Condensation AFTER installing a PIV System

[chuckalicious]

So here’s an odd one. My 1980s bungalow has always had issues with condensation, looking at the water damage to the wooden window sills.

When we moved in, I had mechanical ventilation installed in both bathrooms with humidistats, plus we use a decent dehumidifier in a closed utility room to dry washing if not using a tumble dryer.

Despite all this, the air was still reaching 70ish % humidity.

So I installed a Nuaire Drimaster PIV in the hall. It’s been running for a couple of weeks and the outside temp has dropped dramatically but the condensation has not worsened. Superb.

However I went into my loft today and found a lot of condensation on the roof surfaces at one end of the house. I’ve never had condensation in the loft and my understanding was a PIV would essentially create a constant flow of fresh air into the loft from outside, thus reducing the risk of condensation.

So what is going on? I cannot see why this has suddenly happened. Seems to go against what’s meant to happen.

Any help greatly appreciated.

 

[stem]

The idea is that air is sucked out of the loft and blown into the house, and then fresh air is pulled into the loft from outside to replace it. Ideally from multiple locations all around the roof to ensure through ventilation and not create isolated pockets of air.

So, first of all there needs to be sufficient openings in both number and size into the loft from outside to allow enough replacement air to be drawn in. Most lofts are sufficiently ventilated to allow this, but its worth checking to be sure.

Secondly the same quantity of air being blown into the house by the fan needs to escape, and as it will now be warm and contain moisture this needs to go outside and not back into the loft. If you have double glazing, sealed doors and windows, and no chimney etc., there's no or little exit via them and the air will go elsewhere. So, if have a loose fitting loft hatch, or light fittings that have gaps around them, particularly in rooms where moisture is generated, then the moist warm air you want to get rid of outside, might actually end up back in the loft and cause condensation on the cold surfaces.

I'm not a fan of these units (no pun intended ) bearing in mind that the constant introduction of cold air from outside will drop the temperature in the loft space, so in theory could actually increase the possibility of any warm moist air that does end up in the loft condensing, and could also increase the likelihood of frozen pipes, if there are pipes in the loft.

MHVR systems are more expensive but provide a better solution as they control where the air enters and exits the building and also provide heat recovery.

 

[chuckalicious]

Thanks Stem. I’m unsure how I can figure out if the loft has enough ventilation and that the air is able to escape the house adequately. All windows have trickle vents which are open and functional.

I’ve bought a set of remote temp and humidity sensors to monitor the loft under different circumstances to see if I can see a pattern.

I’ll be gutted if the PIV is a fail because there’s now a massive vent in my ceiling, plus a hefty sum of money shelled out.

 

[stem]

Most lofts are sufficiently ventilated to the point of being breezy up there on a windy day, however, if insulation has been pushed into the eaves it can restrict airflow. Some properties have soffit vents to facilitate ventilation

My house (1960's build) didn't have any and it was highlighted in the survey when I purchased the property, so I added some simple circular vents.

Window trickle vents should assist with letting air out, but it depends upon their size and quantity. I don't know the capacity of the fan unit you have, but if I had to guess I imagine it will introduce somewhere in the region of 250m3 of air per hour, if so, the same amount of air would need to escape to outside.

Perhaps your temperature and humidity monitoring may provide some clues. If you do find any patterns over a period of time, I would be interested to hear what you find. Whilst I have installed and used an MHVR system in a previous property, I've no direct experience of a PIV unit.

 

[chuckalicious]

The loft has not had any condensation issues in the past, which I assume would have happened if the loft was not adequately ventilated (prior to the PIV).

I do have the PIV on quite high just now. Maybe I should slow it down a bit.

I wish I had the funds for a full MHVR system as it’d be dead simple in my bungalow.

 

[chuckalicious]

Ok, just got the monitors and have set them up. How can the loft have a higher % than the house??

 

[pcaouolte]

The loft can easily have a higher percentage Relative Humidity than the house because the temperature is lower. Cold air cannot hold as much water vapour as warm air.

 

[chuckalicious]

The loft can easily have a higher percentage Relative Humidity than the house because the temperature is lower. Cold air cannot hold as much water vapour as warm air.

Of course, I should have realised that.

 

[JohnD]

CONDENSATION IN LOFT (ANSWERS)

This seems to be the time of year for condensation in lofts, so instead of replying to lots of queries I though I would write this "one-size-fits-all" post. Please also see //www.diynot.com/wiki/Building%3ACondensation-in-Houses Condensation in lofts occurs when the amount of water...

www.diynot.com

 

[chuckalicious]

Hi John. I did have a good look through that thread, but didn’t see anything specific to my issues - ie the increase in airflow in the loft and condensation. My understanding is that it’s often poor airflow that causes condensation.

 

[JohnD]

And a source of moisture.

Where do you suppose yours is coming from?

 

[chuckalicious]

And a source of moisture.

Where do you suppose yours is coming from?

This is what I’m trying to figure out. So far the loft monitoring shows a steady temperature and humidity, regardless of what is happening in the house (ie showers with extraction, hob extractor on etc) suggesting there are no major leaks into the loft. I had wondered if the ducting for my extractors was leaking in the loft but that doesn’t seem to be the case.

Interestingly I have turned the PIV down, thus reducing the fresh airflow in the loft and the condensation seems to have improved (not gone though).

 

[JohnD]

Do you have any holes in the ceiling, for example downlighters?

Are there water tanks or plumbing in the loft?

How do you dry your washing?

 

[chuckalicious]

Do you have any holes in the ceiling, for example downlighters?

Are there water tanks or plumbing in the loft?

How do you dry your washing?

Yes there are downlighters throughout the house. No plumbing or tanks or other water sources in the loft. Well sealed loft hatch. Washing only dried in utility extension with no adjoining roof space - dehumidifier always running.

 

[stem]

Yes there are downlighters throughout the house.

If have a loose fitting loft hatch, or light fittings that have gaps around them, particularly in rooms where moisture is generated, then the moist warm air you want to get rid of outside, might actually end up back in the loft and cause condensation on the cold surfaces.