Please help. Why will no one do this job?

[Mike13]

It is saying that with the appropriate vapour control barrier inside and out, that new builds do not need an air gap in the "cavity", it can now be filled with insulation.
You just have to control the temperature and moisture levels so that you do not get condensing humidity in the middle of the wall. That is often a problem with timber frame and celotex fill for example.

 

[opps]

It is saying that with the appropriate vapour control barrier inside and out, that new builds do not need an air gap in the "cavity", it can now be filled with insulation.
You just have to control the temperature and moisture levels so that you do not get condensing humidity in the middle of the wall. That is often a problem with timber frame and celotex fill for example.

Although I gave you a thumbs up, how have you come to that conclusion? I have not seen anything in the post to back up your point.

I am out of my depth in this thread, but I do not understand how you have made a definitive statement.

Can you please show me the previous post that backs up your post?

I am not saying that you are wrong, but based on your post, I can see nothing that holds water.

I don't want to see people visiting this site (either now or in the future) being mislead.

Please feel free to prove me wrong. I genuinely hope that you do. As I said, it is beyond my ken.

 

[freddiemercurystwin]

If it makes it simpler to grasp, we're talking about filling the gap between the inner and outer skins of masonry that form a wall. There is a cavity before it is filled with insulation yes? The cavity is fully filled. Not gonna comment on the timber frame aspect.

 

[mrrusty]

I don't think you should compare modern wall building techniques with retro-fit. Modern walls are built as carefully tested systems where the combinations of materials should not create problems. Older houses often have more porous bricks, more porous mortar, different surfaces internally allowing breathability of room air in to the wall (whereas modern houses are very air-tight).

I'm sure that sometimes retro-fit is fine, and sometimes it is not. What I am also sure of is that the outcome is fairly pot-luck because no-one ever does a scientific study of that particular house to determine if it will be OK or not. If a house was designed and built with a ventilated old-style cavity, that's the way it should remain IMHO.

 

[Mike13]

Exactly, in a new fit, they control the vapour pressure and dew point through the wall to stop condensation. In a retrofit is more difficult to control.
0C and 100 % relative humidity you can carry 0.6% moisture, at 20 C and 100 % relative humidity (like a kitchen or bathroom) you can hold 2.31% moisture in the air. So you need a vapour barrier to reduce the relative humidity that can enter the wall so that condensation cannot occur in the middle.
Look carefully at steam tables for atmospheric pressure to see the limits you have to respect.

Water - Saturation Pressure vs. Temperature

Online calculator, figures and tables with water saturation (vapor) pressure at temperatures ranging 0 to 370 °C (32 to 700°F) - in Imperial and SI Units.

www.engineeringtoolbox.com

third table down, look at the 0-25 numbers and the middle column.
0.0231 at 20C corresponds to 2.31% moisture in the air
the humidity profile in the wall must remain below 100% relative humidity at all times or you will get damp/mould in the middle.

 

[r_c]

Are you suggesting that cavity wall insulation is a problem full stop, regardless of whether it is a new build or not? I ask because the house across the road is being built, and the cavities are being filled with thick wads of what look like regular rock wool (to a layman like me it just looks like loft insulation - no silver foil and not rigid). I cant see a huge difference between insulating this way and blowing/injecting rock wool insulation. What am I not considering?

To answer my own question, it looks like there is a difference between the two:

The cavity wall insulation batts are designed to be as water-repellent as possible, which cannot be said of blown-in cellulose for example. This is probably why issues related to cavity wall insulation are of rare occurrence in new builds but a big problem in retrofitted wall insulation.

Source: https://www.buyinsulationonline.co.uk/blog/problems-with-cavity-wall-insulation