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Does cavity wall insulation cause damp & condensation?

Posted by Tim Pullen on 19 December 2011 at 3:58 pm

See recent article: Can I make a claim for problems with my cavity wall insulation?

Q: I am considering having cavity wall insulation but have read that it can cause damp and condensation. Would be interested in your views.

A: Cavity wall insulation is a great idea if you have a clean cavity. It is true that filling the cavity with insulation can cause damp and this is usually due to lumps of mortar (known, endearingly, in the trade as snots) lying on the cavity tie.

As the wall was built it is possible that mortar fell in the cavity and came to rest on the ties.The process is that rainwater hits the external skin and penetrates to the snot sitting on the tie. That is not a problem in an open cavity as the ventilation will dry any moisture penetration before it reaches the inner skin.

Fill the cavity with non-breathing insulation and that will stop any ventilation. The rainwater will still hit the external skin and penetrate to the snot but now cannot be evaporated away and continues to penetrate to the inner skin and emerges as a damp spot.

Ideally you will check that your cavity is clean before filling it with insulation. Reputable insulation companies will do this for you as a matter of routine and give you an honest answer. Less reputable companies may say they have done it and give a more or less honest answer. Distinguishing between the two is the usual process of asking for and taking up references – from customers that had their cavities filled at least 12 months previously.

Alternatively, get someone like Dyno Rod to check the cavity for yourself (other drain cleaning companies are available). It is a quick, simple process of drilling a few 15mm holes in the wall, sticking in a little camera and seeing what the cavity is like. Companies like Dyno Rod use cameras for checking drains all the time and are set up to do this for you.

As to condensation, this occurs when warm air meets a cold surface – glass of chilled Chablis in a warm pub – the condensation appears on the outside of the glass. It is far more likely to occur with uninsulated walls as they are the cold surface.

Insulating the cavity allows the inner skin of the wall to become warm, essentially pushing the dew-point back into the wall where there is then a possibility of condensation within the brickwork – known as interstitial condensation. If this is going to occur it is generally on the inside surface of the external skin of brickwork.

The effect of interstitial condensation will vary with the insulation. If polyurethane foam is used it becomes bonded to the brickwork and prevents any condensate running down the wall, holding it in the brickwork where frost action could cause damage. Blown fibres tend to overcome the problem by allowing any condensate to trickle down the wall to the damp-proof course and away out of the wall. As a consequence the majority of companies offering cavity insulation use blown fibre rather than polyurethane. Again, a reputable company will sort this out and it is generally not a problem.

If all of this means that cavity-fill insulation is not a good idea for you, think about internal or external insulation. But whatever, insulate the walls - 35% of heat lost from the home is through the walls.

Picture by Joost J Bakker

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More information about cavity wall insulation on YouGen

Definition of hard to treat cavity wall for Green Deal

Insulation information page

Does cavity wall insulation prevent condensation? 

Will insulating my cavity wall cause damp?

Find a insulation installer

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

If you have a question about anything in the above blog, please ask it in the comments section below.

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16 comments - read them below or add one


susanComment left on: 13 December 2017 at 8:44 am

Have had bead and glue insulation installed into wall cavities 6 yrs ago by previous owner. Their conservatory has blocked airbricks from subfloor of bungakow. A builder has added an airbrick to gable end to promote airflow. He has placed it under  visible dpc. However he has not fitted sleeve to go through cavity and just drilled through . Will this be ok

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Gilly Jones

Gilly JonesComment left on: 29 September 2014 at 9:02 am

Hi Jonnyess

We get so many question around this topic we've answered yours in its very own blog: Will insulating my cavity walls cause damp?

Do take a few minutes to read our tips

Gilly, YouGen team

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 16 September 2014 at 10:56 am


From what you say, the rubble is all below the DPC and there will probably not be a problem. The bigger problem will be snots on the wall ties. If you are 100% sure they are all dealt with then you should not have a problem.  


If the insulation installation company have done their scoping and are happy then that reinforces the idea that you will be OK.

I am guessing you have thought about internal or exterrnal insulation and discounted them for very good reasons. If there is any doubt over the quality of the cavity then one of those would be the natural choice.

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JonnyessComment left on: 13 September 2014 at 12:58 pm

I have a 1930 s bungalow. Brick as the outside skin and breeze block as the inner skin. Following a visual scoping by the prospective cavity insulation installers, I have removed some bricks and manually removed rubble and snot high in the wall and above the damp proof level.  I can.t easily get to the rubble at the very bottom of the cavity, which is sitting three or four courses below dpc. As this is so far below damp proof, can I assume that it will not cause a damp bridge type problem as any damp across the gap will stil stay below damp proof? 

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Emmaparker_apexComment left on: 14 December 2013 at 10:26 am

Around a third of all the heat lost in an un-insulated home is lost through the walls.Cavity wall insulation is one of the best ways to reduce the amount of energy you need to heat your home. It could save you around £180* a year on your fuel bills. It also lower the carbon emission from your home by around 610kg* and improve the energy rating on energy performance certificate.

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Gilly Jones

Gilly JonesComment left on: 25 November 2013 at 10:24 am

And here is Tim Pullen response: The reader does not state how old the house is, which may have a bearing, but does state that the cavity has not been inspected.   In my view a proper inspection is essential.   Warmfill is a polystyrene bead insulation usually combined with a bonding agent (glue) to prevent the bead settling down after being injected to the cavity and leaving a cold spot at the top of the wall. It is a good product for this situation as it acts to prevent moisture passing from the external brick skin to the internal skin. But that is only true if the cavity ties are free from mortar drops and likewise the bottom of the cavity. That can only be checked by visual inspection, usually with a tiny camera. The insulation company should do that to enable their guarantee to stand up, if not get someone like Dyno Rod to do it.   If the cavity is clean then Warmfill is great for the job. If not think about internal or external wall insulation.

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Tasha Kosviner

Tasha KosvinerComment left on: 22 November 2013 at 10:59 am

Here's another question from a concerned reader. Tim, what do you suggest?

"I have just signed up to have my walls filled with Warmfill insulation but am not sure if I am doing the right think now after reading the above comments. I have no idea if the company doing the work have checked the inside of the walls and my building is definitely exposed to wind blown rain during the winter. I would appreciate any advice before I commit to the work being carried out. Thanks!"

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ROBINOFSHERWOODComment left on: 18 November 2013 at 8:30 am

The funds spent on cavity wall insulation are wasted. If the top line of bricks (the wall plate) was partially sealed leaving a 5mm running gap, then this might be 'a much better idea', and tied in with reducing the number of air brick holes to a single line of 3 x holes in each air brick, better still new home builders should reduce the cavity width to around 20-25mm.

We are trying to create a warm house, and at the same time having to choose between adequate wall ventialtion (in practice over ventilation in the wall cavity) and heat retention, and the two goals are 'at war with each other'; has anyone ever heard a client state "How wonderfully warm the house has been", after having had it pumped full of fibreglass or rockwool waste, pellets. I haven't?

The answer is Housewrapping, air envelopes, plugging gaps, in older houses simply sealing the doors beats cavity wall insulation any day, and lets say goodbye to letter boxes, and disuded chimneys whose flues remove the heat out of a home quicker than an open door. 

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AnonymousComment left on: 17 June 2013 at 7:19 pm


Insulation process.....   Here are some important things to be taken into account when having cavity wall insulation installed, otherwise it could result in the insulation needing to be removed/extracted. Survey ;  Are there gutter problems or finlock gutters? Property exposed to wind driven rain? Condition of the render and brickwork; all fuel appliances inc. fossil fuel, gas, oil and type of flue. Open flue fossil flues and oil over fifth required vent - open flue gas fuels over 7kw requires a vent. All flueless appliances and backboilers require a vent, but balance flues generally do not. This is only a rough guide. Make sure there is good access around the property for ladder, also getting over conservatories / sheds could these pose a problem? Gables however can be drilled and filled from inside the loft if necessary. Check sash windows are boxed in. Check every elevation for rubble and debris in the cavity and that it's not already profiled. Check air bricks to see if they are sleeved and how many need to be repaired. If a barrier needs to be fitted because your neighbour has not had their insulation done, ie semi or mid terraced houses. Unfortunately, surveyors are mostly salesmen and are commission based, they will probably complete a survey in just 10 minutes.   Installation ; the installers should be checking everything the surveyor has previously checked. Change and sleeve subfloor vents for wooden floors, the same with room vents, especially kitchen and bathroom vents, as these are the rooms which produce a lot of moisture. Check the appliances for ventilation requirements, most fires and boilers have a plaque on them with the fire/boiler ratings. This also applies to appliances on an inside or a outside wall, as when you have cavity wall insulation installed, the air flow will change around your property. An appliance on an outside wall will require a smoke check, or flue test - before and after installation so not to fill up the flues. Barriers to be fitted if needed (by drilling a hole at the top and a hole at the bottom) dropping a long chain to hook out of the bottom hole to allow a large brush to follow, staying in the walls so the insulation doesn't fall into the next door neighbours property! Some mispractices can include;   Stopping the insulation machine too early - causing voids, not having the machine set up right - blowing material in too fast. A box test or flow rate check (depending on the material used) must be carried out before every job, recording the time and weight. Not changing the subfloor vents for wooden floors - they may put a brush over them or blow air brought them, this is not acceptable. Not fitting a cavity brush. Incorrect drilling pattern. Not smoke checking appliances before and after (outside wall appliances) THIS IS PARAMOUNT. Check inside your loft for any overspill, check they have stapled a flue test certificate in the loft. Nobody should be installing the insulation below where someone is above them drilling holes, as drilling will cause shards of brick/debris to fall on the insulation in the cavity - causing it to bridge and cause you problems.

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Lawrence Simmons

Lawrence SimmonsComment left on: 30 April 2013 at 10:05 pm

Be warned, cavity wall insulation may be likened to turning a house into a big thermos flask. It can trap cold air in the house in cooler times of the year / day when heating hjas not bee on or has been too low and trap warmer air in hotter periods. Apart from during warmer summers, when it will attenuate cooling in the night, any savings from CWI in the rest of the year will be counter-balanced by this effect.


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Flood Restoration

Flood RestorationComment left on: 16 February 2013 at 10:58 am

Well said, home restoration is really very tough job, so it needs experts so that we can prevent our home from any kind of damage. Thanks for posting it.

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Tony Johnstone

Tony JohnstoneComment left on: 22 February 2012 at 11:28 am

To pick up on damp at the base of cavity walls:-
if snots don't catch on a tie they will continue to the bottom.
if the damp proof course is badly installed they will lodge above it.
if they lodge above it, water on the inside face will enter the inner leaf.

I have been racking my brains to solve just this problem for a 20's dwelling where the cavity is ony 25mm.

If someone has developed a robot capable of working in 25mm to grind and remove mortar I would be very pleased to make their acquaintance.

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 22 February 2012 at 9:28 am

Clearly, cavity wall insulation is a controversial issue. Adrian at enact seems to have installed insulation to 150,000 house, and there are some millions more across the UK. Yes, we do get reports of problem houses but very few reports of successful installations My own home is half-way up a mountain in West Wales, very exposed and we do get a bit of rain. We had the cavity filled with injected foam some years ago and have no damp penetration problems.   

It does not suit every house. All the things said by various contributors about checking the cavity are true. Equally true is the need to check the quality of the installation company.  

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Anthony525Comment left on: 21 February 2012 at 4:10 pm

When John Prentice was Deputy Prime Minister the government issued a warning saying. Cavity wall insulation should not be installed in properties in exposed locations, these to include the west of England and Wales.

The problem has always been that wind driven rain can and does find its way through walls both solid and cavity.

Cavity walls are designed to enable the rain to run down the inside of the outer layer of bricks/blocks whatever. Leaving the inner skin dry.

Fill the cavity with anything that is not waterproof and you have a problem.

Cavity wall insulation should only be installed inside a wall that is know to be dry and will remain so for the life of the building.

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@adrianenactComment left on: 26 January 2012 at 12:40 pm

Some fairly negative comments above!  Often the damp material at the bottom of the cavity is caused by a similar problem of waste and 'snots' falling to the bottom of the cavity when the wall is being built.  A reputable cavity wall installer should drill and scope at least 1 wall (ideally each elevation) to check for this.  Despite a scoping of the wall being best practice, unfortunately there are no standards which dictate that this happens and the industry really should look at this.

 Our company, Enact,  has installed cavity wall insulation into at least 150,000 homes and to date we have never had to remove the material due to damp although I have known cases where the guarantee association has paid to do this.  The key things to watch out for are cracked walls and render and to avoid properties which face onto a high exposed position.  Some people refer to high exposure zones but these encompass tens of thousands of homes on on exposed housing estates which are perfectly fine to insulate despite misinformation on certain TV programmes! 

Of interest we offer cavity wall insulation free of charge, these grants will most likely be gone by the end of this year and then you will have to pay £500+ so if you are considering loft and cavity wall insulation, this year is the time to do it! 

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D Boreham

D BorehamComment left on: 25 January 2012 at 5:50 pm

Some of Tim Pullen's observations are incomplete, leading to dangerously misleading conclusions.

If penetrating damp was caused by a build-up of insulation material on mortar snots bridging the cavity, why are there so many reports of damp having been introduced at the very base of walls, and not randomly distributed wherever ties have been used?


There are many reports of householders, seduced by misguided government subsidies and dubious "consultants", finding that the bottom few feet of their wall cavities are full of sodden clumps of insulation fibres, oozing water as well as leaking heat.  These areas of damp are frequently very extensive - way beyond any "spot" around a faulty wall tie.


If you are in this unfortunate position, neither the installer nor the so-called guarantee will rectify the problem free of charge.  Instead, issues like your lifestyle and lack of ventialtion will be offered as the cause.  Your cost of removing large quantities of soggy fibres will vastly exceed any heat savings you may have dreamed of.


The statement that 35% of a dwelling's heat may be lost through its walls does not predict that any form of retro-fit cavity insulation will reduce that loss.  In fact Mr Pullen omits reference to any independent research to show that blown fibre insulation helps at all.


If a sensible, helpful contribution could have been made in this piece, then a firm prohibition on retro-filling any cavity wall exposed to wind blown rain would certainly have been appropriate.  As would the suggestion of considering the use of 2 to 3mm loose blown polystyrene spheres as the material of choice for those determined to take the risk, even if this material doesn't attract any subsidy at present. 


As for injected foam systems - does no-one remember the fiasco this produced in the 1970s?


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