Part O no!
When I began my career as an architect in 1971 the Building Regulations comprised a single small-format paperback volume about 10 or 12mm thick and gave pretty clear guidance on the rules and ways of complying. I am now retired and very pleased I no longer have to deal with the current regs (shorthand for brevity). Over the years the scope of the regs has expanded to include topics such as energy conservation, electrical safety and, most recently, broadband provision and electric car chargers. The PDF file for the current regulations is more than 28MB and includes 18 volumes from Part A to Part S (that's the one about chargers). Not only are there far more rules than there were, covering many more factors, but their complexity has (in my opinion I must add) grown in proportion. Where there were once simple formulae for angles of light to windows or tables of limiting spans for floor joists there are now lists of other documents - British Standards and the like - which designers have to buy and wade through, and increasing dependence on computer software (more expense and expertise required) to comply with such issues as energy efficiency. The whole thing has grown to the point where it acts as a hindrance and disincentive to construction. Now for the real subject of this post, one of the more recent and, in my opinion (again) misguided 'approved documents', Part O.
Part O deals, appropriately enough, with the problem of overheating. This in itself is perfectly reasonable. This last summer has seen a lot of overheating. My last post was about measures that we have taken to reduce the problem in our own home. So it is right that the regs should address the problem but has it been done the right way?
Part L deals with energy conservation and has evolved from a few U-values for walls, roofs and windows to very complex and comprehensive rules and provisions covering fabric losses, ventilation losses, solar gain, orientation, exposure, water heating, incidental heat, and more besides, and requiring computer software to demonstrate compliance. Part O is aimed at limiting overheating of buildings by summer sunshine. Part L already deals with solar gains and allows passive solar heating to play a part in reducing energy usage in winter. The document already has a section titled 'Limiting heat gains and losses' and includes guidance on comfort cooling and mechanical ventilation. Large buildings in Britain, with high volume /surface area ratios, glass curtain walls, hundreds of occupants each with a computer, and lights burning all day have required cooling as much as heating for thirty or forty years. Now, with global warming starting to seem very real rather than just a theory, summer overheating is becoming a problem in our homes as well as our office buildings. Air conditioning in homes is commonplace in places like the US and Japan and is starting to appear in home here too. AC controls overheating and consumes energy and it is quite right that comfort cooling and mechanical ventilation are covered in Part L. So why not deal with overheating there too?
We didn't need a Part O but we have it, so let's look at it. There's not an awful lot to look at, thankfully. It targets summer overheating by solar gain - trying to limit it and to deal with it if it occurs. There are two approaches. One is termed 'dynamic thermal modelling' and follows the methodology of CIBSE's TM59. I haven't dug down into this but I imagine more computer software, training and fees will be involved. The other approach is the 'simplified method' which sounds more attractive. This really is simple and aims to limit solar gain by limiting window size and removing any excess heat mainly by opening windows.
size matters
Two tables give limits on window size depending on location (basically inner London or everywhere else, though Manchester gets a special mention), orientation, whether there are opening windows on opposite walls (cross ventilation) and whether the room has more windows than other rooms in the house (??). The limits vary from 11% to 37% of the floor area. The 37% figure is for north or east-facing windows in cross-ventilated homes which is sensible. The reasoning behind some of the 11% limits is less transparent (in my opinion of course).
Now, Part L already has rules which tend to encourage house-builders to fit the small windows seen in most recently-built housing, as windows leak much more heat then well-insulated walls. I seem to remember the regs used to have rules designed to ensure good levels of daylighting in homes, but this highly desirable aim seems to have been abandoned and rooms in modern houses are often gloomier than those my generation grew up with.
As well as limiting window sizes, the simplified method requires windows in inner London to be glazed with low-g glass or shaded by shutters or, for south-facing windows, overhangs. Don't ask me why inner London windows require more protection from the sun - all I can suggest is that large numbers of air-conditioned office blocks and hotels are shifting heat from indoors out and causing the whole city to overheat.
an open and shut casement
So, we have shaded our little windows to ensure the summer sunshine doesn't cause overheating, but if it does then two more tables set out minimum areas of opening windows. Again, location is a factor and cross-ventilation helps, but the minima range from a 4% to 13% of floor area or 55% to 95% of window area. Part O describes this approach as 'passive'. It is worth noting that a sliding sash window cannot even achieve 55% open area and even a hinged casement or top-hung window would struggle to give a free area of 95%.
Now, when Part O was introduced this might have made some sense, but this year, when temperatures were reaching 40 degrees C and exceed 30C for days on end, we were advised to keep our windows shut! If the outside is hotter than the air inside the house, opening windows is going to make things worse, not better. With the severe overheating we had a taste of this year and which is like to be worse in future, passive measures - simple ventilation - is not going to cut it. Part O says mechanical cooling should only be used where passive measures are not enough. I suggest that before many more years that will be more often than not. A further problem with relying on opening windows to deal with overheating is that this can create issues with noise, security, safety and pollution, Part O requires fall protection for windows above ground level (though this actually contradicts guidance in Part K which already deals with fall protection!) but noise, security and pollution may present insoluble problems.
sunny side up
Few house-builders pay attention to orientation. Look at a recent housing development and houses will usually be facing in all directions. Look at traditional homes - cottages, farmhouses and the like - and you will often find the principal rooms and most windows face south(ish). In winter the sun rises in the south-east, is still low in the sky in the south at midday, and sets in the south-west. Windows facing north, east or west will see little or no sun. In the middle of the day, the sun is strongest and, being low in the sky, penetrates to the backs of rooms, lighting and warming them. South-facing glazing gives useful passive solar gain when it is useful. In summer the sun rises in the north-east, rises high in the southern sky and sets in the north-west. It is low but quite strong in the east and west and strong and high in the south and overheating becomes a problem. designing homes so most glazing is on the south makes the most of passive gains in the heating season, saving energy. Windows facing other directions are not such a good idea, leaking heat in winter and risking overheating in summer.
give us a break
Part O makes no mention - none - of one of the most useful measures to avoid excessive solar gain: brise soleil - literally broken sun. This takes the form of slats or louvres arranged to shade glazing from the sun without causing too much loss of light. My last post described retractable brise soleil we have added to give shade to our south-facing kitchen windows in summer then fold out of the way to allow full daylighting in winter. The louvres extend around 60cm above the window giving full shade when the sun is at 60 degrees elevation at midday, midsummer, but, even if deployed would still allow the low winter sun to warm the space. We have other south-facing glazing but this is shaded by roof overhangs or trees. The most energy-efficient solution is to concentrate glazing on the south side to maximise passive heat gain in winter and to use brise soleil to minimise unwanted heat in summer, but Part O has nothing to say.
Given that we were advised to keep our windows shut and draw the curtains when things got bad, how do we cope when they get worse? Traditional middle eastern homes use features such as mashrabiya - screens to give privacy and shading and allow ventilation - wind-catching towers for through-ventilation, and shady courtyards with pools and fountains to cool the air. A few modern buildings incorporate ventilation 'chimneys' using natural convection - passive stack ventilation. These approaches have potential: using water for passive cooling is worth looking at. Beyond these we are faced with active cooling - air conditioning, basically.
We are moving quickly away from gas boilers to using far more efficient and climate friendly heat pumps to heat our homes in winter. But a heat pump is basically an air conditioner in reverse. Air conditioners usually cool the air but will also heat it. Similarly, heat pumps can be effectively reversed to provide cooling. A single solution can provide complete climate control - less energy-efficient than passive methods but effective in occasional extreme need. The most energy-efficient homes - designed to PassivHaus and similar standards - use heat recovery ventilation (mechanical ventilation with heat recovery - MVHR) rather than relying on extract fans and opening windows. Quiet, draught-free and very economical on power, MVHR recycles the heat that would otherwise be thrown out by extract fans (or passive stack ventilation). These low-energy homes need very little extra heat in winter and this could be provided by a low-power heat pump extracting even more heat from exhaust air to warm the fresh air supply. Making this heat pump reversible would allow it to cool the incoming air in hot weather, dumping the heat to the outside, just like air-con. More energy-efficient still would be passive water-cooling of the supply air, possibly using cool water from an underground water-harvesting tank.
I think I have thrown in enough ideas to show that Part O is desperately limited and, although relatively new, already out of date. It should be scrapped and overheating and cooling strategies should be integrated with the energy conservation and heating rules and guidance in Part L. If simple tables, formulae and rules of thumb are inadequate, easy-to-use software should be made available for free, Some computer-aided design solutions already incorporate or link with environmental design software.
Summing up, the Building Regulations have evolved to the point where rather than enabling safe, healthy, energy-efficient, future-proofed buildings they complicate and hinder the design process. Part O in particular is unfit for purpose and a rethink is needed of all aspects of the regs that impact climate control and energy conservation in order to encourage, not hinder, the design of low-energy, comfortable, well-lit homes.
Comments
Post a Comment