this, I try to picture 8m3/hr cubic meters and head to our small bathroom to measure it. I guessed right; it is just over at 8.25m3.
That seems like an awful lot of warm air to lose every hour of the day and night when trying to not spend too much money on fuel bills.
The BRE, the people who write the Building Regulations, suggest that a house achieving airtightness of 5m3/hr will use 40 per cent less energy on space heating than a house built to 2005 standards of 10m³/hr.
For comparison to what you might have in your own home, my colleague Tim tested his own leaky Victorian house and it was 18.0h-1 before they started out on their retrofit project.
More questions answered:
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This column is here to give you the benefit of some of the things we have learned over the years. If you have a project you would like to discuss in more detail, do get in touch or see our getting started package.