# Cavity Wall Insulation - Does it work?



## Shane007 (10 Sep 2011)

Whilst I completely agree with maximizing insulation to minimize heat loss from the fabric of a building, does, in particular, cavity wall insulation really benefit?

Here's where I am coming from: For me in a typical cavity wall construction, there is an inner skin of blockwork with 50mm of polystyrene insulation adjacent this skin within the cavity. Then there is the cavity and then the outer skin of blockwork with a nap finish or similar. The outer skin of blockwork is open to the elements and can be soaked with driving rain, etc. My understanding is that one of the functions of the cavity is to allow air flow behind the wall to ventilate and assist the outer skin of blockwork to dry out.

If we pump the cavity full of beads, does this not restrict the air flow from behind the outer skin and hamper the ventilation/drying process for this wall and therefore do we not end up with a damper external wall? If so, what could the long term effects be on the integrity of this structure?

Would external insulation or internal insulated dry-lining be much more beneficial, although more expensive and more disruptive?


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## onq (10 Sep 2011)

Hi Shane,

The cavity in use may be found empirically to have assisted the outer leaf to dry.

However the original function was to help to create an isolated outer  skin which would "catch the weather" and prevent water being transmitted  to the inner skin.

In other words the outer leaf was always intended to "get wet" and durable materials such as brickwork and concrete were chosen for this  purpose, for much the same reason that various metals skins, slates and  tiles were selected for roofing materials.

-----------------------------------

As the conception of the cavity wall and its construction changed over  time, details were developed that relied on structural isolation of the  outer leaf so that almost all of the load-bearing of the wall was  carried on the inner leaf - how could one transmit a compression load to  an outer leaf with only flexible ties? Thus the outer leaf helped with  stability issues, as opposed to directly taking loads.

As understanding of insulation factors progressed, cavities were  supposed to be "closed" to restrict the movement of air within them to  improve insulation. But in an Irish climate this proved unworkable,  since it is so wet that _weep holes_ were needed over every  head and at any stepped flashing or cavity tray to allow water throw out  towards the outer leaf to "get away".

This was done to avoid saturating the lowest brick on the way causing  "salts" to leave their whitish deposit on the surface, disfiguring the  elevation. Weep holes that let water out by definition let air and  insects in - thus one sees the plastic insect guards in open vertical  joints in brickwork and blockwork (the perpend joints).

More recently, timber frame construction uses vent holes both below and Damp Proof Course level and just under the eaves to actually promote ventilation in the cavity. This is not for  the benefit of the outer leaf but to ensure water vapour that has migrated through the timber frame construction and condensed on the outer surface of the breather paper (facing into the cavity) can evaporate away.

-----------------------------------

To address the risks for increased insulation AND reduced cavity ventilation on outer leafs we need to realize that high quality concrete, brickwork and mortar is not unduly affected by water but is affected by frost.

Drying out a wall depends on two factors - ventilation and heat. An  outer leaf is only four inches or 102mm thick, and will dry out in time  with a temperature gradient and an occasionally dry outer surface  letting evaporation of the water occur, even without a cavity behind it. Once the temperature drops below zero, water in the outer leaf will freeze, as opposed to evaporate.

The question you raise may be less an issue of whether sealing the  cavity will restrict the outer leaf in drying out as whether putting  more insulation between the warm interior and the potentially freezing  exterior will reduce the rate so much that frost could  freeze water in the outer skin leading to degradation of the mortar and  brickwork - this is known as "flaking" or "spalling". Concrete tends to be less affected than brickwork.

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The current thinking of super-insulating houses in winter climates that  are becoming ever colder raises questions about the longevity of mortar, brickwork and  render on exposed elevations in wet winter climates where prolonged  freezing conditions follow periods of intense and persistent rain. The  outer leaf is kept colder for longer because of the insulation and the filling of  the cavity means there is little ventilation on that side.

The result could well be an increased incidence of brickwork flaking and spalling  and the external render cracking off due to frost action occurring within saturated outer  leafs and their coverings.

Regarding your specific query, external insulation protects the structure be definition, but what  protects the external render covering the outer leaf? A medium solution  may be to insulate internally to the level required by the regulations  and apply moisture repellent render to the blocks and paint it regularly  with exterior grade paint. This returns to an ancient technique of  sealing involving whitewashing walls with lime.

I am monitoring several properties I have completed to see how -


Full fill in-cavity insulation
Internally insulated single leaf with rendered blockwork
Internally insulated cavity walls with rendered blockwork
 - solutions are weathering. 

Most of these sites are not too exposed and the quality of workmanship and materials was high. All employed traditional detailing with adequate roof overhangs and/or sustantial parapets. To date all forms of construction  seem to be holding up find so far. Touch wood.


ONQ.

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## Shane007 (10 Sep 2011)

Hi ONQ,

Thank you for taking the time to write such a detailed reply. I suppose only time will tell. 

I have spoken to many people who have had cavity wall insulation carried out and many see no difference and only a few seem to notice the house warmer. The latter were mainly in houses with no insulation at all so any insulation would make a significant improvement.

Anyone I have spoken to who have singled out north facing rooms and insulated with insulated plasterboard internally all seem to have very noticeable improvements. The other advantage with this option is that it can be carried out room by room as budget allows and much lower U values can be achieved. 

Finally, do you know of any other cavity wall bead that offers a better U value than the current graphite bead with a U value of approx 0.035?


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## onq (10 Sep 2011)

Hi Shane, you're very welcome.

Its difficult to prove that bead insulation has filled every space in the cavity.
The insulation that's there can sometimes be dislocated off the wall and be part of the problem. With no insulation to start with I'd expect to see a better result.

North-facing rooms will see the greatest result.

Internal insulation leaves a lot of cold bridges unless you considering all the awkward details, ground floor 1st floor and eaves.
Insulating without sealing fireplaces will have little effect in windy weather.
Insulating without double- or triple-glazed windows will have limited effect.

Doing all that without reducing infiltration gains/leaks will not improve the position as much as it can, but then you need an MVHR system.
You need to take an overview and work towards it - window size and orientation matter a lot.

I don't recommend cavity beads for reasons discussed elsewhere in this thread.


ONQ.

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## RMCF (10 Sep 2011)

onq

I hope you don't mind me asking a question here, as you do seem to know your stuff.

I too have recently been considering cavity wall insulation, but I have not yet spent a winter in the house (only moved in Feb this year) and was thinking about waiting until I see how I fare this winter so I can make a better decision next year.

Anyway, I have noticed that my back bedroom (downstairs) is the coldest room in the house. The rear of the house is generally north facing, although not directly. This bedroom sides on to the driveway and the rear of the house. It is also the childrens room, which worries me a bit for a possible cold winter. The opposite bedroom on the same wall but towards the front of the house (south) is a lot more cosy, despite also backing on to the driveway.

After reading this thread I was thinking that if the house feels generally ok over the winter maybe it would be better to do internal insulation on that cold bedroom only? Surely that would be a more cost effective option that cavity wall? 

Perhaps you could give a rough idea of how much it would cost to internally insulate one bedroom with 2 external walls? I think the room is approx 12ft x 12ft.

Thanks.


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## roker (11 Sep 2011)

I had a new bungalow with cavity insulation, in the winter I would still get condensation running down the end walls in the bedrooms. I would personally recommend insulating the inner wall not the cavity.
It never made sense to me having the cavity insulated, because there is a mass of blocks to heat up on the inside which takes time, and the blocks also transmit heat to the cold floor/foundations


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## Shane007 (12 Sep 2011)

onq said:


> Doing all that without reducing infiltration gains/leaks will not improve the position as much as it can, but then you need an MVHR system.


 
I understand the principles of MVHR, however my concern is the bacteria build up within the filtration system. How will this pan out over the years as more and more systems get installed? How often should the filters be changed, 3 months, 6 months, 12 monthly intervals?
How will it be monitored and will, for example, tenants in a rented property with MVHR installed change the filters at the appropriate intervals?

I think it was last year on Duncan Stewart's programme where the lady with a lung disease was not told to change the filter and it was tested after 12 months and found to have millions and millions of excessive bacteria blowing through the home.


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## onq (13 Sep 2011)

RMCF said:


> onq
> 
> I hope you don't mind me asking a question here, as you do seem to know your stuff.
> 
> ...



No problem at all, although you'll have to ask a builder for costings on each method.

The problems with retrofitting internal insulation centre on cold bridges at the opes and junctions of walls and ceilings/floors and eaves.

With a timber floor, its possible to lift the boards and insulate the perimeter of the floor.

No point insulating the bedroom and not the room below and the attic above.

The problem with cavity fill is checking to see the entire cavity is treated.

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon                       as a defence or support - in and of itself -  should       legal        action    be      taken.
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## onq (13 Sep 2011)

roker said:


> I had a new bungalow with cavity insulation, in the winter I would still get condensation running down the end walls in the bedrooms. I would personally recommend insulating the inner wall not the cavity.
> It never made sense to me having the cavity insulated, because there is a mass of blocks to heat up on the inside which takes time, and the blocks also transmit heat to the cold floor/foundations



Whether you insulated in the cavity or the room, you run the risk of condensation, but what you describe shouldn't have happened.

Insulating in the room runs the risk of condensation behind the insulation, especially if you leave a gap, i.e. with dabs as opposed to tight mechanical fixings.

Insulating in the cavity should have avoided this, but what you describe suggests the cavity was still "cold" and allowing the inner surface of the wall to stay cold - IOW, the Cavity insulation was not effective.

That having been said I have seen several other reasons why walls stream with condensation - its usually a combination of

- lack of insulation

- poor ventilation

- higher than normal moisture content in the air and

- relatively low heating.

The issue of problems with heating the inner wall (slow response) arises where the space is intermittently heated.

However if you super-insulate the outside of the building with external insulation, you'll heat the walls once and then the trapped heat is a heat store, topped up each time you heat the building.

Cavity insulation doesn't do as good a job for the reason mentioned above 

- you cannot guarantee it will fill the cavity

- you're limited to 50-60mm of original cavity insulation (which may have been poorly fitted) and 25-40mm of cavity insulation

- which variation arises because it may not be properly dispersed.

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon                       as a defence or support - in and of itself -  should       legal        action    be      taken.
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## onq (13 Sep 2011)

Shane007 said:


> I understand the principles of MVHR, however my concern is the bacteria build up within the filtration system. How will this pan out over the years as more and more systems get installed? How often should the filters be changed, 3 months, 6 months, 12 monthly intervals?
> How will it be monitored and will, for example, tenants in a rented property with MVHR installed change the filters at the appropriate intervals?
> 
> I think it was last year on Duncan Stewart's programme where the lady with a lung disease was not told to change the filter and it was tested after 12 months and found to have millions and millions of excessive bacteria blowing through the home.




This is a huge concern for me also.

Will we see diseases like Legionnaires Disease in Irish Homes?

Not enough data to say.

A further concern is sound transmission between rooms.

A more serious concern is the transmission of cold smoke, fumes and gases around the house to the sleeping occupancy.

No fatalities have occurred thus far that I know of, but these issues are foreseeable.

Architects who are designing or certifying systems should consider them.

To answer your query I'd say one month, but each home will be different and only testing can answer this.

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon                       as a defence or support - in and of itself -  should       legal        action    be      taken.
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## Guns N Roses (13 Sep 2011)

onq said:


> The problem with cavity fill is checking to see the entire cavity is treated.


 
Firstly I would like to congratulate you on your comprehensive reply.

In you opinion ONQ, would a thermal camera survey be able to determine if the cavity has been filled?

Also why do you not favour bead insulation?


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## Mpsox (13 Sep 2011)

I can't add to the technical discussion here, however, a few years ago, we lived in a 70s built bungalow which had poor insulation and a very low slope on the roof meaning it was physically difficult to insulate the attic. We got the cavity insulation done and it did make quite a difference.


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## onq (13 Sep 2011)

Guns N Roses said:


> Firstly I would like to congratulate you on your comprehensive reply.
> 
> In you opinion ONQ, would a thermal camera survey be able to determine if the cavity has been filled?
> 
> Also why do you not favour bead insulation?



I try to be competent, while not holding myself out to be an expert on building matters

Both your questions answer each other.
There are many factors which mitigate against it ending up being evenly deployed in a cavity including

- displaced insulation board
- "snots" of mortal on wall ties
- wall ties themselves
- stepped and/or dislodged Damp Proof Courses and Cavity trays

I don't favour bead insulation because there are very few ways to check  if its been carried out properly apart from using a thermal camera.
I am also very wary that a mass of such insulation spreading from wet  external leaf to the wall ties, will conduct moisture to the inner leaf.
I have seen no empirical follow up studies showing the thermal  efficiency of the material, covering the known issues and exposing  problems.

This is a prejudice on my part born out of a knowledge of where things go wrong in the building industry but I cannot prove its a bad job.
Equally, where its an addition to existing cavity insulation board, it can only improve the insulation, assuming it does what it says on the tin.

However one thing this aftermarket solution cannot do it improve the performance of cold bridges, which are one of the main causes of thermal dysfunction in cavity walls.
The warm bits get a bit warmer and the cold bits stay the same because the nature of cold bridges is that they don't have hollows in them which the beads can fill.
If cold bridges had such voids as opposed ot solid "bits" that transmit heat from a warm place to a cold place, they wouldn't be cold bridges in the first place!

In terms of a quick fix with

- no interior or exterior mess
- no loss of room area
- no increase in wall dimensions
- no changes to external elevations
- no changes to external finishes
- no changes to services

...cavity bead insulation has its attractions.

However it cannot be taken as offering 

- improved sealing
- reduced cold bridging
- significant improved thermal performace

...because its usually limited to being only 2"/50mm of granular fill insulation within an existing partially insulated cavity.

By its nature its very limited in what it can do.


ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon                        as a defence or support - in and of itself -   should       legal        action    be      taken.
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## onq (13 Sep 2011)

Mpsox said:


> I can't add to the technical discussion here, however, a few years ago, we lived in a 70s built bungalow which had poor insulation and a very low slope on the roof meaning it was physically difficult to insulate the attic. We got the cavity insulation done and it did make quite a difference.



(nods)

Such walls were often totally uninsulated.

Thus the cavity fill insulation in such a situation might have had a three or four inch (75-100mm) cavity to fill.
This larger width could have helped it migrate to fill the entire void properly as opposed to getting "caught".

In terms of total insulation this would have meant you'd be looking at a 3-4" (75-100mm) total fill.
There would have been a huge difference between this and an uninsulated cavity wall.

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon                         as a defence or support - in and of itself -    should       legal        action    be      taken.
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## Shane007 (13 Sep 2011)

onq said:


> A more serious concern is the transmission of cold smoke, fumes and gases around the house to the sleeping occupancy.
> 
> No fatalities have occurred thus far that I know of, but these issues are foreseeable.
> 
> ...


 
This one I had not thought about. I was more concerned about bacteria and health issues, but now that you mention it, this is a far more serious concern.

Why is it promoted then as the ultimate in home energy saving and I stand to be corrected, but is it either to be a building regulation requirement or is a building regulation requirement? 

Surely your concerns should be dealt with first, for example, all systems must have as standard built-in carbon monoxide and smoke alarms that would automatically shut the system down upon detection or better still divert such gases, etc. to the exterior!


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## Guns N Roses (13 Sep 2011)

onq said:


> I am also very wary that a mass of such insulation spreading from wet external leaf to the wall ties, will conduct moisture to the inner leaf.


 
I also have wondered about this. If you believe the product literature, it suggests that any moisture will work its way down the cavity through the beads.

I'm booked in to have my own house done with cavity wall insulation next week. I've taken the day off to inspect the workmanship. Will let you know how it goes.

As regards checking to ensure that the cavity is adequately filled, is it not a matter of calculating the void to be filled and comparing it with the amount of material used.


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## onq (13 Sep 2011)

@ Shane,

I'm not trying to raise fears unnecessarily, but a colleague an I came separately to these concerns and that's what worries me.
What quality of people do we have invoking building standards in this country - politicians of the technical ability of John Gormley?
The sad truth is that when I looked at the approved details on the Environment website they all seemed to be one-line solutions.

From what I recall it seemed that they addressed air-sealing by drawing a line through a wall - wall junction and dealt with threshold/rising wall cold bridges by declaring the rising wall to have a u-value.
The implication was that someone without knowledge of building materials or systems worked on one problem at a time and addressed sealing and insulation, but little else.
One building control officer I know - with a young family - has realized that the proposed "sealed" lifestyle just doesn't work in a household with kids and pets.


I've started to engage with a passive house Group on http://www.LinkedIn.com to see what those in the industry may say.
I'm currently putting one house certification on hold and developing workarounds for some of the issues as I understand them.
I don't have a timetable for completing all of this research, and it remains unresolved, but the important thing is to start asking the questions.

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon                          as a defence or support - in and of itself -     should       legal        action    be      taken.
Competent legal and building professionals should be asked to advise in                          Real Life with rights to inspect and issue   reports    on     the         matters    at      hand.


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## onq (13 Sep 2011)

Guns N Roses said:


> I also have wondered about this. If you believe the product literature, it suggests that any moisture will work its way down the cavity through the beads.
> 
> I'm booked in to have my own house done with cavity wall insulation next week. I've taken the day off to inspect the workmanship. Will let you know how it goes.
> 
> As regards checking to ensure that the cavity is adequately filled, is it not a matter of calculating the void to be filled and comparing it with the amount of material used.



I look forward to hearing about it.
Perhaps you might consider taking a few photos and putting them on Flickr or Facebook or whatever suits.

Someone suggested that the insulation forms an impervious mass.
Water meeting the top of this mass, which may be curved in profile or lumpy, could dribble down either side, or could go down the outside and find a hole and head inwards.
This hole could occur at or under a wall tie, and a wall tie splits board insulation to either side, so depending on the workability of the drip detail, there could be a penetration at that point.

If the insulation isn't impervious, then I fail to see an impediment to the passage of moisture, so I'd be really interested to learn more about how the product performs and installs in the real world.


ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon                           as a defence or support - in and of itself -      should       legal        action    be      taken.
Competent legal and building professionals should be asked to advise in                           Real Life with rights to inspect and issue    reports    on     the         matters    at      hand.


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