I had a brilliant mentor when writing Beginner’s Guide to Eco Renovation and an early eye opener for me was the day she told me:
‘If you insulate your house without also making it airtight, you will be disappointed.’
Like wrapping up to go for a walk
Think of the process you go through before going on a winter walk:
- Put on your woolly jumper – really cosy, but it will still let the wind pass through the gaps in the knitting and you’ll feel cold
- So you put on a windproof jacket – now you are really warm and cosy at the same time.
- As you walk, you heat up and get a bit sweaty, so you open up the jacket to let some fresh air in.
You need the cosy ‘jumper’ around the external walls of your house, but if that’s all you do you will still feel the cold. Air is so clever – it gets everywhere. It can find the tiniest crack and opening. And however hard you try, ensuring there are no minuscule gaps in your insulation is pretty much impossible.
Airtightness is your belt and braces
Include airtightness and you ensure a warm and comfortable home. The airtightness membrane essentially wraps up the external walls of your home. It covers all joins and possible gaps, making sure that no uncontrolled air can get in.
The airtightness membrane marks out the Thermal Envelope – the areas of your house that wrapped up against outside.
What does airtightness require?
The purpose is to stop uncontrolled air coming into your house. This means covering every join or junction that could allow air to sneak in. This is achieved by installing the airtightness membrane then sealing it in place with a very sticky airtightness tape:
- Every piece of membrane is sealed to the next
- Membrane is sealed into the windows/doors as they are installed, so there are no gaps
- Tape seals up the gap between wall and floor / wall and ceiling
- Sticky tape is put in wherever pipes or wires come into the thermal envelope
- Ceiling lights have special containers to stop air flow
What is this membrane?
The airtightness membrane feels a bit like magic! It is both airtight and breathable. Now that sounds contradictory until you remember that breathability is actually about being vapour permeable. So it’s quite possible for the membrane to stop air, while being open to vapour.
It’s exactly like your jumper and windproof jacket. The membrane is equivalent of your jacket being a breathable brand (like Gore-Tex) which allows the sweat (water vapour) to pass through the material leaving you warm and dry. In just the same way, your house stays warm, draught free and dry with the airtightness membrane
I wish I’d understood this better
It took a while for the penny to drop about airtightness. The architect had included it in the design for our extension, but I wonder if our builders understood the importance of sealing it altogether and ensuring it was sealed up around every inlet. If I was starting again now, I’d be on this daily.
This is the perfect example of why we need to be informed as homeowners and customers. Builders don’t understand this or they don’t want the extra work involved. So it’s up to us to make clear that what we want and ensure it’s done properly.
It’s all about being nit picky
It’s not a difficult job, just time consuming and detailed. You need to get your head into it, looking for any place where air can sneak in.
We would go around the house at the end of every day to check what had been done and to make sure it was the standard we wanted. But there were moments when we missed a trick.
It was the windows that got us
The perfect example was when our windows were installed. We assumed, wrongly, that the fitters from a passive house window company would understand the whole process. Instead they asked us where we wanted the windows to be – set back or forward. We had no idea and – instead of holding fire and calling the architect to check – we got bumped into giving and answer. We said to match the rest of the windows in the house.
Unbeknown to us this meant that they were put too far forward, leaving us with a gap between window and the internal insulation. So a major cold bridge and air gap.
Of course, it could all be sealed up and made good, but it was a lesson to me. Make sure you know the basics and don’t trust to fitters or builders to have the same standards as you do.
So get nit picky – and make your home is airtight
It will pay off in comfort and cost. Not to mention the climate emissions you’ll safe
Just don’t forget ventilation
Of course you then need to attend to ventilation – and bring in controlled air, but that’s another blog