I went to a very good AECB event yesterday where the focus was the consideration of new silver standard details. One of the ways in which better thermal performabnce is achieved is by having a high level of air tightness. In theory, fully filling a masonry cavity wall will eliminate so called 'thermal bypass' which is the circulation of air around and in between poorly fitted partial-fill rigid boards.

One concern with this, is the possibility of water ingress into the insulation through the outer leaf of brickwork/blockwork.

The counter-argument is that as cavity widths increase to say 150mm or more, the proportion of insulation which may get wet, will be less and can be considered sacrificial.

What do we think?

The idea is that it doesn't matter whether the outer skin is brick, rendered block or slate - the first little bit of insulation is said to be sacrificial. Intuitively, I don't like this, but I have never used full-filll in my life for fear of it getting wet. Easier to change your mind on these things if others show confidence that it is okay.

Yes it can, and yes, I think it is. What you describe makes perfect sense and reflects the Victorian [and earlier] house building method. The only way to render is using lightweight low density mixes. In my area, where we have high rain exposure, the best way to render a solid wall is with porus render. Just use three coats instead of two and never paint it.

Mark I have seen lots of walls demolished and 'break throughs' for extensions, windows made larger etc.

the BRE produces a guidance note on cavity fill which will probably explain all (phone me if you can't get one: 01424 752311).
yes, you certainly do get water in cavities but it does not hang about waiting for holes in the wall to be made but will be there when its raining heavily and especially if accompanied by strong winds. ask anyone who has put on extension without a cavity tray and you will usually find that water gets in sooner or later. mineral (rock) wool is the best fill .
however, cavity fill alone will not give the sort of u-values we should be producing for minimal energy use and you will either have to insulate outside (best because of thermal storage) or inside. do not forget to do a dew-point calculation to ensure that condensation is on the out (cold)side of the vapour barrier.

Mike, should we not be asking a ourselves nowadays if we go to the expense and trouble of incorporating a cavity with all the detailing, workmanship, structural issues and potential latter day problems then why fill it?
Cavity fill is an expression originally derived from a remedial process.
Surely we have two choices/opportunites in new build. Internal insulation giving a rapid response to heating or external insulation with the benefits of a thermal store and minimal risk of interstitial condensation.

Hi Saint,

I can see where you are coming from with this, and there is an argument for doing as you say. However, I'm not entirely convinced about either for the following reasons

Internal insulation, While a rapid response may be desired by some in the winter, summer temperatures can be less comfortable.
External insulation seems ideal but a I am concerned about the durability of comparitively high density rendering systems on low density backgrounds. I wonder if there is a real risk of cracking of materials with disproportionate thermal expansion and contraction.

Maybe my concerns are unfounded but I mentioned this on an earlier thread about passiv-haus and no-one seemed to contradict me other than to say there are other methods than render, which is of course true.

Hi guest, Can I ask if your house in an area which is severely exposed or sheltered?

Thanks Guest that is good to hear. I assume you are nice and warm with all of that insulation and that your energy bills are low. Do you know what type of foam was used?

I am very interested in this. I have recently bought a 1930's semi brick house with tile cladding just below the bay windows. I would like to have cavity wall insulation to help reduce our energy usage. The site is very exposed and has an open aspect to both front and back. The back faces SouthWest which is the prevailing wind direction, and it only needs to be slightly windy elsewhere in town for it to be very windy at the house. The kitchen extension has either been built without a cavity tray, or the drains are blocked as when its wet and windy the water gets in, a good few litres from one nights rainfall. Some of the window frames also leak in the wind-driven rain.

Due to the fact that there is obviously moisture on the inside of the cavity, I have been put off the idea of cavity wall insulation as it may defeat the purpose of the cavity as a moisture barrier and cause damp issues.

So should I worry about the damp getting in, or will the moisture not get past the first bit of insulation?

I don't want to use and exterior render instead as its a nice brick house and it would spoil the feel of the property.

Thanks for any comments! :)

Is it not still the convention to have a cavity in most roofs as well as walls for the same reason- to safeguard the building through extreme weather conditions,states of repair,and over a long lifespan? Why alter this? This approach does not limit the amount of insulaton or air tightness the building can have.
More problematic is the possibilities for bringing the current buildng stock up to todays (and future) U value requirements.I have read that replacement of same at todays rate of build would take 1,000 years -surely not an option!
So, what is the likely hood of an exposed, 25year old ,stone/cavity/block house experiencing damp problems caused by bridging across the infill material?
Would some materials be more likely to allow any moisture to run down (harmlessly ?) than others?
Has any one any experiance of using "perlite" as cavity fill in existing buildings ?

With regards to the brick built 1930's house - does it have a cavity or is it solid?

I had my cavity walls filled with Rockwool 30 years ago. The heating bills dropped but the act of blowing the Rockwool blew off plaster in three places. I have had no damp problems.

PDobson (in the process of registering)
I am building a new house in Surrey and I am interested in upping the wall and roof insulation - The construction is block & brick with a 100mm cavity & 150mm rafters (warm roof). All space in side is at a premium because the planners made us downsize. Any advice on what insulation I should use - I have looked at Tri-iso super 10 on the roof in conjunction with something like 70mm celotex but NHBC wont touch it - They are OK with Thinsulex ? apparently - anyone got experience of that? The full fill interlocking EPS system looks interesting - haven’t got a definitive answer on that from NHBC yet.