Actually, it seems as if we’ve pretty much skipped spring and gone straight to summer. We had a week away, and got back in the evening on Easter Saturday to find the living room at almost 23 deg C, and the thermal store at 68 deg C! Upstairs, our max/ min thermometer showed the temperature had been up to 28, so obviously there had been some sunny days whilst we were away. We opened some windows and pretty soon the temperature was more comfortable, but it did make us wonder a bit about leaving the house for long periods in the summer – it isn’t really a serious problem, but it would be a pain to come back to melted candles!

Part of the issue may be that we had turned the MVHR down to its “unoccupied” setting, so there was less air circulation than normal. The MVHR has a “summer bypass” – which means that, when the extracted air exceeds a set temperature, it by-passes the heat exchanger, and just brings cold air into the house – so maybe, during the warmer seasons, we should just leave the MVHR running as usual and take advantage of its cooling potential. And maybe we should store the candles somewhere cooler.

And this has made me think a bit about the way we feel temperature. The air temperature in our caravan was frequently above 23 deg C, and it never really felt uncomfortable, in fact there were times when it still felt quite cold at that temperature. On the other hand, the house feels pleasant at around 19, which would certainly have been too cold to sit around in the caravan.

There are three factors at work here. Air temperature is important, and easy to measure, but the temperature of the surfaces, and the velocity of moving air, are also significant. By “surfaces” I mean mainly the walls, windows and floors, all of which radiate, so just as it can feel warm in sunshine on a cold day, it can feel cold in warm air if the house itself is cold. This is a big problem in solid-wall houses, and also a problem with big, cold, windows. The point about air movement is maybe more obvious, it’s the same effect as blowing on your soup to cool it down. But both the temperature of the surfaces and the air movement are major issues in Passive House construction, and as a result we are comfortable at slightly lower temperatures than we had expected. Maybe there’s also a lesson here for non-Passive houses (active houses?) – cheaper and better glazing has made big windows increasingly popular, but not only do they lose more heat relative to their area than the walls, the internal temperature is also lower. I’m beginning to think the normal standard for all new houses should be triple glazing – the extra cost is now quite modest, and the benefits are greater than might be expected.

Finally, a while back I touched on the embedded energy of construction. Traditionally, this wasn’t that big an issue, the operational energy needed to heat houses was much greater than the energy used to build them. But as we reduce the operational energy load, the construction energy becomes proportionally higher, and we felt there were probably things we could have done to use lower-energy materials. One example is the insulation materials – we used mainly mineral wool in the walls and roof, and polyisocyanurate (aka PIR, aka Celotex/ Kingspan) under the floor. These are all pretty good insulators, but some of the more natural products, especially those based on wood fibre or cellulose, have absorbed CO2 as they grew, so they help reduce greenhouse gases twice, once when they are growing and again by reducing heating needs.

So I was interested to hear about a Passive House in Fife which was built using Warmcell insulation in the walls and roof – see Warmcell is made from cellulose – mainly recycled paper, so it’s one of the materials that locks away CO2. The grade of Warmcell they used had a thermal conductivity of 0.04 Wm2/K, compared with 0.032 for our mineral wool – so I suspect it would only be realistic with a “blockier” shape of house than ours. Anyway, it just goes to show, there are always ways to do things better!



I’ve been waiting for the really cold weather to write this post, but the Passive House winter already seems to be drawing to a close, so (tempting providence?) here goes.

The 2013/ 14 winter has been wet and windy in the Highlands – nothing like as wet as in the rest of the UK though, we’ve very rarely had standing water around us, and then only for a day or so. It hasn’t been very cold, but then neither have there been any intervals of really mild weather, generally day-time temperatures have been consistently around freezing to 5 deg C. I’m not quite sure how that affects the average household’s fuel use, there’s a trade-off between the high winds and mild temperatures. On balance though, I suspect most folk will have lower fuel bills than in a normal winter.

The Passive House experience has been similar. Wind and cold still have an impact on us, although far less than for most houses. Levels of direct sunlight affect us much more though.

We’ve had a few short breaks away from the house during the winter which illustrate the impact of the sun. The first, over hogmanay, meant the house was without any active heating for 4 days. We left it with an internal temperature of 19 C, and when we returned it was 17 C – much to the surprise of the guests who came back with us. On reflection I think there must have been some sunshine during the period, as the second time we were away for a similar period, and the temperature on our return was 13 C. This is still quite encouraging – one of the nagging doubts we had was whether we needed some kind of frost protection, and based on these experiences we think the only way water would freeze in the house would be if we were away and external temperatures were sub-zero for a prolonged period, with no sunny spells. This seems pretty unlikely, really low temperatures generally come with clear weather.

Apart from these few spells away, we’ve been in and around the house pretty much all winter (we both work from home) and have fallen into a routine where we light the stove in the late afternoon or evening, and burn it quite hard for a couple of hours. That cheers up the living room a bit in the evening, and charges the thermal store, which in turn gives us space heating and hot water until we light the stove again the next day. This is all very comfortable, but the worry has always been, just how much firewood would we get through? And as the pile in the log shed has diminished, when will the heating season end?

Well, this may be premature, but this week (mid-February) we had our first day when we didn’t need to light the stove. And then the next day, the internal temperature was still very comfortable, but we lit the stove briefly to provide some hot water. Could this be the end of the Passive House winter? If it is, it can only be because of the lengthening days, the temperature outside is still under 5 C, and it’s still windy. We’re still keeping our fingers crossed (maybe not so tightly!) but so far we’ve only used about 2/3 of the firewood store.

It’s interesting to wonder how well our approach would suit people living a commuter life, with a more traditional need for heat first thing in the morning and again in the evening. I think for most people the temperature would still be very comfortable when they got in from work, but the fire would need lighting reasonably soon to top up the thermal store and give some hot water. It would still be nice and warm in the morning, and there would be plenty of hot water for a shower. And, of course, if the hassle of lighting the stove was too much, people could always install a gas or oil boiler instead of the log stove, they’d still get the benefit of tiny bills.

This is a question we’ve been asked quite a lot – and one we’ve been asking ourselves! We thought we knew the answer, but in last week we were able to test things out. We don’t use a lot of electricity (about 6 – 7 kWh/ day) but we use it for quite a lot of important functions. Some of these are common to most folk, particularly where there’s no mains gas – our cooking is all electric, we need electric pumps to run the heating system, and of course we use electricity for lighting, computing and all the gadgets associated with modern life. But, unlike most households, we also use a small amount of electricity for the ventilation system.

One of the late changes to the design of the house was to position the stove immediately below the thermal store. The hot water from the stove’s back boiler is pumped to the store under normal circumstances, but we wanted to retain the option to use the stove if there was a power cut, so we figured if we put it directly under the store, then convection would transport the hot water to the store even if the pump wasn’t working.

Cooking is a bit more difficult. Most wood-burning stoves get hot enough to boil a kettle, but we had to chose one that puts very little heat into the room. In fact, even with the stove roaring, the casing is no more than warm to the touch. So our options are limited to either using a camping stove or lighting the more conventional log stoves in the workshop.

Non-electric lighting is provided by candles and torches, like anyone else, and the other gadgets have to be seen as non-essential. But the ventilation system was always a bit of a nagging concern. Sure, if necessary, we could always open a window, but that isn’t ideal in the middle of winter. And, of course, the ventilation system is our main heat distribution mechanism.

Anyway, at some stage on Wednesday night the power cut out. We get short power cuts from time to time, but this time the BBC was reporting up to 150,000 homes cut off due to high winds, so it sounded as if we could be without power for a while. And in fact, it was 8.00 p.m. before the lights came back on. Soon after sunrise it started to snow, and the temperature dropped as the day went on. Nevertheless, the house was reasonably warm to start with, and it wasn’t until mid-afternoon that the temperature dropped below 19 deg C, and we felt the need to light the stove.

That part worked quite well. We needed a bit of a change of attitude though – normally we burn the stove very hot and for quite short periods – so we get the most efficient use of fuel, and the cleanest flue emissions. Now though, we actually wanted the heat in the room rather than the thermal store, and in any case, using convection to transmit heat to the thermal store was going to be less effective than the normal pump. So, we damped down the stove a bit and closed the hall door to keep the heat in. The result – a nice warm living room, and the thermal store was easily hot enough for showers. In fact, the rest of the house was still at about 18 deg C by the time the electricity came back on.

The other big success was that we didn’t really notice the lack of a ventilation system, maybe just opening the door a few times to feed the hens, etc. was enough to introduce a bit of fresh air.

We did, however, notice the fumes from the camping stove, and I wondered whether it would have been safer to use the stove outdoors (not very enticing though!). And a camping stove is fine for camping, but somehow it felt a bit spartan using it in the house. Still, we managed to produce hot drinks, a hot meal, and a hot thermos flask for one of the neighbours, so I think even the cooking was a qualified success.

Overall, we were pleased to find that the house is habitable for a day without electricity – although the feelings of withdrawal from the lack of phone and computer are another matter! – and, actually, not wholly unpleasant.

The summer seemed to go on for ever, we were wearing shorts well into September, but in the last month or so autumn has finally arrived, with day-time temperatures averaging about 10 degrees C, occasional frosts and even some snow on the hills. So here are some reflections on living in the house now it’s a bit cooler outside.

We now seem to be lighting the stove for an hour or two, probably two or three times a week. So far, that’s mainly been driven by the need for hot water, rather than for space heating – but of course, just lighting the stove does heat the space a bit, so the two can’t be completely separated. We haven’t yet felt the need to use the heated towel rails, or the post-heater on the MVHR unit, which is actually just as well, as we’ve discovered a leak in that plumbing circuit.

There’s no obvious correlation between the interior and exterior temperatures, inside it continues to be around 20 to 22 deg C, irrespective of the temperature outside. Last weekend we were away and fully expected to find that the house had cooled down a bit after two days with no cooking and no body heat, but it was still 20 C when we got back. So, we swan around inside in T-shirts, even when there’s a frost outside. As you can imagine, this is a very satisfactory experience!

The one thing that does make a noticeable difference to the temperature is the presence of guests in the house. I suppose that shouldn’t be surprising, each body generates about 100 W of heat, so if the house is comfortable with just 2 of us, having 3 guests staying over means opening windows to stay cool. But even though it was what we expected, it still seems a bit strange. One effect has been that we’ve delayed the house warming party until the weather is a bit colder!

One minor source of irritation is that the thermal store doesn’t seem to store heat as well as I had expected. Partly I think that’s because the insulation of the pipework in the plant-room isn’t up to the standard of the store itself, and all the bends where pipework is attached to the store are uninsulated. So far I haven’t been motivated to sort that out, most of our hot water has been provided by the solar panels, so the loss of heat didn’t make much difference to us. Now, however, the solar panels only really give a bit of pre-heating most of the time, so I ought to try to improve things.

On the other hand, the PV array is doing better than we had thought, in the first 12 months since it was connected the output was about 10% higher than the forecast provided by the installers.

We also had an export meter installed in order to keep an eye on how much electricity we’re using. This is a topic which is rarely discussed, for some reason! Most houses now have digital electric meters, which means they measure the electricity imported. Once a PV array is added, the electricity imported is no longer the same as the electricity used, because some of the electricity generated by the PV is used on-site. The PV array comes with a generation meter, but that still doesn’t let you work out consumption – to put it into a formula:

Usage = imported + generated – exported

Most households with PV know the figures for imports and generation, but not exports, and consequently not usage. There is a financial implication – the householder is paid for 50% of the electricity generated, on the assumption that half is used locally and half exported to the grid, because apparently it’s uneconomic to measure the amount actually exported. This seems very odd to me – I got a reconditioned kWh meter for £15, and it took an electrician a few minutes to wire it in. The electricity companies say that to meet OFGEM requirements for billing, there would have to be an expensive maintenance contract on the export meter, but I can’t remember anyone ever doing maintenance on our import meter, and these days no-one even comes to read it. So I don’t really understand why there’s such a resistance to installing export meters. But hang on, now we have the meter we see that we only use about 15% of the electricity we generate, so we’re exporting 85% and being paid for 50%. Could that be the real reason no-one wants to talk about export meters?

And I almost forgot, the extra meter tells us we are using about 6 kWh/ day for the fridge/ freezer, cooking, computing, lights, and of course the woodworking machines in the workshop. This is about the amount we expected, and between half and two thirds of the amount we generate.

We’ve been living in the house for just over a month now, which has coincided with an unusually long, settled spell of hot weather – virtually unheard-of in the Highlands, and certainly not what we had in mind when we were designing the house.

When we first moved in, we played around with the ventilation system for a few days. There’s a summer by-pass, meaning that when the extract air gets above a set temperature, the heat exchanger is by-passed, so the incoming air is no longer pre-heated. We quickly found that, even with the by-pass temperature reduced from the factory setting, we were too hot at night, so we simply switched off the MVHR and opened some windows. Once we’d done that, we found we could keep the temperature comfortable by opening the windows wider on particularly hot evenings and closing them a bit when it was a bit less hot. Just like most householders, in fact, so not exactly revolutionary.

It did get me thinking though, that a number of friends have said they couldn’t countenance living in an airtight house, and some have commented they don’t like the idea of a mechanical ventilation system because they like to sleep with a window open. And one of our builders asked me why we had so many opening windows when we have a mechanical ventilation system. By now it will be pretty clear where I’m heading with this post – the house works perfectly well with the MVHR switched off and the windows open, in fact the Passivhaus Institut encourages designers to put opening windows in all rooms, precisely so that the MVHR can be switched off when it isn’t needed, saving some electricity. This may not be a good enough answer for the die-hards who like their bedroom windows open in the depths of winter – at that point, you’d have to question whether a MVHR was a sensible investment! But at least, with the structure of the building airtight, we can decide whether or not we want draughts, and chose to have them on hot, sultry nights, but not when there’s a northerly gale piling snow against the windows.

Maybe the word “airtight” is a big part of the problem, it certainly doesn’t sound all that inviting. I read an interesting comment by the architect of a social housing Passive House development that he had found tenants more receptive to phrases like “less leaky”, so maybe that’s something for the Passive House community to take on board. I also think people generally have no idea of how little ventilation is needed – we have had former GPs asking if we’d die of asphyxiation if there was a power cut during the night (the answer is that the atmosphere in the morning may be slightly stuffy, but we certainly won’t have pegged out!).

Apart from the revelation that window opening works, the big success over the last few weeks has been the solar thermal system. Intellectually I find this quite hard to justify on cost grounds – the system might cost £5,000, or more to retrofit, and it saves about 50p/ day of electricity for maybe 5 months of the year – so the payback is way beyond the life of the equipment. We justified it on the basis that £5,000 is a drop in the ocean of new house construction, but that’s hardly scientific. There is, of course the possibility of claiming the Renewable Heat Premium Payment, and the Renewable Heat Incentive (in due course, if the Government doesn’t spend all the money on fracking in the meantime). On the other hand, there is a great joy in hearing the solar thermal pump kicking in at 9.00 on a sunny morning, and even more joy in knowing, in the evening, that we have 750 litres of water at 60 degrees C, far more than we could use before the sun comes up again.

“Finishing” is a funny word. What we originally envisaged when we thought of a finished house is probably years away – the mature hedges and garden will take ages. At the stage we move in, even the house will still need curtains, lampshades, some additional furniture, and no doubt other bits and pieces. But for us at the moment, “finishing” means getting everything to a state where we can comfortably move in. That’s still quite an imprecise definition – for instance, we feel we really need bathroom doors, but we could do without locks on them for a while if need be.

Finishing individual bits of work has a fractal nature – the closer you get to it, the more you see that needs done, so it really is a matter of judgement when something’s finished, . We had great celebrations when we finished the staircase, weeks ago – but we only just added the bottom step, and there are still some plugs needed to hide the screw holes under the bannister.

"Finished" staircase

“Finished” staircase

But for our current purposes, that’s finished.

And finishing refers to the last part of the journey, not just the arrival at the destination. We feel as if we’ve been finishing since the beginning of January, working pretty much full-time on the staircase, internal doors, floors, and of course the bathroom.

A bath with a view

A bath with a view

Wash basin and unfinshed WC

Wash basin and unfinshed WC

Which itself is not quite finished.

In woodworking terms, “finishing” refers to the long process of scraping, sanding and oiling to give the best possible finish (there’s no getting away from the word!). Connoisseurs may be able to see that we’re mostly working with Scottish hardwoods – meaning on the positive side, loads of character and attractivefigure, but on the negative side, grain running in all directions, making it particularly difficult to get a good finish. So finishing means hours of tedious, noisy work.



...and scraping

…and scraping

And a bit like the other sort of finishing, after a few days sanding doors it’s very tempting to re-define what exactly is finished!

Anyway, the main point of this post was just  to show that we haven’t been idle during the last few months when I haven’t been keeping the blog up-to-date. Tim has also been busy over the winter on stonework around the base of the building, steps down from the doors, and the front path – complicated by needing to be wheelchair-accessible to meet building regulations.

House showing the front path

House showing the front path and stonework below cladding

Back of house showing stone steps and cobble path

Back of house showing stone steps and cobble path

In fact, we’ve almost finished!

The structure of the house has been complete now for quite a few months – although we still don’t quite have all the insulation installed, and the house still isn’t occupied. Nevertheless we’ve been building up a rough idea of how the house seems to react in different conditions.

Towards the end of March we spotted a week where the forecast was for day-time temperatures around zero C, and took the opportunity to run the stove and heating system for a few days to check whether everything was working. It took a day or so before the structure of the building was at an ambient temperature, but once there we found that, on a day with sunny intervals and outside temperature hovering around zero, and the stove not lit, the house actually increased in temperature! Once the sun went down the internal air temperature was still maintained, but the thermal store cooled as it pumped heat into the ventilation system. The next day was overcast and still cold, and we lit the stove in the morning, letting it go out early in the afternoon. That was enough to re-charge the thermal store. So, the conclusion from this imperfect experiment was that we should be able to heat the house comfortably by running the wood-burning stove for a few hours a day, even on a cold, cloudy winter day.

Something else we discovered from this was that with the heating system running, it’s impossible to maintain any significant temperature difference between the top and bottom of the thermal store. This was a bit alarming at first, as the conventional wisdom says the heat should go in at the top, and cold water at the bottom, so the store is thermally “stratified”. But with our system, the temperature lost in the heating circuit is so modest that the water coming back to the store, after heating the air in the ventilation system (and the heated towel rails, mustn’t forget those), is almost as hot as it was when it left the store – so any stratification there may have been is almost immediately lost when the heating system is switched on. That means there’s a danger that we could have a fair amount of heat in the thermal store, but in the form of a lot of luke-warm water, i.e. not much use for anything. We’re going to see if we can tweak things to reduce this problem, but we may just have to accept that, in winter, we need to keep the whole store at a temperature of 60 C or more most of the time – much lower than this and the heating system is ineffective. This would make the useful capacity of the thermal store quite a lot less than if we could keep the top at 60 C whilst the bottom gradually cools. Another issue with the loss of stratification is that it would pretty much rule out the use of the solar thermal panels during the winter – the panels need to be hotter than the bottom of the store, so if the bottom of the store is never cold, the panels never work. We never expected to get much contribution from the solar thermal in winter, but nevertheless, it would be a shame.

More recently (i.e. May – but one of the coldest Mays anyone around here can remember, we’ve had snow, hail and frost) the house seems to keep its internal temperature fairly consistently at about 19 C. Considering how well it worked on a sunny day in winter, it seems surprising the house doesn’t over-heat now. I think it’s partly due to the height of the sun and the thickness of the walls – the windows are set back further from the outside wall than most modern houses, which means when the sun is high (i.e. in summer) the glazing is partly shaded by the external wall. In winter, on the other hand, the sun is lower and shines more directly through the windows.

In this summer regime, where we don’t need to use the heating, the thermal store is used just for hot water. Mains water runs around a coil inside the store, going in cold at the bottom and coming out hot at the top. This means the bottom of the store is cooled faster than the top, so (a) we get hot water for longer, and (b) we have cold fluid from the bottom of the store to send round the solar thermal panel. Most days this month the solar thermal system has provided enough water for a couple of showers; on the odd cold, dull day, when there wasn’t enough sun for a shower, we lit the stove for an hour or so to give loads of hot water.

So the lessons we’ve learnt so far are:

1. Direct sunshine makes a huge difference to the house – both to space heating in the winter, and to hot water in the summer.

2. We’re still confident the house will need hardly any heat, and the decision not to include radiators or under-floor heating is looking like a good one.

3. We need to play around with the heating a bit. We think, if we can increase the temperature drop around the heating circuit, that will improve the stratification when in winter mode. But it’s going to be difficult to know if the tinkering has worked until winter!

And I suppose the answer to the question in the title is, “Yes, it definitely works, but we’re still not sure quite how well.” Sorry it isn’t exactly headline-grabbing, but we’re very happy with the way things are looking.