Cavity Insulation - Truth of moisture bridging the cavity?

David_Aldred

Hi again,
The traditional external cavity wall to the majority of the UK housing stock, which includes a void or cavity between the inner and outer leaves of the cavity wall is an improvement of the solid walls constructed prior to this which were prone to penetrating dampness, uneconomic to build and comparatively often had poor insulation properties.

Given the rise in heating costs attempts have been made to improve the thermal performance of the traditional cavity wall by using different materials / thickness of materials to build all or part of them with other than simple clay / concrete bricks and providing insulation externally, or within the cavity of the wall, or internally. It may be realised that inserting cavity wall insulation is the option that visually and dimensionally is the least noticeable.

Insulating modern properties where the insulation is built into the wall as work proceeds is obviously far more easier to ensure it is fully effective than inserting the cavity wall insulation blindly to properties already built and where that cavity and indeed the wall itself may be compromised in some way. Modern construction often makes uses of a wide cavity typically of 100mm where the insulation is held back to the inner leaf of the cavity wall and a 50mm cavity remains, though some full cavity fill insulation is still sometimes used.

Various materials have been used to provide this retro fit cavity wall insulation to existing properties, with foams, fibres, balls and chips being just some of them. Given the insulation cannot effectively be held back to the inner leaf of the cavity wall and the cavity width is usually but not always in the order of 50mm the systems currently used generally rely upon fully filling the cavity void.

The majority of cavity wall insulation that is retro fitted to properties is trouble free so one has to put any problems into perspective. However in some cases retro fitting cavity wall insulation is not trouble free and there is correlation between the installation of the cavity wall insulation and increases of internal dampness with resultant degradation to the plaster finishes and decay of timbers. What follows is a discussion of such problems and implications of cavity wall insulation and to the majority of retro fit cavity wall insulation the matters discussed may not apply but which may be of interest.

As with a lot of things when an event goes critical and things do wrong it often due to a number of factors that on their own may be insufficient to cause a significant problem but when combined have a cumulative effect such that failure occurs.

Poor workmanship can happen in any industry and cavity wall insulation installers are no exception. Poor workmanship can be minor or major. Patch areas of missing insulation can be minor yet significant and these will be colder than areas that are insulated or I have seen complete wall areas failed to be cavity installed sometimes because access may have been difficult and the operators may have thought nobody would check, for instance with former council houses. Where insulation is missed the cold spot is prone to condensation which may be more acute in that one area than it was prior to the property as a whole being insulated.

To sub floor voids there is a reliance upon the ventilation provided by the external vent bricks to remove moisture laden air from the sub floor void before it can decay ground floor timbers and cause dampness above to walls irrespective of the dpc in the walls being effective or not.

When installing retro fit cavity wall insulation some installers fail to place dams around the sub floor vents such that the cavity wall insulation compromises or entirely blocks those vents so that dampness with decay follows and for which the installer is usually entirely quite rightly liable.

Some types of cavity wall insulation slump over time such that what was once a fully filled cavity becomes over time one where voids to the insulation are created. Again where there is no insulation these areas are cold compared to areas that are cavity wall insulated and again condensation to such areas can be more acute at such points than it was prior to cavity insulation being fitted.

The reason for recommending any issues regarding cavity wall ties be addressed first before retro fitting cavity wall insulation if such a choice is available are clear. From the above it may be seen that corrosion of ferrous ties is likely to be significant enough to warrant remedial action to a majority of properties to which retro fit cavity wall insulation is being considered.

Given such cavity wall tie work may require opening up of the cavity wall then doing so to a property that is already cavity wall insulated presents considerable difficulties to the wall tie contractor of making good that cavity wall insulation with the same material and ensuring no voids to that insulation have been created and for which they could be held liable especially if dampness / degradation were to follow as a result. One can imagine the difficulties for a contractor opening up a wall containing loose polystyrene type ball insulation on scaffolding in winter with it blowing a gale as the material pours out of the wall creating voids to unknown areas above.

In addition it must be realised that hammer drilling and where applicable hammer chiseling through the brickwork to install or address cavity wall ties inevitably leads to debris being deposited into the cavity. The brick is strong in compression and weak in tension and as the operative hammer drilling or chiseling reaches the rear of the brick face the section of brickwork remaining is placed in a very aggressive state of fluctuating tension such that the rear of the brick which is often weakened in any case through age and erosion of wind blown debris swirling around within the cavity breaks away. The operative may do their best to lessen the number of such occurrences but even with a caring operative this may be unavoidable and depends upon a considerable number of factors not least of which is just how bothered somebody is employed to be on the end of a hammer drill all day for often minimal pay during often foul weather. It is horrible work.

The brick debris falling into the cavity can be large sharp awkward chunks quite sufficient in size to bridge a minimal width cavity and depending upon how they fall they can slope down into the property promoting penetrating dampness. With no cavity wall insulation in place the debris may fall upon lintels over windows or doors, onto other ties compromising them, or may fall all the way down to the base of the wall where it can be removed. However it should be appreciated that with retro fit cavity wall insulation in place the debris will fall onto the insulation and cannot be removed without opening up the wall. If there are voids to the cavity insulation then that section of broken brick may be some unknown distance from where it actually broke away.

Although mechanisms for leaks are various including capillary action, wind pumping, gravity, kinetic energy, air currents etc, the primary reason for penetrating dampness is pressure difference. That is to say you could have a wall made of cloth and without a pressure difference between one side and the other you will have no penetrating dampness through it but a wall of considerable thickness may leak severely if there is enough pressure difference and hence why high rise buildings are so prone to leakage.

The outer leaf of a cavity wall is usually only around half a brick thick and is often inadequate to hold back wind driven rain penetration such that this wind driven rain will penetrate the outer leaf of the cavity wall and actually trickle down the inner face of the outer leaf of masonry. If the situation is transient such that the moisture can vent outwards and the wall recover without such moisture migrating inwards through any deficiencies to the cavity wall, then the wall recovers during more favourable weather. However if there are moisture pathways such as debris upon ties, types of ties lacking drips at mid point, debris in the base of walls, areas lacking cavity trays or upon services within the cavity then moisture may permeate inwards to some degree and depending upon the volume and period to which this occurs depends upon whether dampness is seen internally in such circumstances.

Condensation does not only occur upon internal surfaces to rooms, it can and does occur within the depths of walls themselves and the amount of condensate that forms can make walls very damp indeed. Statistically condensation either on surfaces or within the depth of the building construction is estimated to account for around 70% of all dampness seen within properties so statistically it is the primary moisture source.

The way walls to older housing stock and many new builds are originally designed to be a breathable structure is sometimes inappropriately altered albeit with the best of intentions. An example of this may be the application of hard dense cement renders that are far less breathable than true lime renders, re-pointing breathable lime mortars with hard dense cement mortars and painting of walls externally in an effort to prevent wind driven rain penetration all of which may be done to entire elevations and part elevations. Such inappropriate ‘improvements’ to older weakened substrates are inherently prone to premature failure, they often trap a degree of penetrating dampness within the wall and significantly increase the amount of condensate occurring both within the depth of the wall and upon wall surfaces due to the reduction in the way the wall vents externally.

It should be realised that moisture that occurs within the cavity of the wall be that penetrating dampness or condensation etc should be vented outwards and hence the requirement for effective cavity trays over lintels to direct this moisture outwards via vertical open weep joints at set intervals to the outer leaf of the cavity wall and similarly such weep joints being present to the base of walls. In older properties these weep joints may have not been provided, or be covered over such that there is no means for this moisture to vent or drain outwards.

Vents from rooms into the cavity void only allow warm moist air to enter the cavity and condense out forming a damp spot on the internal leaf of the cavity wall. Such vents should always be ducted through the entire depth of the cavity wall to the outside.

Installing retro fit cavity wall insulation involves closing up any vents to the cavity wall itself to prevent the material falling out.

Upon installing the retro fit cavity insulation the point at which condensation may have been occurring within the depth of the cavity wall prior to such insulation is often shifted due to the cavity becoming warmer until the cold side of the insulation.

The full fill cavity insulation does not stop air moving in the cavity but it will significantly tend to reduce the number of air changes per hour especially where previous wall vents to that cavity are blocked up to stop the insulation falling out.

The sun shining upon damp masonry can drive moisture inwards by a process known as summer or reverse condensation especially where the outer leaf of cavity wall is less permeable than the inner leaf of the cavity wall. Warm front condensation can occur sharply if after a cool spell the weather warms up but there is a lag for the building to catch up such that it takes several days longer to come up to a temperature past the dew point (the temperature at which condensation will occur).

As the number of condensation / mould / damp posts on this forum will confirm, the inadequate removal of moisture laden air from problem areas results in higher relative humidity until dampness, rot and degradation follows. In a semi closed cavity wall void of 50mm or sometimes less, filled with retro fit insulation, variable degrees of penetrating dampness and increased levels of condensation that has shifted where such condensation will occur, it is not difficult to see how some retro fitted insulated cavities can become a saturated environment. A previous post on here stated the cavity wall is like double glazing but that simply is not true as the cavity wall is not a closed void but a semi closed void and anybody who has seen a double glazed window when it becomes only a semi closed void will notice the condensate that occurs within it.

This condition may be transient where the moisture reservoir is matched by a moisture sink but if the moisture reservoir exceeds the moisture sink then the reservoir will accumulate until it is sufficient to sustain the problem to a point where the wall as a whole becomes saturated and the dampness consequentially takes the easier path of migrating inwards through the more permeable clay common brickwork / permeable original mortar, porous plaster; than attempting to vent outwards through the hard dense cement render / dense face brickwork, cement re-pointed mortar, painted masonry moisture reservoir within the wall to become a permanent situation.

Under such circumstances it is not surprising to see correlation between retro fit cavity wall insulation and the acceleration of ferrous cavity wall tie corrosion.

Properties vary considerably in their design, construction, location and the lifestyles of their occupiers. Simply walk within a mile radius of your own home to see how diverse homes and they way they are managed / maintained in addition to how varied exposure to wind driven rain they are. Some properties are severely exposed such as on hillsides or higher compared to surrounding properties and this may be compounded by some properties being coastal (within 8km of the coast). The extent of this classification of being severely exposed to properties within the UK is considerable and may be seen within BS5534part1.

Clearly given all of the above some properties are in more danger than others with regard to going critical and it is reasonable to suggest that a majority of properties are some way off this limit but others of which the number is certainly significant are for a number of reasons even where the owner keeps them well maintained bordering upon the brink, where the moisture sinks are just about matching the moisture reservoirs that is until the installation of the retro fit cavity insulation. Given the changes to the way the wall will behave as described above once retro fit cavity wall insulation is attempted along with its inherent limitations when inserted blindly, it may be seen that this insulation is often all that is required to provide the trigger for moisture reservoirs to exceed moisture sinks. The change may be sudden and dramatic or may be gradual taking well over a year to fully occur such that with the latter the homeowner may not correlate the retro fit cavity insulation as being the actual trigger until they stop and think about it.

The type of material used to retro fit cavity wall insulation does seem to have a bearing upon the matter to some degree. Experience reveals the foam materials can have fissures occur within them providing moisture pathways and the blown fibres although more popular possibly due to cost are more troublesome than the polystyrene type chips and balls. The balls / chips appear to penetrate all areas easier whilst the blown fibres have hairs that probably provide a better medium for the unwanted propergation and retention of water droplets.

For properties that do suffer increased dampness as a result of going critical following retro fit cavity wall insulation the problem can be minor or chronically obvious, effecting every external wall in the house over its full height. For a person or family to have their pride and joy of a home go from beautifully presented decor to damp degrading plaster, loss of finishes, mould and structural decay with no other correlation than cavity wall insulation being retro fitted is completely heartbreaking. What I find even worse is attempts by cavity wall installers to deny liability until the facts of detailed investigation show both their initial survey and workmanship to be negligent in all of the cases I have been asked to become involved with. It is for these people where their houses have gone critical as a direct result of the retro fit insulation being installed that these comments are made because believe it or not this is a forum to help people who are in a mess through no fault of their own, other than saying yes to somebody who was pushing something at them for their own profit without consideration of the consequences. For the majority of those with cavity wall insulation that is fine I wish you all well and the same to all those in the cavity wall insulation industry who do a very good job of inspecting and installing where appropriate.

People upon this forum have had a harsh go at somebody who declared they knew a decorator who did not approve of some properties being cavity wall insulated saying they were unqualified to give such an opinion and then when I posted a comment of stating that in some cases it is indeed inappropriate qualifying that I was employed as an expert witness in matters of dispute for dampness that was shot down too. May I conclude that it is easy to have a cheap pop at both an expert witness and a decorator until you find yourself utterly in the poo as some of those that have had chronic problems with retro fit cavity wall insulation are, at which time you will quickly discover you need the best one you can get.

Kindest regards to all, David Aldred CSRT, AInstSSE, MIWSc, CSSW, Independent damp and timber surveyor

27col

A very interesting and informative post David. It would be interesting to know your opinion on the efficacy of the various silicone wall coatings such as Thompsons Water Seal type products.

David_Aldred

Hi 27col,
Most water repellants are based upon silicone resins, stearates, siliconates, and silanes. Silicone resins are usually dissolved in white spirit at around 5-6% with curing of the resin by elimination of water from the molecule or by hydrolysis then elimination of the water.

Performance varies with silicone resins. The simple resins work well on brick but not so good on limestone, concrete or cement renders due to the alkaline issues. Siliconates are water soluble and once applied they react with carbon dioxide from the air to form effective water repellants. Silanes usually dissolved in miscible alcohols have the advantage of usually being able to be applied to damp substrates where they condense to form silicone resin. Poly-oxo aluminium stearate is a non silicone water repellant diluted into solvent that when cured forms a resin film that works well on limestone but has mixed results on brickwork.

These water repellants work by lining the surface pores of the masonry to resist water being taken into them. Whilst they are very effective on the fine pores of the masonry they will not help in situations of large cracks to masonry which obviously would need to be repaired prior to application of the material.

What people might not be aware of is water repellants have quite a few advantages other than simply reducing penetrating dampness, such as they reduce organic growth externally and keep the building cleaner but most importantly they keep the building warmer because if the pores contain air instead of water the thermal conductivity is drastically reduced.

For example a detached house can loose around 25% of its heat through the walls and if these walls are wet then the thermal conductivity goes up by 100% which can double the heat loss through the walls. Since brick walls only require around 3-4% moisture content for this to happen you can see how easily this will occur and so if the walls are wet you are going to have to spend a lot more on heating to keep the house at the same temperature as if the walls contained air within their pores.

It should also be realised that if the masonry is wetter it will be more prone to freeze / thaw damage where the outer face of the masonry is broken away when the water molecules expand as they thaw after a hard frost and water repellants significantly reduce this problem too.

Whilst water repellants are claimed by their manufacturers to remain effective for 20 or more years they may be eroded long before that time, for example by sand blowing in the air to properties classed as coastal such that the products need to be re-applied more frequently and local to us we have seen the necessity to re-apply the water repellants in around 7 years so obviously things vary a lot depending upon exposure where you live.

Of note water repellants should not be applied to walls that have just been re-pointed since that mortar will require a significant period to cure if surface disruption where the repellant has been applied is to be avoided. It should be remembered that the material is a pore lining material and not a pore blocking material such that whilst it may aim to hold back around 90% of wind driven rain they will be unlikely to hold back water that is under significant hydrostatic pressure.

The water repellant materials will often cause problems to paints that are inappropriately applied over the top of such treated walls so if you are going to apply water repellants to brickwork don’t go changing your mind and decide to paint the walls the following year.

In conclusion therefore water repellants when used appropriately have considerable advantages in addressing penetration of dampness into walls, reducing heating costs and keeping the building cleaner with minimal organic growth. Following the manufacturer’s recommendations for suitability and instructions for use is obviously essential for effective application in consideration of the fact that water repellants are not all the same and substrates do vary which will have an impact upon how appropriate such materials are for that particular situation as indicated by the above.

Hope this helps, kindest regards, David Aldred Independent damp and timber surveyor.

27col

Many thanks for your reply David. Very informative as usual.

What would you recommend as the method of application.

Any preference as to maker/brand.

David_Aldred

Hi 27Col,
As indicated by the above, given the fact that not all water repellants are the same you really need to be enquiring direct from the manufacturer regarding their data sheets and whether they are suitable for your particular situation. Having the data and health and safety sheets in your hand I always find reassuring so you have something in writing to refer to if things do go wrong but I found telephoning the manufacturer and speaking with their technical staff always to be most helpful if I was unsure about anything.

I know everybody goes on about Thompsons because it is the one folk see on their TV but there are plenty of others out there who have a lot of experience in the manufacture and use of these products backed up by a highly skilled and knowledgable technical lab, not least of which are the chemical dpc manufacturers given the relationship between the products.

I have listed three such companies below for your consideration. From experience when applying these products you need to mask anything and everything you don't want to have this material land upon as it is not the easiest thing at all to remove which is of course one of its design functions.

Given the carrier for the water repellant is often solvent then if you don't want to watch your tarmac drive / asphalt roof / bitumen dpc melt into an unrecovarable mush you need to be oh so careful with this tin of disaster juice. Definately one to be kept under lock and key away form the kids and if you want a trip down to A & E you can always try lighting up a cigarette whilst decanting it for that Guy Fawkes look or blinding yourself for life if you haven't thought ahead for eye washes / first aid.

Masking windows, doors, etc, removing / protecting the garden and anything living including the cat if you don't want that waxed fur look should be high on your tick list and you can imagine what happens if you drop some into yours or even worse your neighbours fish pond or anything else that happens to be in the way. Gravity don't you just love it.

Another classic is applying the material on a windy day at high level only to find yours and your neighbours car never need waxing again for the next 20 years but seriously it can be a nightmare if you allow the material to go anywhere other than where you and your neighbours agreed was intended. The cost of paying somebody to do it seems rather small compared to the bill from next door for a re-spray to their Ferrari and £5000 worth of belly up Koi Carp - you have been warned and you seriously might want to pay somebody to apply it who is fully insured to take the liability for a worst case scenario.

Given the above if you do fancy impressing your family and friends with how easy it is to apply you really do need to follow the instructions from the manufacturer to the letter and I do mean to the letter with regard to how you transport, store, apply, retain, dispose of, address any spills, accidents with this type of material and usual rules of try it where nobody can see it first and see how it looks dry before you turn your lovely des res into the most talked about house down your street for all the wrong reasons.

I have seen the professional operatives apply the product with the woodworm spray pumps using a coarse spray so as not to atomise the material but even this can cause equipment hassles for any solvent reacting with plastic so as I say you are best sticking to the method and precautions clearly stated on the data sheets for that particular product and that way if there is a problem later at least you can go back to the manufacturer rather than having to move house. Suppliers as follows:

Safeguard Chemicals - Redkiln Close, Redkiln Way, Horsham, Sussex, RH13 5QL (tel 01403 210204) www.safeguardeurope.com

Wykamol Group – Unit 3, Boran Court, Network 65 Business Park, Hapton, Burnley, Lancashire BB11 5TH tel 0845 4006666)
www.wykamol.com

Sovereign Chemicals – Park Road, Barrow-In-Furness, Cumbria. LA14 4QU (tel 01229 870800 )
www.sovereignchemicals.com

Hope this helps kindest regards, David Aldred Independent damp and timber surveyor

MiserlyMartin

Ebe_Scrooge wrote: »

If it's any help, my brother works for a CWI installation firm. They do private houses, but the bulk of their work comes from the local council - insulating schools, hospitals, council offices, etc. I would have thought that installing CWI must save money and not cause problems, otherwise the council wouldn't be able to justify spending the money on it - and I assume they have their own structural engineers who say it's OK. I don't know for certain, but that's my view for what it's worth :-)

Its a local council we are talking about here - usually famous for finding new and spectacular ways of wasting our tax money especially Lib Dem ones.