Cavity wall insulating quandary

[zetland76]

Hi,

A young lady has just advised me that I could get a grant for cavity wall insulation on my semi and it would only cost me £150.
I have a few questions.

From my school Physics I thought that “air” had one of the best thermal insulation properties. So why fill up the air gap in the cavity?
If I go ahead, how do I choose a good contractor? I have read some horror stories on this forum where a contractor has made the holes in the walls but not bothered filling them.
What material should they use? (Fibre or balls).
Should they take a before and after thermal image of the wall?

Any other tips gratefully received.

Thanks Mike

 

[stuart45]

Air is a good insulator. However a large gap such as a 2-3 inch cavity sets up convection currents which cool the wall. The thermal properties of insulation materials depend on their ability to trap small pockets of air.
If you have got an older house with a brick/ brick cavity wall it will have a U value of about 1.7. This can be reduced to around 0.6 after filling.
There have been some problems with CWI in the past. Most of these were down to poor workmanship and poor surveying of the property. eg. solid walls built in stretcher bond being mistaken for cavity and filled.
Nowadays the width of the cavity is checked as well as the condition of the ties and if the cavity is clean. Some of the older cavity walls are tied with headers and these will probably not be suitable.
After the job is completed a thermal image will show up any hot spots where the fill is missing. If the cavity is on the narrow side beads will be used.

 

[JohnD]

I found it made a tremendous difference to heat loss in my house, if I went out to work at 8 and came home at lunchtime, after the heating had been off for 5 hours, it would be cold. Once I had the cavities done, it would still be warm. Also my old 15 kW boiler used to struggle to keep the house warm in bitter frost, and if you ran a bath you could feel the house cool down. Afterwards, the boiler was just idling. The cost of installation is repaid in gas savings in a year or two (at your price, perhaps in the first winter)

contractors vary, so IMO a way to get a reliable one is to ask your electricity or gas supplier for a price. They usually have special offers, they sometimes subsidise them, and they will know how to get any grants you are entitled to. I am a bit suspicious of unknown companies who knock on my door.

the major energy companies are very jealous of their reputation, and should back any guarantees you get, and you can be sure they will not go bust or run off with your money. In the event of any problems they will lean on the contractor to get a good result.

Mine was done in fibre with a moisture-repellent treatment and seems fine. I have heard that the polystyrene balls can run out of cracks, or if you drill a wall or take a window frame out.

BTW mine was installed by Saxon as subcontractors but I ordered it from British Gas and I was very pleased with the neatness and care they took, including sweeping up all the dust and matching the mortar colour used to fill the holes. A chap from BG came round to check on quality of the job too.

I have no connection with BG or Saxon but have worked for other utility companies.