A very EnerPHit Christmas?

Now into our second heating season since the refurb, we are definitely reaping the benefits in terms of comfort. We have a house that is quick to heat up and slow to cool down (Hooray!)

Are we anywhere near our aspirational design target - EnerPHit?

The five little radiators (2 more than last winter) are easily enough to heat the house up quickly after a week away and the washing dries in no time. Dealing with children in the middle of the night is less of a rude awakening now the night-time temperature is over 18 degrees C rather than below 12 degrees C.

And it has been even more comfortable than November and December 2012, when we had just moved back in. Back then, the house had stood empty for a year and had just been replastered throughout - so a fair bit of of drying out and heating up was required. This year, the summer warmth held in the walls meant that no heating was needed in October, and we put the heating on in November more to keep the walls up to temperature rather than because anyone felt less than warm.

What about the energy use?

November 2010 gas use (heating plus hot water and cooking) was over 2600kWh for the month. November 2012 dropped to ~1000 kWh for the month (heating and hot water only - cooking now electric), and November 2013 came in at 475 kWh.

So in terms of space heating, we used 3 times more gas in the first November after the refurb compared with the second. Perhaps not surprising, but more significant than I had imagined.

And what about EnerPHit?

Taking total gas use for 2013 (well, 21st December 2012-13) and subtracting the hot water component gives us about 3400 kWh space heating for the year - or 27 kWh / m2 / yr for our 125m2 house.

(Using the typical method of measuring floor area, the house is 140m2, but for Passive House calculations a different method is used, hence the lower figure of 125m2).

So we are somewhere close to the EnerPHit target of 25 kWh / m2 / yr.

However, it must be said - we don't run at 21 degrees C day and night (we like the bedrooms at 18-19 degrees at night, and the downstairs at 20 degrees in the day). That said, the first few months of 2013 were colder than the average winter, and it was still our first heating season post-refurb.

And the building fabric? While the insulation and cold bridge detailing were built very much as per the spec, the air tightness is not below 1 ACH at 50 Pa at the moment. At the end of the build it stood at 1.08 ACH in spite of a very leaky door seal, subsequently resolved. However, 12 months of DIY have taken their toll and we now have some work to do fixing the leaks created by "him indoors"!

Even so, it's high on comfort and low on energy use. And it is very cozy at Christmas - having family and friends round is not only fun and sociable, but also keeps the house very toasty without any heating thanks to the increased occupancy!

Are super-insulated homes too hot in summer?

It depends....

With various newspaper articles recently on the subject of well-insulated homes overheating in hot weather, I thought it was time to reflect on how my superinsulated home responds to high outside temperatures.

The short answer for my house is "no". We've had some very hot weather during July and August 2013 but inside the house I've not seen a thermometer show higher than 25 degrees C, and usually less than that.

This is not a big surprise. Passive House design means that overheating must be considered, and the temperature should generally remain below 25 degrees C.

The well-insulated roof and walls help to keep heat out of course, and we can add to this effect by closing curtains as the sun moves round the house if we wish. However, our secret weapon is the thermal mass - the solid brick walls, covered internally with a dense render before the plaster skim, soak up heat when the air temperature is high, and release it in the evening when the air cools.

We can maximise this effect in the evening by opening patio doors downstairs to bring cool air in, and opening the attic windows to allow warm air to rise and escape (drawing more cool air in). This leaves the house at a cooler temperature for the night and into the next day.

Even if we keep the windows shut to keep the hot air out, the ventilation system with heat recovery does bring warm air in from the outside. However, when outgoing air is cooler than the incoming air, the heat recovery unit will to help cool the air brought in from the outside until indoor and outside air reach the same temperature.

In contrast, many modern cavity wall insulated houses are not designed with overheating in mind. All sorts of elements can contribute to overheating. Large areas of glazing, particularly if in a sloping roof can allow excessive solar gain. Loft rooms are often not that well insulated, turning them into an oven on hot sunny days. Theoretically, the inner skin of brickwork could provide thermal mass to soak up the heat, but it is commonly of lightweight blockwork or is covered in plasterboard (which prevents heat from the air from being absorbed by the brickwork behind it).

So, as usual, the devil is in the detail. A lot can be done to reduce overheating even on a limited budget. Fancy automated external blinds may be the ideal, but careful use of curtains (e.g. leaving them shut before going out for the day) can make a significant difference. Likewise opening windows on the cooler side of the house / at cooler times of day is worth doing, with or without a ventilation system.

Climate scientists are predicting a greater frequency of heatwaves in future years, so it is worth keeping overheating in mind when buying or improving a home...

Thermal imaging gives the thumbs up!

For the last few weeks, I've been waiting impatiently for cold weather! Why? For the thermal imaging....

When the temperature is cold outside and the house is warm on the inside, any heat escaping through gaps in the building fabric can be seen by a thermal imaging camera. This shows up as hot spots (when viewed from the outside) or as cold spots (when viewed from the inside).

David Hill of Carbon Legacy visited with his thermal imaging camera and we went round the whole house looking for cold spots. We examined window reveals, door reveals and thresholds, wall-roof junctions, and every possible weak point we could think of.

To my huge relief, the results were very good.
- The insulation of walls, floor and roofs is clearly doing its job.
- Junctions between walls and roofs or walls and floors all looked pretty good.
- The window reveals and window sills were my main area of worry, but they were all very respectable too - the worst showed about a 2 degree difference between the cold spot at the corner of the window sill and the adjacent wall, the others were all below 1 degree difference.
- The windows themselves were very good (as you might expect of Green Building Store's triple glazed windows with insulation within the frames and Fakro's quadruple glazed roof-light).
- Where the air tightness work around the front door has not been completed yet, this showed a few cold patches - no surprises there.

Getting such a good result depends on a number of factors, for example: good detailing by the architect (thank you Gil!), the right products (e.g. pro-Clima tapes and membranes), a good builder (McCane Construction), quantity and quality of external wall insulation (Westville using the Permarock system) and the watchful eye of the insulation and air-tightness champion (me).

Of course, the ultimate measure of success for a low energy home is the low energy bill! But first the final bits and pieces have to be completed and then I'll want to see 12 monthly readings for gas and electricity (to compare with the pre-refurbishment readings). However, the signs so far are promising... gas use in December 2012 was a quarter of what it was in December 2010, in spite of the fact that the floor area has increased from 95 to 140 m2.

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....

We have a roof!

The house is starting to look more like a house than a ruin now that it has a roof. The benefits of the insulation on the roof and dormer walls won't be fully appreciated until a few more windows have gone in!

Refurbishments based on passive house principles don't usually have roof lights, but we wanted a bit of extra light in the loft room. However, as roof lights go, the quadruple glazed Fakro rooflight with insulated flashing is probably as well insulated as it is possible to get. The challenge will be to make it completely air tight around the opening, and we plan to add extra insulation around the edge on the inside to minimise cold bridging.

The dormer is the first part of the external insulation work to be completed, and it's looking very smart. It took a bit of coordination and thought to get the vertical insulation to meet the roof insulation all round (battens etc. can potentially get in the way), but we think we've got it well and truly covered!