Retro-fit Cavity Wall Insulation

[monday]

I'd be grateful for some advice on this issue.

Detached house was built 1996-7 under a version of the Building Regs that permitted use of thermal block for the inner skin rather than cavity wall insulation. As a result we have been seized on by the retro-fit industry for free retro insulation under the current Government/utility co scheme.

The insulation proposed is the glued polystyrene bead and neighbours have already had this done, without problems so far. I am aware from this forum and other sources that the "glued bead" method is more satisfactory than "loose bead" or "fibrefill". I was brought up to believe that the cavity is there for a purpose (I am a civil engineer rather than a builder) but I am not in an exposed location (urban Surrey) so am not too worried about rain penetration (or should I be?)

These houses are on a former industrial site with possibility of pockets of landfill gas; for this reason they have suspended precast concrete floors with underfloor ventilation via brick-sized grilles just below dpc. Can anyone advise me of the typical detail here: is there any possibility that the cwi could end up blocking the underfloor ventilation? Is the cavity normally concrete filled up to dpc?

Also I have seen on this forum a calculation of the change in U-value resulting from cavity wall insulation on a brick/cavity/thermal block wall. The improvement was small and brought the U-value up to not much more than the equivalent of a traditional cavity wall WITH cwi.

So all in all, I have lived in this house for nearly 20 years and am wondering if it actually worth it? Any views?

 

[^woody^]

Where there needs to be protection from land gases, there is normally a membrane across the bottom of the cavity to separate it from the under floor void. Similar concept to a cavity tray. The installers will assess this.

As for the insulation, if done properly there should be no problems. I know of whole estates with poly beads in place for decades with no issues. There is a benefit in heat retention and energy savings, see the many calculations.

 

[monday]

Many thanks.

 

[theprinceofdarkness]

I had poly beads sprayed in with glue. 99% effective for 30 years. Slight problem is that the glue did not work 100%, so that around badly made hole for pipe some beads fell out, sort of like half a bucketfull as opposed to 6 cu. m. they put in. One corner of a square bay filling dropped about a foot in fill, which I made good, myself.
My air bricks were all smashed out, the cavity stuffed around the aperture with glass fibre and new air bricks put in. The making good of the big holes for the blowing-in, was fantastic, even though I knew roughly here they were there were very difficult to spot. Very much down to the skill of the operative!
In my Daughters fibre fill, they " lined" the air bricks by knocking in plastic pegs in the outer lines of holes, so reducing the free air area.
Frank

 

[monday]

Reassuring, thank you. As always, I guess it comes down to workmanship. If I get the same crew as neighbouring houses, should be OK although you can spot houses that have been done from the filled holes, which are neat enough but not necessarily a good colour match.

 

[brotherdeal]

Reassuring, thank you. As always, I guess it comes down to workmanship. If I get the same crew as neighbouring houses, should be OK although you can spot houses that have been done from the filled holes, which are neat enough but not necessarily a good colour match.

Did you go through with the bead insulation? As I'm considering it on a very similar property to yours.

 

[monday]

I had completely forgotten abut this thread Fortunately the forum auto-advised me of your message.
No, I never did go ahead. I had a contractor to quote but there was difficulty accessing some of the walls from the outside - for example a ground floor extension and an integral garage block access to the 1st floor external wall above. This would have meant working from inside and didn't fancy the mess.

My next door neighbour went ahead with no problems as far as I know.

Good luck.