Guest Blogger – Leonard Smit
Welcome back to the land of the super-insulated home! Hopefully you’ve read Part 1 where I introduced the key principles of the Passivhaus, including its genesis during the ’70s as the Super Insulated house. You have? Congratulations for getting this far, because – for a little while here, it gets harder.
In Part 2 we’re going to look a little closer at some of these principles to gain a sense of what they mean in practise, and why they are so important to your hip pocket.
Before doing that, let’s clear up some of the confusing terminology that unnecessarily complicates this topic. This is not an exhaustive list, nor necessarily entirely accurate. You will also see some of my world view creeping in so it’s quite likely that some may object to my interpretations. I can live with it as long as you can 🙂
Eco, Green, Environmentally Friendly, Sustainable:
These are vague, undefined terms. They can mean nothing or they can mean anything (marketing dream words). In many cases they are used to describe things that are blatantly the antithesis of what a reasonable person would think is meant … a green building in Las Vegas anyone?
A building with a heavy design focus on being kept warm by trapping the heat of the sun without using ‘active’ methods (such as fans or pumps). Passive solar designs are often accompanied by the use of prevailing winds for natural ventilation, a focus on thermal mass, lots of sun oriented glass, stack effect ventilation, etc.
Similar to Passive Solar, but focused on keeping the house cool, rather than warm. This is not a particularly well studied area and good information is difficult to find. Passive cooling is very difficult to do successfully, which is perhaps why it’s been a particular area of interest for me.
A building that generates the same amount of energy as it consumes. It is not necessarily an indicator of energy efficient design, but the two often go hand-in-hand. Many zero energy buildings are built to the Passivhaus Standard.
The Standard created by the Passivhaus Institut, based on a 40 year body of science and engineering.
The Passivhaus Standard includes relevant passive solar and passive cooling design elements such as the heat of the sun, the amount of glazing and its placement, shading, thermal mass, and earth tempered ventilation.
The confusion begins …… some English speaking countries that have adopted the Passivhaus Standard have decided to anglicise ‘Passivhaus’ into ‘Passive House’. But ‘Passive House’ is also quite widely used as the short name for ‘Passive Solar (House)’.
To avoid confusion, I’ll be using the term ‘Passivhaus’ in this article, even though I’m quite fond of ‘super insulated’ because of the richness of its scientific and engineering history.
Tech alert! This section is quite technical. If you prefer to skip it, here is the summary – if you want to build a thermally efficient house (whether Passivhaus or not), and your proposed builder doesn’t know what a thermal bridge is, or tells you they’re not important … find another builder. Seriously. Run. Don’t walk!
OK – still with me? A thermal bridge is exactly what it sounds like it is. Anything that conducts heat reasonably well and is placed in such a way that it connects (or partially connects) the interior of the building to the exterior is a convenient bridge for the heat army to go marching across – pesky little beggars. When it’s cool outdoors the heat leaks out, cooling the interior. When it’s warm outdoors the heat leaks in, warming the interior.
The most common thermal bridges are formed by metal or concrete items – some examples are some types of metal windows, steel items (bolts, nails, wire, brackets, reinforcing, etc.), concrete floors and metal pipes. But even things that we don’t normally think of as conducting heat can form a thermal bridge. So the wood framing of a wood framed building can be a thermal bridge because the wood conducts heat better than the rest of the wall.
Let’s head into the kitchen. Imagine a pea on a plate … now tip the plate slightly to one side; the pea will roll to the lower side. The more steeply the plate is tipped, the faster the pea rolls. The pea is the flow of heat and the angle of the plate is the difference in temperature between the indoors and the outdoors. The bigger the temperature difference between indoors and outdoors, the more heat will flow across a thermal bridge.
Put some gravy on the plate and the pea will roll slower for the same amount of tipping as with no gravy. That’s like a thermal bridge made of something (like wood) that conducts less heat.
So what’s the big deal? Most thermal bridges are such a tiny portion of the structure that they surely can’t have much effect … can they? Anyone remember the bit in Part 1 about physics completely ignoring intuition, perceptions, wishful thinking, politicians, lawyers and optimists? Thermal bridges are a great example. They have an effect vastly disproportionate to their size.
To illustrate, here is an example from our build. It is simplified, but it was a stark lesson in the effect of thermal bridges. The outputs are from a free program named THERM produced by the Lawrence Berkeley National Laboratory in the United States of America. (http://windows.lbl.gov/default.htm).
This is structural design at the roof to wall junction required to resist the large uplift forces created by the moderately high winds in the area (80 to 90 km/hour is not uncommon). The image is simplified as it doesn’t show the angle of the skillion roof. Indoors is the white space at the bottom right, and outdoors is the bottom left and top.
The first iteration of the design used a 10 mm thick structural steel strip across the top of the wall to secure the roof to the wall. This image shows the isotherms (lines connecting places of equal temperature) when it’s 22°C indoors and 0°C outdoors (typical winter temperatures here). It’s a bit difficult to see, but the indoor temperature at the roof to wall junction goes down to 9.2°C, and the outdoor temperature at the junction is 3.2°C.
This shows the same information in a false colour temperature representation You can clearly see how the indoor junction is being pulled well down below room temperature and how the heat flow ‘bulges’ toward the outdoor junction.
Big deal. Nice pictures, but what does it mean? There are two real world problems with this outcome The first is that it’s cold enough around the indoor junction to form a strip of condensation indoors all along the front wall of the house. The second is that the heat being lost here has a direct impact on how much heat is needed to maintain the indoor temperature.
If we do some very simple thermal loss calculations; at 22°C indoors and 0°C outdoors the heat loss through the steel plate is 1.1 kW, which is additional energy needed to be added to maintain the target indoor temperature.
The final design used a different arrangement with the structural strength coming from two steel angles either side of the wall to secure the roof. The 0.56 mm thick steel of the skillion roof is retained without any thermal break for structural integrity purposes, but it carries only a 50 W energy cost.
The indoor temperature at the roof to wall junction now goes down to only 19.2°C (a 10°C difference compared to the first design), and the outdoor temperature at the junction is 0.2°C (almost no difference compared to the outdoor temperature).
And the same picture with false colour temperature representation. Sigh … isn’t it gorgeous?
Let’s go back to the pea on the plate. Remember that for the same amount of tipping of the plate that the pea rolled faster when there was no gravy on the plate? Let’s push that analogue as far as it will go – super glue the pea to the plate. Tip the plate. Nothing – the pea stays exactly where it is and no heat is flowing. We’re now simulating a perfect insulator; something that does not conduct any heat at all. In the real world, we don’t have materials like that, so the ideal analogue would be a pea creeping sloooowly across the plate. Treacle or molasses anyone?
High levels of thermal insulation reduce the heat that flows through the building envelope, in turn reducing to a minimum the amount of energy needed to keep the interior warm or cool. The most important considerations for thermal insulation are how well it resists the flow of heat and how thick it is. Even the best insulators don’t do much if used in thin layers. So why is thickness important?
Time for a science experiment
Paper is only a slightly worse insulator than rock wool insulation (http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html). Take a sheet of paper (newspaper works well) and place it on a cold surface that doesn’t warm quickly (the steel side of a fridge is good). Now press your palm tightly against the paper – your palm will quickly feel the cold coming through. Now pile about 20 sheets of paper on top of each other and do the same. Your palm will initially feel cold, but over a period of a minute or so will warm up. That’s the insulation value of paper coming into play.
It’s exactly the same with a building. The thicker the insulation, the better it performs. The ideal thickness of insulation is a balance between cost of the insulations vs. the cost of the energy that would be needed to maintain comfort levels. The bigger the difference between the indoor and outdoor temperatures (irrespective of whether it’s hot or cold outdoors), the more cost effective it is to add insulation.
But the best insulation in the world is a waste if the building loses energy through thermal bridges and air leaks.
O.K! We both made it this far. Congratulations again. You’re well on the way to understanding why so many home owners are becoming advocates of the Passivehaus system.
Part 3 will, I promise, raise the odd eye-brow as I introduce the concepts of ventilations and air tightness. Yes, they seem to be mutually exclusive but, in fact, work together to create and excellent living environment in all climates.
Once again, please share your thoughts and your questions are, as always, welcome. Just scroll down to the ‘Comments’ section below.
Thanks for joining me. ‘See’ you shortly.