Positive Input Ventilation

Have you considered just open vents above doors/at the base of walls just to allow natural movement of air? In our old house (old granite building with lath and plaster) there were some retro fitted built in cupboards which could get a little mouldy. Just leaving cupboard doors open a little to allow air movement in the colder months was enough to prevent this.

Very happy with the positive pressure unit I fitted. It hasn't completely solved our damp issues but its a lot better. Well worth it.

There have been many threads over the years about them just do a bit of a search

https://singletrackmag.com/forum/topic/positive-pressure-ventilation-loft-units-which/

I would say without a doubt it helped in our house and helped improve our sons health.

Your loft needs to be right for them to work though ie dry with a little ventilation.

Also stop drying stuff on radiators, always use extractor fans etc

As you can see from that thread I bought one with a heater element. In retrospect that wasn’t necessary and I never ended up using the heater

I always end up considering these at this time of year!

However, two queries - firstly, the idea (as I understand it) is that it helps to circulate dry, warm air from the loft. Does this mean you have to have a ‘warm roof’ loft setup? We have a cold roof (ie the insulation is on the ceiling floor of the loft, meaning the roofing felt has no insulation).

Secondly, the whole idea of PPV is to use the gaps and cracks in your house to circulate air. How is that going to work with the drive to heat pumps etc which rely on very insulated houses?

even with the best insulation you can put in a roof, the roof space will still be warmer than the surrounding world as some heat will get up there. So by drawing that dry(er) air from the loft into the house in a slow controlled way it will always be more "efficient" than opening windows and vents. Clearly there is a cost, upfront, and running, but the expectation is that you dont need to run your heating as hard with one of these compared to opening windows to create the same level of air circulation.

If you’ve got a 300 year old house I’d be checking to see if you’ve got modern plaster in it rather than breathable lime plaster, that is the cause of a lot of issues in older houses.

Also if you have a blocked up fireplace make sure there is a vent to that fireplace in the room or it can cause issues down the line.

The positive pressure vents do seem to be very effective though as long as other issues are also sorted out.

I installed one a couple of years ago and am very happy with it. It virtually eliminated condensation on the windows in the morning (and associated mould).

I have mine on a timer and only run it from 8pm to 9am.

I have mine on a timer and only run it from 8pm to 9am.

Do you find that the noise of the fan is an issue?

Do you find that the noise of the fan is an issue?

You really can't hear ours, it's suspended on some wire from one of the roof joists and has some fancy pants sensors in it so it doesn't run if the air in the loft is too hot.

The water pressure booster pump however sounds like a helicopter taking off, that needs sorting!

i have one in my loft, I have another in a box to fit to my mums house this weekend.

I mounted mine to the joists first time but I could hear it at night when it was really quiet, now suspended on a cord.

For the install in my mum's house, I've just bought a wifi temp/humidity sensor so I can remotely monitor the difference before/after install.

Mine is suspended in the loft and I do not hear it. I don't run it during the day because there is more movement in the house and condensation doesn't seem to be a problem during the day (in my house). I found running it 24hrs made the hallway quite cold when the heating was off in deepest winter (when it was subzero outside).

We've got a new-build with one in, and it does an amazing job of keeping the house dry. The shower tray and doors will literally be bone dry within an hour of us using it.

It does however blow in a bit of a cool draft. Our stairs are open plan into our lounge, and the draft coming down the stairs is quite noticeable on a chilly evening. I've just fitted a switch so I can turn it on/off manually.

I'm in process of doing something similar. But we don't have a loft so I'm putting one in the bedroom. I'm a bit stuck because I have knackered my drill coring through the wall.

My girlfriend's house is really damp like the floor boards are collapsing damp!

Have changed the bathroom extractor fan to a trickle one that runs all the time with a humidity sensor to boost in the shower comes on. Hoping to have hot air coming in and then cold air being extracted that way through the house

Your loft needs to be right for them to work though ie dry with a little ventilation.

Also stop drying stuff on radiators, always use extractor fans etc

Yes we do all the right things (we have underfloor heating downstairs so don’t even have radiators to dry things on..!) and our loft is very well ventilated - it’s completely open plan without any party walls along the whole row of 7 cottages. I assume that’s ok when fitting a PIV.

it’s completely open plan without any party walls along the whole row of 7 cottages. I assume that’s ok when fitting a PIV.

hope no one has any extractor fans venting into the loft space. friends bought a newly renovated place and discovered a year or so down the line and at some cost that several extractor fans were just venting into the loft space.

Our loft suffers badly from condensation in the winter, would this help move the air in there or just pump wet air into the house?

Our loft suffers badly from condensation in the winter, would this help move the air in there or just pump wet air into the house?

There shouldn't be any sources of moisture in your loft. If it is damp air coming from the house into the loft then a PIV should help. If its a leaky roof then I'd fix that first. Also, as above, check you don't have an extractor fan venting into the loft

I have a nuaire flatmaster in our house as we have no roof space. House is about 200 years old, stone walls, 'modernised' i.e. uPVC widows and no provision for venting, the vents in uPVC windows are pretty useless in old buildings.
the PIV is good, run it constantly on the low setting, venting to the top of the stairs. the air coming out of the vent creates a bit of noise, but it's not disruptive. I could fit larger pipework to reduce the turbulence.
our ambitious aim for the house is to improve ventilation, air quality, whilst improving heat retention. May end up with a heat recovery system.

only disadvantage with the flatmaster is that when neighbours have bbq's in summer, or in winter when next door fire up their open fire using what I can only think is petrol soaked coal by the smell of it, it does tent to waft in. i have been looking into carbon filters for that sort of thing though...on the long list.

We installed an inline extractor fan for the bathroom.

It is located in the loft but is virtually silent in the bathroom.

It runs for half an hour every time the light is switched on.

I figure it pretty much does the same job as a positive pressure fan. Certainly a lot cheaper and completely sorted the condensation problem.

If it is damp air coming from the house into the loft then a PIV should help

installation recommendations are that any route to the loft from the house is sealed. ie around the loft hatch so you don't just create a feedback loop.

Have you tried or considered trying a good quality dessicant dehumidifier?

They are about £180 and very effective for preventing black mould IME.

Plus they provide a bit of heating and are portable, so can be used around the house and garage.

We installed an inline extractor fan for the bathroom.

We already have them in the utility room, bathroom & shower room. They run on for some time after we switch lights off but they clearly don’t cope with the rest of the house.

Our loft suffers badly from condensation in the winter, would this help move the air in there or just pump wet air into the house?

Ours did in previous years; I don't think it's coming from the house, I think it's just when it's damp outside and the air in the loft cools down at night it condenses out. I put soffit vents in, made sure the insulation wasn't blocking airflow at the eaves and put some lap vents in where the layers of felt overlap; it's much better now.

We already have them in the utility room, bathroom & shower room. They run on for some time after we switch lights off but they clearly don’t cope with the rest of the house.

Are they inline ones? As it might just be a question of flow.

There is no inherent reason why a single fan can't work for the whole house. It has to draw the air from somewhere.

The Mannose one I linked to above has a flow rate of 245m^3 per hour. That is around 3 times the rate of a standard fan.

Then because it is very quiet (because there is only a vent in the bathroom with the motor in the loft) it can be left running for longer without causing annoyance.

So assuming a 30 minute run time, compared to 10 minutes normally, that could equate to a 9 fold increase in air flow.

To me, it makes sense to have the extraction in the most humid room of the house [the bathroom]. My concern of positive pressure ventilation is that you are bringing house fulls of cold air into the house. Whereas to me it makes more sense to extract at source and stop the humidity spreading to the rest of the house.

As @b230ftw said above, look at the causes first.

Have a read of this:

https://www.heritage-house.org/damp-and-condensation/managing-damp-in-old-buildings.html

I am not sure I buy all this "old buildings need to breath" stuff.

Old buildings "breathed" in the past because they were badly sealed. So there was always a stiff breeze blowing through the house. Moisture transfer through the fabric of the building is negligible.

Once sealed, as is the trend, they are badly insulated boxes with cold surfaces that attract condensation and damp.

... Improved ventilation/extraction and dehumidifiers are the way to go.

The people who designed these buildings had no idea. If they had, they would have put insulation into new builds at some point before the 1980s. Not to mention, that most old houses had fires in every room with 80% of the heat going up the chimney.

@jkomo loft condensation can normally be sorted reasonably easily. As someone has mentioned, soffet and lap vents can improve it drastically.

installed a basic drimaster last weekend. took me ~2 hours. data so far from the merros temp/humidity logger I installed in the front room. house has been empty all week.red dashed line is when I switched it on.

We have 40 years of bungalow experience with drimaster and similar piv, and they are very simple and effective. Interesting that your logger shows house is both warmer and drier.

Any links to your logger? I use "proper" stuff at work but they would be a bit spendy for home use. Can you add other channels (eg outside ambient temperature)?

£18, seems ok so far. all sorts of accessories.

https://www.amazon.co.uk/meross-Thermometer-Greenhouses-Instruments-Collectibles/dp/B08PF3CB59/

the app is a bit clunky but you can download the data in csv form, that plot is from a couple of lines of python.

Finally got round to finishing the PIV job (had a few issues with cutting the core for the vent - involved a wee trip to a&e!)

No loft space, so this one is wall mounted, cost £300 fro the unit ( https://www.fastlec.co.uk/vent-axia-lo-carbon-pozidry-compact-pro-piv-with-integral-heater2?gclid=Cj0KCQiAkZKNBhDiARIsAPsk0WgpgETB5AU3DaqSc_VNq4fszvRuFv1bpN082bLpkveCd_tzMZyAYgQaAtAJEALw_wcB)

So far, according to my Nest thermostat it has dropped the internal humidity from 70% to 50%. Could definitely tell as my skin felt all dry!

Hopefully running this over winter can solve the damp/mould issues in the house and we don't end paying £20k for having to dig out all round the outside of the house and re-do DPCs etc

i ran the report again at my mum's house. just over two weeks since i installed it. I thought it had stabilised at about 60 but the cold dry air at the moment has dropped it even further.

I have been convinced that we need one of these in the house for the last few years, but am now not so sure.

We get condensation overnight on the windows, even though we had new units fitted throughout a couple of years after moving in.
We also have a couple of cold areas that have evidence of mould gathering.

- We have no door on the kitchen - it's not practical, although I am considering getting a carpenter in to make a folding door, or see if they have some other pearls of wisdom how to solve that particular problem. As a result, although we have an extractor hood, it seems to me that a lot of the cooking moisture ends up in the rest of the house.
- We end up doing a lot of clothes drying indoors over the winter. We have a tumble dryer, but there's still loads of stuff that can't go into it.
- Although we have a bathroom extraction fan (inline 4" fan mounted in loft), it has never seemed particularly effective at removing the moisture from the air.

Last time I dipped into this thread a month or so back, I decided I needed to take start doing something about it.

I bought a couple of humidity sensors that log upstairs & downstairs. I did this so we could see if we actually have a problem & how much fluctuation we get - they just record max & min values - no data logging. One is at the top of the stairs while the other is in the living room downstairs.

We also bought a de-humidifier so we can dry the clothes in our spare room with this running. I was hoping this would have two benefits; 1 - we can get through the pile of washing quicker as it will dry faster, and 2 - It would stop the moisture in the clothes getting into the air.

I also looked at the vent on the extractor in the bathroom. The centre of it can be wound open or closed. Opening it makes it slightly louder as more noise from the ducting makes its way into the bathroom. I had it quite closed down (although still with a decent gap) , so I opened that right up.

So far....
- humidity in the house seems to have varied between 41% & 54%. Annoyingly, the humidity sensors seem to keep the min/max for the last 24hrs which I didn't realise it did. I assumed it would just keep max & min until being reset. But, better than having no idea at all.
- The de-humidifier definitely dries the clothes quicker than leaving them on the drier on the landing. It's hard to know how much difference it makes to moisture in the house, but at least 2.5 litres needs emptying from the tank every time it's been run, so that must be helping.
- anecdotally, the moisture in the bathroom seems much less. My Wife shuts the window & it ends up like a sauna in there, but since adjusting the vent it does seem to be extracting better.

The house doesn't appear to be as moisture ridden as I thought it was.
Given that the humidity seems to be under control it seems like a PIV might be overkill for a bit of condensation on the windows overnight. My main concern with getting one is the reports of a cold draught where the vent it sited.
My Wife moans enough about how cold the house is, without adding another source of cold air!

if you have condensation on inside of the windows and mould spots on the walls, I don't believe those humidity sensor readings...

jam-bo
Full Member
if you have condensation on inside of the windows and mould spots on the walls, I don’t believe those humidity sensor readings…

Hmmm, yes I was surprised it was so low. I have no real way of assessing the accuracy of the devices I bought, but...

The Nest thermostat in the living room also displays humidity & while not always displaying the same number as the sensor I bought, it is generally in the same ballpark (difference of perhaps 5%).
When the dehumidifier is turned on (in an upstairs room), within a minute or so it reads a very similar value to the sensor at the top of the stairs.

The mould is mainly in the bathroom on the exterior wall grouting.
There is the inside of a cupboard that sees a bit if we put things right against the wall, but this is where an old chimney runs so I think it gets particularly cold here.

I might investigate that logging device you posted though, so thanks for that!

Been running that wall mounted Ventaxia PIV unit and the trickle extractor fan (with humidistat boost function) in the bathroom for about a month now, and internal humidity is down to 40%, from 70% before we fitted them. First day of running them was down to 50%, so it's dropped further. External humidity is really high just now too (80% or more according to the BBC weather).

Not seeing any condensation issues which is pretty good for winter!

Picked up a couple of Lidl humidity/weather stations to compare humidity in the bedroom and bathroom with outside (they have an external temp/humidity sensor). Readings are a little off from the Nest thermostat in the bedroom (3% difference, but these things are +- 5% accuracy anyway) but trends are all matching.

Sorry to resurrect an old thread but it seems to be the most recent on the subject.

I'm about to buy either Nuaire or Vent-Axia, don't have a particularly bad condensation issue but do have corners that develop black mould, nest stat shows the house at about 65%.

Reading the comments above about ensuring existing extraction is effective does make me wonder if I need a £300 PIV unit, or would a silent running standard extractor fan 'venting' the loft into the house with a filter achieve the same thing?! It wouldn't turn off when the loft was hot but I'm sure it wouldn't be hard to make that happen...

It would certainly help fund the belt drive upgrade for my bike 😋

It should do much the same, but you'd lose choice of fan speeds, the option to add heating, and the multi-directional install vent. The Drimaster filters are very large, I think you'd have to replace anything much smaller very regularly. There's a long thread running on PistonHeads in the home, gardens & DIY section, with some good price offers direct, if you scan back a few pages.

Thanks I'll take look. I also found a document about building regs and it states they don't recommend PIV as it. Can potentially direct warm moist air through uninsulated voids and potentially cause damp issues that you won't know about until it causes rot.

I'm wondering about dMEV now which would basically be replacing my bathroom extractor (currently triggered my the light switch with a 20 minute over-run) with an always on extractor with a boost mode for humidity, and I could probably convert my cooker hood to do the same thing.
None of my windows have trickle vents hopefully the older side and rear units are not metal reinforced as it appears quite easy to retrofit...I think I would need them for PIV to work anyway. And some of my internal doors don't have sufficient undercuts to allow air to move between rooms so that's an easy fix.

I put a Nuaire Positive Vent in the top of the stairwell, it's fantastic. The only downside is it does very slightly drop the temperature of the house slightly. My house has concrete floors and suffered from condensation, the Nuaire has been brilliant (run for about 5yrs now). They also make the house air feel cleaner and dryer.

Think I need to sort the trickle vents first in the rear rooms that get mould, otherwise I suspect the PIV will just move air from the hall into the other three rooms (one with a ducted cooker hood, one with a ducted ceiling extractor, and one with a wood burner and flue). When I think about it, the two problem rooms just don't have any permanent ventilation!

The thing with the PPV is making sure your loft is a dry air space first. If there is any condensation or damp up there it won’t work.

Other than that go ahead and put one in asap. They made an amazing significant difference to our 3 bed stone 150 yr old within a week, no need for extra vents etc.

Re a drop in temp, we noticed it if you stood directly under it, but the drop in temp feel in the house is prob more a difference in feel due to the drop in humidity.

We bought one with an air warmer and never used the warmer bit once

How do you know the heater has never come on? We left ours set to the stock '10 degrees' setting but I am assuming the air temp has been much lower than that in the loft during the -5 or whatever. I don't have any way of knowing how much it has been on for though.

Our house is a bit older at just over 200 years, average humidity was around 75% before fitting, and is 55-60 now. Only had it through this winter. The vinegar is coming out in spring to scrub all the problem spots and see if they come back. It seems to be working well despite the loft being quite wet - it's really well ventilated but the house vents up the gaps by the walls so much that all the humid air condenses on the lower beams/back of the wooden boarding. I was hoping the Nuair would help with this by drying the house out a bit and pulling more fresh air through the vents. So it's a negative input ventilation for the loft, and positive for the house.

We really need to re do the loft insulation but if you add anything over the main living space it makes the boards soaking, I'm assuming by keeping the loft space colder directly above.

Next stage is buy or rent a thermal camera and try to find out where the warm air is coming up.

I did buy a Vent Axia Lo-Carbon PoziDry Pro PIV with Heater before Christmas, to fit in our house. But then we sold the house before I could fit it 😂

So if anyone would like to buy a brand new PIV then hit me up. Based J29 of the M1, if anyone is interested.

We fitted one of these some years ago and it made a huge difference to damp in the house and made the house feel warmer. It had to come out when we had a loft conversion (which seems to have been equally helpful for damp etc). That was in a '30s semi with cavity wall insulation, double glazing and no trickle vents.