Technical Introduction

 

Whenever we have talked to people about what sort of house we were going to build most people have asked if it was going to be an ‘eco-house’, a phrase often bandied about on programmes such as Grand Designs. The reply of course was yes! But what exactly is an ‘eco-house’? Well Wikipedia says this: 

 

An Eco-house (or eco-home) is an environmentally low-impact home designed and built using materials and technology that reduces its carbon footprint and lowers its energy needs.

 

And it turns out that we have been lying a bit because in reality we need to own up to the fact that this is not what we are building because, whilst we will try to keep the construction materials as environmentally friendly as possible, we have to be realistic and we cannot get away from the fact that we are going to use rather a lot of concrete which can never be described as environmentally friendly. I will go into more detail about our method of construction and rationale for this later.

 

So what are we building? Well we are building a house that, once built, will have not just a low carbon footprint but hopefully a negative one so that we can make amends for the concrete used in construction. In order to do this we plan to build a Passivhaus.

 

What is Passivhaus?

 

Passivhaus is a German voluntary standard for energy efficiency in a building developed by a Swedish chap called Bo Adamson and a German called Wolfgang Feist who together looked at the science of thermal efficiency in buildings and worked out the best way to build them. There is a lot of very boring mathematics and a very complicated spreadsheet into which all the thermal properties of everything used to build the house is input. This throws out two figures - one for the total annual heating requirement (heat demand) and a second number for how much heating is required to maintain the house at a comfortable temperature (heat load). Those figures are quoted per square metre of building and, if you really want to know, to qualify as a Passivhaus they must be equal to or less than 15kWh/m2 and 10W/m2 respectively. There is one other figure you need to know which relates to air leakage in the house (i.e. draughts!) which says that the air leakage should be less than 0.6 x the volume of the house per hour when the wind outside is blowing at 20mph (Force 4/5). To put all that into context the heat demand for an average UK house is more like 150kWh/m2 with air leakage of 20 x house volume per hour.

 

The attention grabbing statement is that a house built to Passivhaus standards needs no central heating and all its heat demand can be satisfied from heat given off by electric appliances and the occupants of the house. This statement is not entirely true but it’s not far off and it’s safe to say that heating bills for a house built to Passivhaus standards are about a tenth of those for a normal house.

 

So how do you make a house energy efficient? 

 

Well that’s obvious isn’t it? Insulation! Well that’s what I thought but it’s only partly true, the first priority is not insulation but airtightness. Consider this: My mate Pete is a cyclist who feels the cold, so what better than a pair of tight-fitting cycling shorts made from a yard of merino wool? On the other hand my mate Brian is a Scotsman who is proud to wear his kilt in the traditional manner, so what better way to keep warm than a kilt of the same merino wool. As I’m sure you know there are about 8 yards of cloth in a kilt, i.e. 8 times the insulation of a pair of cycling shorts. So in a howling, cold northerly wind which one will end up with the frozen assets? Despite having far less insulation I think Pete would be by far the happier of the two and that is because cooling due to air movement far outweighs heat retention from insulation.

 

Movement of Air

 

One of the most surprising facts that I’ve learnt from looking into Passivhaus construction that illustrates the importance of controlling air movement is that in an average house, if you add up all the ‘holes’ in the fabric of the house, whether it be from badly sealed windows, air bricks, letter boxes, cat flaps or just poorly mortared brickwork or unplastered concrete blocks (concrete blocks are not airtight), then you will have a hole about a metre square. If you add up the holes in a certified Passivhaus you will have a hole about the size of a credit card.

 

Consider the number of hourly air changes in an average house which I said was about 20 per hour? What does that mean for the house? Well the good thing is you have a house with lots of fresh air! But from a heating point of view it’s not so good because what it means is that you put your heating on to heat the air in your house to 21ºC and every 3 minutes that air is replaced by air from outside which might be at 1ºC which you then have to heat again. Your radiators are constantly chucking out heat which is constantly being lost to the outside. With a Passivhaus only having 0.6 air changes per hour those 3 minutes become 100 minutes so the heating requirement is much less, and that is without insulation!

 

There is a second aspect of air movement that has nothing to do with leaks but still contributes to the heating requirement and that is down to convection currents. What does that mean? Well, in short, hot air rises and cold air falls so anywhere where there is a particularly warm place or a particularly cold place there will be air movement and in a normal house that means radiators and windows. Just the movement of air, even if it is at room temperature, will cause cooling - if you blow on your skin your breath is a warm 37ºC but it makes your skin feel cool. The bigger the difference in temperature the bigger the air movement and the colder it will feel. This brings me on to windows and the rather contentious subject of triple glazing. There is a school of thought that says that triple glazing is a waste of money because if you compare cost savings purely on the insulation properties of triple to that of double it would take many years to recoup the extra outlay.

 

But it’s not that simple, not because of insulation but because of air movement. People find draughts uncomfortable and if you sit in a room with a large window in cold weather you will feel a draught because of the convection currents set up by the cold surface of the window compared to the air temperature of the room. Studies have shown that if you have an area of a room with a temperature that differs from the normal ambient temperature by more than 3 degrees you will feel thermal discomfort and you are likely to turn the heating thermostat up 2 degrees in order to compensate for the ‘draught’. So what has that got to do with triple glazing? Well if you have a single glazed window and the outside air temperature is at 1ºC then the inner surface of the window, in a room heated to 21ºC, will be only about 5ºC, leading to a cold ‘draught’. With modern double glazing the inner surface may be as high as 15ºC but this is still lower than the ambient temperature of the room and will still produce a 'draught'. The big advantage of triple glazing is that the inner pane is almost the same temperature as the air in the room therefore no draught, meaning the ambient temperature can be lower and the room will feel much more comfortable. I’m sure we all know how cold the glass on the inside of a window normally is but if you get the chance feel the glass on the inside of a triple glazed window, it’s quite remarkable.

 

So airtightness and movement of air within a house are very important to ensure good thermal efficiency of a house but there is an elephant in the room, namely suffocation. It’s all well and good having an airtight house, you’ll be lovely and warm for a few hours before you collapse from lack of oxygen. The question is how do you get fresh air into the house without losing all the heat? That’s where the saviour of Passivhaus comes in, in the shape of a thing catchily called Mechanical Ventilation with Heat Recovery, or MVHR for short. MVHR is basically a big extractor fan which, through a network of ducts, sucks air out of rooms where the air is likely to be stale, such as kitchens and bathrooms, and blows fresh, filtered air from outside into the other rooms. The really clever thing about it though is that using a passive heat exchanger the fresh air that is brought in to the house is almost the same temperature as the stale air extracted. These units can operate at 96% efficiency so hardly any of the heat from the house is lost to the outside. So there it is a constant, plentiful supply of warm fresh air giving the house a superb level of air quality. It is important to stress that this is not air conditioning, it is just warmed, filtered, fresh air. You may say that surely MVHR causes draughts if it is constantly blowing air into the house but it operates at a low level and only changes the air more like once every 3 hours not once every 3 minutes so the air movement is hardly noticeable.

 

Insulation

 

We have seen that air movement acts in two ways to increase heating bills by sucking air out of the building through holes and by induced draughts around cold surfaces, but insulation also has a major role to play.

 

Whatever sort of heating a house has, be it traditional radiators, log burning stove, storage heaters or underfloor heating, in the main they all do the same thing in that they heat the air in the room and that air in turn heats the contents of the room along with walls, floors, doors and windows. What happens to the heat absorbed by the fabric of the room rather depends on what it is made of. Glass, plaster and metal are very good conductors of heat and the heat absorbed will be conducted away quickly to anywhere where the temperature is colder. Wood and fabrics are poor conductors so they won’t absorb much heat and will not conduct it away. Any part of a house that allows heat to be conducted from the inside direct to outside (termed a cold bridge) needs to be insulated. For a Passivhaus this means that the entire envelope of the house has to be a continuous wrap of insulation. Walls, doors, windows, floor and roof must all have laid down minimum insulating properties with no breaks or gaps at the intersections. And not just any insulation, size does matter. Passivhaus insulation is likely to be double what is found in a modern house and 4x that which UK building regulations required just 10 years ago. To be fair though UK building regulations are slowly getting the idea and the minimum amount of rigid insulation to meet their current standard is now (in January 2020) 100mm for a standard brick/block cavity wall. However, houses built to Passivhaus standard using brick/block cavity construction tend not to use rigid insulation and for a very good reason, that being our old friend air movement. Remember the cycling shorts and kilt? In order for the insulation to be effective it must not allow any movement of air between it and the substrate it is meant to be insulating, even small gaps will make the insulation ineffective. The big problem for cavity wall construction are things called ‘snots’, chunks of mortar that squeeze out of the blockwork as it is being built and either fall into the cavity or set bulging out from the joint, making it almost impossible to install the insulation tight up against the blockwork. This means inevitably that there will be movement of air between the blockwork and the insulation with a corresponding detrimental effect on the insulation. For this reason Passivhaus insulation in this type of construction is more likely to be a natural fibre/rockwool type or full-fill blown insulation which will stop air movement in the cavity and therefore be more effective. It may be that rigid insulation has better thermal qualities but unless it is installed very carefully it is unlikely to be as effective as fibre or blown insulation. As a result Passivhaus cavities are larger and usually incorporate between 200mm and 300mm of insulation.

 

That deals with the walls but, as I said, the insulation envelope must be uninterrupted and in fact the required insulating properties of the floor pad and roof must be at least the same standard as the walls and of course triple glazing is a must!

 

That completes a broad introduction to the type of house we are planning to build. Next time I will go into more details on the construction method that we have chosen and why.

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