Should an MVHR Exhaust vent into a cavity wall or directly outside?

492800

Hi all,

We recently discovered that our MVHR system vents into a cavity wall, with the air then escaping through an airbrick on the exterior.
We’re wondering if this setup is correct or if it could be contributing to a damp issue we’ve been struggling with.

Background:

• We moved into our 10-year-old flat just over two years ago and have had an occasional damp spot in our bedroom, in a corner by an external wall (the wall has the airbrick)
• Engineers have been investigating for some time, including water tests and cavity wall inspections, but the cause remains unclear.
• The issue occurs year-round.

What we’ve recently noticed:

• The MVHR exhaust blows directly into the cavity wall, which then vents through an airbrick.
• There’s presently significant condensation on the cowl (which we recently added to try and reduce water ingress).

Questions:

• Is this a normal setup for an MVHR exhaust?
• Could this be contributing to our ongoing damp problem?
• could condensation build up and cause this (but then this shouldnt be the case in summer would it?)

Here’s a video showing the airbrick and cavity wall.
And a photo of the airbrick (now with the recently added cowl).

Would really appreciate any insights!

ComicGeek

The MVHR exhaust duct should be directly connected to the airbrick - it shouldn't discharge into the cavity, and then hope that it finds its way through an airbrick. 

The air exhausted is going to be cool (but not colder than the outside) but with high relative humidity. So when it touches a cold external surface (like your new external cowl) there will be condensation on the surface of it. The MVHR exhaust should be pushing the air out fast enough so it doesn't cause a problem - if the duct isn't connected directly to the outside cowl/air brick then the air slows down and is uncontrolled. That could certainly create an issue throughout the year, with the cavity not drying out fully over the summer months. 

If the moist, cool air is just being discharged into the cavity, then the same condensation could form on the outside of the foil backed insulation within the cavity. That shouldn't create an internal damp issue normally as the water runs off, but the corner adjacent might affect air flow. It's hard to see on the video, but it does also look like the foil on the insulation board to the right of the vent has been ripped in places, exposing the internal insulation - it's the foil that stops the moisture penetration, so could be that poor installation has created moisture routes that normally wouldn't be an issue, but are made worse by the MVHR exhaust.   

At the moment the cowl probably makes the situation worse, as it restricts air flow through the air brick and is also creating additional cold surfaces to form condensation on. Problem might be fixed by just extending the MVHR exhaust ducting to connect onto the air brick, ideally in insulated ductwork right up to the air brick. 

Slinky

Depending on the age of your house, if it's old, that moist air could be rusting your wall ties.

492800

ComicGeek said:

At the moment the cowl probably makes the situation worse, as it restricts air flow through the air brick and is also creating additional cold surfaces to form condensation on. Problem might be fixed by just extending the MVHR exhaust ducting to connect onto the air brick, ideally in insulated ductwork right up to the air brick. 

Thanks for your detailed reply - really helpful!

The engineers only did a water test on the airbrick a couple of months ago, despite spending two years trying to fix the problem (why this wasn’t done earlier is beyond me). They found rainwater was getting in and sealed the area with silicone, but a month later, after more rain, the dampness returned. That’s when we added the cowl, thinking it might help. Given your point about airflow restriction, maybe we should remove it?

That said, we think this isn't just an MVHR or condensation issue. The dampness mostly appears after rain and happens year-round, not just in cold months. So while the MVHR setup might contribute, there’s likely another underlying issue. But it's good to get more facts from you guys about whats what

Really appreciate your insights - thanks again!


 reply Slinky said:

Depending on the age of your house, if it's old, that moist air could be rusting your wall ties.

The flat is only 10 years old

ComicGeek

492800 said:

The engineers only did a water test on the airbrick a couple of months ago, despite spending two years trying to fix the problem (why this wasn’t done earlier is beyond me). They found rainwater was getting in and sealed the area with silicone, but a month later, after more rain, the dampness returned. That’s when we added the cowl, thinking it might help. Given your point about airflow restriction, maybe we should remove it?

Just to understand 100% - is the MVHR exhaust duct at the same height and position as the airbrick, and just not extended across the cavity to meet the air brick? If so, the first step should be fixing that - at the moment you are constantly dumping all of the moisture from your bathroom/kitchen into the cavity, day after day.

Rainwater getting in where? Through the natural slots in the air brick?? Connecting it to the ducting would solve that, the exhausted air flow would deal with any rain coming through. At the moment the rain is just coming into the cavity, which it shouldn't be - the cowl might currently help to reduce this part of the issue, but should be removed IMO once the ducting is fixed. 

The cavity should also be ventilated behind the cladding, otherwise you can get build up of moisture and damp.... That's regardless of the issues with the MVHR vent and air brick, and is a larger question. There is a real tipping point with moisture levels in building fabric - that's why these cladding systems have to follow specific design parameters. The higher relative humidity (and higher temp) of the outside air during rain events in the summer could be enough to create a moisture gradient from outside to inside, where the cavity is already damp. That cavity is actually there to protect the inside of your flat from wind driven rain and damp, but can't do its job if it's not drained, not ventilated, or blocked in any way.

492800

Thanks again for your reply – really appreciate the help and insight!

To be honest, we don’t actually know exactly where the MVHR exhaust duct is. The unit is in the hallway, about 7 meters away, with a bedroom and bathroom in between.

All we do know is that a lot of air comes out of the airbrick (especially when the MVHR is on boost), so we assumed the exhaust duct vents into the cavity, and the air eventually escapes through the airbrick. But there’s no visible exhaust duct in the cavity – as you can see from the video, it’s just an open cavity wall, yet air is clearly coming out. That doesn’t seem right, so something is definitely amiss.

Regarding the rainwater ingress (which may or may not be related): The damp spot is at the bottom of the bedroom wall, which is the same wall with the airbrick. Engineers recently did a spray test and found water was getting in around the plastic edges of the airbrick vent in the cladding. They sealed it with silicone, but we’re still getting damp issues.

Here’s the engineer’s report, with photos which might shed more light.

We added the cowl as a last-ditch attempt to help, but it hasn’t made a difference. Now we’re trying to figure out whether the MVHR exhaust setup is part of the problem, or if this is a wider structural issue with drainage/ventilation in the cavity itself.

If we can confirm that an MVHR exhaust should never vent into a cavity wall like this, then that’s a definite issue, but if that would fix the damp problem too that may be another question.

Would love to hear any thoughts on what the next steps should be!
thanks once again

Chickereeeee

"That said, we think this isn't just an MVHR or condensation issue. The dampness mostly appears after rain and happens year-round, not just in cold months. So while the MVHR setup might contribute, there’s likely another underlying issue"

It is the temperature difference that is important, and it may be that the rain cools the surface enough to cause condensation in the summer. Especially as the expelled air is likely to be warmer and carrying more moisture) then.

ComicGeek

So the photos suggest that it's an upper floor balcony, so is going to be more exposed to driven rain than a ground floor wall.

100% the MVHR exhaust shouldn't ever be vented into a cavity - it must be taken to an appropriate external louvre/grille/vent/cowl, appropriate for the air flow rate and exposure.  

The vent is labelled 'Titan', which is one of the main fan/MVHR manufacturers. So very likely that this was supposed to be part of the MVHR system - the fact that it wasn't secured in place suggests that the installation was never completed.  Are there identical flats above/below that could be looked at and compared against? Very likely that the same system was installed in each flat, so can compare what's different between your flat that has the issue and other flats that hopefully don't.

There is a lot of water at the bottom of the wall cavity, so doesn't look like it is being adequately drained or ventilated. It doesn't look to me that the high level vent is there because of the cavity, but that would need someone to look at the cladding product details to confirm - it looks to me more likely that either they didn't complete the ducting, or ran the ducting somewhere else and didn't seal this up. 

If you have recessed spot lights in the bedroom, then you might be able to poke a camera into the ceiling void and have a look internally at this corner.

grumpy_codger

For those who haven't watched the video it's worth saying that it's not a proper "cavity". It's a gap between thermal insulation boards and some sort of metal(?) cladding.

492800

Some good points there thank you. a few things for us to consider. I really appreciate your help here.