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|Advanced Building Insulation|
Why insulation is important in buildings
Buildings insulation are a major part of the overall energy equation, not just for individuals concerned with domestic heating bills, but for governments seeking to formulate a national energy strategy. And it is easy to see why. U-Values are how many watts will transfer across 1 square meter for each degree of temperature difference. These values do actually depend on the temperatures involved but for 'ordinary' temperatures, parhaps 25C on the inside and maybe -10C on the outside, these values can be considered as constants. A building will lose heat through all surfaces including the floor. Not just the walls, the doors and windows and the roof. If we say a typical house has a footprint of 7.5 meters by 10 meters and a wall height of 6 meters, we have 210 square meters of wall/door/window, 75 square meters of floor and 75 square meters of roof. We can ignore the roof's pitch if we assume the attic or loft is not heated and the floor of the loft is insulated. For such a house, the total surface through which heat can be transfered is 360 square meters. If we have an average U-Value of 1.0 the total energy transfered is 360 watts per degree. If the temperate outside is say -2C and inside is 23C, that is 9KW. The equivilent of a liter of gasoline per hour!
On top of this one has to take account of 'air changes per hour'. A cubic meter requires roughly 360 Joules (J, a watt for a second), to heat by one degree C. Our example house has a volume of 450 cubic meters so it will take 162KJ to heat though one degree and 4.05 MJ through 25 degrees. One KWHr is 3.6MJ. A house with typical occupancy needs about one air change per hour in order to avoid becoming 'stuffy'. Houses that are drafty have as many as 3 or even 4 air changes per hour, considerably adding to heating bills as well as discomfort.
Taken over a whole year in a climate such as that of the UK or the northern United States, it is easy to see how a house might use as much energy as a car. In our pages on solar thermal panels and heatpumpswe make the point that burning fuel to heat buildings can and should be largely avoided. The solar thermal panels provide a lot of energy for space heating while heat pumps minimize the energy needed to make good any shortfall in winter. Insulation is important because it cuts the problem off at the root. The better the insulation the lower the energy needed to manage the building's temperature however that energy is provided.
Insulation strategies for new build
Brick is a good structural material but it is a poor insulator. Breeze blocks used for the inner wall are not significantly better. Both materials are inherently thick leaving only a small thickness available for an effective insulator such as polystyrene. The walls could be thicker but only at a cost of losing internal space or external land. Wood is a far better material from an insulating prospective as it is a much better insulator than brick (0.12 to 0.15 W/mK) and can be made thiner allowing a lot more space for an insulating cavity. However brick is a more popular finish. One solution would be to replace the inner breeze block wall with blocks of foamed glass. These are structurally close to breeze blocks but are about 15 times better at insulating with a thermal conductivity of 0.045 W/mK.
Advanced window designs
Windows in their simpest form are a major weak point and are thus where the focus toward greater energy efficiency in buildings often lies. Glass is a poor insulator having a typical thermal conductivity of around 0.8 W/mK, slighly worse than brick. The problem is compounded by the fact that in general, windows tend to be thin. Glass is also a good radiator so a window pane warmed by the interior is very effective at broadcasting the heat to the outside world. But for a number of saving graces, a 5mm thick single sheet of glass would have a U value of 0.8/0.005 or 160, an absolutly outrageous value. Our not having frozen to death before we invented double glazing is largely down to the air that surrounds the glass. At 0.03 W/mk air is about 26 times better than glass as an insulator. Providing the air is reasonably still, the effective U value of the window is cut from 160 to something like 5. This effect is repeated on the outside of the glass if there is no wind but is largely destroyed even on the inside, by drafty window fitting. The emissivity of glass is not important if the glass is at a similar temperature to the outside which with its overal poor U value is inevitably the case. Another saving grace of windows is the greenhouse effect. Glass lets in the visible light but reflects back (some) the infrared from the warm interior.
Double glazing helps because the albeit thin layer of air between the panes is by and large, still. In terms of pure conductive heat loses the U value of two 4mm sheets of glass separated by a 3mm air gap would be about 9. Significantly lower than 160! Using a gas such as argon (0.016 W/mK) lowers the conductivity even further to around 5. Now however we have a warm inner pane radiating out to the outer pane and beyond. And this is why low emissivity or 'low e' coatings are important. Glass has an emissivity in the region of 0.8, making it 80% as good a radiator as a perfect black body. A low e coating can cut this to around 1.0. Such coatings not only stem radiation losses but also improve the extent to which infrared is reflected back into the room. Low e also works to our benefit in the summer, reducing the extent to which infrared from the hot outside overheats the interior.
One can now get evacuated window units. Instead of an air or argon filled gap between the panes, these have a vacuum like a thermos flask. Together with the low e coatings these windows achieve U values of the order of 0.3 - comparable to that of a foam insulated wall. To solve the obvious problem of imploding under the crushing atmospheric presure (10 tons per square meter), small steel spacers are placed ever 2cm or so. These are about 0.2mm in diameter and from a distance of 1 meter, are invisible even to those with 20/20 vision. These windows are so good that the weak point is no longer the pane(s) but the conduction paths in the frame. The tripple pane version (two vacuum layers) is even better.
If we take the UK as an example, building regulations presumably aimed at improving overall energy efficiency and environmental impact of buildings, shows limited thinking. Government agencies are in a very previleged position and can do things we would all love to do. But they miss almost every opportunity. And it is nearly always the same mistake. There is an understandable reluctance to mandate expensive new technologies when there are much cheaper, firmly established technologies that are 'reasonably good'. But an obvious question is not being asked. Is the new technology inherently expensive (i.e. contains expensive materials or has is difficult to manufacture or both) - or is it expensive purely because it does not yet benefit from economies of scale? In the general case it is the latter in which case the process should be simple. The agency asks the manufacturers how much could the new product be produced for assuming economies of scale and when is the earliest date wholesale production could be brought on stream. The agency then has a consultant engineer verify the manufacturers assesments and then a deal can be done. The goverment will limit any patents but mandate the technology the day the production lines start rolling.
Among the technologies that all new build should incorporate are roofs entirely comprising a mix of thermal vacuum and voltaic panels, argon or preferably vacuum double/tripple glazing and heat pumps. Would this cost money? Of course it would. But would it render houses unaffordable? Don't make us laugh! The high house prices in the UK today have almost nothing to do with building costs. Think about it. The modern house is different to houses built in the 50s, 60s and 70s - but not by much. Basically they contain more plastic and less brick than they did back then. Houses used to fetch around three or four times the average salary and back then just as now, builders made a profit out of building them. The cost of building materials have fallen behind salaries so houses should if anything be cheaper. The problem is land but not because land is running out. You only need to look at the map to see that. What is in short supply is land with planing permission - another grand failure of regulation.
Mandating the technologies we have listed above would not increase house prices by a single penny. When builders buy land they do their sums backwards. They start with the target price of the houses they intend to build, work out how much the building will cost, factor in a profit and then use the remainder as their maximum price to bid for the land. If all builders are mandated to spend a few thousand more on each house all that would happen is land prices would fall by a few thousand. So what have we got instead? Restricted window sizes and 'combi' boilers. Oh dear!
But new build is only a small fraction of the overall picture and would be even if the planning departments actually functioned. Although it is always going to be a political albatros to force owners to spend money on their own properties, much more could be done in this area. Certainly it would be politically easier to compell landlords to raise insulation standards and remove at a stroke, hundreds of thousands of tennants from 'energy poverty'. As for owner occupiers, the existing grant system could be massively extended by discounting property taxes to those with a certificate of energy efficiency. We still have a large number of houses that are poorly insulated. Many do not have double glazing or even loft insulation. If government is serious about the Kyoto protocol, such measures alone would provide about half of the CO2 cuts needed under that agreement. They need to do more than promote the idea of windfarms only to mire them in the red tape of their own planning laws. Our message to the UK government is simple. Be as bold with home owners as you are with car owners and take a carrot and stick approach to reduce emissions. See those things that look like dots? Well they are dots and you are supposed to join them up!