Antinuclear

With climate change, all glass building facades have had their day

All-Glass Facades Won’t Exist in Sustainable Cities, Davis Baggs, Sourceable, 13 Oct 15  “……..Glass alone cannot cover all contingencies, and the hotter the summer sun, the less successful it is. In winter in cold and even temperate climates high transmissivity (untinted) highly insulated glass units with Low-E coatings are typical and work very well. Problem is, in summer unshaded glass lets a lot of heat in, and the very factors that work for the building and occupants now works against them letting heat in and keeping it in, very efficiently overheating buildings and people all the better for the air conditioning engineers (whose fees are often based on the size of the air conditioning plants) and suppliers (who designed some of the major software tools engineers commonly use to design buildings).

So to overcome this, heat absorbing and reflective glasses were developed and to greater or lesser extent are used in all glass facades in climates where there is any bite at all in the summer sun.

Herein lies some of the problem; heat isn’t just ‘heat’. Solar energy does enter buildings by conduction (efficiently offset by insulated glass units or IGUs) from the atmosphere but the majority enters via direct and diffuse solar radiation.

If we make the glass reflective enough to stop enough solar radiation to avoid overheating of unshaded all-glass facades, we create strong rogue reflections that cause accidents and literally melt cars. So we wind the reflectiveness back and bump up the tinting levels to absorb the radiation in the façade and this is where the next problems begin…………..

The human body is up to four times more sensitive to radiant heat than any other form of heat. In fact, we are more than two times more sensitive to radiant heat than all of the other heat loss or gain pathways (convection, conduction, respiration, and evaporation) combined. The body is highly sensitive to even small changes in radiant temperatures; that’s why we love radiators, heated floors and mass surfaces in buildings to retain heat and keep us both warm and cool.

But as you tint glass, it progressively absorbs more and more heat itself and it becomes in effect a large plate radiator. The darker the tint, the more heat it both absorbs and then eventually re-radiates even when it’s double-glazed, such as in IGUs. Its important to note that this doesn’t cause the room air to heat up much, so its not picked up by the room temperature sensors of the HVAC, but it does heat people up easily and effectively, even through clothes.

Even a few degrees of radiant temperature can make a room very uncomfortable. A radiant temperature increase (or decrease) of  five degrees can make the room feel eight degrees hotter (or colder) without changing the temperature in the room…….

Let me share my experiences of a building I visited in mid-winter (January) in the desert, midway between Dubai and Abu Dhabi. It was comprised of two 12-storey office towers of azure blue (heat absorbing) reflective (silver blue mirror look to the outside) double glazed IGUs linked by a three-storey entry podium of clear IGUs……

At midday, things got even more interesting. The outside temperature had risen to 35 degrees, the inside air temperature was still 21 degrees, but the outside skin of the building in the sun measured 65 degrees and the inside skin temperatures of the blue IGU in the offices were 35.4 degrees. Interestingly, the clear insulated glass podium was an astounding 37.2 degrees – because it was not reflective like the blue IGUs, it absorbed more heat……the solar intensity in UAE at that time of year is similar to Australia in summer….

Unshaded all glass buildings as we know them now, have no place in sustainable cities of the future. https://sourceable.net/all-glass-facades-wont-exist-in-sustainable-cities/#

October 14, 2015 - Posted by Christina Macpherson | Uncategorized