Draughts in the cavity walls?

Of course a cavity is draughty - it's purpose is to keep a house dry by keeping damp out through physical separation.

Cavities are awful from a thermal performance point of view - and arguably don't work in many cases to keep moisture out unless built in a complex, labour intensive and detail heavy way. They have to have airflow, hence all the weep holes, vents, open tops etc.

Some 'cavities' are full fill with insulation, but that insulation has to be either waterproof or vapour permeable.

The drafts behind dry lining are due to the crappy nature of having thousands of blocks/bricks with mortar joints. Every single mortar joint, service entry or penetration by a floor or bolt etc leads to the potential for airtightness being lost - and therefore drafts around the inner wall lining. Again, reducing the effectiveness of so much of the insulation and effort of the outer wall.

FWIW, far more houses are built around the world with solid walls. We in the UK have a building supply chain and skills base which is highly developed and historic around cavity walls - but that doesn't mean it is a good system.

And as you are finding, it leaks heat.

How do you know the draughts at the windows and extractor vent are due to air coming from the cavity? Is it not more likely that its wind coming directly from the outside due to poor sealing?

If draughts are coming through plasterboard then you have a firepath from poor construction methods.

I'd say spread of fire through the cavity wall would be of little concern compared to the combustable floor, coverings and joists. He doesn't live in a tower block.

How do you know the draughts at the windows and extractor vent are due to air coming from the cavity? Is it not more likely that its wind coming directly from the outside due to poor sealing?

It's not clear, but I think the top of the cavity wall is open (by design, apparently) so this is likely to be a far bigger gap. By the sound of it this is normal.

When I say I've noticed draughts I don't mean that the house is draughty in general. We have patio doors on the first floor (fire thing, I think) and the threshold for it is wooden and had warped. This gap let in a draught, so I pulled it up to insulate under it on a breezy day and I could feel loads of air moving around in the cavity. First floor of course so there were walls underneath where the threshold was. I think I am going to use expanding foam to seal around that area.

If draughts are coming through plasterboard then you have a firepath from poor construction methods.

It's only where the house has settled and joints have opened up. One of the jobs I have is to go over everything with polyfilla and seal it up.

I’d say spread of fire through the cavity wall would be of little concern compared to the combustable floor, coverings and joists. He doesn’t live in a tower block.

Yeah, it's a 3 storey house but everything combustible is on the inside.

Building Regs changed in 2002 to introduce insulation in the cavity.

I know this because our 2004 property had empty cavities.

And yes, the cavity is open at the top and bottom to promote airflow and thus remove moisture. Problems arise where they vent into a loft space that doesn't have vents of its own.

If you're insulating with solid insulation (I would) then make sure you batten out a 5mm gap on the cold side to retain air flow.

Hey there,

Airbricks springs to mind 😉 Seriously though they need to. I as told years ago by a builder mate that he was fed up removing sopping wet cavity insulation after it had been inserted after the fact. The issue being damp course of course and you actually needing airflow. My house has 800mm thick walls and sold stone so don't have the issue 😉

JeZ

A lot of more modern builders seem to think plasterboard and skirting boards are the airtight layer. They shouldn't be. The blockwork behind needs to be parged in some way either by cement that is brushed on to try and seal any cracks/holes or you can buy a specialist paint - Blowerproof or Soudal Soudatight. These are either brushed or sprayed on. I have painstakingly fixed a lot of the issues in my house that was also built in 2007. Plasterboard off, and fixed all the gaps that range from absolutely massive to tiny. All the tiny holes add up still. This was all at great expense in terms of time and money. I regret going down that path. It has saved on gas usage but not sure I will ever recoup the cost of the work though. I also had to fix all the cavity closers, had lots of draughts coming in from poorly fitted or none ever fitted, windows are also never sealed properly, usually some silicone gunned in at the front and if you are lucky some foam between frame and wall. It should also be taped and sealed on the inner frame, this helps with noise and draughts.

Foam will work on the gaps, you can also use some building adhesive like Evo-Stick Sticks like or Sikaflex EBT (filler/adhesive/sealant) on top of it to really try and stop any air coming through.
You can add a plaster filler on top of foam too, so if you need to rake out any areas for sealing, foam behind it and then when it's dry cut it back if required and filler on top, sand back and paint.

If you’re insulating with solid insulation (I would)

There's polystyrene sheets in the cavity, probably 30 or 40mm thick.

The blockwork behind needs to be parged in some way

There's some sort of grey hard stuff on the inside of the Thermalite blocks behind the plasterboard, which is then held on with dot and dab. I don't know what the coverage is like mind.

Can you take any pics of the sloping ceiling where the joins are etc? Maybe also around the extract vents and window frames? You could temporarily bag/tape up the extract vent(s), if there is still a draught then the draught is coming in around the vent housing where it touches the blockwork and plasterboard. Same with the windows, are you able to feel for draughts around the frame/wall junction, wetting the back of a hand or something to see if there is a draught? It may also be your window seals too that's at fault.

Draughts are what make me feel the cold more than anything.

The windows are definitely hugely leaking. They don't shut properly. I replaced the hinge but it didn't help that much there was still play. I'm going to try again with a new hinge and some closing blocks at the same time.

I think of good insulation like wearing a down jacket. It's toasty warm if it's zipped up and there's no air circulation.
Unzip it and let air circulate all around and you may as well not be wearing one.

Voila, the UK method of installing expensive insulation into buildings....

There’s polystyrene sheets in the cavity, probably 30 or 40mm thick.

Sorry, I was referring to your sloping ceiling there!

I think of good insulation like wearing a down jacket. It’s toasty warm if it’s zipped up and there’s no air circulation.
Unzip it and let air circulate all around and you may as well not be wearing one.

Voila, the UK method of installing expensive insulation into buildings….

Sorry, I was referring to your sloping ceiling there!

Yeah I don't know if there's insulation. I am going to cut a test hole and find out.

I pulled up he threshold and had a good look. The door/window frame was sitting on the outer bricks only, and the cavity was open to the void underneath the threshold. This gap then let cold air into the gap between ceiling and floor. So I've filled it with expanding foam and I'm watching it set now. Even with everything pulled apart the entire room feels significantly warmer. The floor and window frame by the window was always cold, now it's all at room temperature.

All the houses on this street were doubtless built the same way. How much gas has been wasted making heat that exited this way? I bet I'm the only person in the street in a position to fix it too.

House builders are bastards.

House builders are bastards.

House builders want to make as much money as possible. Why would they change practice?
I think the cat is out of the bag with what has been going on for the last decade in UK energy policy. Who are the real barstewards .....?

Moving air in the cavity is fine, it is your inner layer you want as airtight as poss. That means paying close attention to gaps around skirting boards, windows and doors, switches etc. Fillers, foams and airtightness tapes etc can help.

I’m going to try again with a new hinge and some closing blocks at the same time.

I know you posted this a couple of weeks ago...
But you have adjusted the cams on the closing side right? They just look like round studs with a cross head in them, but they are eccentric. If you adust them towards the inner face they pull the window closed tighter.

A fundamental flaw with cavity walls is builders do not understand they need wind tight layer on the outside of the cavity insulation, or no gaps, and then an airtight layer internally. Standard practice is to have the wind blowing between gaps in the insulation and rely on the plasterboard or some crappy badly fitted sheet of plastic for the airtight layer. You end up in a plasterboard tent with the wind blowing through every penetration as the op has discovered. A blower door test and a thermal imaging camera will show you where the air is pouring in behind your plasterbaord. I will warn you seeing the results is likely to make you very unhappy. Houses built in the last 20 years are generally crap with late 90’s early 2000’s being really bad.

What can be done then? Is it possible to seal the top of the cavity via the soffits?

I suspect a main culprit is the vents in the outer brick wall for the extractor fans. I don't think they are actually mates to the ducts.

There are also air brick vents low down; I think it's a radon area.