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...

Warmest Winter in Years!

This winter has seemed never-ending, but I have to say that braving the cold outside has been much more pleasant when not also having to brave the cold inside.

And now that it finally feels like spring, it is very comfortable indeed indoors. The heating has not been on at all for weeks, and the temperature sits at a balmy 20 degrees (+/-1). The sun comes in through the windows and the warmth stays with us for the evening and through to the next morning. The chill blast that awaited on emerging from the duvet is a thing of the past. It is most civilised.

Without draughts and cold spots, 20 degrees (or even 19) feels a whole lot warmer than it used to before the house was refurbished. I can remember sitting huddled in the warmest room at about 22 degrees and feeling just about ok until I moved. Now the whole house just seems to find its own temperature and it never feels cold.

My conclusion is that the Passivhaus methodology (for new build and refurbishment) definitely works. The house is behaving as it was designed to do in terms of comfort. Energy readings and temperature measurements so far back this up (but a year or so of further measurements will give a clearer picture).

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.

It's cold outside...

... but fortunately seems to be ok on the inside even when the temperature drops to -5 on the outside.

Strange but true - at 19 degrees C, the house feels warm (whereas it felt much less comfortable at the same temperature before the insulation and draught-proofing).

The plaster is still drying out and the brickwork is still warming up after a year of exposure to the elements, so the gas boiler is working harder than it will need to next year, all other things being equal.

The 3 tiny radiators are not really enough to keep every room at 19 or 20 degrees C under these circumstances, but we are happy with bedrooms dropping to 17 or 18 degrees C at the coldest point in the night. (It beats the 8-12 degrees C that was the midwinter norm in 2010 and 2011).

The other striking difference for me is the complete absence of joint pain, which was a permanent feature for at least 4 months through previous winters. I had a hunch it was down to cold and damp in the house and I'm really delighted to experience such a spectacular difference this year. (Of course, for those who haven't got an extensive collection of injured joints, it won't make much difference).

The energy bills will also have a story to tell, but I don't have enough measurements yet (and there are further efficiency improvements to make anyway before the house reaches its "as designed" state).

One thing you do have to get used to is condensation or ice on the outside of the windows (because heat loss through the triple glazing is so low that the ice can still be found on the outside of the windows for hours - until the sun melts it off). I'm not complaining - so much nicer than having the ice on the inside as was the case for days on end in winter 2010 and 2011.

Moving in!

Finally, we have moved back in! There is still a fairly long list of things yet to be done (e.g. installing a kitchen) but great progress has been made in the plumbing department since moving in 2 weeks ago.

So what's it like? Thermally, it is fantastic - warm, comfortable, the temperature is stable, no hot and cold spots, no draughts. The thermometer sits around 19-20 degrees all the time. It is a completely different house from the one we moved out of a year ago.

We have 3 tiny radiators, which are currently on to help dry out the plaster, but we've had to turn some of them down or it gets too hot. So it looks as if the heating system is about right for the house. We'll know by the end of the winter if the system is adequate in really cold weather.

Since the MVHR was switched on, air quality has also been very pleasant (much better than having a window or two open, which did not promote air flow in every room).

It is still something of a DIY zone, but we are getting there. Not quite ready to commission the MVHR system and carry out the final air tightness test...

Open house visits in the Nottingham area

This house and other low energy new builds / refurbishments will open their doors to people who are interested during September.

For details, go to http://wbecohouses.co.uk/

Air tightness definitely heading in the right direction!

Now that the attic roof and the extension roof have been "taped and membraned" to the hilt, we've had an air tightness test to see whether we are anywhere near the EnerPHit target of 1 ACH @ 50 Pa.

The average of the de-pressurisation and pressurisation (sucking air out and blowing air in) values came out at 1.078 ACH @ 50 Pa, which is certainly cause for optimism! We are extremely grateful to our merry band of Eco House Group helpers who came along and tracked down draughts during de-pressurisation, or went round with tape and sealant blocking up the draughts identified. While some draughts are easy to spot and solve, it is truly amazing how some draughts can be felt, but no obvious hole or fault in the taping can be seen.

In the days following the test there were some worrying moments when the plasterboard went on as this can stretch or puncture the membrane if it is a little tight in places (almost inevitable in a complex roof structure like this one). However, improvements elsewhere cancelled out any small issues which we could not detect behind the plasterboard, so we are still on track.

As long as the remaining refurbishment work results in a few improvements and no mistakes, we should be able to get below the magic figure of 1. But success is by no means guaranteed so we'll be waiting until the very end to find out if we have made it.

For those more familiar with the other way of expressing air tightness (air permeability), Midland Energy Services has also provided that figure: 1.07 m³/h.m² @ 50 Pa.

Air Permeability and Air Changes per Hour do not necessarily match, but in a medium sized house like this one, they can do. For a small flat or a large hotel, the two figures would be very different.