Another little power station goes live in West Bridgford

Although I've been somewhat preoccupied with air tightness of late, other things are happening. Today, the new PVs were hooked up to the electricity meter and we are now generating energy and feeding it to the grid. Now all we need is some sun....

Compared with all the other work we've had done, the 2 visits from the team at Carbon Legacy to install the PVs have been most straightforward. What is there to say? Day one and the panels appear on the roof, day two and the cables link to the inverter and from there to consumer unit and meter.

The amount of electricity which we hope the system will produce in a year should be a little over 3,000 kWh.    This will hopefully equate to a significant proportion of our total annual energy use (electricity plus gas) once the low energy refurbishment is complete. (I'm expecting total energy use to be between 6,000 and 9,000 kWh a year - watch this space!)

For comparison, the original estimate for annual energy use in the "as bought" state (based on a SAP energy model) was 36,000 kWh. In fact, energy-saving habits meant that we only used about 24,000 kWh of gas + electricity in a year.

This is like a mini version of the UK's energy challenge. The Department of Energy and Climate Change anticipate that the UK could face energy shortages by 2015. Hence the interest in generating more renewable energy in one form or another. However, there's a very long way to go to meet ever-growing demand for energy, especially with certain older power stations reaching the end of their lives soon.

However, if homes, business etc. were very energy efficient and used MUCH LESS energy (through additional energy-saving habits, insulation, more efficient appliances and equipment, etc), then the scale of the challenge would be reduced - and the chance of power outages in 2015 would also be reduced.

The gentle art of extreme air tightness

Now that we have a roof, windows and externally insulated walls, it is time to tackle the air tightness to make sure that this low energy refurb really does end up with low energy bills.

For this refurb, the air tightness layer is on the inside: the plaster on the brick walls, tapes around the windows, and a breather membrane across the rafters in the attic.

Today I was most fortunate to have 2 volunteers to help me with taping up the joins in the Intello. (Thank you Debbie and Elaine!)

After a little trial and error, I think I have found a way round some of the more complicated details (with the help of the most flexible type of tape in my tool box).

The moment of truth will be the air tightness test that will follow in a week or two. The preliminary test (before the plastering and the internal tapes and membranes) was respectable - better than the average new build. But once the internal air tightness barrier is in place, we are aiming for a value below 1 ACH at 50 pascals. Will we manage it? Place your bets now....

Cold bridge detailing

Ultra-low energy refurbs should come with a health warning - you need lots of stamina to keep plodding on!

The insulation has taken longer than anticipated. This is largely because of the "complicated bits" which arise when trying to achieve a refurb at the passive house end of the spectrum. 

Installing a lesser thickness of insulation would normally be quicker and easier, but putting on 2 layers of boarding in most places and being extremely careful at junctions between wall and roof, window and wall, etc takes more time. 

(The greater the insulation thickness, the more any "thin" areas matter - hence places like our eaves junction have taken a lot of effort to achieve what we hope will be a good result).

This week, the boarding is finished and the render coats are well under-way - the end is in sight!

Already, it feels pleasantly warm inside - even with some windows open and no heating system installed. Of course, it should be warm in the house in July - but it has been a rather cold July so far, and it is noticeable that the house is warmer than it was before the insulation went on.

The gas boiler arrives

After the recent post on gas boilers, Alan Clarke gave the following pointers relevant to our ultra low energy refurb...


With a small  radiator system it isn't critical to have an ultra small boiler, but roughly speaking the capacity of the radiators ought to match the minimum boiler output - though this will be higher than your heat loss (smallest system boiler is 3kW min I think).

If you'll be having a combi, these have to be reasonably powered to heat water quickly enough for a shower - and hence have a higher minimum output too at 6-7kW - so will need more radiator capacity.

Then I think it is important to have good controls to control the radiator temperature, to derate the output to suit the low heat load of your house.

I like the rehema avanta boilers for dealing with low heating loads this way, these have either a simple room-compensating controller, or a fancy programmable thermostat, which also offers weather compensation. But having used one of these with a local builder in a passivhaus he's asked to use Worcester Bosch for the next job as they are easier to deal with for spares and servicing - and Worcester Bosch now also do boilers with "intelligent controls".


In the event we have gone for an Atag - but it may be some time before know whether this was a good choice or not. Hopefully we'll have moved back in and be using it for hot water in the autumn, and won't need to heat any radiators until November / December....

Air tightness

Just getting to start the external wall insulation took some effort - making sure holes, vents etc in the wall were blocked up (or in some cases filled with foam), and making sure key areas were reasonably air tight with blue felt and blue tape.

The main air tightness barrier will be on the inside - the plaster on the brick walls, and a breathable membrane in the attic room.

However, we want to achieve a pretty good level of air tightness on the outside so that air doesn't get into the insulation layer from the outside and "wick" heat away from the house.