Ventilation Strategy for Managing Condensation : Part 2

Looking at Ventilation Strategy & Options

Mould caused by ineffective ventilation and poorly insulated loft space.
Mould caused by ineffective ventilation and poorly insulated loft space.

In the absence of a modern whole house MVHR (mechanical ventilation and heat recovery) system, which few of us have, then your choices for how best to ventilate your property fall to single room options such as extractor fans or vents, or a PIV system may be recommended. The vast majority of properties that we deal with will rely on standard extractor fans, which more often than not are poorly chosen, poorly installed and poorly understood.

General Principles for Ventilation

In part one we explained why opening windows is a terrible idea for managing condensation damp and why is was crucial to place equal emphasis on managing heat losses as well as air changes. With that principle in mind, where extractor fans or single room heat recovery fans are installed, they should be wired to run continuously on trickle speed 24 hours a day with boost speed wired to the bathroom lighting circuit or pull cord. In our view fans with humidistat switches or over run timers are ineffective and we never specify them. Humidistat switched fans are particularly unreliable as the humidistat sensor often gets fouled with airborne debris.

It is absolutely critical that there are no open windows, wall vents or trickle vents in any room containing an extractor fan because if there is, then the fan will simply draw air from this open vent, short circuiting the extraction process and preventing air changes in the property. The key is to ensure that air is being drawn from other rooms so ideally trickle vents should be open in other rooms not containing a running extractor fan.

Ventilation Options

Positive Input Ventilation (PIV)

PIV system in loft space
PIV system in loft space

Until recently very little research had been done to prove the effectiveness of PIV and yet it widely specified. The BRE recently set up a parallel study to investigate the performance of PIV systems and carried out trials in their Watford test house and field studies in 16 Welsh properties. The study concluded with the following key findings:

  1. PIV did not directly save any energy but may save a little when compared to conventional extraction providing the same level of ventilation exchange. This is because roof space temperatures are usually a minimum of 3OC higher than outside.
  2. Input ventilation was found to be effective in reducing relative humidity levels by around 10%RH in the test house, even when internal doors were closed. Vapour pressures reduced overall by 0.2kPa. The unit was shown to be more effective upstairs than downstairs.
  3. In the field monitored houses input ventilation was not consistently effective in reducing relative humidity. When internal humidity levels over those outside was examined, PIV was found to be effective in the most humid houses but did little in the dryer houses. Even in the cases where it was effective there were often inconsistencies between rooms in the same house.
  4. In both the test and occupied houses, the roof space was consistently more humid than outside (excess vapour pressure of about 0.1 kPa), implying that moisture was being transmitted to the roof space from the rooms below. The results showed this moisture transfer regardless of whether input fan was operating or not. This demonstrates that PIV may actually recycle higher levels of RH back into the habitable space.

I’d be understating the case if I said that results were not particularly encouraging and an interesting point that was made  is that the occupants perceived improvements or benefits were far greater than were actually proven. Clearly for some occupants there was a psychological benefit or placebo affect taking place. The last time I wrote about the proven poor performance of PIV I had damp industry salesmen stating their disagreement and commenting about how great PIV was and how they’d had a ‘masterclass,’ not just a class, but a masterclass in ventilation from someone at the BRE involved in this study but this doesn’t change the findings and we simply don’t ever specify PIV and have never needed to.

Positive Input Ventilation (PIV) or Expensive Fan Heater? 

When PIV was first developed, it operated on the principle that air was slightly warmer in the roof space than would be found outside the property. This warmer air was recovered by the PIV system and recycled back into the habitable space, so creating alleged heating efficiencies and savings. In practice, this was rarely found to be the case and the poor effectiveness of PIV systems saw an alleged improvement in design, with some of the newer PIV fans now having integrated electrical heaters installed .  This resulted in the PIV fans blowing warm air into the building, which of course would show a reduction in internal humidity levels, and raise surface temperatures to the building fabric. Under these circumstances, PIV would show a significant improvement over their original design principles but is it now a ventilation system or a very expensive fan heater? 

I’ve no doubt that removing the PIV and running a fan heater at first floor level would show similar reductions in condensation damp and mould at first floor level but once home owners or tenants wise up to the cost of running these PIV heaters then they tend to turn them off. If you do choose to install PIV (and I wouldn’t recommend it), then check whether it has an integrated heater and ensure you understand the running costs. 

Passyfier Vents

Passyfier vent
Passyfier vent

Passyfier vents are a relativity new product, again of unproven reliability. Essentially they are an improvement on a standard open wall vent in that they are packed with rockwool which is moisture permeable but retains heat and prevents drafts  within the property. The tubing connecting the inner and outer face of the vent is sloped to the outer face of the wall to allow for drainage  of any moisture that is collected in the rockwool packing. In their own right we cannot believe that these vents will provide an effective ventilation strategy but they have to be an improvement on a standard open wall vent since drafts and heat loss will be massively reduced. In fact where open wall vents are installed we have frequently packed these vents with rockwool  insulation and received very similar benefits at very low cost.

Single Room Heat Recovery Fans

Single room heat recovery fan
Single room heat recovery fan

These are our preferred option for ventilation but are often ruled out due to cost. A  good standard centrifugal fan may cost circa £70.00 whilst a single room heat recovery fan may cost around £250.00. However they do merit one or two words of warning. 1. Some units  come with heat exchangers that occasionally require them to be removed and cleaned in Miltons fluid or similar; this may be a prohibitive requirement in specifying for the social housing sector. 2. Manufacturers claim amazing performance of up to circa 90% heat recovery but they fail to mention that higher efficiencies are only achievable at low trickle speeds. Heat recovery is incredibly poor at high speeds because the air moves across the internal heat exchanger too quickly. However given that we generally recommend continuous running at trickle speeds then this principle is perfectly geared towards installing, and getting the best from single room heat recovery fans.

Single Room Extractor Fans

Crude but effective test for extractor efficiency
Crude but effective test for extractor efficiency

These can still be fairly effective in managing condensation so long as you avoid the pit falls that most installers fall into and follow our general principles for running extractor fans… silent & continuous running at trickle speed. We commonly see cheap axial fans installed to ceiling mount locations, yet generally, axial fans are not powerful enough to move air through the length of ducting in the roof space.  We generally specify a centrifugal rather than an axial fan (though high powered axial fans are available) because we know it will be effective for ceiling, as well as wall mounting.  A crude but effective test we often carry out is to simply see if the running extractor holds a sheet of paper, if it doesn’t then in all  likelihood it is ineffective. It is also critical that silent running fans are installed because if they are noise intrusive in operation then they will be turned off. We commonly see  cooker hoods installed to deal with extraction at first floor or kitchen level but these are ineffective purely due to the noise they produce. They’ll be ran infrequently during cooking and often are not even piped to an external wall so actually contain nothing more than a charcoal filter to deal with cooking smells. They should not be viewed as a suitable replacement or alternative for a silent and continuous running extractor fan.

Ensuring that Extractor Fans can Work Effectively

It is absolutely critical that extractor fans, or single room heat recovery fans are located in a room that has no other open air vents or open windows. If you open a window next to a running extractor fan, or there is an open air brick or trickle vent in the same room as the extractor fan, then the fan will simply draw air from the open vent/window, as opposed to the room, These open vents effectively ‘short circuit’ the extraction process and you will completely negate any benefit that would otherwise be gained from a running extractor fan. Incredibly, this is such a common issue and the one which most explains why many homes see little benefit from an extractor fan installation.

6 responses to “Ventilation Strategy for Managing Condensation : Part 2”

  1. Daniel avatar

    Could your recommend a specific fan?

    1. Joe Malone avatar
      Joe Malone

      Try a Vectaire ELX 1003. It’s a good quality fan, costing circa £70-80. This should be wired to run continuously on trickle speed, with boost speed wired to the bathroom lighting circuit.

  2. Abbie avatar

    Another site I’ve been researching on recommends these – – do you have experience of these and their effectiveness (or lack of it!)? What of the above categories do they come under? Thank you!

    1. Joe Malone avatar
      Joe Malone

      I have no particular affiliation when not comes to fans Abbie, but in general terms would recommend that you buy a good quality fan (Expect to spend circa £70-80.00. Ideally, centrifugal, rather than an axial fan, and have it wired to run continuously on trickle speed with boost speed wired to the bathroom lighting circuit. You’ll need a fan that runs silently on trickle speed. The Vectaire ELX 1003 is a good option.



  3. Bob Noise avatar
    Bob Noise

    My roof space is consistently humid, with condensation forming on the old-school non vapour permeable roofing membrane whenever it is cold outside. When in the attic I can always feel a breeze through the gaps under the tiles. Would I be right in thinking that mechanically ventilating the attic wouldn’t significantly reduce condensation?

    1. Joe Malone avatar
      Joe Malone

      Hi Bob,

      In my experience, mechanical extraction to the loft space would have little effect or benefit. You should focus on the following:

      1. Recognise that much of moisture is migrating through from the habitable space so ensure you have adequate and effective mechanical extraction installed to the kitchen and bathroom. Install silent fans which run continuously on trickle speed. Managing moisture in the habitable space will often significantly improve interstitial condensation to the roof space.
      2. Ensure that there is a good seal to the loft hatch. Many hatches are very poorly sealed and allow moist air to migrate into the roof space.
      3. Ensure a robust supply of natural ventilation to the roof space, by way of airbricks, soffit vents or ventilated ridge or roof tiles. You could first try installing lap vents, 6 to either side of the roof, these quickly slot in at joints between the sarking felt. Make sure they are staggered as opposed to being installed in line with each other. I’ve inserted a link below.


      Joe Malone

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