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humblebee hive – retrofit #2

Cavity Wall Insulation (CWI)

Delays with the Planning Application process – and more on this in a later post – meant we couldn’t start the “big works” until this month (March 2022). But one of the things we could get on with and commission, with relatively little pain, was the cavity wall insulation (CWI).

Having established we had a 50mm wide cavity (the minimum spacing you can feasibly fill) our patient Retrofit Coordinator – Richard Shears at the Retrofit Hub – advised us of a company that he trusted to do the job right so we were on it back in November 2021, which had been our original retrofit start date.

With our plan being to externally insulate too, questions about whether to fill or not to fill, and with which material, were less critical than if we were going for internal wall insulation, but it’s certainly recommended to go for a hydrophobic material and not one that will absorb water like a sponge. Retrofit advice is generally to go with grey EPS (polystyrene) bead or closed cell (expanding PU) foam insulation (we went for the former), but in any case you want to make sure all the gaps are properly filled, which you can do with a thermal imaging camera.

If old mineral wool or cellulose insulation is causing you problems because it’s gotten wet, you can also have this sucked out: the company that we used for our CWI – Duoserve – will do this for you one day, let it settle, then come back to fill with EvoBead the following day.

Grey EPS beads – visible now that we’re putting new windows in!

Polystyrene in an eco-build? It’s a tricky balance, but choosing the right material to do a particular job is equally important. Given the idea is to extend the life of the building to last another 100 years or so, we finally came to terms with it, and with a thermal conductivity* of 0.033 W/mK, it will be the most insulating 50mm of the wall buildup: it immediately brought the external wall U-value down to 0.51 W/m2K from 1.65 W/m2K.

I only recently found out that the beads now get coated with adhesive, so that when you do works to the walls / drill holes, they pretty much stay in place (and don’t spill out like they used to) and it prevents “settling” over time.

“Brushes” to line the floor vent and new air brick

Brushes installed in cavity to hold back the beads

The suspended timber floor still needs ventilation, so our jolly installers Shaun and Brad showed us the brushes they use to line the opening within the brickwork to stop the beads from spilling out (and presumably from filling up the floor void, which wouldn’t be much fun).

Picture courtesy of Wikipedia

Tip: you can tell whether your cavity has been filled by looking for mortar-filled holes (size of a 2p piece) every few brick lengths along the brick face.

*Insulation materials range in thermal conductivity from 0.020-0.050 W/mK, generally synthetic materials are at the lower end, mineral wool in the 0.035-0.040 range and natural materials upwards of 0.038.

Costs #2

Grey EPS EvoBead CWI, including installation, £1,620.00, VAT £81.00, Total £1,701.00 (£15.50/m2 GIFA).

Total Insulation Retrofit to date £4,400 (approx. £40/m2 GIFA) – note I had to correct my previous post figures on Loft Insulation costs.

Target filled wall U-value: 0.51 W/m2K (final target is 0.16-0.18 W/m2K).


New energy efficient appliances

Part of our decarbonization strategy is to get off gas where we can. And that also means being economical with electrical energy, given current rates. So energy efficient appliances were going to feature at some point, even though we’ve always been keen to repair and extend the life of equipment wherever possible.

Part of the whole house insulation plan is also to remove the chimney top and fill the void so one prep job was to decommission the largely redundant gas fire in the living room.

With a main oven that conked out less than a couple of years after we’d had Bosch replace the fan, it was an opportunity to a) have a more efficient, working oven b) replace the gas stove with an induction hob. While we were at it, we wanted to have the oven at a height that was helpful as we aged but generally safer and more convenient to operate.

And so we were kindly given contact details of a friendly, local gas safe plumber John (Wilmslow Gas and Heating) – gold dust around here – and got him round to service the boiler which we hadn’t bothered to do for a while, disconnect the gas fire and also the hob.

Having pored over Ethical Consumer and various energy efficiency comparison websites for months prior, we finally took the plunge, and ordered what we thought was best value for highest efficiency from the selection at John Lewis. Our ever-optimistic outlook meant we hoped that the JL installation fee would cover wiring in the oven in its new location. Sadly, when they tested where the power was coming from at the socket we’d intended to use – which involved a plug-in gadget that made a piercing noise until the power got cut off at our old fuse box – it turned out it was on the main power ring main and we were told they couldn’t install the oven because it needed its own separate feed. JL kindly reimbursed the installation fee.

After a mild panic to find an electrician in double quick time, the lovely Simon (Henbury Electrical, recommended on a local Facebook group) responded to my message the same day. He came round to quote that very evening and we agreed it was probably time to get the old-style fuse box replaced with a modern consumer unit. A more expensive call out than we’d budgeted for, and notifiable works, but really we were grateful to have a safer installation which would stand us in good stead in the long run.

We had already replaced the fridge-freezer with a Miele A+++ model the year before, and our failing kettle had already been swapped for an insulated Vektra. So now we were also the proud owners of an AEG induction hob and Miele waist-height oven. And boy, does it make a difference to have a modern oven and hob – the speed!

Appliance and Services Costs

I won’t include these in the running total of retrofit costs but you may find these useful if you’re trying to budget – all include VAT:

Miele KFN28133D Freestanding 60/40 Fridge Freezer, A+++, John Lewis, Stainless Steel, £999.00, Delivery £19.95.

Vacuum Eco Kettle – 1 Series – 1.5 Ltr Brushed Stainless, Ethical Superstore, £69.98.

Miele H2265-1B Built-In Single Electric Oven, A+ Energy Rating, Clean Steel, John Lewis, £539.00 (Recycling of old appliance, £20, installation would have been £110)

AEG IKE64450FB 59cm MaxiSense Induction Hob, Black, John Lewis, £619.00 (Recycling of old hob, £20, Installation £90)

Gas disconnection works, Hob £45.00, Gas Fire (includes testing for leaks) £50.00

New electrical consumer unit (and new oven 16A feed), including Building Control notification, testing and certificate, £620.00.

Our new Green Building Store Windows – sneak preview!

Sneak peak for the next instalment… our windows arrived last Friday. Installation and the trench for the EWI works are underway! I promise much “ooh-ing and ah-ing” and possibly some hilarity…



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Welcome to my blog where I download some of my latest thoughts and musings, talk about experiences, write up my biggest personal project - my own home retrofit - and generally use it for catharsis.

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