Hi, I'm about to start a retrofit with an extension on my 1930s semi. The plan is to eventually insulation the whole house externally but before I do that I'm going to have the 50mm cavity insulated with bonded graphite EPS beads.

The front and some of the side of the house cavity was filled with white fluff (mineral wool?) around 10 years ago by the previous owners but they only did half the job because of difficult access.

To have this existing insulation taken out so the whole cavity can be filled with the EPS beads will nearly tripple the cost of the work. The company says they can leave it in and just fill what isn't already filled if I want but I'm wondering if it's a bad idea in the long run?

Could the white fluff slump over time? It doesn't seem to have so far but if I'm externally insulating and covering I guess it will be very alkward to fix if it does slump.

Is it worth doing just from a u-value point of view or is the money better spent elsewhere?

Thank you!

I certainly wouldn't bother removing fluff, then refilling with beads. It is very important to fill the cavity before EWI though - any air leaks would reduce the EWI effectiveness. We have a mix of different CWI fills - mostly fluff done professionally over 10 years ago. I diy bead filled lots of places I found unfilled a couple of years ago when I EWI-ed. Our house originally had some brick, some render, and some trim panels - when I took the trim away (slatted white boards) there were huge gaps visible into the cavity. I foam filled these, drilled holes, then blew in beads. The rendered areas were unfilled too, a lot of them I had easy access too as I moved windows out.

Most relevantly, I closed the top of our cavities - our house is a 1963 build, and none of the cavities had any closers at the top - which would have been a horrible thermal bypass especially with EWI. I had 2 layers of scaffolding fitted when doing the EWI- the upper layer was there so I could lift tiles and fit closers (I used 50mm XPS that I just happened to have, foamed into place, angled so water getting to the felt would drip away. As the felt had sagged so much over the years, I think if we had have ever actually had water getting to it, it would likely have ended up in the cavity :-( But it never happened :-)

Perhaps a bit gung-ho, I foam filled the apex-es of our house, drilling lots of holes a foot away from the top, squirting in water, and foaming. I wanted to reduce the amount of air escaping at the top due to the complete lack of cavity closers and low fluffing there, and I didn't fancy removing those tiles - I'm not great with heights and didn't fancy clambouring around on the roof.

Unless you DIY I think the biggest problem you'll have may be getting a mainstream CWI co. to do something 'non-standard'. When I was trying to get a firm out to fill my (Victorian, and variable) cavities prior to EWI the first thing I did was tell them I did not want a CIGA guarantee, because I knew it would be so far outside the standard parameters to be a no-no for a guarantee.

No real point in doing EWI if there is a possibility of draughts in the cavity, therefore it should be filled prior to EWI and thermography used to find any bits that have been missed. I worry about draughts getting into the cavity from below the g/f, service ducts, under sills, joist ends, porch or garage, eaves box.

Re closing the cavity at the top, I join wall insulation to loft/roof insulation seamlessly. Closer seems to me to be leaving out wall insulation above it.

Here's a pic I took of our roof with a few tiles taken out. You can see the felt is a bit knackered, especially at the bottom, with lots of holes.

Piece of XPS. It was foamed into place, joined up with white insulation above that had been stuffed in from loft side ages ago, with a 50mm gap above everything to felt above. I pushed any leftover bits of fluff I had into the top of the cavity just before I did this, last chance!