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.

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.

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Dealing with Construction Moisture

Why Construction Moisture Can Severely Delay a Development Programme.

Multi-Million Pound RC Framed Residential Development

We dealt with two issues this week relating to problems caused by construction moisture. A plaster beetle infestation and an investigation into the reasons why anhydrite floor screeds were not drying out within a multi-million pound residential development block in London. More on the plaster beetle infestation in a separate blog to follow quite soon.

Our client, was understandably concerned that pumped anhydrite floor screeds, which on some floors had been pumped in 6 weeks earlier, had failed to dry. The installation process for this particular screed required that the laitance be sanded from the surface of the floor once dry, this usually occurs at 3-7 days after installation, however any attempt at sanding the floor immediately resulted in a clogged rotary sanding pad.

Laitance occurs on the surface during settlement/compaction. During this compaction process, bleed water migrates to the screed surface. This brings with it fine particulates within the screed. Laitance is subsequently formed as a result of the evaporation of the bleed water, and once hardened can impede the drying process .

The advantages of laying a pumped anhydrite screed, as opposed to laying concrete is that you can generate quite significant savings on labour and time but only if optimum environmental conditions for drying the screed are present.

Other benefits of using  pumped calcium sulfate screeds are that they are self compacting, have very little shrinkage and rarely require movement joints. They are particularly suited to underfloor heating systems because they dissipate heat far better than a concrete floor slab would.

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Wall channel between anhydrite screeds to each room.

Pumped anhydrite screeds do not provide a wearing surface so are overlaid with a suitable wearing course; this may be a compatible smoothing compound, carpets, floor tiles or any other finished flooring. Many floor screed failures that we investigate concern buildings with underfloor heating systems and a latex wearing screed. There can be a number of complex issues involved relating to material compatibility and poor commissioning of the UFH system but this particular issue was far simpler to identify. The developer had long since overrun the usual 3-7 day drying and six weeks later some floors were still soft on the surface and therefore unsuitable for sanding to remove the surface laitance. In anhydrite screeds, the binder reacts with the water in order to produce gypsum crystals. Around 80% of the anhydrite is converted to gypsum and this reaction uses a large proportion of the mixing water but the remaining moisture is lost by evaporation.

The block, containing around 56 flats over 4 floors, had almost all windows and doors installed but no heating or MVHR system, which would be installed too late in the programme to be of any help. What we consistently found is incredibly high internal relative humidity of around 87% and therefore very little difference between ambient temperature (15 degrees centigrade)  and dew point temperature (13.6 degrees centigrade).

Floor temps below dew point temperature

Scanning for relative moisture content across slab.

Floor temperatures were consistently recorded as being below dew point temperature, therefore proving that condensation damp was constantly rewetting the anhydrite floor screeds. To rule out any other potential moisture sources we like to scan the whole floor for relative readings, which shows up any unusually high peaks in moisture that may suggest another source of moisture, but we found consistent levels of moisture across all floor slabs on each storey. We tested the floor slabs for total moisture content (TMC) over all levels using calcium carbide and found readings ranging from 0.2% TMC at first floor level to 2% TMC on other floors. You would ideally look for a TMC of 0.3%, but no more than 0.5%.

Interestingly, we were able to prove that the simple action of opening windows made very little difference to this problem by recording internal relative humidity two hours after opening windows in the block. This did nothing to reduce relative humidity and of course there was no difference between external and external temperatures so even ambient temperature remained the same.

There may sometimes be a case for installing dehumidifiers and maybe even supplementary heating but in this particular case we do not think that was the right strategy, since no internal doors or partitions had been installed and doors were often wedged open for trade activities. You need a sealed environment to stand any chance of success with a dehumidifier otherwise you are attempting to dry the world. In this particular case we recommended both positive pressure and negative pressure ventilation. We calculated the cubic volume of the block and recommended fan that would give us 12 air changes in the building over a 24 hour period. Fans would set up to force air in at one end of the building and extract air from the opposite end of the building. As a general principal, we do not believe that you should expect these high levels of construction moisture to take take care of themselves and a supplementary airflow is often needed if you are to keep your development programme on target. We make precise recommendations based on construction progress, material testing and environmental conditions found on site.

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Cutting Corners – Builder Short Cuts

A Number of Builder Short Cuts Adopted by Builders

Builder Short Cuts – We come across so many short cuts adopted by builders that I thought I’d start a regular post highlighting some of the strange decisions made by site trades people to save on time or money. As you’ll see, some of the decisions taken make no sense whatsoever. See for yourself.

Short Cuts 1

New consumer unit

New consumer unit

Why not use the circuit labels supplied?

New Ground Spike

New Ground Spike

Laid directly over the patio as a trip hazard.

Need a matching slate?

Need a matching slate?

This will do fine!

Need to remove downpipe?

Need to remove downpipe?

No need, just render around it.

Use of plumbers mate to stop leak

Use of plumbers mate to stop leak

Quicker to tighten unions on valve?

Need a roof vent?

Need a roof vent?

Just hide duct behind rafter.

Need another roof vent?

Need another roof vent?

Just hide duct behind rafter.

Double glazing in timber frame

Double glazing in timber frame

Needs installing on glazing bars.

Installed drains in wrong place?

Installed drains in wrong place?

Just grind a hole in your Aco channel.

Not maximising access costs

Not maximising access costs

Ignoring other defects is not smart.

No frost protection on condensate

No frost protection on condensate

A critical omission

Need a newel post fixing kit?

Need a newel post fixing kit?

Just nail them on!

 

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Not a fan of installations done this badly

Screen Shot 2015-07-19 at 8.46.32 AMMore new build woes unfortunately, this time relating to poor installation of extraction systems. Multivent installations are becoming increasingly popular in new build properties due to concerns relating to air tightness; they are an improvement on single room extraction fans and significantly cheaper to install than whole house MVHR systems.

This particular fan was continuous running and designed to be ultra quiet which is great because I generally find that fans get turned off wherever the noise levels are intrusive. I’ve been recommending and specifying continuous silent running centrifugal fans for quite a few years now.

Unfortunately there was clearly something wrong because I checked the ceiling mushroom outlets and they were not drawing any air whatsoever. I asked the client about this and they explained that they had turned the extract fan off about a week previously after noise levels were so high that it kept them up all night. In fact after a sleepless night the frustrated home owner went into the loft space in the very early hours of the morning and adjusted the motor speed control to its lowest setting to stop the noise. They complained about this to the developer the next day who claimed that not all their properties had fans installed but because they lived so close to the train tracks it had been installed especially to help deal with the noisy trains. I pointed out to my client that the developer was required to install an extraction system in all properties under the approved documents and the fan should be quiet.

A closer inspection of the installation was warranted because I know these systems are generally very quiet when correctly installed. On accessing the loft space I found that the developer had laid an old piece of wood across the bottom chords of the trussed rafter and simply sat the extraction unit on top of the loose piece of wood.

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Poorly installed multi vent fan

They can be mounted horizontally or vertically but in this case the extraction unit should have been fixed to a secure plinth with the special installation screws that come supplied with the unit. Motors are finely balanced in these units but they will move around under the centrifugal forces generated if not properly secured. Moreover the isolation screw installation is critical to prevent sound transmission through the building.

The other thing I really do not like to see is ducting secured with cable ties and in this case there was already a section of ducting disconnected due to this poor fixing method. The consequences of having an extractor fan pumping moist air into the loft space could have an extremely deleterious effect on the roof timbers and consequently end up being an extremely expensive latent defect for the developer.

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