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Does cavity wall insulation cause damp & condensation?

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

Cavity Wall insulation house

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

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.

Comments

5 comments - read them below or add one

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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Anthony525

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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@adrianenact

@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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