Sunday, 17 July 2011


Could THIS simple idea be the answer to avoiding losing greenhouse glass in high winds?
Read on!!!!
                                                   (Click on picture to enlarge)

When we bought our first greenhouse, about 7 years ago, we selected a good quality one that was classified as suitable for ‘windy’ locations, and we opted for the toughened glass option, (mainly because we have grandchildren and didn’t want any risk of injury in the event of them falling against standard glass). We didn’t consider polycarbonate glass at the time as, in our experience, it was prone to going ‘milky’ in colour, and we also felt that it was probably too flexible for our windy location.

Guidance from the greenhouse supplier suggested that it was a good idea to apply a generous dollop of silicone mastic over some of the glass retaining clips, as this would provide extra stability for holding  the glass in place in wind prone areas, (as shown in the following picture).
                                                        (Click on picture to enlarge)

So far so good but, despite following all the advice AND doubling the number of glass retaining clips that would normally be used, we lost 4 full panes of glass, (1.350 x 610)-(approx 53” x 24”) during the very first autumn/winter gales. Toughened glass is all well and good, but it shatters into millions of tiny glass fragments, and it took nearly two full days to clear up the glass which appeared to have ‘exploded’ across the garden for a distance of over eight feet.
We duly replaced the glass, but in the next high winds we lost the same panes again.

Fed up with spending hours on hands and knees extracting glass fragments from the adjacent vegetable plots and flower borders, we relented, and decided to replace the shattered panes with polycarbonate sheet, which was easily obtained from Screwfix. It was even the correct width (610mm), and only needed a few inches cut off the length to allow it to fit.
In the following year back came the gales, and sure enough out popped the new polycarbonate sheets!
BUT.... at least we were able to pick up the polycarb panels, (that were still in one piece), and clip them back almost immediately. Job done.. clickety click, or should I say clippety clip!

What was becoming clear, was that the panes appeared to be blowing ‘outwards’, and it was mainly the panes on the corners of the greenhouse. So why was this? 
During further strong winds, I spent time in and out of the greenhouse observing what was happening to the replacement ‘polycarb’ sheets, and it soon became obvious that they were in fact blowing outwards, even on the side that the wind was coming from, due to TWO main factors –

1.       Air pressure was building up inside the greenhouse, and
      2.       External 'suction', being caused by the wind creating a low pressure at certain points around
                the corners of the greenhouse.

No greenhouse is 100% airtight and, in high wind conditions, if air is ‘forced’ into the greenhouse, through any gaps in the glazing or through top windows or louvre vents, pressure can build up within the greenhouse and this then tends to force the glass outwards. If there are also external points of ‘suction’, formed by wind turbulence, particularly around the corners, the combined result is ‘pop’ – lost glass.

I decided that it if I could create a ‘safety valve’ to allow air pressure to escape from the greenhouse, and also act as a way of reducing ‘external suction', it might just reduce the occurrence of lost glass/polycarb, and my solution has resulted in NO lost glass for the past 5 years, despite the most severe gales. (and the polycarb is as clear as the day we bought it)
On the panes that were blown out the most regularly, (now polycarb), I cut two 100mm holes, one near the top and one near the bottom, and installed Tumble Dryer vent flaps (£1.87 each from Screwfix), as per the pictures below;        (Click on pictures to enlarge)

     Place one vent near top and one near bottom.   
     Inside, the vent is held by four small nuts & bolts

The principle is quite simple......... if air pressure builds up inside the greenhouse, the vent flaps open automatically to allow the air pressure to escape (as seen in the first picture) and, in the case of external 'suction' the low pressure can't 'grab' the glass to pull it out because the flaps open and cancel out the suction effect. In normal conditions, the flaps remain closed.

Ok, so the polycarb isn't cheap, and if you haven't got some basic DIY skills you may have to get a friend to cut the holes and fit the flap vents, but when compared to the price of regularly replacing broken glass, to say nothing of the inconvenience, this has to be a very cost effective option.

Many newer greenhouses use 'glazing bar capping' rather than glazing clips, and this may be more effective at keeping the glass in place during windy conditions, but if it doesn't, installing the vent flaps may also help even if using the bar capping glazing method.