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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Collyweston Stone Slate

Possibly the best material you could have on your roof. 

Leicestershire property with Collyweston stone slate roof.

Leicestershire property with Collyweston stone slate roof.

We do a lot of conservation and heritage work and we surveyed another old grade 2 listed historic building this week, which was particularly fascinating for its roof covering of Collyweston stone slate. Indeed, the roofing material is probably at least partially responsible for the buildings grade 2 listing. Walls  to this building were  circa 500mm thick, with the original part of the building being around 400 years old and constructed of random rubble limestone. Collyweston stone slate gets its  name from the village in Northamptonshire, which is where these slates are made and it is a  material whose use is generally restricted to areas running along the limestone belt so can be found in Northamptonshire, South Lincs, Rutland and Cambridgeshire; the property in question is in Rutland. Collyweston stone slating has never been a large industry but it is now extremely rare and we believe that there are only two roofing businesses operating that specialise in this material.

About Collyweston Stone Slate

Collyweston slate is not actually slate, in fact it is limestone dating from the jurassic period that splits naturally along its bedding plane to form slates. Making these stone slates is incredibly labour intensive and skilful. In the 19th century the process was known as ‘foxing’ and involved a miner laying on his side and tapping away at the overhead seam with foxing picks. At some point the overhead seam would fall and miners would build up temporary supports for the seam using columns of waste stone. If the seam did not fall by the days end then a ‘lions tail’ would be used to lever the seam down; it would hopefully smash into manageable pieces when it hit the floor and hopefully not land on a miner. These pieces were known as ‘logs’ and it was important for the logs to remain damp because they were then left out in the open on a bed of shale so that freeze/thaw action could initiate splitting of the log into slates.

Diminishing courses on Collyweston stone slate roof

Diminishing courses on Collyweston stone slate roof

Even today, slaters rely on frost to split the log. Slates are dressed into various sizes and when you view a Colllyweston slate roof you’ll immediately notice that the slates are laid in diminishing courses towards the roof peak. To accomplish this, the  underlying timber laths, usually 0.75″ x 0.75″ sections are also laid in diminishing courses. They may be spaced at around 6″s near the eaves slate and decrease to a lath spacing of around 2.75″ near the roof peak. The slates are secured with oak pegs fixed through a hole in the head of the slate. These days the hole is drilled but traditionally they were made with a bill and elves.

Life Cycle Costs

Incredibly, Collyweston stone slates are capable of almost continuous reuse, which makes them possibly the cheapest roof covering you can buy if you calculate life cycle, rather than upfront costs. It is the oak pegs or underlying timber laths that are likely to be the weak link so for very old roofing it is not unusual to have to strip and relay the roof to renew oak pegs or laths. This roof was stripped and relaid around 1989 but all the slate was reused.

Erosion to limestone parapet copings

Erosion to limestone parapet copings

Substantial oak frame takes the load

Substantial oak frame takes the load

As you can imagine, there is substantial weight in a Collyweston stone slate roof so the underlying timber frame has to be substantial and you will generally find an impressive and substantial oak frame taking the load, as can be seen in this case.

OPC mortar fillets in remarkably sound condition.

OPC mortar fillets in remarkably sound condition.

There was very little wrong with this roof, bar the heavy erosion seen to the edges of some limestone parapet wall copings and the fact that the base of the chimneystacks and parapet walls flashings had been filleted with OPC mortar. We’d have preferred that NHL 5 lime mortar was used but since both the slates and the parapet walls are limestone then there is little cause to worry about differential expansion and subsequent cracking to the mortar fillets.  The fillets were in remarkably sound condition and generally where we encounter OPC mortar fillet roof flashings, they have generally cracked or failed altogether.  After some relatively minor repair work to the parapet copings and occasional ongoing maintenance work, we feel pretty sure that the roof will be good for another 400 years.

As a postscript to this piece, I noticed that it was found and retweeted by the last company still mining Colllyweston stone slate, Claude N Smith & Co. That in turn led me to one of their Youtube videos, which is absolutely fascinating if you can spare a few minutes to watch. Mining Collyweston Slate

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What Causes Render Damage – Cementitious Render Failure (Part Two)

Causes of Failure in External Render & What to do About it.

When I started writing this blog I originally assumed that would be a two part blog but as it transpires it will need to be in three parts since the scope of issues under discussion is so broad.  In part three I will discuss specification and application of renders in more detail. In part one of this article I discussed the items that should be checked on site when investigation the failure of external cementitious render and before I examine potential remedial works I’d first like to discuss modes of failure.

Render can fail in a number of ways and failure need not be isolated to one particular mode of failure. It is not unusual to see more than one failure mode, particularly if you are dealing with more than one elevation of a building.

Failure modes may fall into the following categories:

Shelling or Debonding

Cement render debonded from whole front gable

Cement render debonded from whole front gable

In this particular image the render was thinly applied in one coat and the render had debonded from the whole front gable. A few very light hammer blows were all that was required for the render to fall away from the building. In this particular case the render was serving a weatherproofing function as the underlying brickwork was roughly  constructed and poorly pointed. The building was suffering from rainwater ingress due to complete failure of the external render system.

When shelling or debonding occurs, the render becomes structurally unsound due to not being adequately bonded to the underlying substrate, this happens  for a number of reasons.

  1. High suction in substrate not recognized and addressed during installation.
  2. Sulphate attack: Ettringite formation between render and masonry blows render bond with wall.
  3. Freeze/thaw action: water gets in behind render and freezes at the render/wall interface. Since water expands as it freezes the resultant hydraulic action blows the bond between the render and the substrate.
  4. Differential movement/expansion: Render characteristics not matched to the underlying substrate and substrate moves or expands and contracts at vastly different rates to the render. It is impossible to maintain a durable bond under these circumstances.
  5. Substrate poorly keyed to in preparation for the render and fully reliant on an adhesive bond, whereas a mechanical keyed bond is also required.

Cracking

The render exhibits signs of cracking that will allow rainwater ingress through the cracks and consequently a risk of penetrating damp. The render may crack for a number of reasons.

  1. Shrinkage cracks due to render being applied in poor weather conditions.
  2. Chemical action: Sulphate or chloride attack.
  3. Differential movement/expansion: Render characteristics not matched to the underlying substrate and substrate moves or expands and contracts at vastly different rates to the render or junction detailing with incompatible materials fails to account fore differing rates of expansion.
Movement in steel lintels not accounted for and subsequent cracking to render

Movement in steel lintels not accounted for and subsequent cracking to render.

 

4. Hardness and subsequent inflexibility. Cement render is essentially a large inflexible sheet that cannot accommodate movement over large areas.

5. Junction detail failure. There is a frequent failure to adequately seal junction details with a suitable mastic or sealant. Rainwater then enters at these junctions where it can freeze and cause debonding and cracking.

Protective film not removed from windows and unsealed junction with render. A direct pathway for rainwater ingress.

Protective film not removed from PVCu windows and unsealed junction with render. A direct pathway for rainwater ingress.

6. Structural cracking. If the building is affected by subsidence or other structural or impact damage then it is inevitable that this damage will be mirrored to a greater or lesser degree in the render. Render can be repaired or replaced so long as you are satisfied that the underlying cause of failure has been addressed.

Cracked render caused by underlying structural movement. Note spreading roof tiles near roof verge.

Cracked render caused by underlying structural movement. Note spreading roof tiles near roof verge.

Consequences of Cracking

We have something of a chicken and egg situation when it comes to cracking. Is the cracking the primary cause of render failure or has the cracking resulted from from the primary cause of failure? Either way, cracking is not just an aesthetic consideration and cracks will form a direct moisture pathway for rainwater ingress behind the render system. Once there moisture can cause direct penetrating damp and  further cracking via hydraulic freeze thaw action and subsequent debonding. This process is self perpetuating and the trick is to establish whether cracked render is recoverable or whether it should be written off.  This can be a very subjective assessment but it needn’t be if a cost benefit analysis is carried out to establish repair versus renewal costs. Of course, you should really only consider repair if you are satisfied that the render is compatible and correctly specified in the first place.

Erosion

We were commissioned to investigate why this render had failed prematurely

We were commissioned to investigate why this render had failed prematurely

Surface erosion is a fairly uncommon form of failure when dealing with cementitious renders. The accompanying image shows severe erosion in an external render system that was incorrectly specified to a London Docklands commercial property that was converted for residential use. The building was circa 200 years old and constructed with lime mortar, so to replicate the breathability and the required degree of softness the installer blended an extremely weak render mix comprised almost entirely of sand with very little Portland cement added. The surface was highly friable as a result and was literally being washed away by rainfall. It was also generally saturated at depth, which was causing secondary erosion through hydraulic freeze/thaw action. The system was also causing a number of problems with internal penetrating damp. Of course what should have happened here is that the installer should have specified a lime render system but wrongly assumed that limes characteristics could be replicated in a weak OPC render mix. The entire  system had to be removed and replaced with lime render.

 

Material Incompatibility

Hard OPC based render on historic property.

Hard OPC based render on historic property.

Old and historic buildings may be constructed of softer gauged brickwork and lime mortar, they are meant to breathe and will go through seasonal wet/dry cycles as they manage moisture; this is precisely what they are meant to do and applying hard cement renders will completely interfere with this process and cause a number of unintended consequences.  If buildings predate the Victorian era and originally had render applied this is likely to be a weak natural hydraulic lime or a non=hydraulic lime system; both of which allow the building to breathe and are soft enough to accommodate some small movement in the underlying substrate. Lime renders of this sort even have the ability to self heal where small cracks occur.  If original render fails, which of course it will over time, then it should be replaced on a ‘like for like’ basis. If you are using a non-conservation specialist then watch them like a hawk wherever lime renders are specified because they sometimes like to sneak a bit or portland cement into the mix in the mistaken belief that this will improve the mix. In fact it will virtually nullify any benefits that would have been gained from using the correct lime mix.

In this image we were dealing with a very old property in Leicestershire that was suffering from a number of issues with internal penetrating damp. The render was applied in an Ashlar finish but was severely cracked on all elevations and testing a small area at the corner of the building confirmed our worst suspicions that a very hard OPC render mix had been used that was completely incompatible with this building. It is one of those occasions were you hope that a very poor bond has been achieved with the underlying substrate but of course it was firmly bonded and incredibly difficult to remove.

Failure to Replicate Underlying Movement Joints

Cracking caused by failure to replicate underlying movement joints.

Cracking caused by failure to replicate underlying movement joints.

It stands to reason that if the underlying substrate has inbuilt movement joints then these need to be replicated directly above in the render coat. If the render coat cannot accommodate and mirror the underlying substrate movement then it will crack. The form of cracking seen is regular horizontal or vertical cracks often seen at regular centres.

The attached image shows one of our previous  investigations, a commercial high rise block in London that was renovated and converted for residential use. The render was applied for purely aesthetic reasons but failed within 12 months of being applied. The situation can be rescued by retrofitting movement joints but given access costs this will prove to be a costly oversight.

Inadequate or Absence of   Sealant to Critical Junction Details

This is without doubt of the most common causes of external render failure we see. Once newly applied render is complete then it is critical that junction details are sealed with a high quality mastic and that will mean attending to window and door frames, boiler flues, soffit to wall junctions, pipe and cable penetrations and basically anything else that forms a junction with the finished render. If this is not done then the render system will take in rainwater from pretty much day one and premature failure is guaranteed.

We are even seeing million pound external wall insulation schemes to high rise blocks that have had no sealant applied whatsoever to any of the critical building junctions and why would you risk having a £1m scheme fail all for the want of some sealant? I was shocked recently to find that a bricklayer working on one of my projects had never used a sealant gun and when I did ask him to seal around windows etc, he made a complete hash of it and it all had to be done again. Plasterers & renderers often see sealant application as work to be completed by others though rarely will they make a point of asking the client to ensure that everything is sealed once the render is dry.

Adhesive failure of recently applied sealant to window reveal

Adhesive failure of recently applied sealant to window reveal

Even choosing the correct sealant is fraught with pitfalls and I recently noticed that a client of mine specified low modulus silicone for everything and whilst low modulus silicone is the best choice for sealing UPVc windows and doors, there are better products applicable to the wide range of situations that you will encounter. I will no doubt write a blog on sealants in the not too distant future because the scope is too broad to include here. Even where sealant is applied it is not uncommon to see adhesive failure of the sealant due to wrong material selection or simply that it’s been applied to a poorly prepared or dirty surface.

Improper Flashing Installation at Critical Junction Details

IMG_2233

Those who apply cementitious renders are often confused as to how flashing details should be dealt with. Where flashing details pre-exist for low level roofing then it is best to stop the render short of those flashing details and provide a bell cast drip detail running parallel and just above those flashings. In this image we can see where original stepped flashings were removed so that the render could be extended down to meet the roofline. Once the render was dry a channel was cut into the render and an apron, as opposed to the correct stepped flashing, was then installed. Lead has a high coefficient of expansion and therefore can crack render. Moreover when you consider that lead needs to be pegged to firmly secure it in place then how do you secure the lead when any attempts at pegging it would undoubtedly lead to cracking of the render?

Chemical Attack

Crystalline ettringite structure. Crystal formation is expansive.

Crystalline ettringite structure. Crystal formation is expansive.

External cementitious render can come under chemical attack, the most common of which is sulphate attack. This occurs when the tricalcium aluminate present in ordinary portland cement reacts with any sulphates present to form ettringite.  Ettringite formation is an expansive reaction so it can cause cracking, bulging or debonding in the render. Sulphates may be present for a number of reasons,  from traffic pollution, contamination of the render mix or most likely the sulphates are present in the underlying masonry. The reaction is expedited where permanent or intermittent saturation of the render occurs so failure to deal quickly with water ingress can lead to sulphate attack writing off the render system.

In part 3, I’ll deal with correct specification of cementitious renders.

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Ventilation Strategy for Condensation Management (Part 1)

Ventilation Strategy for Condensation Management.

A functional extractor fan with common wiring arrangements fails to prevent condensation in this concrete block.

A functional extractor fan with common wiring arrangements fails to prevent condensation in this concrete block.

I have previously outlined my views as to whether or not residents are responsible for condensation damp and we believe that they very rarely are. Residents cause humidity but generally speaking it is buildings that cause condensation and so it is critically important to understand ventilation strategy. That statement will make uncomfortable reading for those landlords relying on spurious claims that condensation damp is an occupancy issue. A potential client called me just this week and told me that her landlord, a large local authority in London, had refused to deal with dampness in her flat because it was proven to be condensation and therefore it was her problem and not theirs. Quite an outrageous position to adopt and one I’m sure will end up costing them considerably when they land in court. I’m sure that the flawed logic behind their position is that the occupant created the humidity and they simply have to manage that humidity by opening their windows and using their heating; advice that is clearly contradictory because it is akin to puncturing your petrol tank just after you’ve filled her up.  You’d really like to keep that expensive petrol in the tank but you’ve been advised that the tank is safer with less petrol and more air so puncturing the tank works a treat.  I hope my analogy illustrates the that fact that opening your windows in the middle of winter to manage condensation is nuts, and if anyone else ever gives you that advice you should tell them so.

I gave  a talk on dealing with damp in old and historic buildings at the RICS conference in Loughborough this week and there was some focus on ventilation strategies because by far the number one damp problem that we see in old solid walled properties is condensation damp. You  could argue that this is simply because we always house really bad tenants in old solid walled properties, irresponsible tenants who are incapable of managing their own humidity but of course the idea is ridiculous; could it simply be that old solid walled properties are thermally inferior to modern properties? It’s a rhetorical question because  we rarely encounter ‘occupancy’ related condensation damp and where we do then it is generally caused by over-occupancy and is a housing management, rather than a building technical issue.  Please review  The Condensation Trap

Why Opening Windows is a Very Bad Idea

Air temperature versus Maximum Moisture Content

Air temperature versus Maximum Moisture Content

We know that old solid walled properties are thermally inefficient and suffer from thermal bridging. They are prone to suffering condensation damp because thermal bridging causes cold internal wall surfaces that are often below dew point temperature. Therefore warmth and heat retention is an equal consideration to achieving air changes within the property. We know that warm air holds more moisture than cool air so an increase in ambient temperature will immediately reduce the internal humidity levels. Lets say that we have a room temperature of 20°C and an internal humidity of 80 percent, if we turn the heating up to 25°C then the internal humidity immediately reduces to circa 60 percent thereby reducing the risk for condensation. The secondary benefit is that this increase in ambient temperature will raise internal wall surface temperatures and may well raise them above dew point temperature, which is another substantial risk reduction, and bear in mind we’ve not even thought about air changes yet.

If we adopt the more common approach of opening windows to achieve air changes something quite different happens… Again imagine that we have a property  with a room temperature of 20°C and an internal humidity of 80 percent. It’s chilly outside with external temperatures of only 3°C but we open the window anyway and this allows cold air to immediately enter the property, reducing the ambient temperature and further chilling the building fabric possibly below dew point temperature; this in itself is an increased risk for condensation. Air at 20°C can hold 15 grams of water vapour per kilogram of air but since the ambient temperature has now reduced, lets say to 5°C, then the air can now only hold 5 grams of water vapour per kilogram of air. So what happens to the missing 10 grams of water vapour? Well quite simply it is immediately given up as transient condensation within the property, the cooling effect of incoming air is so quick that humidity does not have time to exit the building. Transient condensation is what frequently occurs in bathrooms when occupants open windows after a shower and the immediate effect of transient condensation convinces everybody that the steam exited the window but in fact they have been fooled. If it is cold outside and you open a window it will cause transient condensation and of course also results in very high levels of expensive heat loss.

Why We Need an Effective Ventilation Strategy

In simple terms, uncontrolled ventilation (open windows or air bricks) is a very bad idea whilst controlled ventilation is a very good idea. Efficient ventilation and air changes are important in a property but it should not be achieved at the expense of substantial or acute heat loss within the property. There needs to be a balance between achieving the required air changes within the property and a parallel objective to manage or spread heat losses over a longer time period so as to significantly reduce the risk for transient condensation. An effective ventilation strategy will account for heat losses within the property as well as air changes.  In part two I’ll discuss the types of ventilation available, and how best to use the more commonly available methods of ventilation.

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